A Stellar Explosion Seen Through a Lumpy Cosmic Lens

Sometimes in astronomy, never-before-seen phenomena are predicted years before they are observed.  Using Hubble to observe one of the Frontier Fields, astronomers spotted such an event in November 2014. Light from a distant, dying, massive star, known as a supernova, was observed in four locations on the sky due to the light-bending effects of gravitational lensing. This is just over 50 years after a Norwegian astronomer, Sjur Refsdal, predicted this phenomenon in 1964. To honor this pioneering astronomer’s prediction, the supernova has been named supernova Refsdal.

Hubble image of the galaxy cluster MACS J1149 in visible and infrared light.  Inset: The spiral arm of a distant spiral galaxy is lensed multiple times, not only by the collective mass of the galaxy cluster MACS J1149, but also by a single ellilptical galaxy in the cluster.  The supernova is highlighted and observed in four different locations on the sky.  Credit:

The Hubble image of the galaxy cluster MACS J1149 in visible and infrared light. The distant spiral galaxy is lensed multiple times by the collective mass of the galaxy cluster MACS J1149, but a small part of it — namely the spiral arm in the distant spiral galaxy where the supernova exploded [inset image] — is also locally lensed four times by a single elliptical galaxy in the cluster. The supernova, highlighted by arrows, is observed in four locations on the sky.
Credit: NASA, ESA, and S. Rodney (JHU) and the FrontierSN team; T. Treu (UCLA), P. Kelly (UC Berkeley) and the GLASS team; J. Lotz (STScI) and the Frontier Fields team; M. Postman (STScI) and the CLASH team; and Z. Levay (STScI).

The lumpy cosmic lens

After the light left the distant supernova, it traversed the cosmos until it reached the gravitational influence of the massive galaxy cluster MACS J1149. The extreme mass of MACS J1149, most of which is in the form of invisible dark matter, curves or bends space. Light generally follows a straight line, but in the presence of curved space light will follow the curvature. Much like the way a glass lens redirects and amplifies light, gravitational lensing from the curvature of space also redirects and amplifies the light from distant objects. We observe the four images of the same supernova on different parts of the sky because the light from that supernova took slightly varying paths to reach us. Some of the light from the supernova was originally traveling in directions that would never reach Hubble’s mirror, but the curvature of space redirected those light paths towards the telescope.

But wait, it gets even stranger!

The light from the distant supernova is traversing various paths through the curved space of MACS J1149. Those paths have slightly different lengths. The light from the four observed images of the same supernova traveled for about 9.3 billion years, only to arrive at Hubble’s mirror a mere days or weeks apart.

That is not all. The four observed images of the supernova appear on just one of multiple gravitationally lensed images of the background host spiral galaxy. That particular image of the distant spiral galaxy happens to fall directly behind an elliptical galaxy that is a member of the MACS J1149 galaxy cluster (the yellow-white elliptical shape in the center of the inset image above). The elliptical galaxy further lenses the supernova into the four versions we observe. This is a commonly observed effect of gravitational lensing that depends on the observer’s view of the gravitationally lensed light, and is often referred to as an Einstein Cross.

But there are additional lensed versions of the distant host spiral galaxy in the image. Did we observe the same supernova in those other lensed versions of the host galaxy? Astronomers believe we may have missed the supernova from one of the lensed versions of the host galaxy by about 20 years. Due to the curvature of space, its path was slightly shorter. However, they expect that we should observe the supernova in another lensed version of the host spiral galaxy some time within the next five years. The image and accompanying video, below, highlight the varying light travel times of supernova Refsdal.

In this Hubble image, the  expected arrival time of the light from the supernova is highlighted in the lensed versions of the background spiral galaxy.  Credit:

Shown here is the combined visible and infrared view of the galaxy cluster MACS J1149. In this Hubble image, the lensed images of the background spiral galaxy are highlighted. The expected arrival times of the light from the supernova are also shown.
Credit: NASA, ESA, and S. Rodney (JHU) and the FrontierSN team; T. Treu (UCLA), P. Kelly (UC Berkeley) and the GLASS team; J. Lotz (STScI) and the Frontier Fields team; M. Postman (STScI) and the CLASH team; and Z. Levay (STScI).


The video above illustrates the varying light-travel times of the distant supernova as the light traverses around the lumpy space within the galaxy cluster MACS J1149.  Credit: NASA, ESA, Ann Field and G. Bacon (STScI).

