Spotlight on Rachael Livermore, Postdoctoral Fellow, the University of Texas at Austin

This occasional series focuses on members of the Frontier Fields team. It highlights the individuals, their jobs, and the paths they took to get to where they are today.

Portrait of astronomer Rachael Livermore

Astronomer Rachael Livermore answers questions about her role on the Frontier Fields program and the path she took to get there.

What does a typical day on the job entail?  What are your responsibilities?

There’s really no such thing as a ‘typical’ day, and that’s what makes it interesting! I travel frequently, whether to observe on ground-based telescopes, meet with collaborators, present at conferences, or give talks in other departments. My position is 100 percent research, so when I’m in the office in Austin I can spend almost all of my time analyzing data, reading and writing papers, with occasional meetings with other researchers in the department. The travel is fun but can be very tiring, so when I’m in the office I enjoy being able to put some music on and get absorbed in solving problems.

 What specifically is your educational background?

 I have a BSc in Mathematics, Physics, and Astrophysics from King’s College London, an MSc in Astronomy from the University of Sussex, and a PhD in Astronomy from Durham University, all in the UK.

 What particularly interested you in school or growing up?  What were your favorite subjects?

I was always good at math; it just came naturally and I never had to think about it too much. It didn’t interest me, though, math at school being pretty dull. It wasn’t until towards the end of high school when I started reading popular science books that introduced the more esoteric aspects of math—the nature of pi, why prime numbers are so cool, etc.—that I started to realize there was more to it than the mechanical tedium of solving equations. Math is the most fundamental thing there is, and I wish we taught more kids that.

 How did you first become interested in space?

 My mum was completely fascinated by the Moon landing, and that fascination with space must have filtered down. I remember the first time I saw the Moon through my grandfather’s telescope: seeing it as this whole other world with its hills and craters really brought home that there are entire other worlds out there.

Like many people in the field, science fiction also played a huge role in feeding my interest in space. I was a voracious reader as a kid, so my mum would pick up whatever used books she could find for me at charity shops. The first science fiction book she bought me completely blew my mind: it was Nightfall, a collection of short stories by Isaac Asimov. Still one of my favorite science fiction stories!

Portrait of astronomer Rachael Livermore as a child

As a child, Rachael was influenced by her mother’s fascination with space. Seeing the Moon through her grandfather’s telescope was a pivotal point in her young life.

Was there someone (parent, teacher, spouse, sibling, etc.) or something (book, TV show, lecture etc.) that influenced you in developing a love for what you do, or the program you’re a part of?

I was not the most well-behaved kid in school, so I have to give huge kudos to my math teacher Graham Curson for recognizing I was bored and lending me the books that started to make it interesting. It was from those that I moved into reading Stephen Hawking and Brian Greene, which is what got me into physics, a subject I had never enjoyed at school. And science fiction is what drove me towards astronomy in particular. As well as Isaac Asimov, Arthur C. Clarke was a huge influence on me. They’re both very good at highlighting how so much of what we take for granted about the world (gravity, the Sun, etc.) would be so different on any other planet, and running with those ideas to talk about how these differences would affect life. When most people think of 2001: A Space Odyssey they think of the big themes of evolution and the dangers of technology. What sticks in my mind is a throwaway line in which a young girl who grew up in a low-gravity environment expressed distaste for Earth because falling down hurts.

 Was there a particular event (e.g. lunar landing; first Shuttle flight etc.) that particularly captured your imagination and led to life changes?

 I was actually an accountant for several years before I switched careers to astronomy, and while working as an accountant I was also Treasurer of the Tolkien Society in the UK. In 2005 we held a large conference celebrating the 50th anniversary of The Lord of the Rings and one of the speakers was Kristine Larsen, an astronomer, talking about Tolkien’s lunar creation myth. Meeting someone who studied space for a living really made me reevaluate my life. Two weeks later I quit my job and moved to London to start my undergraduate degree, and I haven’t looked back.

 How did you first get started in the space business?

 Unlike a lot of people in the field, I didn’t come in with a long-term plan, having started out on a whim. But when I finished my undergraduate degree I knew I had only scratched the surface, so I applied for a Masters in Astronomy. The exposure to research with real data was what got me hooked, and before I knew what was happening I was being nudged towards applying for PhDs.

 What do you think of the Hubble results, or the impact that Hubble has on society? 

 I’m exactly the right age to have been young and impressionable when the first Hubble images came out, and they were mesmerizing! The exquisite quality of the images has captured the public’s imagination like nothing else, and it’s also turned out to be (by some metrics) the most scientifically productive telescope ever built.

 Is there a particular image or result that fascinates you?

Since I work on gravitationally lensed galaxies, I think the most fascinating image is Abell 370, the first strongly lensing galaxy cluster discovered. It was discovered about 50 years after Fritz Zwicky had suggested the idea of using galaxy clusters as lenses. It was fringe science, not something anyone expected to ever be able to observe in practice. Then in the 1980s, along came sensitive CCD cameras, and there it was. The fact that it was included in the Frontier Fields means we now have really exquisite images of this incredible cluster with its famous prominent arc. The discovery of this cluster brings together so many things: a theoretical idea proving to be right, the way developments in technology drive scientific progress, and gravitational lensing itself, which is inherently fascinating. How incredible is it that the fabric of space works in such a way as to provide gigantic natural telescopes for us?

Wearing an astronomically themed dress of her own creation, Rachael poses in front of a large picture of Abell 370.

Wearing an astronomically themed dress of her own creation, Rachael poses in front of a large picture of Abell 370, the first strongly lensing galaxy cluster discovered. It was the last of the Frontier Fields galaxy clusters to be imaged. Her dress is actually made up of the Frontier Fields, and the top front piece is Abell 370.

Are there specific parts of the program that you’re proud to have contributed to?

My main contribution has been finding the faintest, most distant galaxies. It turns out that’s really hard, because although the clusters magnify the images, you have this gigantic, super-bright cluster in the way. To find the faint background galaxies I had to develop a whole new technique for subtracting the cluster, but it turns out it works really well and I was able to find the faintest galaxies ever seen in the early Universe. 

What outside interests—e.g., hobbies, service, dreams, activities—could you share that would help others understand you better?

 I sew, and have become known for my space dresses; I’ve had several Hubble images printed on fabric and turned them into clothing – including two Hubble Frontier Fields dresses – that I wear for outreach events and at conferences. I’m also still interested in science fiction, so I make costumes for science fiction conventions, and sometimes run the conventions themselves. When I moved to the US four years ago I was excited to discover renaissance faires, which seem to be huge outdoor costume parties with jousting and fried food.

Wearing a Star Trek uniform she sewed herself, Rachael steps onto the bridge of the starship Enterprise.

In a Star Trek uniform she sewed herself, Rachael steps onto the bridge of the Starship Enterprise.

Is there anything else that you think is important for readers to know about you?

Since it was a public outreach talk that started my career in science, I try to pay that forward by doing a lot of outreach myself. I give talks in schools and co-founded Astronomy on Tap in Austin, Texas, which is a series of monthly talks in a bar. One of my favorite things to do is use science fiction as a hook to talk about science; I run a blog critiquing the science in Star Trek in excruciating detail, and I do regular movie screenings with the Alamo Drafthouse cinema at which I’ll analyze the scientific accuracy of the movie (some of these are on YouTube). Using popular culture as a starting point is a great way to get people thinking about science, and it’s meant I’ve been able to talk about Star Trek as part of my job, which is pretty great!

