Friday, December 21, 2012

Life as an Observer

The Keck Telescopes on Mauna Kea
Courtesy W. M. Keck Observatory
One of the best aspects of being an astronomer is actually using a telescope and collecting data. Tonight, I have the opportunity to use one of the best telescopes in the world along with an exciting new instrument. Fellow CANDELS team member Mark Dickinson and I are observing on one of the Keck telescopes atop Mauna Kea on the Big Island of Hawaii. Mauna Kea is one of the best observing sites on the planet because the peak of the mountain where the telescopes are located sits above a substantial portion of the atmosphere. Because of this, the atmosphere has less of an effect on the images of astronomical objects than it would from an observatory at sea level. While the Keck telescopes themselves are at the summit of the 14,000 foot mountain, observers work from a remote observing facility located in Waimea. Being at this lower altitude makes it much easier to work and all of the instruments can be controlled remotely. 

The instrument that we are using is called MOSFIRE (Multi-Object Spectrometer for InfraRed Exploration) and is very new. It is a top of the line instrument that allows us to obtain sensitive, high resolution near-infrared spectroscopy of many objects at the same time. Most spectrographs in the near-infrared observe one object at a time. There are now several available that can observe many objects at once but they are often difficult to use for very faint galaxies. MOSFIRE is still brand new but so far has been working well. CANDELS team member Jonathan Trump recently published a paper on some first results! 

Mark Dickinson and I observing in the Keck I Remote Observing
Control Room
We spent the last several days carefully selecting targets to observe in two of the CANDELS fields, GOODS-S and COSMOS. Our primary targets of interest are distant luminous infrared galaxies detected by the Herschel Space Observatory at z~2. At this redshift, many of the interesting optical emission lines (such as the Hydrogen line known as H-alpha) are shifted into the near infrared. We can use these various lines to measure precisely how far away the galaxies are, whether or not an AGN might be present, how important that AGN is to the energy output of the galaxy, how much star formation is taking place in the galaxy, as well as many other things.

In addition to these targets, we are also observing AGN selected in other ways, such as through X-ray detections or from the shape of their SED in the near-infrared. If there is any space left we are also looking at other types of galaxies at these high redshifts to see if we can detect lines

Once our target selection was complete, we waited anxiously to see what the weather would do. The forecast called for clear skies but starting last night the summit of Mauna Kea became foggy. We awoke this morning (morning for an observing astronomer is really about 2 PM) to find that the fog had not cleared and some clouds had rolled in. We were really starting to get nervous! However, we proceeded as planned and began our afternoon setup. Around sunset, things were really looking dicey and it started to snow! Luckily for us that didn't last too long and the road to the summit remained clear for the night crew to head up and check things out. After a couple of hours, the fog cleared and the humidity dropped to a level low enough to open. We had a good time learning how to operate this new instrument and at 9:30 PM we started our first exposure on a set of galaxies in GOODS-S!

video 
Video from a webcam at CFHT (the Canada-France-Hawaii Telescope) on Mauna Kea showing the clouds roll in Wednesday night and ice starting to form on the camera itself. Video courtesy of CFHT Observatory


Over the course of the night clouds have come and gone but the weather has steadily been improving. Right now we are observing galaxies in COSMOS and things are looking good. We are anxious to analyze our data and see how many of our galaxies have been detected. With any luck, we will soon be writing a blog post about our results! 

This is our last blog post before the holidays but we look forward to discussing more CANDELS science in January. Happy Holidays! 

Follow the Keck Observatory on Twitter!  

Monday, December 17, 2012

Astronomer of the Month: Timothy Dolch

Each month we will highlight a member of the CANDELS team by presenting an interview introducing them and what it's like to be an astronomer. This month's Astronomer is Timothy Dolch.


Tell us a little about yourself!