Probing a galaxy cluster’s dark matter

These observations are not just a validation of some obscure prediction in the scientific literature. Computer models of the mass distribution of MACS J1149, particularly the mass in the form of dark matter, are providing the estimated arrival times of the various supernova light paths. Further study and analysis of the supernova Refsdal light paths will allow for the improvement of those models and a better understanding of the distribution of dark matter throughout MACS J1149. In addition to a better understanding of how dark matter is distributed in galaxy clusters, these results will provide astronomers studying this Frontier Field with a better tool to confirm the distances to far-away lensed galaxies.

Building upon a historic scientific legacy

This is a fortuitous time in astronomy and for the Hubble Space Telescope. The paper describing supernova Refsdal, led by Dr. Patrick Kelly of the University of California, Berkeley, is being released this month in a special issue of the journal Science. This special issue of Science is commemorating the 100th anniversary of Albert Einstein’s Theory of General Relativity — the very theory that led to the prediction that distant supernovae could be gravitationally lensed by foreground galaxies or galaxy clusters. In addition to this confluence of events, it is also Hubble’s 25th anniversary. It is not lost on astronomers that it took many years and many people, including the brave astronauts of five servicing missions, to repair Hubble and upgrade Hubble’s instruments in order for such a discovery to take place. The new technology on Hubble is truly enabling ground-breaking science to this day.

Dr. Lawton would like to thank Dr. Patrick Kelly (University of California, Berkeley) and Dr. Steve Rodney (Johns Hopkins University) for help in creating the content for this post. Supernova Refsdal was discovered using data from the Grism Lens Amplified Survey from Space (GLASS) Hubble program. Follow-up Hubble observations from the Frontier supernova (FrontierSN) team confirmed that the light observed was from a supernova.

You can learn more about this amazing discovery on the recent Hubble Hangout.

The Marvel of Gravitational Lensing

A Giant Lensed Galaxy Arc

A Giant Lensed Galaxy Arc
The view of a distant galaxy (nearly 10 billion light-years away) has been warped into a nearly 90-degree arc of light by the gravity of the galaxy cluster known as RCS2 032727-132623 (about 5 billion light-years away).
Credit: NASA, ESA, J. Rigby (NASA GSFC), K. Sharon (KICP, U Chicago), and M. Gladders and E. Wuyts (U Chicago)

One of the coolest marvels in the universe is a phenomenon known as “gravitational lensing.” Unlike many topics in astronomy, the images are not what makes it appealing. Gravitational lensing produces streaks, arcs, and other distorted views that are intriguing, but don’t qualify for cosmic beauty pageants. What makes these images special is the intellectual understanding of how they are created, and the fact that they are even possible at all. The back story takes an ordinary, everyday process, and transforms it into cosmic proportions.

Most of us are familiar with the workings of a glass lens. If you have ever used a magnifying glass, you have seen how it changes the view of an object seen through it.

The glass lens collects light across its surface, which is generally much larger than the pupil of a human eye. Hence, a lens can amplify brightness. In addition, the path of a light ray is bent when it passes through the glass lens. [To be specific, the path bends when the light crosses from air to glass, and again when it crosses back from glass to air.] This bending is called refraction, and the common lens shape will focus the light to a point. When we view that collected light, our view of the object can be bigger or smaller depending on the distances involved, both from the object to the lens and from the lens to our eyes. In summary, a glass lens can amplify and magnify the light from an object.

Glass lenses, however, are not the only way that the path of light can be changed. Another way to redirect light comes from Einstein’s theory of general relativity.

My three-word summary of general relativity is “mass warps space.” The presence of a massive object, like a star, warps the space around it. When light crosses through warped space, it will change its direction. The result is that light that passes close enough to a massive object will be deflected. This deflection by mass is similar to refraction by glass.

Clusters of galaxies are huge concentrations of mass, including both the normal matter we see in the visible light from galaxies and the unseen dark matter spread throughout. Many galaxy clusters are massive enough to produce noticeable deflections of the light passing through or near them. The combined gravity in the cluster can warp space to act like a lens that gathers, amplifies, and magnifies light. Such a gravitational lens will be lumpy, not smooth, and will generally create distorted images of background galaxies seen through them. Also, this lensing often produces multiple images of the same background galaxy, as light from that galaxy is re-directed toward us along multiple paths through the cluster.

