Author: Dr. Brandon Lawton

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.

September 30, 2016

It Takes a Team

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

Standing on the Shoulders of Giants

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

Still from Hubblecast episode 89: Edwin Hubble

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

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

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

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

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

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Picture of astronomer Henrietta Swan Leavitt taken before 1921.

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

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

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

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

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

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

hubble-frontier-fields-aura-award-recipients

2016 AURA Team Award - STScI Frontier Fields

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

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

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

Science truly is a team sport.

June 29, 2016

Telescopes Team up to View Cosmic Collisions

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

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

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

MACS J0416

Chandra_draft_image1

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

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

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

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

MACS J0717

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

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

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

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

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

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

November 12, 2015 Video

‘Hubble’s Views of the Deep Universe’ – Public Lecture

On November 3, 2015, I gave a presentation called “Hubble’s Views of the Deep Universe”.  This presentation was to commemorate some of Hubble’s most influential observing campaigns during this 25th anniversary year.  Of course, I could not get to all of Hubble’s programs that observed the deep universe in just an hour.  For additional information, check out the science articles on the Hubble 25th website and, of course, keep checking back to this blog.

Dr. Brandon Lawton
“Hubble’s Views of the Deep Universe”

November 3, 2015

For two decades, the Hubble Space Telescope has pioneered the exploration of the distant universe with images known as the “deep fields”. These deep fields have given astronomers unprecedented access to understanding how galaxies form and develop over billions of years in the history of our universe, from shortly after the Big Bang to today. Join us for a retrospective view of Hubble’s contributions to the investigation of the deep reaches of the cosmos and some fresh glimpses of what Hubble is currently doing to further our understanding of the most distant parts of the universe.

This lecture is part of the monthly public lecture series at the Space Telescope Science Institute in Baltimore, Maryland. Each month addresses a different cosmic topic, usually related to Hubble, but always venturing to some fascinating part of the universe. For more information, check out the web page on HubbleSite:
http://hubblesite.org/about_us/public_talks/

August 31, 2015

Astronomers Gather from Around the World

From August 3-14, thousands of astronomers from around the world gathered in Honolulu, Hawaii, to discuss the latest astronomical discoveries at the International Astronomical Union (IAU) General Assembly. The Frontier Fields had a highly visible role during this two-week meeting, including a fascinating three-day focus meeting where all things Frontier Fields were discussed, including recent science results and the future of the Frontier Fields. In this post, I will highlight just a few of the Frontier Fields highlights at the IAU General Assembly.

 

The Frontier Fields was highlighted with a 3-day focus meeting at the International Astronomical Union general assembly meeting in Honolulu, Hawaii.

The Frontier Fields was highlighted with a three-day focus meeting at the International Astronomical Union General Assembly meeting in Honolulu, Hawaii. The focus meeting was kicked off with a great introductory talk by Dr. Jennifer Lotz (Principal Investigator of the Hubble Frontier Fields program).

A Wealth of Science

The Frontier Fields focus meeting covered much of the latest and greatest science results coming from the Frontier Fields program. Some of the new results included deeper understandings of galaxies in the distant universe, more complete pictures of the massive galaxy clusters, and the searches for exploding massive stars, called supernovae. Some big points of discussion at the focus meeting included the methods by which astronomers obtained and studied the Frontier Fields data. These methods included the analysis of the images and spectra as well as the development of physics-based models of gravitational lensing around the Frontier Fields galaxy clusters. The modeling efforts continue to be incredibly important because they tie our physics-based understanding of how gravitational lensing works with the observations of gravitational lensing, and they allow astronomers to accurately search for and study extremely distant and lensed galaxies.

We will highlight some of the new results in future blog posts.

As for the Hubble Frontier Fields, it was nice to see the progress on the observing campaign. Hubble is two-thirds of the way through its Frontier Fields observing campaign, having completed observations of four out of the six massive galaxy clusters and their four associated parallel fields. The completed Hubble Frontier Fields images are shown below.

Shown on the left is the galaxy cluster Abell 2744. Shown on the right is the adjacent parallel field.

Shown on the left is the galaxy cluster Abell 2744. Shown on the right is the adjacent parallel field. This was the first completed pair of targets in the Hubble Frontier Fields program.
Credit: NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

 

Shown on the left is the galaxy cluster MACS J0416. Shown on the right is the adjacent parallel field. These were the second completed targets of the Hubble Frontier Fields program.