You can follow Rachael on Twitter at @rhaegal.

On Beginnings, at the End

by Ray A. Lucas

We recently finished taking data for the Hubble Frontier Fields project, and we’ve learned many very useful and exciting things, both about the universe and the objects in the images. We’ve also learned much in terms of technical issues in the images as well.  The recent STScI Newsletter article by Jennifer Mack and Norman Grogin recounts much of the latter. For me, however, the most fun in projects like this is usually the beginning, with the back and forth of discussions both scientific and technical about exactly what to do and how to do it. I think it is the speculative nature and the ideas flowing back and forth in the developmental phase of programs such as this that most whets my appetite for finding what we ultimately see in the images.

Having been a member of all of STScI’s internal working groups for the various community service deep field projects we have done here since the very first one, and even one of its important predecessors, it has been very interesting to me, and the most fun, to be a part of the debate, discussions, and activity involved in designing and planning these observations, and the way that process has taken place over those years. So, in this blog article, I’d like to share my own longer, and more personal context for the Frontier Fields program and those which went before.

In the beginning, the Servicing Mission 1 Early Release Observations (SM1 ERO) were a demonstration of the power of the new, optically corrected Wide Field and Planetary Camera 2 (WFPC2) and Hubble combination. One of the major goals of the particular SM1 ERO program in which I was involved was simply to go as deeply as we could in 10 orbits in a redder WFPC2 filter in what, for then, was viewed as a very deep—perhaps the deepest ever—detailed, high-resolution image of the night sky. This was in the area of a cluster at a redshift of ~0.4, and a quasar beyond it in the same field of view with a possible more distant cluster around that at a redshift of ~2.055. (Quasars are incredibly bright objects thought to be powered by supermassive black holes.) This image of galaxy cluster Abell 851, or CL0939+4713, showed that Hubble and the then-new WFPC2 were superb tools for revealing the shapes of very distant galaxies in the early universe.

After the first servicing mission to Hubble in December 1993, the newly installed Wide Field and Planetary Camera 2 (WFPC2) imaged the central portion of a remote cluster of galaxies called Abell 851, or CL 0939+4713. At the time this observation was taken, though only of 10 orbits depth in one filter, it was one of the deepest detailed optical images ever taken of the night sky. This observation was a precursor to and helped inspire the later Hubble Deep Fields and Frontier Fields and other similar work. Credit: Alan Dressler (Carnegie Institution) and NASA.

Even with the success and revelatory power of that image, it was still viewed as a very risky thing of possibly dubious value to commit many more HST orbits and staff time and effort to try a significantly even deeper field. STScI’s then-Director Bob Williams convened a panel of community experts who debated whether such a thing should be attempted, and if so, what type of field should be targeted. The idea that something should initially be tried in a generic, nominally empty deep field eventually came to the fore, but it was still seen as a possibly big gamble that might not live up to its potential for the great amount of time required. It took a courageous decision by Williams to go ahead with the project, committing a significant portion of his Director’s Discretionary time to the project.

A number of people rightly felt that they already had significant work of their own which needed pursuing and finishing and, when asked if they would be willing to take part in this original HDF experiment, declined.  However, there were still some relatively few of us who had been discussing the possibilities of this informally.  In my own case, having helped design and set up the SM1 ERO observations of CL0939+4713, I was eventually asked if it was technically feasible for us to even attempt such deep field observations.

Some members of the original Hubble Deep Field team looking at HDF images in December 1995 or early 1996. Left to right: Ray Lucas, Richard Hook, Harry Ferguson (at computer), Marc Postman, and Hans-Martin Adorf.

We performed many experiments helping to define what kinds of possibilities existed. Our experiments were, fortunately, successful, and the ultimate success of the original Hubble Deep Field ushered in a new sociological phenomenon in the field: [professional astronomical] community-service projects with high-level science products quickly released to the astronomical community, with prohibitions on internal staff use of those data and catalogs for their own scientific use for some pre-determined time.

The original 1995/1996 Hubble Deep Field WFPC2 image covered a speck of the sky only about the width of a dime at 75 feet away, or a grain of sand held at arm’s length. In this small field, Hubble uncovered a bewildering assortment of thousands of galaxies at various stages of evolution. Credit: R. Williams (STScI), the Hubble Deep Field Team and NASA/ESA.

In 1998, the Hubble Deep Field-South targeted a quasar with both imaging and spectroscopy, and included many more flanking fields and much deeper parallel observations—all in multiple cameras spanning wavelengths from long UV to infrared, including both the newer Space Telescope Imaging Spectrograph (STIS) and Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) instruments, as well as WFPC2.  Even today, I think these HDF-South observations have been underutilized, although they have now been targeted by, for example, the Multi Unit Spectroscopic Explorer (MUSE) at the Very Large Telescope (VLT) of the European Southern Observatory. This underutilization came as the community gravitated more to observations of another southern-hemisphere field, the Chandra Deep Field-South, which by then had deeper X-ray observations. Hopefully the HDF-South and its Flanking Fields will still be exploited more fully in the future.

This 1998 Hubble Deep Field-South (HDF-South) WFPC2 image was similar to that of the original Hubble Deep Field (also called Hubble Deep Field-North, or HDF-North) in many ways. This was reassuring, although it was not the deepest of the images in the HDF-South. The deepest HDF-South image was actually taken by the CCD camera on the newer Space Telescope Imaging Spectrograph (STIS) instrument, and it had a quasar in the center of its field of view that was also observed with the STIS spectrograph. Other deep observations were also simultaneously obtained with the newer Near-Infrared Camera and Multi-Object Spectrograph (NICMOS) instrument, which was also installed in 1997 at the same time as STIS. A fairly deep “STIS-on-NICMOS” image was also taken on top of the area of the deep NICMOS image, and many parallel Flanking Fields were also imaged in WFPC2, STIS, and NICMOS to shallower depth. Credit: R. Williams (STScI), the HDF-S Team, and NASA/ESA.


The HDF-South was much more complex than the original HDF-North, consisting of a very deep field centered on the quasar in the smaller field of view of the STIS CCD camera (and quasar spectra taken with the spectrographic mode of STIS), plus parallel deep WFPC2 and NICMOS images. Fairly deep images were also taken with STIS superimposed on the deep NICMOS image, with similarly deep parallel images in NICMOS and WFPC2. Finally, shallower Flanking Fields were taken with WFPC2 around and between the STIS, WFPC2, and NICMOS deep fields, and parallel images of similar depth in STIS and NICMOS were taken simultaneously, such that the entire HDF-South consisted of ~30+ fields imaged in 3 cameras across optical and infrared wavelengths, as well as quasar spectra. The STIS Deep Field with quasar in the center is near the top arrow. The NICMOS Deep Field is at lower left, and WFPC2 Deep Field is at lower right. STIS images were also taken of the NICMOS Deep Field at lower left. The extreme lower left image was the resulting NICMOS parallel, and the image at bottom center was the resulting WFPC2 parallel, all of medium depth. All the other images were shallower WFPC2 images and their associated STIS and NICMOS parallels. Credit: NASA, ESA, and Richard Hook (STECF)

Astronauts installed the Advanced Camera for Surveys (ACS) in 2002. Under then-Director Steve Beckwith, we designed the Hubble Ultra-Deep Field (in the middle of the Chandra Deep Field-South) around use of the ACS and using WFPC2 and NICMOS in parallel, creatively making the pure parallel operational system give us the then-deepest-ever detailed UV and infrared observations.

