Discoveries

March 8, 2017

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.

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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

Shown here is NASA’s Hubble Space Telescope, which can observe ultraviolet light, visible light, and near-infrared light. Credit: NASA
Shown here is NASA’s Spitzer Space Telescope, which can observe infrared light. Credit: NASA
Shown here is NASA’s Chandra X-ray Observatory, which can observe high-energy X-rays. Credit: NASA

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
A Sneak Peek at the First Billion Years of the Universe: Galaxy cluster fields extend the reach of the Great Observatories by allowing astronomers to use a technique called gravitational lensing, which magnifies background galaxies that are otherwise presently unobservable.
The observing plan of the NASA Frontier Fields program, using the galaxy cluster Abell 2744 and its adjacent parallel field as an example. Director’s Discretionary (DD) hours were allocated for Hubble and Spitzer observations. DD time comes directly from an observatory’s director, who has a set number of hours to allocate every year.

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.

Processed original black-and-white images of galaxy cluster Abell 2744. Credit: Chandra – NASA/CXC/SAO; Hubble – NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI); Spitzer – NASA/JPL-Caltech/P. Capak
Processed images of galaxy cluster Abell 2744, with color added (X-ray light – blue, visible light – green, infrared light – red). Credit: Chandra – NASA/CXC/SAO; Hubble – NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI); Spitzer – NASA/JPL-Caltech/P. Capak
Processed composite image of galaxy cluster Abell 2744. Credit: Chandra – NASA/CXC/SAO; Hubble – NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI); Spitzer – NASA/JPL-Caltech/P. Capak

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.

Mathematical models of the mass distribution of a galaxy cluster provide magnification maps that pinpoint the locations of greatest magnification due to gravitational lensing. These are where astronomers search for the most distant and faintest galaxies. Shown here are Hubble imagery of galaxy cluster Abell 2744 (green); distribution of mass for the Abell 2744 galaxy cluster (blue); and locations of greatest lensing for background galaxies with a redshift of 9 (pink). There are different magnification maps for background galaxies at different distances. Credit: J. Richard (CRAL Lyon), CATS team, and D. Coe (STScI)
Using mathematical models, astronomers can remove the foreground light from galaxies within a galaxy cluster. By removing the large-scale foreground light, astronomers are able to identify small-scale structures of background, faint, lensed galaxies. Shown here is galaxy cluster Abell 2744 before foreground light subtraction (left) and after foreground light subtraction (right). Multiple distant, faint galaxies become visible using this technique. Those in the circles are background galaxies that are possibly very distant, i.e., those with possibly very high redshifts. Credit: Livermore, Finkelstein, & Lotz 2016

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.

 

Shown here are observations of the Frontier Fields galaxy cluster MACS J0717, taken by Chandra, Hubble, and the Jansky Very Large Array. Diffuse blue colors (Chandra X-ray Observatory) are from the light emitted by gas with temperatures of millions of degrees. Red, green, and blue (Hubble Space Telescope) colors are from galaxies. Diffuse pink colors (Jansky Very Large Array) are from excited gas from shock waves and turbulence due to merging galaxy clusters (middle-top and lower-left), as well as a foreground radio galaxy (center left). Credit: NASA, ESA, CXC, NRAO/AUI/NSF, STScI, and R. van Weeren (Harvard-Smithsonian Center for Astrophysics)
Shown here are gravitationally lensed galaxies in galaxy cluster MACS J0717, as observed by Hubble (left) and Spitzer (right). Credit: Hubble – NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI); Spitzer – NASA/JPL-Caltech/P. Capak
Shown here is supernova Refsdal (four points of light in the spiral arm of a distant background galaxy) multiply lensed by the foreground galaxy cluster MACS J1149. 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)

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.

Shown here are simulated Spitzer (left) and JWST (right) images of distant galaxies in infrared colors. These were constructed from a computer simulation of the deep universe. Credit: G. Snyder & Z. Levay (STScI)
Shown here are Hubble’s observations of Abell 2744 and parallel field (inset boxes), located inside the footprint of the WFIRST Wide Field Instrument. Credit: FF – NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI); DSS – STScI/NASA; Z. Levay (STScI)

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.

November 14, 2016

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.

January 29, 2016

Predicted Reappearance of Supernova Refsdal Confirmed

Hubble has captured an image of the first-ever predicted supernova explosion.

In November 2014, Hubble’s Frontier Fields program caught sight of a supernova called “Refsdal” while examining the MACS J1149.5+2223 galaxy cluster. Astronomers spotted four separate images of the supernova in a rare arrangement known as an “Einstein Cross” around a galaxy within the cluster.

The four images of the same supernova result from the way light from distant objects is not just magnified but bent by the immense mass of the galaxy cluster. (Link: https://frontierfields.org/2014/07/09/seeing-double-or-more-in-frontier-fields-images/)

Seeing such distant, gravitationally lensed objects is, of course, the point of the Frontier Fields project, but this one had a special quirk. By studying different models of just how mass is positioned in the galaxy cluster, astronomers could predict one more light path for Refsdal, one that would delay the light reaching the telescope until late 2015 or early 2016. This means they could predict when and where in the field the image of the supernova would appear next.