Shown on the left is the galaxy cluster MACS J0416. Shown on the right is the adjacent parallel field. This was the second pair of completed targets in the Hubble Frontier Fields program.
Credit: NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

 

Shown on the left is the galaxy cluster MACS J0717. Shown on the right is the adjacent parallel field. These were the third pair of completed targets of the Hubble Frontier Fields program. This marked the halfway point of the Hubble Frontier Fields observing campaign and were completed in the Spring of 2015, around the 25th anniversary of the Hubble Space Telescope.

Shown on the left is the galaxy cluster MACS J0717. Shown on the right is the adjacent parallel field. This was the third pair of completed targets in the Hubble Frontier Fields program. This marked the halfway point of the Hubble Frontier Fields observing campaign. The MACS J0717 observations were completed in the spring of 2015, around the 25th anniversary of the Hubble Space Telescope.
Credit: NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

 

Shown on the left is the galaxy cluster MACS J1149. Shown on the right is the adjacent parallel field. These were the fourth pair of completed targets of the Hubble Frontier Fields program.

Shown on the left is the galaxy cluster MACS J1149. Shown on the right is the adjacent parallel field. This was the fourth pair of completed targets in the Hubble Frontier Fields program.
Credit: NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

A Truly Multi-Mission Effort

Perhaps the most exciting aspect of the Frontier Fields focus meeting at the IAU was hearing from the multitude of ground- and space-based missions investigating the Frontier Fields. These observatories cover a wide range of the electromagnetic spectrum, from high-energy X-rays to low-energy radio waves. Scientific results were mentioned during this focus meeting from data obtained by the Hubble Space Telescope, the Chandra X-ray Observatory, the Jansky Very Large Array, the Very Large Telescope, the Atacama Large Millimeter/submillimeter Array, the Keck Observatory, the James Clerk Maxwell Telescope, the Herschel Space Observatory, and others. There was also a discussion of how the future James Webb Space Telescope will help us understand the cosmic frontier probed by the Frontier Fields.

With so many telescopes staring at these 12 patches of the sky, a wealth of data is being accumulated and studied that will keep astronomers busy for years to come. We truly expect the science from the Frontier Fields to redefine our understanding of massive galaxy clusters and the distant universe.

Sharing the Story

The Frontier Fields were highlighted in many other venues at the IAU meeting, not just during the Frontier Fields focus meeting. The Frontier Fields were a part of a Hubble 25th anniversary image gallery exhibit in the main concourse area of the convention center. A presentation was given to the astronomy education and outreach community about how the Frontier Fields are being incorporated into education and outreach products by the Office of Public Outreach at the Space Telescope Science Institute. Frontier Fields materials were available at the official NASA exhibit during the IAU meeting.

Perhaps the most stunning display of the Frontier Fields occurred at NASA’s hyperwall. The hyperwall is a high-definition video wall that provides a large and clear view of astronomical images and visualizations. Dr. Rachael Livermore (University of Texas, Austin) gave a visually stunning tour through Hubble’s Frontier Fields, including visualizations that highlighted the effects of gravitational lensing. Dr. Christine Jones (Harvard-Smithsonian Center for Astrophysics) gave a truly spectacular multiwavelength, multi-mission view of the Frontier Fields that included data from the Hubble Space Telescope, the Chandra X-ray Observatory, and the Jansky Very Large Array.

NASA’s hyperwall is always a big draw at professional astronomy meetings, public outreach events, and informal education venues. I highly encourage you to attend a hyperwall talk if you happen to be in the neighborhood of an event that has the NASA hyperwall.  You can follow NASA’s hyperwall on Twitter – @NASAHyperwall .

 

The Frontier Fields were featured, in high-definition, on NASA's Hyperwall. Top - Rachael Livermore presents the current status of Hubble's Frontier Fields. Bottom - Christine Jones-Forman presents a multiwavelength view of the Frontier Fields.

Images from the Frontier Fields were featured, in high definition, on NASA’s hyperwall. Top: Dr. Rachael Livermore presents the current status of Hubble’s Frontier Fields. Bottom: Dr. Christine Jones presents a multiwavelength view of the Frontier Fields.