The simple idea of a glass lens becomes both cosmic and complex in gravitational lensing. Imagine a lens stretching millions of light-years across (many million million millions of miles). We don’t need to construct such a lens, as nature has provided a good number of them through the warping of the fabric of space. These lenses allow us to see very distant galaxies in the universe, some of which could not otherwise be observed. That’s the marvelous reality of galaxy clusters acting as gravitational lenses.

Celebrating Hubble’s 25th Anniversary

In April, Hubble will celebrate a quarter-century in space. The telescope, launched into orbit in 1990, has become one of NASA’s most beloved and successful missions, its images changing our understanding of the universe and taking root in our cultural landscape. Hubble pictures have not only expanded our scientific knowledge, they have altered the way we imagine the cosmos to appear.

pillars 1

Hubble took its iconic “Pillars of Creation” image of these star-forming clouds of gas and dust in the Eagle Nebula in 1995. Credit: NASA, ESA, STScI, J. Hester and P. Scowen (Arizona State University)

Hubble’s prolonged success has been a testament to its innovative design, which allowed it to be periodically updated by astronauts with new equipment and improved cameras. Hubble  has been able, to an extent, to keep up with technological changes over the past 25 years. The benefits are evident when comparing the images of the past and present.

pillars 2

This new image of the Eagle Nebula’s “Pillars of Creation” was taken in 2014 to launch Hubble’s year-long celebration of its 25th anniversary. The image was captured with Wide Field Camera 3, an instrument installed on the telescope in 2009. Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

Hubble’s new instruments — specifically, the near-infrared capabilities of Wide Field Camera 3 — are what makes the Frontier Fields project possible. The faint infrared light of the most distant, gravitationally lensed galaxies sought in the Frontier Fields project would be beyond the reach of Hubble’s earlier cameras. Frontier Fields highlights Hubble’s continuing quest to blaze new trails in astronomy — and pave the path for the upcoming Webb Space Telescope — so it makes sense that its imagery is included in a collection of 25 of Hubble’s significant images, specially selected for the anniversary year.

The immense gravity in this foreground galaxy cluster, Abell 2744, warps space to brighten and magnify images of far-more-distant background galaxies as they looked over 12 billion years ago, not long after the big bang.  This is the first of the Frontier Fields to be imaged.

Abell 2744, the first of the Frontier Fields to be imaged, is part of Hubble’s 25th anniversary collection of top images. The immense gravity of the foreground galaxy cluster warps space to brighten and magnify images of far-more-distant background galaxies. Credit: NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)


The 25th birthday is a significant milestone, so Hubble is throwing a year-long celebration, with events happening in communities and online throughout 2015. Last week, Tony Darnell hosted a discussion of the beauty and scientific relevance of the Hubble 25th anniversary images, one of the many anniversary-themed Hubble Hangouts he’ll be doing as the months go on. To keep an eye on upcoming events, see the images, and learn about the science, visit our special 25th anniversary website, Hubble25th.org.

MACS J0416 Data is Complete

Observations of another Frontier Fields galaxy cluster and parallel field are complete. This time, we have new images for you of MACS J0416.1-2403. Here’s the galaxy cluster:


And here is the parallel field:


Beautiful, aren’t they? This is the second Frontier Fields cluster and parallel field to be fully imaged. You can see the first here.

Remember that to maximize scientific discovery, Hubble is using two of its instruments simultaneously to examine both the cluster and the parallel field, then observing the same areas again with the instruments switched.

Hubble takes two sets of observations, called epochs, in order to thoroughly examine the two areas. During the first, Hubble spent 80 orbits with the Advanced Camera for Surveys (ACS) pointing at the main galaxy cluster, and Wide Field Camera 3 (WFC3) looking at the parallel field. ACS provides a visible-light view, and WFC3 adds near-infrared light.

During the second epoch, Hubble spent 70 orbits targeting WFC3 on the main cluster and ACS on the parallel field.

Scientists are poring over the new data, and one result is already in. Expect to hear more about these observations in the near future.

Frontier Fields Hangout: Hubble Finds Extremely Distant Galaxy in Gravitational Lens

Peering through a giant cosmic magnifying glass, NASA’s Hubble Space Telescope has spotted one of the farthest, faintest, and smallest galaxies ever seen. The diminutive object is estimated to be over 13 billion light-years away.
This new detection is considered one of the most reliable distance measurements of a galaxy that existed in the early universe, said the Hubble researchers. They used two independent methods to estimate its distance.