My name is Tim Dolch, and I’m a postdoctoral research associate, or postdoc, at Oberlin College in Oberlin, OH. (“Postdoc” refers to the phase in a research career between graduate school and a permanent position, during which a professor or full scientist hires a recently graduated Ph.D. While postdoctoral appointments are typically only funded for two years - and astronomers often do at least two separate postdoc jobs - the benefit is that you can devote most of your time to research. Finding a permanent position is, obviously, nice and permanent, but the downside is that a lot more of one’s time goes into grant writing and other administrative matters.) Born and raised in the vicinity of Cleveland, OH, I attended the California Institute of Technology in Pasadena, CA where I majored in physics, although taking as many astrophysics classes as possible. I did my graduate work at Johns Hopkins University and the Space Telescope Science Institute  in Baltimore, MD. There I did my thesis research with Harry Ferguson, and became a co-investigator on the CANDELS project.
 
What is your specific area of research? What is your role within the CANDELS team? 

For my postdoctoral research I’m working with the NANOGrav collaboration detecting gravitational waves with pulsars. Within the CANDELS group my research was, and continues to be, on the extragalactic background light or EBL. (See my July blog entry for a more detailed explanation.) During the 20% of time I have as a postdoc to work on personal research outside of the NANOGrav collaboration, I have been leading the EBL group’s telecons and working on several papers that, together, will be a sequel to my Ph.D. thesis.

What made you want to become an astronomer? At what age did you know you were interested in astronomy? 

My family went to science museums and astronomy club events frequently when I was growing up, and so I don’t really remember a time I wasn’t interested in astronomy. I have always been an avid science fiction fan, which led me to read a book called “Asimov on Physics” when I was fourteen years old. It’s a beautiful little book that traces the history of physics starting with, I think, Archimedes, and ending with the Uncertainty Principle. After that I was sure I wanted to work in some area of physics, and naturally this led me to astrophysics.

What obstacles have you encountered on your path to becoming an astronomer and how did you overcome them? 

During graduate school I realized experientially how important the writing process is to being a scientist. While I had known this fact theoretically, I was surprised how much time it was taking me. Eventually I took the advice of friends and colleagues and started writing for three hours every morning (even if I didn’t know exactly what I was going to say), and then made a lot of progress. I discovered that one shouldn’t simply think up ideas first and write about them later – writing itself is part of the scientific thought process. It’s not as though you have an insight about nature one day and then the next day have to pay the price of writing about it. The encounter with nature can happen the moment you touch your fingers to the keyboard.

Who has been your biggest scientific role model and why? 

Although he was primarily a mathematician, Jacob Bronowski has become one of my heroes because he is filled with wonder at the very fact that  science works at all. I read his book “Science, Magic, and Civilization” during graduate school and suddenly found myself not taking the profound nature of scientific reasoning for granted. I remember being taught in elementary school that the scientific method consists of forming a hypothesis, doing an experiment to test it, and then modifying the hypothesis according to the results. While this is not false, it’s grossly incomplete. We go through that process in our heads all the time with or without science. Cavemen went though that thought process. What is the insight that occurred during the scientific revolution? According to Bronowski, it was the conviction derived from Platonic thought that nature operates linguistically. The language nature speaks is the same mathematical language we know. The book helped me to see that this strange correspondence between ourselves and the universe (strange because we can understand it at all!) is a reason to always expect surprises from nature.

What is it like to be an astronomer? What is your favorite aspect? 

It’s a lot of work and personal sacrifice – for example, you have to be willing to move anywhere in the country, if not the world, at multiple junctions of your career, especially during the postdoctoral phase. But the reward is all the beauty you get to see which you wouldn’t otherwise know about. My favorite aspect is the way that, as time progresses, I’m more and more amazed by subtle, more hidden phenomena that I wouldn’t have known about as a general reader. Recently I learned that millisecond pulsars which have a double-peaked pulse are that way not because they are double pulsars, but often because they are single pulsars rotating at speeds approaching that of light – and thus their apparent pulse profile undergoes a relativistic transformation.

What motivates you in your research? 

I wait expectantly for those moments when something in nature becomes, as it were, unveiled. This happened most strikingly for me when I was looking at a plot, and saw that one of my background predictions lined up unexpectedly with a published measurement.

What is your favorite astronomical facility? (This could include telescopes or super computers, for example) 

For me it’s a tie between the Hubble Space Telescope (HST) and the Arecibo Observatory in Arecibo, Puerto Rico. HST needs no further justification after one sees the Ultra Deep Field for the first time. Arecibo (featured in the movies Contact and Goldeneye) is the observatory with which I have a lot of tangible experience – it’s breathtaking to enter the coordinates of a pulsar in the control room and see the Gregorian dome 450 feet above you move and track the source.