 

 

 

 

 

April 22, 2015

Taking Stock During this Hubble Anniversary Week

This is a big week for the Hubble Space Telescope. Twenty-five years ago, on April 25, 1990, the Hubble Space Telescope was released into orbit from the Space Shuttle Discovery. Astronomers from around the world are taking stock of the amazing achievements of Hubble over the past 25 years: observations that continually challenge our view of our own Solar System, discoveries of extrasolar planetary systems, a more complete view of star and planet formation, understanding how galaxies evolve from just after the Big Bang to the present day, putting constraints on the nature of the enigmatic dark matter, and even helping to discover that the majority of the mass-energy in the universe is in the form of a mysterious repulsive force known as dark energy. To top it all off, thanks in large part to five servicing missions, Hubble is a more powerful telescope today than at any point in its history.

Astronomers are not only celebrating Hubble’s iconic achievements of the past, they are looking forward to what Hubble can accomplish over the next five years. This anniversary week at the Space Telescope Science Institute (STScI) in Baltimore, Md, a symposium is being held called Hubble 2020: Building on 25 Years of Discovery. STScI is the science operations center of the Hubble Space Telescope, so it is a fitting location for astronomers to gather to discuss the past and the future of Hubble science. For the adventurous out there who would like to test and strengthen their astronomy acumen, watch the astronomy symposium online, where astronomers discuss science results with other astronomers.

For other events celebrating Hubble’s 25th anniversary, you can click here.

Hubble Frontier Fields Update

Part of the conversation happening around the past, present, and future science of Hubble focuses on Hubble’s exploration of the deep universe. As it so happens, April 2015 is also the month where the imaging and processing of the Hubble Frontier Fields data are half-way complete. Of course, astronomers will be pouring over the images for years to come — the science results from the Frontier Fields are just beginning.

Shown in the images below are the first three completely imaged Frontier Fields galaxy clusters (Abell 2744, MACS J0416, MACS J0717) and their respective neighboring parallel fields.

Shown here are the first three completed Frontier Fields galaxy clusters and their associated parallel fields. Labeled, from the top, are galaxy cluster Abell 2744, the neighboring Abell 2744 parallel field, galaxy cluster MACS J0416, the neighboring MACS J0416 parallel field, galaxy cluster MACS J0717, and the neighboring MACS J0717 parallel field. The MACS J0717 galaxy cluster image and its associated parallel field are still being processed, so we expect another version of these images shortly.

Shown here are the first three completed Frontier Fields galaxy clusters and their associated parallel fields. Labeled, from the top, are galaxy cluster Abell 2744, the neighboring Abell 2744 parallel field, galaxy cluster MACS J0416, the neighboring MACS J0416 parallel field, galaxy cluster MACS J0717, and the neighboring MACS J0717 parallel field. The MACS J0717 galaxy cluster image and its associated parallel field are still being processed, so we expect new versions of these images shortly.
Credit: NASA, ESA, and J. Lotz, M. Mountain, A. Koekemoer, and the HFF Team (STScI)

Astronomers are already looking forward to the future of deep-field science. While much of the discussion this week is about Hubble, astronomers generally acknowledge that to truly build off of Hubble’s discoveries, we need the next-generation Great Observatory, the James Webb Space Telescope (JWST). JWST is scheduled to launch in the fall of 2018. I think it goes without saying that the participants of the Hubble 2020 symposium are incredibly excited at the prospect of these two behemoths of science — these machines of discovery —  exploring the universe at the same time.

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.

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 28, 2014

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.

Zitrin_etal_2014_Abell2744_Models_3_v2

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.

March 28, 2014

How Were the Galaxy Clusters Chosen?

The 12 Frontier Fields will greatly expand upon our knowledge of the earliest galaxies to form in the universe. These images of the distant universe (in space and time), will provide us with a sneak peek at the first billion years of the universe. So how were these fields chosen?

The Frontier Fields program was sketched out by the Frontier Fields team in the earliest phases of a recommendation process. Much can change in the process of going from an initial recommendation to a final program. The final program hinged upon finding the best galaxy clusters to anchor the Frontier Fields program. Team members deliberated between several different galaxy clusters, nominated by both those directly involved in the program and the broader astronomical community, before settling on the final candidates.

Special consideration was given to galaxy clusters that

  1. maximize magnification and fit within Hubble’s view;
  2. were located in “clean” locations on the sky;
  3. were observable by ground-based observatories in the Northern and Southern hemispheres.

The Frontier Fields team, with input from the broader astronomical community, was able to narrow down the galaxy cluster candidates to the six chosen for the program. Although it was not possible to select six clusters that met all of the criteria, most of the clusters satisfied most of the criteria. Let us explore the three criteria in a little bit more depth.