The original 2004 Hubble Ultra Deep Field, taken with the even newer, more sensitive ACS camera with a larger field of view, revealed thousands more galaxies than the earlier WFPC2 Deep Field images, in an even “deeper” core sample of the universe, cutting across billions of light-years. The snapshot includes galaxies of various distances, ages, sizes, shapes, and colors. In vibrant contrast to the rich harvest of classic spiral and elliptical galaxies, a zoo of oddball galaxies also litters the field. Some look like toothpicks or tadpoles; others like links on a bracelet. Some also appear to be interacting. These galaxies chronicle a period when the universe was still younger and more chaotic. Credit: NASA, ESA, and S. Beckwith (STScI) and the HUDF Team.

Subsequent observations with ACS and the even newer WFC3 camera have given us even greater depth and wavelength coverage at higher resolution, particularly in the infrared channel of WFC3. This led to the GO program—not an STScI community service program, but done by external observers adding to the HUDF via approval by the international peer-review committees which review proposals and recommend observations to be done—called the Extreme Ultra-Deep Field. This program combined all existing archival imaging with still more new, deep infrared observations to try to look even farther back in time.

Called the eXtreme Deep Field, or XDF, this photo above was assembled by combining 10 years of NASA Hubble Space Telescope photographs taken of a patch of sky at the center of the original Hubble Ultra Deep Field. More than 2,000 images of the same field were taken with Hubble’s two premier cameras – the Advanced Camera for Surveys and the Wide Field Camera 3, which extends Hubble’s vision into near-infrared light – and combined to make the XDF. The new full-color XDF image reaches much fainter galaxies, and includes very deep exposures in red light, enabling new studies of the earliest galaxies in the universe. The faintest galaxies are one ten-billionth the brightness of what the human eye can see. Hubble pointed at a tiny patch of southern sky in repeat visits for a total of 50 days, with a total exposure time of 2 million seconds. Credit: NASA, ESA, G. Illingworth, D. Magee, and P. Oesch (University of California, Santa Cruz), R. Bouwens (Leiden University), and the HUDF09 Team

All of this gives context to the observations which we have just recently finished: the Hubble Frontier Fields. Again convening a panel of community experts, and building on the success of large observing programs such as CLASH and CANDELS, STScI’s then-Director Matt Mountain explored a brilliant idea to use the gravitational lensing effect of massive galaxy clusters to magnify galaxies in the early universe beyond them, and to also provide a baseline for searches for higher-redshift supernovae. The plan was to come as close as is possible for Hubble to come to the bread-and-butter observations of the much-anticipated, soon-to-be launched James Webb Space Telescope in searching for some of the earliest galaxies in the distant, early universe.

The panel recommended that a group of six galaxy clusters and six adjacent parallel fields be targeted.  That was a very important development, because it also addressed in a major way a phenomenon known as cosmic variance. In this phenomenon, the large-scale structure of the universe affects observations, so that a measurement of any region of sky may differ from a measurement of a different region of sky by a considerable amount. Because the size of the fields of view of Hubble’s cameras are roughly the size of a grain of sand held at arm’s length, we’re talking about deep line-of-sight “pencil beams” in the sky when we talk about these deep fields. With the superb resolution of Hubble’s cameras, incredible detail is attained, and we can see thousands of galaxies in unprecedented detail all across their fields of view.

But given what we now know about the larger-scale structure of the universe, when it comes to the matter which we can detect, at least, there are longer filaments and areas where they intersect, and voids in between. Sometimes, the small field of view of a camera may land on a filament of galaxies, and other times in a void between filaments, or partly on a filament and partly off. Therefore, the more deep fields we observe in various different places around the sky, the more we statistically beat down the perhaps unusual or anomalous statistical effects of any one particular local environment in the area of that particular deep field as we attempt to identify the more general nature of the universe across filaments and voids, etc.

A major feature of the Hubble Frontier Fields program is the use of two fields in parallel, on-cluster and off-cluster, for each of the galaxy clusters targeted in the program, giving us both a cluster-centric and a generic parallel field at some much larger distance away from the cluster, for each cluster.  So, in effect, we get 12 fields for the price of six.  Six on-cluster fields are dominated by each galaxy cluster’s environment—something very different from a traditional deep field in terms of the physics and dynamics affecting its galaxies, and also somewhat peculiar to that cluster— and six are off-cluster, parallel fields that contain thousands of field galaxies not particularly in any cluster environment.  Given the relatively small angular size of each individual parallel field, this larger number of parallel fields especially helps to minimize the effects of cosmic variance when measurements from all other similar deep fields are combined or considered together.

This image illustrates the “footprints” of the Wide Field Camera 3 (WFC3) infrared detector, in red, and the visible-light Advanced Camera for Surveys (ACS), in blue. An instrument’s footprint is the area on the sky it can observe in one pointing. Adjacent observations were taken in tandem, or parallel. In six months, the cameras swapped places, with each observing the other’s previous location.

When Matt Mountain’s committee recommended a study of six galaxy clusters and six parallel fields, we still had to work out which clusters to observe. Under the overall leadership of Jennifer Lotz, we conducted a trade study, a common tactic in situations such as this. Various factors about each potential cluster and their advantages and disadvantages as potential targets were examined in greater detail. We tried to keep in mind anything which might bias our selections in various ways. The number of clusters was gradually winnowed down as we discussed each of them, until we had our final six. After that, we prioritized them, planning to do an initial set, and then the remainder if a mid-course review by the external panel felt that it was warranted to continue and complete the program based on results to that time.   

A professional astronomical community program of improving gravitational lensing models was also put in place, with competitive proposals for grant funding to do the work and share the improved resulting models with the community.  Also, having seen the power of public outreach in our other efforts, we involved those at STScI who are best at bringing our work to both the wider astronomical community and the public to allow them to help more meaningfully and widely bring our efforts to light. We also reviewed our prior experiences and policies and precedents from the various earlier deep field programs and debated whether any needed adjustment.  

So, now, we can also say that we’ve been lucky. The Hubble Space Telescope and the science instruments have performed well, getting us all the data we had hoped and planned to get. As we continue to work on the data, we’re now seeing the more refined versions of the Frontier Fields images which we will release to the community soon. They are indeed beautiful and interesting, and they will help the community—all of us—to better prepare for the soon-to-come James Webb Space Telescope observations.  