The galaxy was detected as part of the Frontier Fields program, an ambitious three-year effort, begun in 2013, that teams Hubble with NASA’s other Great Observatories — the Spitzer Space Telescope and the Chandra X-ray Observatory — to probe the early universe by studying large galaxy clusters. These clusters are so massive that their gravity deflects light passing through them, magnifying, brightening, and distorting background objects in a phenomenon called gravitational lensing. These powerful lenses allow astronomers to find many dim, distant structures that otherwise might be too faint to see.

Mapping Mass in a Frontier Fields Cluster

The Frontier Fields project’s examination of galaxy cluster MACS J0416.1-2403 has led to a precise map that shows both the amount and distribution of matter in the cluster. MACS J0416.1-2403 has 160 trillion times the mass of the Sun in an area over 650,000 light-years across.

The mass maps have a two-fold purpose: they identify the location of mass in the galaxy clusters, and by doing so make it easier to characterize lensed background galaxies.

Mass map of galaxy cluster MCS J0416.1–2403

The galaxy clusters under observation in Frontier Fields are so dense in mass that their gravity distorts and bends the light from the more-distant galaxies behind them, creating the magnifying effect known as gravitational lensing. Astronomers use the lensing effect to determine the location of concentrations of mass in the cluster, depicted here as a blue haze. Credit: ESA/Hubble, NASA, HST Frontier Fields

Astronomers use the distortions of light caused by mass concentrations to pinpoint the distribution of mass within the cluster, including invisible dark matter. Weakly lensed background galaxies, visible in the outskirts of the cluster where less mass accumulates, may be stretched into slightly more elliptical shapes or transformed into smears of light. Strongly lensed galaxies, visible in the inner core of the cluster where greater concentrations of mass occur, can appear as sweeping arcs or rings, or even appear multiple times throughout the image. And as a dual benefit, as the clusters’ mass maps improve, it becomes easier to identify which galaxies are strongly lensed, and which galaxies are farther away.

Stronger lensing produces greater distortions. Astronomers can work backwards from the distortions to pinpoint the greater concentrations of mass responsible for producing such altered images.

Stronger lensing produces greater distortions. Astronomers can work backwards from the distortions to pinpoint the greater concentrations of mass responsible for producing such altered images. Credit: A. Feild (STScI)

The depth of the Frontier Fields images allows astronomers to see extremely faint objects, including many more strongly lensed galaxies than seen in previous observations of the cluster. Hubble identified 51 new multiply imaged galaxies around this cluster, for instance, quadrupling the number found in previous surveys. Because the galaxies are multiples, that means almost 200 strongly lensed images appear in the new observations, allowing astronomers to produce a highly constrained map of the cluster’s mass, inclusive of both visible and dark matter.

The dark matter aspect is particularly intriguing. Because these types of Frontier Fields analyses create extremely precise maps of the locations of dark matter, they provide the potential for testing the nature of dark matter. Learning where dark matter concentrates in massive galaxy clusters can give clues to how it behaves and changes. And as the mass maps become more precise, astronomers are better able to determine the distance of the lensed galaxies.

In order to obtain a complete picture of MACS J0416.1-2403’s mass, astronomers will also need to include weak lensing measurements. Follow up observations will include further Frontier Fields imaging, as well as X-ray measurements of hot gas and spectroscopic redshifts to break down the total mass distribution into dark matter, gas, and stars.

Frontier Fields Finds Faint Light of Homeless Stars

The Frontier Fields’ project has detected the glow of about 200 billion freely drifting stars within the massive galaxy cluster Abell 2744. The stars were dragged from their home galaxies by gravitational tides during collisions and interactions over the course of 6 billion years.

As many as six Milky Way-sized galaxies were torn apart in the cluster. The light of the outcast stars is believed to contribute to 10 percent of the cluster’s brightness, though that light is quite faint because the density of the stars is low. The combination of depth and multiwavelength observations provided by the Frontier Fields program makes this study of such dim stars possible.

The total starlight of galaxy cluster Abell 2744 is depicted here in blue in this Frontier Fields image. Not all the starlight is contained within the galaxies, which appear as blue-white objects. A portion of the light comes from stars that have been pulled from their galaxies and now drift untethered within the cluster. Credit: NASA, ESA, M. Montes (IAC), and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

The stars are rich in heavy elements such as oxygen, carbon, and nitrogen, which means they formed from material released by earlier generations of stars. The presence of these elements indicates that the stars likely came from galaxies with similar mass and metallicity to our own Milky Way galaxy, which have the ability to sustain ongoing star formation and thus build populations of such chemically enriched stars. Elliptical galaxies are low in star formation while dwarf galaxies lack the kind of constant star formation that would be essential.