 

Maximize Magnification

Astronomers focused on massive galaxy clusters as candidates because the gravitational lenses they create are likely to provide the greatest magnification of background galaxies, but there were other considerations as well.

Hubble is observing the Frontier Fields with a visible-light instrument and an infrared-light instrument. The fields of view of these instruments, defined to be the area of the sky they can image in one pointing, are relatively small – a box with sides about 1/15 the width of the full moon. Because of the small fields of view, the galaxy clusters need to be relatively compact so that any magnified background galaxy remains within the fields of view.

There is another reason why the galaxy clusters must  be relatively compact in size. For each of the galaxy clusters, Hubble is also imaging an adjacent parallel field. For the goals of the program, the parallel fields need to contain unobstructed views of the early universe, devoid of the metropolis of galaxies that make up the galaxy clusters. Astronomers lose the magnifying power of the galaxy clusters, but gain simplicity. For the parallel fields, astronomers do not require detailed models of how the light from the distant galaxies are lensed by the foreground clusters.

 

Clean Locations on the Sky

Below is a map of the sky showing the locations of the six pointings required for Hubble to acquire the 12 Frontier Fields, labeled in order of when Hubble plans to observe them. The green labels are previous deep-field programs. The map is in right ascension and declination coordinates.

For a map of the Frontier Fields on the sky, with respect to the constellations, see this previous post.  Note: The right ascension of the map in the previous post is flipped with respect to the map below in order to portray the constellations as they appear to us on Earth.

 

Locations of the Frontier Fields on the sky. The colors denote the amount of extinction of background light due to dust - red is greatest dust extinction, blue is least dust extinction. The wavy dust band across the sky is our Milky Way galaxy. Credit: D. Coe (STScI), D. Schlegel (LBNL), D. P. Finkbeiner (Harvard), M. Davis (Berkeley)

This map shows the locations of the Frontier Fields on the sky using right ascension and declination coordinates. The Frontier Fields are numbered in the order of their observations. The colors denote the amount of extinction, or dimming, of light from distant galaxies due to foreground dust. Dark red denotes the greatest dust extinction. Dark blue denotes the least dust extinction. The wavy dust band across the sky is our Milky Way galaxy. The thick purple line is the ecliptic, which is the plane of our solar system. The two thinner parallel purple lines mark 30 degrees north and 30 degrees south of the ecliptic. Previous deep-field programs are labeled on the map in green: HDF-N (Hubble Deep Field North), HDF-S (Hubble Deep Field South), UDF (Ultra Deep Field), UDS (Ultra Deep Survey), COSMOS (the Cosmic Evolution Survey), and EGS (the Extended Groth Strip). Sgr A* denotes the position of the center of our Milky Way galaxy.
Credit: D. Coe (STScI), D. Schlegel (LBNL), D. P. Finkbeiner (Harvard), M. Davis (Berkeley)

 

The two main features to note on the above map are the colors that signify dust that can lessen the light from distant galaxies reaching Hubble’s mirror and the thick purple line that marks the plane of our solar system, known as the ecliptic. The locations of the Frontier Fields’ galaxy clusters were chosen to be in relatively “clean” parts of the sky.  By that we mean that the galaxy clusters are not located where there is a large quantity of foreground dust.

Dust Extinction

The galaxy clusters in the Frontier Fields were chosen to avoid areas of greatest dust extinction. Dust extinction is the scattering or absorption of light by dust. It is problematic because it lessens the light we receive from distant objects. On the above map, dark red denotes areas of greatest dust extinction. Dark blue denotes little dust extinction. The red, high-extinction band in the all-sky map is due to the dusty disk of our own Milky Way galaxy. It appears wavy due to the projection of the sky onto the right ascension and declination coordinate system.

Zodiacal Light

The thick purple line denotes the plane of our solar system, called the ecliptic. Dust within our solar system is clustered around the ecliptic. This dust scatters the light from our Sun and produces a bright haze. It can be very difficult to observe faint objects through the zodiacal light. For this reason, the galaxy clusters were chosen to avoid the ecliptic.

 

Observable from Telescopes across the Earth

Much of what we learn from the Frontier Fields will come from follow-up observations using ground-based telescopes. Most of the galaxy clusters in the Frontier Fields are observable by state-of-the-art astronomical telescopes in both the Northern and Southern hemispheres. These include the new radio telescope in Chile, named ALMA, and the suite of telescopes on Mauna Kea in Hawaii.

For more info, the Frontier Fields galaxy cluster selection was also recently described in a Google+ Hubble Hangout.