The James Webb Space Telescope is a large infrared telescope with a 6.5-meter primary mirror.  Scheduled for launch in October of 2018, Webb will be the premier observatory of the next decade, serving thousands of astronomers worldwide. It will study every phase in the history of our universe, ranging from the first luminous glows after the Big Bang, to the formation of solar systems capable of supporting life on planets like Earth, to the evolution of our own solar system. This illustration shows the cold side of the James Webb Space Telescope, where the mirrors and instruments are positioned. Credit: Northrop Grumman.

Webb’s much greater size than Hubble, and the much greater sensitivity of the new telescope and its detectors, will mean that Webb can make the faster exploration of more deep fields a reality. The resulting statistical advantages will give us greater confidence in the answers we find in our ongoing community studies of galaxy origins, and their formation and evolution to the forms we see in galaxies much nearer by us in space and time. It has taken a lot of work from everyone involved in our various teams of people designing and planning, implementing and scheduling the observations, and processing the data, but it has been, as with the earlier programs, a joy to see the impact spread into the community.

For me, it has been one of the great privileges and honors of my many years here at STScI to have been a part of all of our various extragalactic, deep-field, community-service programs since the original Hubble Deep Field, and to have worked with so many exceptional people who have helped to conduct these programs and produce these science products for the use of the world-wide astronomical community and the public in general.

Ray Lucas is a Research and Instrument Scientist at the Space Telescope Science Institute, where he has worked for about 32 years. His main interests in astronomy are interacting and merging galaxies and galaxy formation and evolution. He has been a member of all of STScI’s community service Deep Field project teams since the original Hubble Deep Field, particularly helping to work out details of the observations in the early stages. In addition to many other smaller programs on various galaxies, he has also been an investigator on a number of large galaxy survey programs like GOODS, HUDF05, and CANDELS, and other projects of his own. Aside from astronomy, among many other things, his passions include music. He plays fiddle, mandolin, and Celtic bouzouki, and some other instruments as well. Here is a link to his web page.

Sharing the NASA Frontier Fields Story

NASA’s Frontier Fields program has reached a critical point.  The observations by NASA’s Great Observatories (Hubble, Spitzer, and Chandra) are nearing completion, and the full data are nearly all online for astronomers (or anybody else for that matter) to study.  To herald this part of the program, the Frontier Fields were highlighted at the January American Astronomical Society (AAS) meeting in Grapevine, Texas, where over 2,500 astronomers gathered to discuss the cosmos.  A new exhibit was displayed to help tell the story of the Frontier Fields program to the science community.  We share that story with you below.


Shown here is the NASA Frontier Fields exhibit at the 229th AAS meeting, in Grapevine, Texas.  Credit: Z. Levay (STScI)


NASA’s Great Observatories Team Up to View the Distant Universe

The Frontier Fields is a program developed collaboratively by the astronomical community.  Despite the fact that observations are coming to an end, the wealth of data being added to NASA archives will ensure new discoveries for years to come.

The NASA Frontier Fields observations are providing the data for astronomers to

  • expand our understanding of how galaxies change with time
  • discover and study very distant galaxies
  • refining our mathematical models of gravitational lensing by galaxy clusters
  • explore the dark matter around galaxy clusters
  • analyze the light from supernovae
  • study the diffuse light emitted from gas within galaxy clusters
  • study how galaxy clusters change with time

Advancing the Deep Field Legacy

Chandra, Hubble, and Spitzer are building upon more than two decades of deep-field initiatives with 12 new deep fields (six galaxy cluster deep fields and six deep fields adjacent to the galaxy cluster fields).

By using Hubble, Spitzer, and Chandra to study these deep fields in different wavelengths of light, astronomers can learn a great deal about the physics of galaxy clusters, galaxy evolution, and other phenomena related to deep-field studies. Observations with Hubble provide detailed information on galaxy structure and can detect some of the faintest, most distant galaxies ever observed via gravitational lensing.  Spitzer observations help astronomers characterize the galaxies in the image, providing details on star formation and mass, for example.  High-energy Chandra X-ray images probe the histories of the giant galaxy clusters by locating regions of gas heated by the collisions of smaller galaxy sub-clusters.

An example of images taken by Hubble, Spitzer, and Chandra of the Frontier Fields galaxy cluster Abell 2744 are shown below.  These images show how astronomers can use color to highlight the type of light observed by each of NASA’s Great Observatories.

Developing Mathematical Models of the Clusters

By discovering background galaxies that are obviously multiply lensed, and measuring their distances, astronomers can use Einstein’s theory of general relativity to map out the distribution of mass (normal matter plus dark matter) for the galaxy cluster.

Once this mass distribution is known, astronomers can go back and look at regions where they expect the largest magnification of distant galaxies, again due to Einstein’s theory of general relativity.  From these calculations, astronomers can develop magnification maps that highlight the regions where Hubble is most likely able to observe the most distant galaxies.  This technique has allowed astronomers to detect ever-more distant galaxies in these fields and has helped astronomers better refine their models of mass distributions.

Initial Discoveries

In the first few years of the program, over 85 refereed publications and 4 conferences have been devoted to or based, in part, on the Frontier Fields, including a workshop at Yale in 2014 and a meeting in Hawaii in 2015.  Three types of science results are highlighted below.


Studying the Histories of Merging Galaxy Clusters

Frontier Fields observations by NASA’s Great Observatories, along with additional ground-based observations, are building our understanding of the physics of massive galaxy-cluster mergers.

Studying Distant Galaxies

By studying Hubble Space Telescope deep imaging at the locations where gravitational lensing magnifications are predicted to be high, astronomers are detecting galaxies that are up to 100 times fainter* than those observed in the famous Hubble Ultra Deep Field. Infrared observations by the Spitzer Space Telescope enable astronomers to better understand the masses, and other characteristics, of background lensed galaxies and those residing within a massive galaxy cluster.

*Author note: this has been updated from 10 times fainter than the Hubble Ultra Deep Field to 100 times fainter than the Hubble Ultra Deep Field due to recent published results you can find, here.

Serendipitous Discoveries

In 2014, a multiply lensed supernova was discovered, providing a key test of the models of gravitational lensing. As predicted by the models, a new lensed version of the supernova appeared in 2015.  Learn more about the appearance of a new lensed version of Refsdal here.

Looking to the Future

Both the James Webb Space Telescope (JWST, scheduled to launch in late 2018) and the Wide Field Infrared Survey Telescope (WFIRST, scheduled to launch in the mid-2020s) will greatly expand our understanding of galaxies and the distant universe.

JWST will build upon the success of Spitzer’s observations of the infrared universe with enhanced clarity and sensitivity, probing deeper into the universe than ever before.  Due to the expansion of the universe, light from the most distant galaxies are shifted to redder wavelengths, moving beyond the visible spectrum and into infrared light.  One of JWST’s primary science goals is to observe these infant galaxies at the edge of the observable universe.

Imagine having a Hubble-class telescope that can observe in the infrared and see greater than an order of magnitude more of the sky with each observation.  WFIRST’s expansive field-of-view – 100 times wider than Hubble’s – will allow for new ground-breaking surveys of the deep universe.

The Hunt for Jellyfish Galaxies in the Frontier Fields

Jellyfish galaxies, exotic galaxies with “tentacles” made of stars and gas, appear as though they are swimming through space. So far, astronomers studying the Frontier Fields have found several of these strange galaxies, and they are currently combing through the mountains of data to find even more.