Where do you see yourself in the future? What are your career aspirations? 

I would like to continue doing research and be heavily involved with teaching and outreach as well. 

If you could have any astronomy related wish, what would it be? 

The Green Bank Telescope in West Virginia in under threat of closure. This would be a tragedy for radio astronomy, especially as far as gravitational wave detection is concerned. So I would like to see that saved. I would also like to see an EBL-specialized camera mounted on a spacecraft flying to the outer solar system. That way the extragalactic (or at least, extrasolar) background light could be observed without obscuration from all the dust in the zodiacal cloud. There are proposals in the works along these lines.

What is your favorite, most mind-boggling astronomy fact? 

Interstellar dust grains migrate into the solar system and occasionally enter our atmosphere. About 22 tons worth of interstellar dust reside in our atmosphere at any given time.

Is there anything else you would like for the public to know about you or astronomy in general? 

I did theater in high school, and recommend the experience for astronomers or any scientists who are starting their careers. It can help you with speaking to an audience later on, which is something you’ll eventually do frequently.

Wednesday, December 12, 2012

Astronomers in a Castle

From Ringberg Castle looking to the mountains. Image Credit: D. Kocevski
On a bluff in the foothills of the Bavarian Alps, overlooking the placid waters of the Tegernsee, sits a castle. Now, castles are not uncommon in Germany, but this particular one is special for astronomers and many other scientists around the world. In 1973, this castle, Schloss Ringberg, was donated to the Max Planck Society and ultimately converted into a location for scientific meetings and conferences. Here, as many as 60 scientists can get together in a picturesque location, away from the bustle of everyday academic life, and put their minds together to tackle interesting and relevant questions in their fields, in a setting that promises good food, comfortable quarters, gorgeous views, camaraderie and colorful insights into the romantic, and only a bit outlandish, inspiration of the castle's creators and architects, the Wittelsbach Duke Luitpold and the early twentieth century artist, Friedrich Attenhuber.

Chess and science in the Duke's office.Image Credit: D. Kocevski
Every room is a testament to Alpine culture and design: warm and dark wood paneling, green spectrum paint, runic filigree embellishments, enormous tiled stoves in each room, pastoral and woodland themed tapestries and murals on walls and balconies. It was to this rather unique setting that a group of astronomers and astrophysicists, including several CANDELS team members, converged on the 3rd of December, a cold and snowy Monday, for a conference to discuss the connection between Active Galactic Nuclei (AGN) and the galaxies in which they are found. This connection is of key interest to astronomers studying how galaxies form and change with time, as discussed in many older posts in this blog. The primary aim of the meeting was to bring together some of the best theoretical models of galaxy evolution and compare them with our fresh and recent understanding of AGN from various observational campaigns, many of which have gained from the superb coverage of CANDELS.

AGN, to this enamored scientist, are some of the most fantastic denizens of our astounding Universe. Our best picture of these beasts is one in which profoundly dense super-massive black holes, millions to billions of times as massive as our Sun, are growing as vast amounts of gas fall into them, in a process that is, as yet, poorly understood. Immense sources of energy, they pump radiation and powerful winds into their environments, and, through this, halt the formation of stars by blowing out or heating essentially all the gas in the galaxies they inhabit. It turns out, however, that it is genuinely very hard to pinpoint the moment in which AGN actually directly affect their 'host' galaxy, mostly because such 'feedback' happens over a very short time, a fleeting heartbeat of an instant in the long eons of cosmic time. Instead, galaxy modelers can tell observers like me what the long-term effects of feedback are and we can go out to our telescopes and vast datasets and try to search for the smoking gun, the imprint of AGN feedback on their host galaxies.