Astronomers began taking snapshots of the predicted area over an expected time period. And sure enough, on Dec. 11, 2015, the astronomers captured the reappearance of the supernova where they had anticipated it would be. The detection of this fifth appearance of the Refsdal supernova served as a unique opportunity for astronomers to test their models of how mass — especially that of mysterious dark matter — is distributed within this galaxy cluster, and they seem to be right on track.

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These images show the search for the supernova, nicknamed Refsdal, using NASA’s Hubble Space Telescope. The image on the left is the galaxy cluster MACS J1149.5+2223 from the Frontier Fields program. The circle indicates the empty but predicted position of the newest appearance of the supernova. The image at top right shows observations taken by Hubble on Oct. 30, 2015, at the beginning of the observation program to detect the newest appearance of the supernova. The image on the lower right shows the discovery of the Refsdal supernova on Dec. 11, 2015, as predicted by several different models.

Credit: NASA, ESA, and P. Kelly (University of California, Berkeley)

Acknowledgment: NASA, ESA, and S. Rodney (University of South Carolina) 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)

March 6, 2015

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.

December 9, 2014

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.

December 2, 2014

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.

November 12, 2014

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!

October 23, 2014

First Galaxy Field Complete: Abell 2744

This past summer, the Hubble Frontier Fields team completed observations of the first cluster on its list: Abell 2744!  The second set of observations — astronomers call them epochs — consisted of 70 orbits and marks the completion of the first Frontier Fields galaxy cluster. During this set, Hubble’s Advanced Camera for Surveys (ACS) was pointed at the main galaxy cluster and studied the visible-light portions of the spectrum, while the Wide Field Camera 3 (WFC3) looked at the parallel field in the infrared.

Remember that Hubble will visit each field multiple times, with Hubble oriented such that one set of observations will point WFC3 at the cluster and ACS at a parallel field adjacent to the cluster (that’s one epoch).   The telescope will then come back and do another set of observations with the cameras switched: ACS pointing at the cluster and WFC3 pointing to the parallel field (that’s the second one).

The Frontier Fields team does this to allow for complete wavelength coverage in both infrared and visible light for the galaxy cluster and the parallel field.

The first epoch, completed in November 2013, consisted of  87 orbits.  This brings the total amount of time Hubble looked at this cluster to 157 orbits.

Here’s the result.  This is the galaxy cluster Abell 2744:

Final mosaic of the Frontier Fields galaxy cluster Abell 2744. This image is the culmination of both epochs totaling 157 Hubble orbits. The numbers prefixed with "F" are the Hubble filters used by the ACS and WFC3 cameras to take the image. The scale bar of 30" is approximately 2% the angular size of the full moon as seen from Earth - very small! Credit: NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

Final mosaic of the Frontier Fields galaxy cluster Abell 2744. This image is the culmination of both epochs totaling 157 Hubble orbits. The numbers prefixed with “F” are the Hubble filters used by the ACS and WFC3 cameras to take the image. The scale bar of 30″ is approximately 2% the angular size of the full moon as seen from Earth – very small!
Credit: NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

And here is the parallel field:

Parallel field of Frontier Field Abell 2744

This is the completed composite mosaic of the Parallel Fields observed with galaxy cluster Abell 2744.
Credit: NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

See? Epic! Er, I mean epoch.

Once the second epoch was completed, some of the faintest galaxies ever seen were measured for the first time.  Astronomers have been working on these images since their release, and we are anxiously awaiting to hear what they find.

February 27, 2014

Galaxy Find Showcases Frontier Fields’ Potential

A faraway object in Frontier Fields cluster Abell 2744 could be one of the most distant galaxies found to date, according to an international team studying a combination of Hubble and Spitzer data on the cluster.

The galaxy, called Abell2744_Y1, is about 30 times smaller than our Milky Way galaxy but is producing at least 8 times more stars. Further observations are needed to verify the galaxy’s redshift, which will help determine how truly distant it is. The highest-redshift galaxies, with values greater than 8 or 9, are so distant we see them as they were about 13 billion years ago. For comparison, galaxies with a redshift around 0 are in our local neighborhood and galaxies with a redshift of around 1 appear as they were halfway back to the Big Bang. Astronomers are hoping Abell2744_Y1 is in the high-redshift range.

hs-2014-17-a-web_printAbell2744_Y1 (right) and its location in the Abell 2744 galaxy cluster (left). Credit: NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI), and N. Laporte (Instituto de Astrofisica de Canarias)

Though scientists are interested in galaxies with redshifts in the lower range, technological advances have made those discoveries far more routine. For many astronomers, the focus today is on galaxies with the higher redshifts. The difference is significant, since lower-redshift galaxies appear in our observations as they were billions of years after the Big Bang, but higher-redshift galaxies can show us the state of the universe just hundreds of millions of years after the Big Bang.