Sometimes also known as “parachute galaxies” or “comet galaxies,” jellyfish galaxies form when spiral galaxies collide with galaxy clusters. When the cold gas from an approaching spiral hits the hot gas from a galaxy cluster, the stars continue on, but the collision blasts the cold gas out of the galaxy in trailing tails, or “tentacles.” Bursts of stars form in these streamers, sparked by the shock of cold gas hitting hot gas. The tentacles, with their knots of newborn stars, trace the path of the colliding, compressed gas. Eventually, these jellyfish galaxies are thought to settle into elliptical galaxies.

Three examples of jellyfish galaxies in the Frontier Fields. In each image, the telltale, trailing “tentacles” of stars and gas are present. The left and right galaxies are from galaxy cluster Abell 2744. The middle galaxy resides in galaxy cluster Abell S1063.

Some examples of jellyfish galaxies in the Frontier Fields. In each image, note the telltale, trailing “tentacles” of stars and gas. The left and right galaxies are from galaxy cluster Abell 2744. The middle galaxy resides in galaxy cluster Abell S1063.

Jellyfish galaxies are sometimes also seen in less massive groups of galaxies. Their characteristic shape is, however, usually much more pronounced for spirals falling into massive galaxy clusters, because the gas they encounter there is denser, and because they move faster due to the stronger gravitational pull of the cluster. The higher speed results in a more energetic collision that, in turn, increases the pressure that strips the infalling galaxy of its cold gas and triggers widespread star formation.

Astronomers have studied similar interactions in detail in nearby galaxy clusters but do not fully understand the much more violent process that creates jellyfish galaxies in very massive clusters. If the cold galactic gas is stripped very quickly these collisions could be the primary way by which spiral galaxies are transformed into ellipticals. Unfortunately, because the phenomenon is over so quickly, it is very difficult to observe. One expert on jellyfish galaxies—Dr. Harald Ebeling of the Institute for Astronomy at the University of Hawaii—explains that this is why astronomers are looking at extremely massive clusters, such as those in the Frontier Fields, in their search for a large sample of these galaxies.

Aside from helping to explain why elliptical galaxies are so common in the universe, jellyfish galaxies capture the process of galaxy/gas collisions in action. Their trailing, star-forming tentacles may also explain the presence of “orphan” stars that do not belong to any galaxy.

The work to uncover the secrets of the Frontier Fields goes on. Stay tuned for more exciting news on jellyfish galaxies and other oddities as scientists continue to study the vast amount of data collected in the Frontier Fields.

A Deep View Down Broadway

Abell 2744 Parallel Deep Field from the Hubble Frontier Fields Project Credit: NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

Abell 2744 Parallel Deep Field from the Hubble Frontier Fields Project
Credit: NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

[Note: this blog post also appears on the Hubble’s Universe Unfiltered blog.]

One of the more philosophical concepts that astronomers have to deal with on an everyday basis is the commingling of space and time in astronomical images.

The underlying idea is straightforward. The speed of light is finite. Light from a star or nebula or galaxy takes a measurable amount of time to cross the space between it and us. Hence, the light we see now left that object at some previous time. We view astronomical objects as they were in the past. As I like to say, looking out in space is also looking back in time.

The implications of this maxim are considerable, especially in dealing with the deep field images from Hubble (see the accompanying image of the Abell 2744 Parallel deep field). Such images contain a wonderful assortment of galaxies, with a few stars here and there. Each object is at a different position in space, both in the two-dimensional sense of a different position within the image and in the three-dimensional sense of being at a different distance from Earth. Further, objects at different distances are seen at different times in the past. Hence, astronomers must examine these deep field images in four-dimensional space-time.

Tackling the expanse of space and time in these images can be mind-boggling. We’ll start with the stars, which are easier to understand. All the stars are local, within our Milky Way galaxy. These stars are generally hundreds to thousands of light-years away. The light we observe today might have left the star while the pyramids of Egypt were being built. Because stars don’t change appreciably on scales of thousands of years, stars in deep fields are just like stars in other astronomical images.

The galaxies, however, stretch much farther into space. The nearest are many millions of light-years away, while the most distant are around ten billion light-years away. Galaxies don’t change much on million-year timescales. For example, it takes over 200 million years for our Sun to orbit once within our galaxy. Even though the light may have left a galaxy when dinosaurs first started to dominate our planet, the same galaxy would look similar today. Thus, the nearby galaxies in these images are comparable to local galaxies.

Given billions of years, however, galaxies do change, and these deep field images provide compelling evidence. Distant galaxies do not have the standard spiral and elliptical shapes. They are often elongated, have bright spots of star formation, and are much smaller in size. We see galaxies as they were before the Sun, Earth, and the solar system formed. We study the development of galaxies over time to see how they form and grow. The perplexing point is that, for any given galaxy in the image, there is no distinct visual indicator of its distance in space or time. The layers of the universe are jumbled together across the image, and it is a grand puzzle of cosmology to sort them out.

The usual method to determine distances, and therefore times, is to measure the cosmological redshift of each galaxy. That concept has been discussed in a Frontier Fields blog post by Dr. Brandon Lawton: “Light Detectives: Using Color to Estimate Distance”. Thus, I’d like to take this essay in a different direction.

The Manhattan Deep Field

When discussing the cosmic mixture of space-time with an artist visiting from Spain, I happened upon a novel idea for a human-centric analogy.

Imagine you are in New York City, specifically Times Square in Manhattan. You look down Broadway to the southern end of the island about 4 miles away. If the speed of light were extremely slow, traveling only one mile per century, what would you see?

Each mile down Broadway would represent one hundred years of New York’s history. Each block would be 5 to 10 years earlier in the development of the metropolis.

A quarter of a mile away, the southern end of the theater district would appear as it did in the early 1990s when “Miss Saigon” came to Broadway. Only a few blocks farther would be the disco era and the civil unrest of the 1960s, then the World War II years and the Great Depression.

The Empire State Building, about a mile away, would vanish, as it was not built until 1931. At a similar distance, Madison Square Garden would be seen hosting heavyweight boxing matches in its original building, before the demolition and re-construction in the late 1920s.

Progressing another mile down Broadway to Union Square would travel back past the Civil War, Tammany Hall politics, economic growth fostered by the Erie Canal, and Alexander Hamilton’s original run on the New York stage.

The mile beyond to the SoHo district progresses through the times of New York as the capital of the United States, the Revolutionary War, the founding of Columbia University, and the importation of slaves by the Dutch West Indies Company.

The final mile to Battery Park leads through the colonial era alternately dominated by Dutch or English foreign powers, past the garrison of Fort Amsterdam, to the island’s Native American roots and the initial explorations by Henry Hudson.

A “slow speed of light” view from Times Square would lay out the entire history of the city of New York in a single view. The commingling of space and time would make it the historian’s exceptional equivalent of the astronomer’s standard observation: a deep view down Broadway.

This idea of a time-warped view of New York provides an analogy to what Hubble uncovers: the history of galaxies compressed and jumbled within each deep field. Perhaps it can help you to look at these images from that requisite four-dimensional perspective. These deep field images are truly a trip down memory lane.