This discovery indicates that a significant fraction of the stars that would otherwise end up in these galaxies is being stripped out in the merger process. Astronomers intend to look for the light of such estranged stars in the remainder of the Frontier Fields galaxy clusters.

A Black Hole Visits Baltimore

[NOTE: This post is the fourth in a four-part series. Previous posts are: 1) Einstein’s Crazy Idea, 2) Visual “Proof” of Gravitational Lensing, and 3) Gravitational Lensing in Action.]

For the final part of this series of blog posts, let’s bring things back to Earth. The demonstration of a physical process will always seem a bit arcane when using unfamiliar objects as the example. Most folks don’t have a working relationship with galaxies, let alone the strange varieties one gets in the distant universe. Instead of taking the viewer into the universe, it can be more intuitive to bring the cosmic phenomenon closer to home.

Suppose that, say, a black hole decided to take a short vacation. Perhaps it got tired of the enormous responsibilities of being such a tremendous distortion of space-time. It needed a weekend off to cool its jets (absurdly geeky pun intended – sorry). Around Baltimore, where I work, the black hole might go down to the Inner Harbor, enjoy the sights and activities, indulge in a crab feast, and leave completely rejuvenated. Now, while I haven’t yet tried to visualize a black hole eating crabs, and the concomitant singularity eruptions due to Old Bay seasoning, we can approximate what tourists might have seen during the visit.

A Black Hole Visits Baltimore

Credit: Frank Summers (STScI), special thanks to Brian McLeod (Harvard).

This scientific visualization presents a black hole of about the mass of Saturn passing through Baltimore’s Inner Harbor. The initial view from Federal Hill shows the usual boats, shops, and office buildings along the water. As the black hole passes across the harbor, the view of the background buildings is distorted due to gravitational lensing. Light is redirected such that, in the region around the singularity, imagery is flipped top/bottom and left/right, with multiple views of the same object. This transformation of a familiar skyline scene can help one imagine the transformation of unfamiliar galaxies in the distant universe.

Note: As in the previous simulated lensing image, a simplified, planar approach of gravitational lensing is used for this visualization. However, in this case, the foreground objects were not removed. The visual distortion of ship’s masts on the near side of the harbor would not occur. We humbly ask your indulgences.

While in graduate school, I had to solve problems using the complex collection of general relativity equations – but only a few times. And all of those instances were for problems with enough symmetry that things could be considerably simplified. I gained an appreciation for the essential character, and some of the beauty, of the mathematics behind it. However, as stated in the first post in this series, the whole concept still has a feeling of weirdness.

Perhaps that notion would have dissipated had I specialized in relativity. Instead, as I developed into a scientific visualization specialist, I’ve gotten to revisit things from a public presentation, rather than research, perspective. The visual allure of gravitational lensing can attract an audience for topics typically mired in equations. It shows how a simple magnifying glass can have a truly cosmic analogue. It helps explore the perspective changing shift in gravity from Newton’s force to Einstein’s geometric re-interpretation. It opens the pathway to deeper philosophical thoughts about the fabric of space-time and the very underpinnings of our universe. Now, that’s quite the opportunity for an outreach astrophysicist like me.

In this case, weird is cool.

Gravitational Forensics: Astronomers Discover a Distant Galaxy in the Frontier Fields

The first Hubble Frontier Fields observations of a galaxy cluster and adjacent parallel field are complete, and interesting results are starting to arrive from astronomers. In this post, we explore how astronomers used the tools available to them to piece together the discovery of a very distant galaxy.

The Discovery

A team of international astronomers, led by Adi Zitrin of the California Institute of Technology in Pasadena, Calif., have discovered a very distant galaxy observed to be multiply lensed by the foreground Abell 2744 galaxy cluster. The light from this distant galaxy was distorted into three images and magnified via gravitational lensing of Abell 2744. This magnification provided the astronomers with a means to detect the incredibly faint galaxy with Hubble.

Astronomers are interested in finding these very distant galaxies because they represent an early stage of galaxy formation that occurred just after the Big Bang. Light from this galaxy has been traveling for quite some time. We are seeing this galaxy as it existed when the universe was only about 500 million years old. For context, the current age of the universe is around 13.8 billion years old.