Astronomers can find these high-redshift objects with modern telescopes, but the ones they see tend to be the biggest, brightest galaxies of the early universe — the unusual objects, the outliers. To see the more normal objects — the typical galaxies of the time — astronomers use natural gravitational lenses in space to extend the reach of telescopes. And that’s where Frontier Fields comes in.

The Frontier Fields project will be looking at galaxy cluster Abell 2744 again in May, and using its natural lens to find more galaxies like Abell2744_Y1. The more ordinary, representative galaxies astronomers encounter, the better they can understand and explain the development of the early universe. Just as you couldn’t draw an accurate portrait of humanity by only examining the tallest people on the planet, scientists can’t build a complete picture of the universe when they can only detect its biggest and brightest galaxies. Astronomers need a large sample of everyday galaxies to learn what that population is really like.

Since 2006, the number of high-redshift galaxies known has increased from a dozen to 236, but only two have been confirmed as actually being high-redshift galaxies. To truly confirm the objects’ distances, astronomers need to obtain spectra of the galaxies. When Hubble looks at Abell 2744, it won’t be taking spectra of Abell2744_Y1, but the galaxy is bright enough to be detected by major ground-based telescopes, so it’s possible an Earth-based observatory may confirm its distance.

The small galaxy is producing stars at a rate of anywhere from eight to 60 solar masses per year, much more than the Milky Way’s star production rate of about one to three solar masses per year. A solar mass is the equivalent of our Sun’s mass. That number may not sound like much, but remember that stars develop over the course of millions of years, meaning that tiny galaxy is pumping out stars at a thoroughly respectable pace. Will other distant galaxies magnified by Abell 2744’s gravitational lens match its production? We’ll learn more as the Frontier Fields data begins to pour in.

January 17, 2014

Cosmic Archeology

Today’s guest post is by Dr. Mario Livio, Hubble astrophysicist and author of the blog “A Curious Mind.” A version of this post appeared previously on Dr. Livio’s blog.

During the Christmas season of 1995, the Hubble Space Telescope was pointed for 10 consecutive days at an area in the sky not larger than the one you would see through a drinking straw. The region of sky, in the Ursa Major constellation, was selected so as to be as “boring” as possible — empty of stars in both our own Milky Way galaxy and in relatively nearby galaxies. The idea was for Hubble to take as deep an image of the distant universe as possible. The resulting image was astounding.  With very few exceptions, every point of light in this image is an entire galaxy, with something like 100 billion stars like the Sun.

Image

The original Hubble Deep Field image.

Detailed analysis revealed that the very remote galaxies were physically smaller in size than today’s galaxies, and that their morphologies were more disturbed. Unlike the grand-design spirals or smooth elliptical shapes that we see in relatively close galaxies, the distant objects look like train wrecks. Both of these observations fit nicely into the idea that galaxies evolve largely by “mergers and acquisitions.” Small building blocks merge together to form larger ones, or cold flows of dark matter along dense filaments fuel the growth. What we see in the distant past are those interacting—and hence smaller and less regular in shape—building blocks.

Since then, Hubble observed even deeper, producing the “Hubble Ultra Deep Field” in 2004, and then in 2009 an image that included infrared observations taken with the new Wide Field Camera 3. This observation allowed astronomers to glimpse the universe at its infancy, when it was less than 500 million years old (the universe today is 13.8 billion years old). The Deep Field observations have also enabled researchers to reconstruct the history of global cosmic star formation. We now know that about 8 billion years ago the universe reached its peak in terms of the new star-birth rate, and that rate has been declining ever since — our universe is past its prime.

 Image

This tiny object in the Hubble Ultra Deep Field is a compact galaxy of blue stars that existed 480 million years after the Big Bang. Its light traveled 13.2 billion years to reach Hubble.

The Chandra X-ray Observatory has created its own Deep Field observations, discovering hundreds of low-luminosity active galactic nuclei, where disks feed mass onto central black holes, and emit copious x-ray radiation.

Infrared observations with the Spitzer Space Telescope completed the picture of the deepest images of the cosmos to date. Together, Hubble, Chandra, and Spitzer have created a detailed tapestry of a dynamic, evolving universe in which some two hundred billion galaxies are within the reach of our present telescopes.

Currently, Hubble is engaged in observing six new deep fields, each one centering on a galaxy cluster whose gravity can deflect, multiply, and magnify the light from more distant objects (the effect is known as “gravitational lensing”). In parallel, Hubble will also observe six deep “blank” fields. The goal is to use those so-called “Frontier Fields” to reveal populations of fainter galaxies, and to characterize the morphologies of distant star-forming galaxies.

The first of these super-deep views of the universe has already revealed almost 3000 of previously unseen, distant galaxies.

To see the very first galaxies that formed in our universe, we will have to await the James Webb Space Telescope, on schedule for launch in 2018. From a cosmic perspective, new discoveries are just around the corner!