Astronomers at the Ringberg AGN meeting, deep in the middle of a discussion session.
Image Credit: D. Kocevski

At the meeting, some spectacular examples were shown of heating and winds from AGN, clearly supporting the notion that feedback from the black hole has its place in the panoply of astronomical processes. However, a number of new studies show a very weak relationship between star-formation and AGN activity -- not what is expected, either by models or if feedback is widely present. We had some interesting and fruitful discussions at the meeting about this apparent mismatch. Is feedback not as efficient at halting star-formation as we had thought? Do we understand well enough the timescales over which feedback acts on galaxies? Are we missing something in the models? A clear picture eluded us, but we moved on with a set of new experiments and concepts with which to tackle these problems.

Another interesting and relevant question that took up a lot of discussion is why AGN turn on in the first place: what triggers them? Bright AGN are rare - another poorly understood aspect of their nature. Their frequency, how common they are in galaxies, has a directly relation to how much they can affect the evolution of their galaxies, especially if their occurrence is tied to an important phase in a galaxy's life. For example, if a galaxy happens to enjoy a windfall of gas from intergalactic space which spurs the formation of stars, some of that gas may fall to the center and light up the black hole. The resultant AGN will then blow out the gas from the galaxy - the ultimate cosmic killjoy. In this case, the trigger is the process that dumps gas into the AGN's host galaxy and the processes that carry it to the center. Other popular triggers are galaxy mergers, vast collisions that roil up galaxies and spur huge bursts of stars and nuclear activity.

At the meeting, we learned about novel and interesting ways to fuel and trigger AGN in the early Universe. Back then, the Universe was a lot denser than it is now, and streams of gas would fall on to galaxies from intergalactic space. In a process similar to mergers, these flows of gas could shake up the galaxy and send a lot of gas into its center. Such a process is not expected to be important in the present-day Universe around us - a reminder of the arrow of time and the finite history of the Cosmos. In more local AGN, a clear sign of the role of galaxy mergers was shown from studies of pairs of AGNs in the Sloan Digital Sky Survey. In addition, a simple connection was shown to exist between AGN and the gas content of a galaxy, which suggested that, while mergers are a good way to trigger AGN, they are probably not the most important process in nearby galaxies. The meeting helped to underline the complex variety of ways that gas feeds the monstrous black holes in the centers of galaxies.

Stories exchanged over beer in the fabulous Hexenzimmer.
Image Credit: D. Kocevski
Over two and a half days of presentations and lengthy discussions, we pondered the physics of unfathomably incredible forces in the far reaches of our Universe as heavy snow piled up on the trees and coated the castle grounds. Over breaks for coffee and lunch, we huddled in smaller groups and debated over some of the finer points of theory or interpretation, skeptical inquiry that defines the method of science. Over big glasses of fresh Bavarian beer in the evenings, we traded our scientist hats for other metaphorical headgear, as we exchanged stories and banter, and got to know each other better on a personal basis. This is probably one of the lasting legacies of a meeting at Schloss Ringberg - collaborations, future plans, friendships. All because a Duke decided decades ago to build a picturesque castle that he couldn't quite afford on a hill in Germany.


Monday, December 10, 2012

The Theoretical Astrophysical Observatory

This post is not going to be about CANDELS directly, but about work that, in the long run, could play an enormous part in helping CANDELS astronomers analyse and interpret their data.

ASwinburne University in Australia, myself and my group are developing a new tool, called the Theory Astrophysical Observatory (TAO), which will make access to cutting edge supercomputer simulations of galaxy formation almost trivial. TAO will put the latest theory data in to the "cloud" for use by the international astronomy community, plus add a number of science enhancing eResearch tools. It is part of a larger project funded by the Australian Government called the All Sky Virtual Observatory (ASVO).

TAO boasts a clean and intuitive web interface. It avoids the need to know a database query language (like SQL) by providing a custom point-and-click web-form to select virtual galaxies and their properties, which auto-generates the query code in the background. Query results can then be funneled through additional "modules" and sent to a local supercomputer for further processing and manipulation. These include the ability to:

  • Construct observer light-cones (i.e. with the geometry of the sky) from simulated data cubes (the default format of the models and which assume a cartesian geometry);
  • Generate complete spectral energy distributions for model galaxies to provide true multi-wavelength galaxy luminosities;
  • Produce custom mock images of the sky in each simulated universe;
  • Add a virtual telescope simulator, through which the theory data can be "observed" by current or future telescopes.