It Takes a Team

There is no denying that the history we tell about science is full of achievements often credited to individual efforts.  The reality, of course, is that scientific achievements are not done alone or in intellectual vacuums.

Standing on the Shoulders of Giants

Astronomer Edwin Hubble, for example, built upon the ideas of other astronomers when he made his landmark discovery in 1923 that the faint spiral nebulae observed in the sky were actually other galaxies outside our Milky Way.  This surprising finding greatly expanded our understanding of the size of the universe.

Still from Hubblecast episode 89: Edwin Hubble

This is a still image from Hubblecast 89, which talks about the life of Edwin Hubble.  Credit: NASA & ESA

Before Hubble’s discovery, scientists were embroiled in a fierce debate about the nature of these nebulae.  Some, most prominently astronomer Harlow Shapley, believed that these nebulae were parts of our own Milky Way galaxy.  Others, like Heber Curtis, posited that the Milky Way galaxy was smaller than suggested by Harlow Shapley, and these nebulae were likely entire galaxies outside of the Milky Way.  This scientific disagreement was brought to the fore during a public debate between Curtis and Shapley in 1920.

It was not until 1923 when Edwin Hubble observed a cepheid variable star in one such nebulae that the debate was quickly settled.  Hubble determined that the cepheid variable he was observing was very far away – much too far away to be a part of the Milky Way galaxy.  In fact, he had discovered the variable star resided in what we now know to be our neighboring Andromeda galaxy.  This put to rest the debate vociferously argued by Shapley and Curtis.

Cepheid variable stars are stars whose intrinsic brightnesses change with time by a known amount. This makes them great “standard candles” to calculate their distances.  If you know you are observing a 60-watt light bulb, you can calculate the distance to the light bulb based on the amount of light you observe – the  fainter the 60-watt light bulb appears, the farther away it is.

The key to Hubble’s discovery was the knowledge that we could determine a cepheid variable’s intrinsic brightness based off of its observed periodicity, which is the amount of time the variable star takes to go from maximum brightness to minimum brightness and back to maximum brightness.  Hubble could not make his discovery without this background information, which, as it turns out, was first published in 1912 by astronomer Henrietta Swan Leavitt.  Henrietta was not given proper credit for this monumental discovery at the time, but there is now no doubt that her efforts paved the way for our modern understanding of stars and distances in the cosmos.


Picture of astronomer Henrietta Swan Leavitt taken before 1921.

Astronomy, like all sciences, is dependent on building upon our scaffolded knowledge to further our understanding into new realms of the unknown.  It also depends upon teams of dedicated individuals working together.  Edwin Hubble, a premiere astronomer of the early 20th century, built upon the discoveries of prior scientists and engineers.  He also depended upon the support of his assistant and the staff of the Mt. Wilson Observatory, where he conducted many of his observations.

The Frontier Fields: A Team of Professionals Building Upon the Successes of Prior Programs

Today, astronomy is increasingly relying on larger projects that require teams of men and women with diverse skill sets, including the Hubble Frontier Fields program.  Frontier Fields was conceived following the successes of prior Hubble deep-field programs.  These include the Hubble Deep Field, Hubble Ultra Deep Field, CANDELS, and in particular, CLASH – which helped build our understanding of gravitational lensing around galaxy clusters.  The general Frontier Fields program also both benefited from, and enhanced, our understanding of mathematical models that predict how light from distant galaxies will be lensed by foreground massive clusters.  Of course, all of the deep-field studies are possible because of the work of prior luminaries such as Edwin Hubble, Henrietta Leavitt, and Albert Einstein.

In July 2016, the Hubble Frontier Fields team was given the AURA team award.  AURA – the Association of Universities for Research in Astronomy – operates the Space Telescope Science Institute (STScI – the science operations center for Hubble) for NASA.

"The STScI Frontier Fields team receives the 2016 AURA team award for its
unparalleled efforts in implementing the Hubble Frontier Fields Director's
Discretionary program and providing rapid [astronomical] community access to
high-level data products generated from the observations." - AURA

The full list of recipients of the AURA award can be found by clicking the link below.


2016 AURA Team  Award - STScI Frontier Fields

Some of the recipients of the 2016 AURA team award.  The team received the award in July 2016 for the Hubble Frontier Fields program, which began in 2013.  Credit: P. Jeffries/STScI.

In addition to the awardees, there is also support from the STScI directorate (Ken Sembach and Neill Reid).

It should be noted that the NASA Frontier Fields program is bigger than just the core Hubble Frontier Fields program at STScI.  There are also teams of people working with NASA’s other Great Observatories, the Chandra X-ray Observatory and the Spitzer Space Telescope, to acquire images of these fields in invisible X-ray and infrared light.  There are teams of astronomers proposing for follow-up observations of the Frontier Fields using many ground-based observatories in radio, millimeter, infrared, and visible light.  In addition, there are the astronomers and mathematical modelers who are taking this publicly available data and using it to broaden our understanding of the physics of the cosmos.

Science truly is a team sport.

Beyond the Frontier Fields: How JWST Will Push the Science to a New Frontier

The Frontier Fields Project has been an ambitious campaign to see deep into our universe. Gravitational lensing, as used by the Frontier Fields Project, enables Hubble to see fainter and more-distant galaxies than would otherwise be possible. These images push to the very limits of how deeply Hubble can see out into space.

Hubble, Spitzer, Chandra, and other observatories are doing cutting-edge science through the Frontier Fields Project, but there’s a challenge. Even though leveraging gravitational lensing has allowed astronomers to see objects that otherwise could not be detected with today’s telescopes, the technique still isn’t enough to see the most distant galaxies. As the universe expands, light gets stretched into longer and longer wavelengths, beyond the visible and near-infrared wavelengths Hubble can detect. To see the most distant galaxies, one needs a space telescope with Hubble’s keen resolution, but at infrared wavelengths.

That infrared telescope is the James Webb Space Telescope, slated to launch in October 2018. It has a mirror 6.5 meters (21 feet) across, can observe wavelengths up to 10 times longer than Hubble can observe, and is the mission that will detect and study the first appearances of galaxies in the universe.full_jwst_hst_mirror_comparison

Figure 1: Webb will have a 6.5-meter-diameter primary mirror, which would give it a significant larger collecting area than the mirrors available on the current generation of space telescopes. Hubble’s mirror is a much smaller 2.4 meters in diameter, and its corresponding collecting area is 4.5 square meters, giving Webb around seven times more collecting area! Webb’s field of view is more than 15 times larger than the NICMOS near-infrared camera on Hubble. It also will have significantly better spatial resolution than is available with the infrared Spitzer Space Telescope. Credit: NASA.

Observations of the early universe are still incomplete. To build the full cosmological history of our universe, we need to see how the first stars and galaxies formed, and how those galaxies evolved into the grand structures we see today.


Looking back in time to the first light in the universe:

Astronomers use light to explore the universe, but there are pieces of our universe’s early history where there wasn’t much light. The era of the universe called the “Dark Ages” is as mysterious as its name implies. Shortly after the Big Bang, our universe was filled with glowing plasma, or ionized gas. As the universe cooled and expanded, electrons and protons began to bind together to form neutral hydrogen atoms (one proton and one electron each). The last of the light from the Big Bang escaped (becoming what we now detect as the Cosmic Microwave Background). The universe would have been a dark place, with no sources of light to reveal this cooling, neutral hydrogen gas.