Like visitors to a nursery, astronomers can see this baby galaxy is much smaller than present-day adult galaxies. In fact, they measure it to be about 500 times smaller than our own Milky Way galaxy. This baby galaxy is estimated to be forming new stars at a rate of one star every three years. That is about 1/3 the current rate of star formation of our own Milky Way, but keep in mind that this infant galaxy is much smaller than the present-day Milky Way. This baby galaxy is not just small but also a lightweight. It has the mass, in stars, of only about 40 million suns. Compare that to the Milky Way, which has a mass of several hundred billion suns. It is also one of the intrinsically faintest distant galaxies ever discovered.

The three lensed images of the baby galaxy are highlighted in the composite image below.

Credit: NASA, ESA, A. Zitrin (California Institute of Technology, Pasadena), and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (Space Telescope Science Institute, Baltimore, Md.) Shown is the discovery of a high redshift galaxy candidate, triply lensed by Abell 2744. The high redshift galaxy candidate's lensed images are labeled as a, b, and c.

Credit: NASA, ESA, A. Zitrin (California Institute of Technology, Pasadena), and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (Space Telescope Science Institute, Baltimore, Md.) Shown is the discovery of a very distant galaxy, triply lensed by the foreground galaxy cluster Abell 2744. The distant galaxy’s lensed images are labeled as a, b, and c.

This is now one of only a small handful — about 10 — of galaxies we have discovered at such great distances. The way the team discovered this distant galaxy is, perhaps, as interesting as the galaxy itself. The team of astronomers used a traditional color-based method for determining that the galaxy is a candidate for being a distant, baby galaxy. They then followed up with a pioneering new technique to confirm the distance via the geometry of gravitational lensing.

Using Colors to Find Candidate Distant Galaxies

Why do we think that the galaxy is very far away? Astronomers used Hubble’s filters to capture the light from this baby galaxy in several different colors. The intensity of light coming from the galaxy at different colors can give an estimate of the galaxy’s cosmological redshift. Cosmological redshift, commonly denoted by the letter “z,” is a number that signifies how reddened a galaxy is due to the expansion of space. A distance can be estimated once a cosmological redshift is measured. Larger cosmological redshifts correspond to larger distances.

Adi Zitrin and his collaborators initially found the distant galaxy (labeled “a” in the figure above) by noticing that it remained when they were looking for only the reddest galaxies. Remember, a galaxy may appear red if its light is redshifted due to the expansion of the universe. The farther the galaxy, the longer its light has to traverse the expanding universe, getting more and more stretched (redshifted) along the way. Astronomers are particularly interested in finding a population of galaxies with large cosmological redshifts — values of z around 10 or greater — because they represent some of the earliest galaxies to form after the Big Bang.

From the colors of the galaxy found in box ‘a,’ the team estimated that the galaxy has a redshift greater than 4, with 95% confidence. In fact, the colors of the galaxy in box ‘a’ highly favored a galaxy around z=10, but they could not discount that what they were measuring was an intrinsically red galaxy at a lower redshift, around a z=2. How do we sort this out?

Deciphering the Geometry of Abell 2744’s Gravitational Lens

Astronomers can do better, and these astronomers have shown that with knowledge of how mass is distributed in the foreground galaxy cluster, it is possible to distinguish between higher redshift and lower redshift background galaxies. Thus, with updated maps of the mass distribution of the Abell 2744 galaxy cluster, astronomers created more precise mathematical models of how light from a more distant galaxy behaves as it passes around the galaxy cluster’s warped space.

The geometry of a gravitational lens is such that the more distant a background galaxy behind the galaxy cluster, the farther from the center of the galaxy cluster we observe the distorted and magnified, lensed versions of the galaxy. This is portrayed in the graphic below, where two lensed versions of the more distant, highly redshifted, red galaxy appears on the sky at larger apparent distances from the central, foreground, lensing galaxy cluster.

Credit: Courtesy of Dr. Dan Coe (STScI). Shown here is an illustration of how the multiple lensing of a background galaxy will show its maximum magnification depending on its distance to the foreground galaxy cluster. More distant galaxies will be lensed such that we observe them further from the center of the galaxy cluster.

Credit: Dan Coe (STScI). Shown here is an illustration of how the multiple lensing of a background galaxy will show its maximum magnification depending on its distance to the foreground galaxy cluster. More distant galaxies will be lensed such that we observe them farther from the center of the galaxy cluster.