TAO is already making it easy for the scientific community to apply the latest theoretical models. With an early TAO prototype, a group of Australian astronomers constructed a representation of the deep night sky that will be seen by the newly commissioned Australian Square Kilometer Array Pathfinder (ASKAP) radio telescope, which is based in the outback of Western Australia.

To do this, the TAO prototype was used to build a light-cone of many hundreds of thousands of simulated galaxies. These galaxies were selected from the much larger millions contained in the TAO database. The selection was based on the neutral hydrogen properties of each galaxy, as predicted by one of my galaxy formation models (Croton et al. 2006). After fine tuning to match the sensitivity of the ASKAP telescope, the researchers were able to reproduce the ASKAP night sky even before a single photon in the radio part of the electromagnetic spectrum had ever been collected by the telescope.

The night sky, as seen by the ASKAP radio telescope, and generated by a new eResearch tool, the Theoretical Astrophysical Observatory (TAO). Similar representations of the CANDELS fields are currently under development.

In the visualisation shown above one can see the light-cone of galaxies produced by TAO. Two surveys were constructed from the simulated data. The first replicates the shallower and wider WALLABY galaxy survey and is expected to find approximately 600,000 galaxies. The second is the much deeper and  narrower DINGO galaxy survey, and will find approximately 100,000 galaxies according to TAO. More information on these surveys can be found at the ASKAP science page.

The work has recently been published in the prestigious UK journal The Monthly Notices of the Royal Astronomical Society. Don't forget to check out the amazing movie here!

These tools are being applied to the CANDELS survey to do similarly exciting science. Watch this space!

Thursday, December 6, 2012

Supernovae Part II – Supernova Environments

In a previous post, we introduced readers to the exciting field of supernovae. Today we are going to delve a little deeper and discuss the environments where supernovae explode. These environments are interesting because they can provide information about the nature of the stellar systems that produce supernovae.  This can help us understand how and why some of the brightest events in the universe are produced.  

Type Ia supernovae are known as “standardizable candles”. Without going too deep into the science, a "standard candle" is an object whose observed brightness only depends on how far away it is. If you hold a flashlight 5 feet from your eye, it looks much brighter to you than if you hold it 100 feet away. By knowing the difference in brightness, you can estimate the distance to the flashlight. Now picture a flashlight with dying batteries. If you only see the flashlight far away, and have no idea how bright it would be up close, you can't estimate the distance. But if you somehow know how much power is left in the batteries, you can use that information to correct your distance estimate. In a nutshell, this is a "standardizable candle". The distance to a "standardizable candle" is not only based on it's observed brightness, but also on intrinsic properties of the object. For supernovae, what this means is that by measuring certain properties of the explosion through our observations, we can estimate how bright it should be if it were nearby, and therefore we can calculate how far away it is. 

In one of the most astounding tenets of astronomy, looking at objects that are farther away is analogous to looking back in time, because the speed of light is finite. It takes a long time for the light from these supernovae and their host galaxies to reach us. Finding supernovae that are farther away therefore allows us to trace the history of the expansion of the universe. The 2011 Nobel Prize in Physics was awarded for the role of type Ia supernovae in discovering dark energy, the mysterious force that is driving the acceleration of the expansion of the universe. Several CANDELS team members played key roles in the discovery, and team member Adam Riess was one of the prize recipients.

Recent research suggests that the brightness (and therefore our estimated distance) of a type Ia supernova depends in some way on the properties of it’s host galaxy. This suggests that the explosion mechanism for type Ia supernovae depends in some way on the surrounding environment. In the CANDELS Supernova Survey, we are searching for supernovae from a time when the universe was only 3 or 4 billion years old (we have measured the age of the universe to be around 14 billion years old). Galaxies from the early universe do not look like our own Milky Way galaxy; they are smaller, bluer (because young, hot stars are blue), and less polluted with heavier elements such as iron which are produced in the interior of stars and in supernovae. Studying the environments of these very far away supernovae is therefore important for tracing the expansion history, but also in our basic understanding of the systems that produce type Ia supernovae.

In the rest of this post, we will show some of the diverse galaxies that supernovae have been discovered in by the CANDELS supernova team.