Some of that gas would have begun coalescing into dense clumps, pulled together by gravity. As these clumps grew larger, they would become stars and eventually galaxies. Slowly, starlight would begin to shine in the universe. Eventually, as the early stars grew in numbers and brightness, they would have emitted enough ultraviolet light to “reionize” the universe by stripping electrons off neutral hydrogen atoms, leaving behind individual protons. This process created a hot plasma of free electrons and protons. At this point, the light from star and galaxy formation could travel freely across space and illuminate the universe. It is important to note here, astronomers are currently unsure whether the energy responsible for reionization came from stars in the early-forming galaxies; rather, it might have come from hot gas surrounding massive black holes or some even more exotic source such as decaying dark matter.

The universe’s first stars, believed to be 30 to 300 times as massive as our Sun and millions of times as bright, would have burned for only a few million years before dying in tremendous explosions, or “supernovae.” These explosions spewed the recently manufactured chemical elements of stars outward into the universe before the expiring stars collapsed into black holes.

Astronomers know the universe became reionized because when they look back at quasars — incredibly bright objects thought to be powered by supermassive black holes — in the distant universe, they don’t see the dimming of their light that would occur if the light passed through a fog of neutral hydrogen gas. While they find clouds of neutral hydrogen gas, they see almost no signs of neutral hydrogen gas in the matter located in the space between galaxies. This means that at some point the matter was reionized. Exactly when this occurred is one of the questions Webb will help answer, by looking for glimpses of very distant objects still dimmed by neutral hydrogen gas.

Much remains to be uncovered about the time of reionization. The universe right after the Big Bang would have consisted of hydrogen, helium, and a small amount of lithium. But the stars we see today also contain heavier elements — elements that are created inside stars. So how did those first stars form from such limited ingredients? Webb may not be able to see the very first stars of the Dark Ages, but it’ll witness the generation of stars immediately following, and analyze the kinds of materials they contain.

Webb’s ability to see the infrared light from the most distant objects in the universe will allow it to truly identify the sources that gave rise to reionization. For the first time, we will be able to see the stars and quasars that unleashed enough energy to illuminate the universe again.


Figure 2: JWST will be able to see back to when the first bright objects (stars and galaxies) were forming in the early universe. Credit: STScI.


Early Galaxies:

Webb will also show us how early galaxies formed from those first clumps of stars. Scientists suspect the black holes born from the explosions of the earliest stars (supernovae) devoured gas and stars around them, becoming the extremely bright objects called “mini-quasars.” The mini-quasars, in turn, may have grown and merged to become the huge black holes found in the centers of present-day galaxies. Webb will try to find and understand these supernovae and mini-quasars to put theories of early galaxy formation to the test. Do all early galaxies have these mini-quasars or only some? These regions give off infrared light as the gas around them cools, allowing Webb to glean information about how mini-quasars in the early universe work — how hot they are, for instance, and how dense.

Webb will show us whether the first galaxies formed along lines and webs of dark matter, as expected, and when. Right now we know the first galaxies formed anywhere from 378,000 years to 1 billion years after the Big Bang. Many models have been created to explain which era gave rise to galaxies, but Webb will pinpoint the precise time period.

Hubble is known for its deep-field images, which capture slices of the universe throughout time. But these images stop at the point beyond which Hubble’s vision cannot reach. Webb will fill in the gaps in these images, extending them back to the Dark Ages. Working together, Hubble and Webb will help us visualize much more of the universe than we ever have before, creating for us an unprecedented picture that stretches from the current day to the beginning of the recognizable universe.


Figure 3: This illustration shows the cold side of the Webb telescope, where the mirrors and instruments are positioned. Credit: Northrop Grumman.



The Final Frontier of the Universe

[Note: this article is cross-posted on the Hubble’s Universe Unfiltered blog.]


Gravitational lensing in galaxy cluster Abell S1063 Credit: NASA, ESA, and J. Lotz (STScI)

Fifty years ago, in 1966, the Star Trek television series debuted. Given the incredible longevity of the franchise, it seems remarkable that the original television series only lasted three seasons.

Each episode famously began with the words “Space: the final frontier.” To me, those thoughts evoke an idea of staring into the night sky and yearning to know what is out there. They succinctly capture an innate desire for exploration, adventure, and understanding. Such passions are the same ones that drive astronomers to decipher the universe through science.

While Captain Kirk and company could make a physical voyage into interstellar space, our technology has (so far) only taken humans to the Moon and sent our probes across the solar system. For the rest of the cosmos, we must embark on an intellectual journey. Telescopes like Hubble are the vehicles that bring the universe to us.

To explore remote destinations, the Enterprise relied upon a faster-than-light warp drive. Astronomy, in turn, can take advantage of gravitational warps in space-time to boost the light of distant galaxies. Large clusters of galaxies are so massive that, under the dictates of general relativity, they warp the space around them. Light that travels through that warped space is redirected, distorted, and amplified by this “gravitational lensing.”

Gravitational lensing enables Hubble to see fainter and more-distant galaxies than would otherwise be possible. It is the essential “warp factor” that motivates the Frontier Fields project, one of the largest Hubble observation programs ever. The “frontier” in the name of the project reflects that these images will push to the very limits of how deeply Hubble can see out into space.

But is this the “final frontier” of astronomy? Not yet.

Abell S1063 Parallel Field - This deep galaxy image is of a random field located near the galaxy cluster Abell S1063. As part of the Frontier Fields Project, while one of Hubble's instruments was observing the cluster, another instrument observed this field in parallel. These deep fields provide invaluable images and statistics about galaxies stretching toward the edge of the observable universe.

Abell S1063 Parallel Field – This deep galaxy image is of a random field located near the galaxy cluster Abell S1063. As part of the Frontier Fields Project, while one of Hubble’s instruments was observing the cluster, another instrument observed this field in parallel. These deep fields provide invaluable images and statistics about galaxies stretching toward the edge of the observable universe.

The expanding universe stretches the light that travels across it. Light from very distant galaxies travels across the expanding universe for so long that it becomes stretched beyond the visible and near-infrared wavelengths Hubble can detect. To see the most distant galaxies, one needs a space telescope with Hubble’s keen resolution, but at infrared wavelengths.

In what may have been an homage to the Star Trek television series with Captain Picard, the project for such a telescope was originally called the “Next Generation Space Telescope.” Today we know it as the James Webb Space Telescope, and it is slated to launch in October 2018. Webb has a mirror 6.5 meters (21 feet) across, can observe wavelengths up to ten times longer than Hubble can observe, and is the mission that will detect and study the first appearances of galaxies in the universe.

In the Star Trek adventures, Federation starships explore our galaxy, and much of that only within our local quadrant. Astronomical observatories do the same for scientific studies of planets, stars, and nebulae in our Milky Way; and go beyond to galaxies across millions and billions of light-years of space. Telescopes like Hubble and Webb carry that investigation yet further, past giant clusters of galaxies, and out to the deepest reaches of the cosmos. With deference to Gene Roddenberry, one might say “Space telescopes: the final frontier of the universe.”

