New Interactive Explorer for Galaxy Cluster Abell 2744

The high-resolution images taken by the Hubble Space Telescope for the Frontier Fields survey have yielded a treasure trove of insights into very distant galaxy clusters.  In addition to providing astronomers with unparalleled views of galaxies that Hubble would not otherwise be able to see, the high-resolution images are providing views of distant corners of the universe that are similar to the famous Hubble Deep Fields.

To give you some idea of just how detailed and rich the Frontier Field images are, an astronomer at the Space Telescope Science Institute has created this interactive viewer to explore them yourself:

Click here to explore the Abell 2744 yourself:

Abell 2744 Viewer

To help you use and navigate the viewer, we’ve created a short video to help familiarize you with the interface and controls.  Over time, we’ll be adding more of the Frontier Fields clusters, so be sure to check back for updates.

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

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

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

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.

What is Dark Energy?

There is a dark side to the universe; in fact, most of what makes up our cosmos is dark. In our post, entitled “What Is Dark Matter?” we introduced this pie chart that shows the relative composition of everything in the universe.


Composition of matter in the universe. These numbers have been revised by results from the Planck mission. More info here. Credit: NASA/ESA

This deceptively simple diagram shows the percentages of everything the universe is made of. Embedded in this uncomplicated, straightforward pie chart is a story full of surprises and anxiety.

Measuring the Universe

With the exception of Einstein’s “biggest blunder,” few prior to the 1990’s had any expectation that a cosmological force, such as dark energy, even existed. It was thought that the universe was solely comprised of normal matter and dark matter. There was much debate on the nature of dark matter.  How much is there? How much is made of exotic undiscovered particles versus the more mundane but visibly dark stuff like planets, small stars, etc.?  Much has been learned, but dark matter is still largely a mystery today.  Theories and experiments abound to find all constituents of the missing dark matter, particularly the exotic variety that does not contain normal matter, i.e., those particles that do not interact with normal matter other than via gravitational force.

Dark matter and normal matter both have one thing in common: gravity. Thus, the expectation for astronomers was that they would observe some decrease in the expansion rate of the universe over time due to the pull of gravity from all of the matter in the universe. In the 1990’s, two groups of astronomers attempted to measure the deceleration rate of the universe independently by looking at a whole bunch of Type 1a supernovae.  Type 1a supernovae are the explosions of a certain type of star, where the explosions themselves all have the same intrinsic brightness. You can determine how far away the star is by how bright it appears to us; the dimmer a Type 1a supernova appears, the farther away it must be.  Just like the equivalent of a standard 60–watt lightbulb, finding these “standard candles” allows astronomers to accurately measure the distances, and thus the time in the past, where these explosions took place.

Click here for more information on how Type 1a supernovae were used to measure distances.

What the astronomers actually discovered was far more surprising, and it was important that two different groups did this because, if only one had done it, no one would have believed what actually happened. These teams of astronomers noticed that distant supernovae, whose light from the early epochs of the universe was just now reaching our telescopes, were fainter and thus farther away than expected. In 1998, these two groups both declared that the universe wasn’t decelerating at all – it was accelerating!

This was a completely unexpected result — no one saw it coming. I mean, the universe is full of normal matter and dark matter, all gravitationally pulling on each other as the universe expands. Shouldn’t that mean the universe is slowing down its expansion? One could hear hyperventilating cosmologists from across the globe.

After everyone started to calm down, astronomers began to ask, “OK, so what does it take to have an accelerating universe?”

The answer is, you need something else besides matter. Whatever that is, we call it dark energy.

But What Does Dark Energy Mean?

After the initial surprise of finding an accelerating universe wore off and people started thinking about it, astronomers did something they rarely do — they accepted the idea rather quickly. Usually, an unexpected result like this generates huge debates among scientists, and this did too. The thing is, the notion of a cosmological force like dark energy now solved a lot more problems than it created. In an uncharacteristically short period of time, people started warming to the idea of dark energy.

As a function of time, galaxies are moving away from us at a faster and faster rate, and that is what is meant by an accelerating universe. The discovery of dark energy has brought the ultimate fate of our universe back into question. Will dark energy continue to increase its dominance over gravity and cause our universe to rip apart — a potential fate known as the Big Rip? Or will the repulsive force of dark energy and the attractive force of gravity balance out so that the universe expands forever at a constant, non-accelerating rate? With the current understanding of dark energy, it seems improbable that gravity will reverse the expansion and collapse the universe back in on itself. However, the nature of dark energy is not well understood yet.


What’s Next for Dark Energy?

Right now, astronomers are making observations designed to constrain some of the many dark energy models that are out there.  The nature of this research is often done from the ground so that wide areas of the sky can be observed for a very long time. This kind of campaign is not well-suited to a high-demand telescope like the Hubble Space Telescope. The idea is to “constrain,” or better understand, the expansion rate of the universe, and measure the growth of large–scale structure (like galaxy clusters).