CANDELS images of supernova host galaxies: images a), b), c), e) and f) are 0.004 degrees on each side. Image d) is 0.008 degrees on each side. For reference each image is approximately the size of a US quarter dollar viewed at 1/4 of a mile away.

a) This galaxy is one of the most nearby supernova hosts that we’ve discovered in CANDELS. The light that reached the Hubble Space Telescope to produce this image was emitted around 2 billion years ago. It is a fairly typical “edge-on” galaxy; we are viewing the galaxy right along the plane of the disk. If you look closely, this galaxy has a faintly visible dust lane; this material blocks the light emitted behind it and is therefore slightly darker.

b) This is a similar looking galaxy, but slightly farther away. The light in this image was emitted about 4 billion years ago.

c) Likely a similar shape galaxy to the previous two galaxies, however this one is rotated 90 degrees so that we’re viewing it from a very different angle. This galaxy is also quite a bit farther away; the light you are looking at was emitted 7 billion years ago! The blue clumps are likely bright, blue, massive, young stars, indicating significant recent star-formation. The supernova that went off in this galaxy may be a core-collapse supernova (core-collapse supernovae are much more likely to be found in blue regions like the ones in this image) instead of a type Ia supernova. A core-collapse supernova is the death of a massive star; when the star runs out of nuclear fuel, the outer regions of the star collapse onto a very dense, compact core, and the rebound of this material causes the explosion that we observe. The CANDELS supernova team is still working on the classification of all of our discovered supernovae.

d) Don’t be distracted by the galaxies to the left; the compact object in the center is the host galaxy of a supernova that exploded 6 billion years ago. This galaxy is observationally very different from the first 3 hosts. It is likely much less massive, and though it is difficult to draw conclusions about current star formation from just this image, it is not very blue, implying that it contains alot of fairly old stars.

e) Again not to be distracted by surrounding objects, the faint blue galaxy in the center of this image hosted a supernova 7 billion years ago. The size and shape of this galaxy is similar to d), but this galaxy is much more blue. It is highly likely that this galaxy has had more recent star formation.

f) These three galaxies all appear to be at the same distance from Earth, and so we believe they may be interacting, merging together at some point to form a larger galaxy. We are seeing them as they were about 8 billion years ago. A supernova that exploded in one of these galaxies is very likely a type Ia, and the environment appears different from some of the nearby type Ia supernovae. 

The CANDELS supernova team is hard at work classifying and analyzing the supernovae that we are discovering. You can see the diverse nature of galaxies that host supernovae; while star-forming (blue) galaxies are more likely to host a core-collapse supernova than old, red galaxies, this does not mean that type Ia supernovae cannot explode in blue galaxies. There are many questions that remain to be answered regarding the role of the host galaxy environment on type Ia supernova explosions. Understanding the environment of each supernova will be an important tool in constraining the nature of type Ia supernova progenitors. As the supernova environment may evolve as we discover them farther and farther away, understanding the role that the host galaxy plays in our distance estimate is a goal of the CANDELS supernova team.

Tuesday, December 4, 2012

Binary Black Hole Workshop

Illustration of binary black holes. Image credit: P. Marenfield (NOAO)
Last week, I attended a workshop on Binary Black Holes and Dual AGN sponsored by NOAO in Tucson, Arizona. This workshop was held in memory of David de Young, an NOAO astronomer who passed away last December. Dave received his PhD in 1967 from Cornell University and joined the NOAO staff in 1980. Dave's own research focused on Active Galactic Nuclei (AGN) and he had over 120 scientific publications over the course of his career. The focus of last week's workshop was on a topic that Dave's research related to in many ways.

In previous posts, we introduced super massive black holes and AGN. This meeting focused on the search for pairs of black holes in galaxies. Why are these interesting? There is evidence to suggest that all massive galaxies contain a supermassive black hole in their centers. When two galaxies merge together, each of the two likely had their own black hole. Eventually, the two black holes will merge together at the center of the coalesced galaxy, but before this happens there should be a time when the two black holes are observable separately. In some cases, both may be active, in which case a dual AGN might be observable. Finding and studying these systems can tell us a lot about what happens in the final stages of a merger and we can learn a lot about black hole physics.