Astronomers can use the computed geometry of gravitational lensing to ascertain the cosmological redshift of the lensed galaxy based on its observed positions relative to the foreground galaxy cluster. If multiple images of the lensed galaxy appear nearby the cluster, it is at a lower redshift. If the multiple images of the lensed galaxy appear more separated from the cluster, it is at a larger redshift.

Finding the Multiple Images of a Distant Lensed Galaxy

With the updated mathematical models of the gravitational lensing by Abell 2744, Adi Zitrin and his team could follow up and look for multiply lensed images of the one potentially distant galaxy they had found, labeled “a”  in the image at top. The mathematical models give them positions on the sky to look for the lensed siblings of galaxy ‘a’ for various redshifts. If the distant galaxy is at a relatively low redshift, multiply lensed images will appear nearer the cluster. If the distant galaxy is at a high redshift, multiply lensed images will appear farther from the cluster.

With the computational tools and mathematical knowledge available to them, the team discovered the lensed versions of galaxy “a” at positions that match a high-redshift solution. In the figure below, they marked the locations of the lensed images, labeled “B” and “C”, along with their best mathematical estimates of redshift for each of them (labeled along the blue- and green-colored redshift lines). What is labeled as the initially discovered candidate galaxy “a” in the image at top is now labeled as “A” in the image below.

Credit: Adi Zitrin et al. 2014. Shown here are the expected positions of the three lensed versions of the newly discovered high redshift galaxy candidate, based on mathematical models of the gravitational lensing from Abell 2744. Galaxy lens A, B, and C are all in positions that match high redshift solutions in the models, i.e. redshifts of around 8 or greater.

Credit: Modified from Adi Zitrin et al., ApJ, 793 (2014). Shown here are the expected positions of the three lensed versions of the newly discovered high-redshift galaxy candidate, based on mathematical models of the gravitational lensing from Abell 2744. The multiply-lensed positions of the galaxy, labeled “A”, “B”, and “C,” match the high-redshift solution in the models, i.e., redshifts of around 8 or greater.

This is but a taste of how astronomers will use the Frontier Fields to combine exquisite imaging with updated mathematical models to detect and study some of the first galaxies to form after the Big Bang. We are just at the beginning of collecting the baby pictures of galaxies in our universe. Stay tuned as we detect more baby galaxies from the dawn of time!

Looking to the Future

The galaxy presented here is one of the least luminous high-redshift galaxies ever detected. This bodes very well for finding future baby galaxies in the Frontier Fields. We also expect that studies of the galaxy clusters themselves, via the new data in the Frontier Fields, will lead to more accurate mass distribution maps and more accurate mathematical models of how light from distant galaxies are gravitationally lensed and magnified.

This really is a new age in using humankind’s most sophisticated telescopes with nature’s lenses to probe deeper into our cosmic past than ever before. Stay tuned for more results from the Frontier Fields.

You can watch a Hubble Hangout of this result here!

Light Detectives: Using Color to Estimate Distance

Distances are notoriously difficult to measure in astronomy. Astronomers use many methods for estimating distances, but the farther away an object is, the more uncertain the results. Cosmological distances, distances on the largest scales of our universe, are the most difficult to estimate. To measure the distances to the farthest galaxies, those gravitationally lensed by massive foreground galaxy clusters, astronomers really have their work cut out for them.

If a massive stellar explosion, known as a supernova, happens to go off in a galaxy and we catch it, then we can use the “standard candle” method of computing the distance to the galaxy. Supernovae are expected to be discovered in the Frontier Fields, but not at the numbers that will help us find distances to most of the galaxies in the images. Without these standard candles, astronomers must use other means to estimate distances.

A Spectrum is Worth a Thousand Pictures

One of the more accurate methods for measuring the distance to a distant galaxy involves obtaining a spectrum of the galaxy. Getting a galaxy’s spectrum basically means taking the light from that galaxy and breaking it up into its component colors, much like a prism breaks up white light into the rainbow of visible colors. By comparing the brightness of light at each component color, a spectrum can give us a wealth of information. This can include detailed information about a galaxy’s composition, temperature, and how fast it is moving relative to us. Because the universe is expanding, we observe most galaxies, and all distant galaxies, to be moving away from us.

When looking at a distant galaxy’s spectrum, the expansion of the universe causes the component colors in the spectrum to be stretched to longer wavelengths. For visible light, red has the longest wavelengths, which leads to the term ‘redshift’. This cosmological redshift can be accurately measured from a spectrum. Astronomers then use mathematical models of the expansion rate of our universe to convert the measured redshift into an estimate of distance. Larger values of redshift correspond to larger distances.