Past surveys like the Sloan Digital Sky Survey have made some progress, and current projects like the Dark Energy Survey (DES) has started its observing runs. DES will observe 5,000 square degrees of the night sky over 525 nights, making measurements that should help us whittle down some of the many dark–energy models presently being considered. Currently being built is the Large Synoptic Survey Telescope, an 8.4–meter ground-based telescope in Chile, which will image the entire sky every few nights at several wavelengths, and will no doubt play a large role in helping us understand dark energy.

Space-based telescopes do have an essential role to play in characterizing dark energy. For example, Hubble has played a key role in getting data on distant supernovae — hence the discovery of dark energy. It is the combination of ground-based large surveys with space-based pointed deep follow-ups that give us our breakthroughs. Future missions are being envisioned to build on the best of both ground-based surveys and space-based observations. The Wide-Field Infrared Survey Telescope (WFIRST) will use a Hubble-class, space-based telescope to survey a large portion of the sky in an effort to better constrain the nature of dark energy through the history of the universe.

Frontier Fields and Dark Energy

While the Frontier Fields were not designed to capture the large numbers of supernovae needed to explore dark energy through cosmic time, the observations of strong galaxy cluster lensing will be used in combination with cosmological measurements from other missions to help constrain the nature of dark energy.  Stay tuned for more!


What is Dark Matter?

hs-2010-37-b-web_printOne of the most novel aspects of the Frontier Fields project is the innovative way in which the Hubble Space Telescope is being made more powerful — without adding a single piece of equipment or changing a single hardware component.

While Hubble itself isn’t altered physically in any way to allow us to peer farther than we ever have into the universe, these observations wouldn’t be possible without one crucial component: dark matter.

Frontier Fields is turbocharging Hubble by looking at the distant universe through gravitational lenses that boost the signal from the feeble light of remote galaxies, essentially making Hubble a more powerful telescope.

For the amateur astronomers out there, these gravity lenses are like adding a Barlow lens to the eyepiece of Hubble.

What creates these gravitational lenses?

Matter, and lots of it.  Thanks to the theory of general relativity, we know that space-time is warped by stars, planets, galaxies, black holes — anything with mass. The light bends as it travels through this warped space-time.

This is exactly what ordinary lenses in a telescope do with light:  they bend it. Hence the term “gravitational lens.”

In order to make a decent gravitational lens that will show you the most distant galaxies in the universe, you need lots of matter.  Among the largest collections of matter in the universe are of galaxies.  Hundreds of billions of stars all grouped together can bend a lot of space-time (and they do). What could be better?

A lot of galaxies all grouped together, otherwise known as galaxy clusters.

We’ve written before about the galaxy clusters that the Frontier Fields team will observe throughout the course of the survey. They were chosen because they made good gravitational lenses.

But while the galaxies in these clusters do have lots of stars in them — hundreds of billions in each one — stars actually are not the major factor contributing to the bending of space-time around the clusters.

The largest contributor to the creation of those gravitational lenses is something we can’t see, smell, taste, hear, touch or interact with in any way: dark matter.

This stuff is all over the universe —  in fact, there is five times more of it in the universe than there is ordinary matter.  Everything we can see in the cosmos — stars, planets, comets, all life on Earth, anything that’s made up of atoms — constitutes roughly 5% of the total matter and energy in the universe.  Dark matter makes up about 24%.


Composition of matter in the universe. These numbers have been revised somewhat by the Planck Space Telescope. More info here. Credit: NASA/ESA

It’s usually at this point the astute person starts asking, “If dark matter won’t interact with us in any way, how do we know it’s there?”

The answer is simple enough. We know dark matter exists because we can see its effects on those things we can see.  We were first tipped off to dark matter in the 1950’s by the motions of galaxies. We noticed that if we added all the mass of all the stars inside of galaxies, something wasn’t right.  The galaxies didn’t rotate the way they should.  Their motions suggested that something else had to be there mixed in all the stars we could see.

What’s more, the galaxies that were gathered together into clusters were short on mass. If just the mass we could observe was all there was, the clusters would fly apart. There wasn’t enough observed mass to make them stay together.

This stuff, whatever it was, was making galaxies rotate as if they had more matter than we could see and was also holding galaxy clusters together.  In astronomy, we are used to investigating celestial objects by the light they emit, reflect, or block. We called this strange new discovery dark matter because it does not interact with light — though clearly it has a gravitational field we can detect.

We’re starting to get pretty good at estimating where the dark matter is in galaxy clusters. We can even make maps of it.  Here is a map of dark matter around the Abell 1689 cluster, home to thousands of galaxies and trillions of stars.