The goal of the workshop was to discuss many aspects of binary black holes, including how to identify them and how they affect their host galaxy. One of the methods used to identify binary black holes is to simply look for pairs of AGN on the sky at the same redshift. This is usually done by selecting AGN identified in the X-ray, but can also be done using a variety of other AGN selection techniques. This method is straight-forward, but it requires that both black holes be active and detectable, and thus finding such systems is very rare. Another common method is to look for spectroscopic signatures of two black holes. Due to the motions of two black holes in orbit around each other, spectra of such systems often have emission lines with two peaks, one for each black hole. However, since other processes can cause these types of emission lines, detailed follow-up observations and analysis is required for these candidates.

We also heard a lot about adaptive optics (AO) observations of binary black holes. Since the two black holes in a merging system are so close together on the sky, it can be very difficult to separate them from each other in images. One technique for obtaining very high resolution images from the ground is called adaptive optics (or AO for short). With AO, images are corrected for the distortions that are caused by atmospheric turbulence so that the final image is much sharper than would have been obtained otherwise. With AO, astronomers are able to peer into the very center of nearby merging galaxies and separate double nuclei and identify the two black holes. In many cases, the surrounding area can be studied in great detail in order to determine how the black holes have affected their surroundings.

Since mergers are important for producing binary black holes, one of the topics of the meeting was trying to understand the connection between mergers and AGN activity. In this context, the recent CANDELS paper by Dale Kocevski came up as an example of the many studies that have tried to investigate this question at high redshift. There is currently a lot of debate about the role that galaxy mergers play in forming AGN in general. The relationship between AGN and their host galaxies is the topic of a meeting taking place right now in Germany and will be discussed more in a future post!

Friday, November 30, 2012

Progress in the Quest for the Most Distant Galaxies

One of the goals of CANDELS is to study galaxies during Cosmic Dawn, when galaxies were just beginning to form. Other teams are also pursuing this kind of research, using the CANDELS data as well as deeper observations from the Hubble space telescope.

There has been some interesting progress in the past few weeks.

The Most Distant Galaxy in the Hubble Ultradeep Field

 

Redshift z>9 candidate galaxy from the
Hubble Ultra-deep field. From Rychard
Bouwens' preprint.
On November 13, Rychard Bouwens posted a preprint that solidified the evidence that a previously-identified galaxy is really very distant. This galaxy is located in the Hubble Ultra-Deep Field, which is located within the CANDELS survey region. But this galaxy is too faint too see with the exposure times that we use for the rest of the CANDELS survey. It took four days of exposure time with the WFC3 infrared camera on Hubble to detect the galaxy. For comparison, the deepest CANDELS exposures are only about six hours. The best guess is that this galaxy is at a redshift of z>9. At redshift 9, the universe was 550 million years old. The light from that galaxy has taken at least 13.1 billion years to reach us.  This particular candidate had been found previously by the same team using about three days of exposure time, so it's nice to see it confirmed with deeper data.

This galaxy, along with the other ones mentioned below, was found using the Lyman Break technique, which fellow blogger Russell Ryan describes in his blog post.  Interestingly, the galaxy is invisible through all but one filter. The very fact that it is visible in the reddest filter, and invisible in all the bluer filters, is the strongest evidence that it is a very high-redshift galaxy.  The interpretation is bolstered a bit by the fact that it is not detected in very deep observations with the Spitzer infrared observatory. It seems pretty likely that this object is not an old or dusty red galaxy at lower redshift.  The source is just a little bit fuzzy in the Hubble images, so it doesn't appear to be a very faint red star in the Milky Way either.

So, it seems like a pretty good candidate. The new observations have made the detection through the one filter and the non-detection through the others more solid. But the evidence that it is truly distant is still quite tenuous, and based largely on ruling out other possibilities.

Finding a Distant Galaxy with a Little Help from a Gravitational Lens


On November 15, Dan Coe and the CLASH team posted a preprint identifying a very solid Lyman-break candidate with a redshift z~10.7. This galaxy turned up in infrared observations that used only four hours of exposure time compared to four days. Hubble got some help for this galaxy from the gravitational lensing effect of a giant cluster of galaxies located between us and the distant object. This boosted the light from the distant galaxy by about a factor of fifteen. It also resulted in three separate images of the galaxy. Each of the images is detected through the two reddest filters, but not detected at other wavelengths. So this is a really solid detection. There are no other objects in the entire CLASH survey (so far) that have similar colors.