This video, developed by the Office of Public Outreach at the Space Telescope Science Institute, gives a demonstration of how light is redshifted as it travels through the expanding universe. Here, the lightbulb stands in place of a galaxy. As the universe expands, it stretches the light traveling through the universe, increasing the light’s wavelength. As the wavelength increases, it becomes more red. Light traveling longer distances through the universe will be stretched/reddened more than light traveling short distances. This is why astronomers use instruments sensitive to redder light, including infrared light, when they attempt to observe the light from very distant galaxies. Watch this video on Youtube.

Larger redshifts not only correspond to larger distances, but they also correspond to earlier times in our universe’s history. This is because light takes time to travel to us from these distant galaxies. The more distant the galaxy, the longer the light has been traveling before we intercept it with sensitive telescopes, like Hubble.

Assuming typical contemporary mathematical models, the universe is about 13.8 billion years old. Galaxies at a redshift of 1 are seen as they existed when the universe was about 6 billion years old.  Galaxies at a redshift of 3 are seen as they existed when the universe was about 2 billion years old. Galaxies at a redshift of 6 are seen as they existed when the universe was about 1 billion years old.  Galaxies at a redshift of 10 are seen as they existed when the universe was only about 500 million years old.

It is notoriously difficult to obtain a spectrum of a very distant galaxy.  They are very faint, and an accurate spectrum relies on obtaining a lot of light.  One is, after all, taking what little light you get and breaking it up further into the component colors, meaning that you start with little light and get out even less light at each component color.  Getting enough light to take an accurate spectrum of a distant galaxy requires very lengthy observations with sensitive telescopes.  This is not always feasible.

Redshifts measured via spectra are called spectroscopic redshifts. Many of the nearer galaxies in Abell 2744 have measured spectroscopic redshifts. There will likely be many follow-up observations from ground- and space-based observatories to obtain spectra of many of the fainter and more distant galaxies in the Frontier Fields. So stay tuned!

I Can’t Obtain a Spectrum!  What to do?

If you do not have a spectrum, are there other ways to estimate the redshift and distance to a galaxy?  Yes!  Just take a look at the galaxy’s colors.

All Hubble images are taken with filters. Blue filters allow Hubble’s instruments to capture only blue light, red filters allow Hubble’s instruments to capture only red light, and so on. By comparing a galaxy’s brightnesses in these different colors, astronomers can estimate the distance to the galaxy. The redder the color, the more likely the galaxy is to be redshifted, and thus, farther away.

This technique of using color to estimate redshift is called photometric redshift. The following two primary methods are used for estimating a photometric redshift:

  1. compare the colors of your high-redshift galaxy candidate to a set of typical galaxy color templates at various redshifts, or
  2. compare the colors of your high-redshift galaxy candidate to a set of galaxies with measured spectroscopic redshifts and, utilizing specialized software, compute the most likely redshift for your galaxy.

In the first case, the photometric redshift comes from the best match between the observed high-redshift candidate colors and the colors of the template galaxies. The template galaxy colors stem from observations of galaxies that tend to be relatively close but are then mathematically reddened over a range of redshift values.

In the second case, astronomers use a set of observed galaxies whose redshifts have been measured spectroscopically, as explained in the prior section. This set contains galaxies at various redshifts. They then use machine-learning algorithms to compare the colors of this set of galaxies with the colors of the target high-redshift galaxy candidate. The software selects the most likely redshift.

Whichever method is used, astronomers are careful to give confidence levels in their calculations. For the computation of photometric redshift, there is typically an uncertainty of around a few percent for high-quality data. In addition, there is the lingering issue of whether the high-redshift galaxy candidate is truly redshifted, or if it is a nearer galaxy that is intrinsically redder. It is not uncommon to read results where astronomers find a galaxy with a probable high photometric redshift and a less probable low photometric redshift, or vice versa.


Credit: Adapted from Adi Zitrin, et al., ApJ, 793 (2014). Shown is a high-redshift galaxy candidate in Hubble’s observations of Abell 2744, discovered using filters. Dark regions represent light in these images. Notice how the galaxy drops out of the image in the bluest filters. This is a hint that the galaxy may be significantly redshifted.

Many of the first results for the Frontier Fields utilize photometric redshifts. In the absence of spectra, photometric redshifts are the next best thing to obtaining estimates of distances for large samples of galaxies. They are readily computed from the current Frontier Fields data.