Dark matter in the massive galaxy cluster Abell 1689, located 2.2 billion light-years away. The cluster contains about 1,000 galaxies and trillions of stars. Hubble cannot see the dark matter directly. Astronomers inferred its location by analyzing the effect of gravitational lensing, where light from galaxies behind Abell 1689 is distorted by intervening matter within the cluster. Credit: NASA, ESA, and Z. Levay (STScI)

Astronomers have gone so far as to map where most of the dark matter is in the universe. Here’s a graphic showing the distribution of dark matter in the universe.


This three-dimensional map offers a first look at the web-like large-scale distribution of dark matter. The map reveals a loose network of dark matter filaments, gradually collapsing under the relentless pull of gravity, and growing clumpier over time. Credit: NASA, ESA, and R. Massey (California Institute of Technology)

Most astronomers believe that dark matter is concentrated in and around small clusters of galaxies.

For the purposes of the Frontier Fields Survey, dark matter plays a crucial role. Without it, these galaxy clusters would have less mass, and space-time would bend less significantly, creating a weaker lens.  By using these powerful natural lenses, the Frontier Fields project will enable Hubble to see galaxies about 10 times deeper than the Ultra Deep Field, the current record holder for the deepest image ever taken.

And that corresponds to 40 billion times fainter than what the human eye can see.

Now the next question you may be asking is, “What’s this dark matter stuff made of?” Astronomers are actively researching that question, but that’s a post for another day — so stay tuned!

Frontier Fields at AAS 224

Frontier Fields had a big presence at this year’s January meeting of the American Astronomical Society.  On Jan. 7, there was a news release announcing the results of the first set of observations of galaxy cluster Abell 2744, along with a gorgeous image of the cluster.  We met with Dr. Jennifer Lotz, the principal investigator for Frontier Fields to get an update and discuss these latest results.

Frontier Fields Hangout Highlights

For those who would rather not sit through the whole hangout (and I really can’ t imagine why you wouldn’t since it was very interesting), I’ve taken the time to index some of the more interesting topics discussed during the hour.  Click on the topic below to go to that part of the hangout.

Please stay tuned for more Hubble Hangouts on the Frontier Fields as the project progresses.  We are planning more hangouts that discuss the role of dark matter in the Frontier Fields clusters, how to get the data yourself from the Hubble archive, and much more!

Supernova Discovered in One of the Frontier Fields

HFF13Zar, Compiled by Steve Rodney

The data had hardly started coming through the pipeline when astronomers made the first Frontier Field discovery: a supernova in the galaxy cluster MACSJ0717, one of the first of the Frontier Fields to be imaged.

The Frontier Fields designation for this object is SN HFF13Zar, and its nickame is “SN Zara.”

Supernovae discovery is an offshoot of Frontier Fields science because Hubble will be revisiting many of these fields several times over the next three years, allowing astronomers to compare recent images with older ones, and look for things that are different.

The supernova is located 1.73 arcmin from the center of the MACSJ0171 cluster and is a whopping 23.53 (+- 0.05) magnitude.

I say whopping, but big numbers on the magnitude scale mean an object is very, very dim. This is definitely a faint supernova, but not out of the ordinary in terms of what Hubble can see. Hubble can see things as faint as 31st magnitude, which is slightly fainter than objects that can be viewed by the best ground-based telescopes.

Without getting too crazy into the magnitude scale topic, suffice it to say for our purposes that

One magnitude thus corresponds to a brightness difference of exactly the fifth root of 100, or very close to 2.512 — a value known as the Pogson ratio. Source: Sky and Telescope

Aren’t you glad you asked?

So the supernova is faint, but Hubble can see it without problems, as you can tell from the right panel, in which a purple circle marks the supernova.  The left panel in this image is a compilation of observations taken in 2006 and prior with Hubble’s Advanced Camera for Surveys (ACS).

SN HFF12Zar was discovered using the F814W filter, known as the i band in the ACS.

The supernova’s home is still unknown — it could have occurred in one of three potential galaxies within 5 arcsec of the stellar explosion. These galaxies are labelled in the above image as A (orange), B (red) and C  (light blue).

The other circles, D (green) and E (yellow) are other galaxies probably not associated with the supernova.

The redshift of the galaxy cluster is z=0.5458 (~10 billion LY away) and according to Dr. Steven Rodney (JHU), Dr. Jennifer Lotz (STScI), and Dr. Louis-Gregory Strolger (STScI), if the supernova is associated with host galaxy candidates A or B, it is a foreground object. If it’s associated with host galaxy candidate C, then it could plausibly be a SN from a galaxy in the outskirts of the cluster.

We’ll be revisiting this cluster again with Hubble in December 2013, as part of the Grism Lens Amplified Survey from Space (GLASS) proposal, but this supernova will probably be faded by the time Hubble looks this way again.  However Steve Rodney and Lou Strolger have a program to search the Frontier Fields data for new supernovae as it comes in; if they find something that is potentially very interesting — very distant and/or lensed by the cluster, they will trigger extra Hubble observations of the supernovae to determine the type of supernova and exact distance.