Three separate images of the gravitationally-lensed galaxy at redshift z~10.7, from the CLASH preprint. The gravitational lens split the light into these three separate images, all of which have the color expected for a very  distant galaxy. The galaxy is invisible at wavelength shorter than ~1.4 microns due to absorption by hydrogen clouds between us and the galaxy. This is known as the Lyman Break, and is the classic signature of a distant star-forming galaxy. The Lyman Break is particularly strong in this one.

The positions of the three images of the CLASH galaxy are marked as JD1, JD2 and JD3 on this image from their preprint. Many of the galaxies that you can see near the center of the image lie in the foreground cluster. This massive cluster acts as a gravitational lens, to magnify the images of the distant galaxy (and split it into several separate images). You can see this kind of lensing effect if you look at a candle through the bottom of a wine glass. As you move the glass around, you will see the image of the candle stretch and sometimes break into several separate images. The curves drawn in the image show the "critical lines" of maximum magnification for background sources at different redshifts. The fact that the JD images lie where they do (particularly JD1 and JD2) adds a lot of support to the interpretation that this is the most distant galaxy yet discovered.
Not only is it a solid detection --  the evidence that it is a high-redshift galaxy is about as solid as it can get without actually measuring the spectrum. The three images of the galaxy lie just about where they are expected to lie based on the models of the gravitational-lens that lies in the foreground. There is some uncertainty in those models, but those models will get better as the CLASH data analysis proceeds, so even without any further Hubble observations we might learn some more about this particular candidate.

Meanwhile, Back In the Ultra-Deep Field

 

Images of some of the new candidates identified in the Hubble 
Ultra-deep Field, from the UDF12-team preprint. The galaxy at 
the bottom is the same one that is shown at the top of this blog 
post, which illustrates how different image processing can produce 
a very different appearance. Nevertheless, both teams agree
that the galaxy is well detected at 1.6 microns and undetected 
through all the filters at shorter wavelengths.
Just today the UDF12 team, led by Richard Ellis, have posted a preprint on their website about new candidate galaxies in the Hubble Ultra-Deep Field. These are the same observations used by Rychard Bouwens mentioned above, but analyzed by a different group. The paper concurs on the interpretation of the Bouwens object, estimating its redshift to be z=11.9. The paper also identifies a half-dozen other candidates at z>8.5. These other candidates are all detected in more than one filter (barely), but are so faint that it would not be at all surprising if a few of them turn out to be either spurious or at a different redshift. For statistical purposes, that's probably fine, because it's possible to estimate with reasonable confidence how many will turn out to be wrong.

The latest CANDELS Sample: A Bit Closer and a bit Brighter

 

Images of some of the brighter, closer, candidates
found in CANDELS, from Haojing Yan's preprint.
Their colors suggest that these galaxies are at
redshifts z>8. 
In this latter vein, Haojing Yan's CANDELS-team study of brighter candidates at z~8 (submitted last December) has now been accepted for publication. The CANDELS observations survey a wider area, but with shorter exposure times. So we can find the rarer, brighter sources, but not the fainter ones.  There are seven decent candidates so far, all at z ~ 8 - so a bit closer than the galaxies discussed above. The evidence that these are distant galaxies is comparable to that presented in the Ellis and Bouwens papers, but not as strong as that in the Coe paper.

What Will it Take to Improve the Evidence that these are Really Distant Galaxies?

 

The quest to find and confirm the most distant galaxy will continue. The gold-standard for confirming that these are really distant galaxies will be to obtain a spectrum and measure the redshift precisely.  The James Webb Space Telescope, slated for launch in 2018, can obtain a low-resolution spectrum for any galaxy that Hubble can see, with just about the same exposure time as the Hubble observations. We might get lucky and confirm some of these distant-galaxy candidates before then, but large-scale studies of galaxies at Cosmic Dawn will really take off when Webb flies.