Telescopes Team up to View Cosmic Collisions

NASA’s Frontier Fields is a program to capture new deep-field images across the electromagnetic spectrum, from X-rays to infrared light.  NASA’s Great Observatories — the Hubble Space Telescope, Chandra X-ray Observatory, and Spitzer Space Telescope — are taking the lead on this ambitious effort. Other observatories around the world, including the Jansky Very Large Array (JVLA) in New Mexico, which observes radio light, also contribute considerable time to observing the new deep fields.

A new Frontier Fields release from the Chandra X-ray Center highlights the energetic chaos that occurs when massive galaxy clusters collide. The two new images combine data from the Chandra X-ray Observatory, the Hubble Space Telescope, and the JVLA radio dishes. Astronomers are interested in understanding how merging galaxy clusters grow with time and what happens to the galaxies, their gas, and the associated, enigmatic dark matter.

The images of these galaxy clusters (MACS J0416 and MACS J0717) are described below. Read the full release from the Chandra X-ray Center here.

MACS J0416


The galaxy cluster MACS J0416 seen in X-rays (blue), visible light (red, green, and blue), and radio light (pink), taken by the Chandra X-ray Observatory, Hubble Space Telescope, and Jansky Very Large Array, respectively.  Credit: X-ray: NASA/CXC/SAO/G. Ogrean et al.; Optical: NASA/STScI; Radio: NRAO/AUI/NSF.

The object known as MACS J0416 is actually composed of two clusters of galaxies that will eventually merge to create a single larger massive galaxy cluster.  The image of MACS J0416 contains Chandra X-ray data (blue), Hubble Space Telescope data (red, green, and blue), and a halo of radio light imaged by the JVLA (pink).

According to a paper published in The Astrophysical Journal, the cores of the two galaxy clusters have likely not passed through each other yet, indicating an early phase of their merger.

Astronomers discovered this by studying the cluster’s appearance in visible and X-ray light.  Hubble’s visible-light images show both the galaxies themselves and their gravitational lensing effects, helping us pinpoint the location of dark matter in the cluster. X-ray observations from Chandra show us the location of the heated gas. In MACS J0416, the galaxies and their dark matter are still lined up with the heated gas, meaning their merger has not progressed very far yet. In other observations of merging galaxy clusters, such as the Bullet Cluster, gas heated by the shock of collision eventually separates from the dark matter.

MACS J0717

Galaxy cluster MACS J0717.5+3745 with dark matter map

The galaxy cluster MACS J0717 seen in X-rays (blue), visible light (red, green, and blue), and radio light (pink), taken by the Chandra X-ray Observatory, Hubble Space Telescope, and Jansky Very Large Array, respectively.  Credit: X-ray: NASA/CXC/SAO/G. Ogrean et al.; Optical: NASA/STScI; Radio: NRAO/AUI/NSF.

The massive galaxy cluster MACS J0717 results from a merger of four clusters of galaxies. The image of MACS J0717 contains Chandra X-ray data (blue), Hubble Space Telescope data (red, green, and blue), and JVLA radio data (pink).  Unlike MACS J0416, MACS J0717 appears to have been merging for quite some time. The evidence of merging includes the separated knots of X-rays (blue) formed by the collision of high concentrations of gas, and the giant arcs of radio emission (pink) stretched and distorted by the merger.

MACS J0717 is also the largest known cosmic lens, and thus a prime candidate for observing distant objects magnified by gravitational lensing. The galaxy clusters in MACS J0717 are still merging and are not yet confined to a smaller area — leaving a large total mass over a relatively large area of the sky. This large gravitational lens can magnify and uncover galaxies of the early universe, a key goal of the Frontier Fields project.

Often, observations of these distant, young galaxies only capture the brightest objects. But observations of MACS J0717 demonstrate how Frontier Fields can be used to view some of the universe’s more ordinary early galaxies. In a paper published in The Astrophysical Journal, astronomers discovered seven gravitationally lensed radio sources in MACS J0717. Many of these galaxies would not be observable without the benefit of magnification due to gravitational lensing. The gravitational lensing of massive clusters in radio waves provides a new view of these radio sources, which are thought to be common — but not well-studied — star-forming galaxies in the early universe.

Hubble has also observed distant galaxies using gravitational lensing.  An example is noted here. By using a combination of telescopes, and a combination of different wavelength observations, the Frontier Fields project is providing a broader and deeper view into the galaxies of the early universe.

The Whirlpool Galaxy Seen Through a Cosmic Lens

The Frontier Fields images, while beautiful, aren’t all that easy to comprehend to eyes outside the astronomy community. Look at them and you see streaks of light and blurry smudges mixed into a field of obvious galaxies. It can be difficult to interpret the distortions that occur as light from distant galaxies becomes magnified and bent by the vast mass of the Frontier Fields’ galactic clusters.

So here’s an interesting thought experiment. What if we could take a well-known galaxy and put it behind one of our Frontier Fields galaxy clusters? What would that look like?

Thanks to Dr. Rachael Livermore of the University of Texas at Austin and Dr. Frank Summers of the Space Telescope Science Institute, you can see for yourself. In this video simulation, the Whirlpool Galaxy, also known as M51, sweeps behind the Frontier Fields galaxy cluster Abell 2744. As it moves, the gravity of the galaxy cluster distorts the light of the Whirlpool, warping and magnifying and even multiplying its image.

Obviously, this isn’t a realistic video — galaxies don’t just take jaunts through the cosmos. But it illustrates how our image of the Whirlpool would change depending on where it was placed behind the galaxy cluster. Livermore used the Whirlpool Galaxy for this video because it’s a well-known, popular Hubble image, easily recognizable through the distortions that happen at different locations in the lensing cluster.

Take a look. After the intro, the image on the left of the dotted line shows the location of the Whirlpool behind the cluster, while the image on the right shows the lensing distortion underway.

In this simulation, we’ve moved the Whirlpool to a distance astronomers refer to as redshift 2. That far back, it would be so distant that the light we’re seeing from it would have started traveling away from the galaxy when the universe was just a quarter of its current age. If the Whirlpool were that far away in real life, its light would take 10 billion years to reach Earth.

Note that this isn’t how the Whirlpool would really appear at that distance. At such a distance, all we would be able to make out is the vivid central bulge of stars. But for the purpose of this illustration, the whole galaxy has been kept artificially bright.

The most impressive distortions occur as the Whirlpool passes behind the center of the galaxy cluster, with multiple, stretched, distorted images of the galaxy appearing. At this point, the light of the Whirlpool beaming toward Earth bends to go around the cluster, but can go either left or right. There’s no preference, so some of it goes one way, and some goes another, and we get many images of the same galaxy.

This location is ideal for astronomers, because as you can see in this illustration, the images become both stretched and magnified, allowing the galaxy structure to be seen in greater detail. Furthermore, because a gravitational lens acts much as a telescope lens, more light is focused our way, making the galaxies brighter.

This, Livermore notes, is a primary reason why astronomers are interested in these galaxy clusters – the chance to see the distant background galaxies in so much greater detail than Hubble would be able to produce on its own.