Building the Hubble Image Mosaics for CANDELS

When we use Hubble to observe the CANDELS fields, there are quite a few steps needed to turn the original ("raw") data into the final nice combined multi-wavelength mosaics that we use for science and for color images - it's not exactly an instant process!

First, the observations for each CANDELS field are spread over many months, and each Hubble observation consists of many exposures - an "exposure" is the basic building block for how we construct the CANDELS observations. Since the galaxies we're studying are so faint, each exposure typically needs to last for about 15-20 minutes just to be able to collect enough light, and even then many of the galaxies still look very faint on a single exposure, so we need to combine many exposures of each portion of the sky in order to see these galaxies and even fainter ones. In addition, the Hubble cameras cover a fairly small portion of the sky (only about 2 - 3 arcminutes, or about 1/10th the diameter of the full moon), so we need to obtain many adjacent exposures to cover the CANDELS fields. Finally, since each Hubble camera can only observe in one filter (or one color) at a time, we need to take multiple exposures in different filters to build up the multi-color mosaics. Over the 3-year lifetime of the CANDELS project, we end up obtaining over 7,000 exposures in total!

Once we have all the different exposures for a CANDELS field, including multiple exposures in different filters and at different pointing centers across the field, we are ready to start working with them. The first steps involve "calibration", where we remove electronic artifacts that are produced by the cameras, and ensure that we are left only with real objects in each exposure. Related to this is the step of removing bad pixels that can sometimes be present on the detectors, along with "cosmic ray" hits, which are small bright random spots on each exposure caused by high-energy particles, known as "cosmic rays", hitting the detector. These can be identified and removed by comparing a sequence of exposures of the same part of the sky - the real astronomical sources (stars, galaxies) will show up at the same locations in all the exposures, while the cosmic rays will be different in each exposure and can then be identified and excluded later on.

Hubble mosaic of GOODS-S, one of the CANDELS fields. Each point of light is a galaxy, with over 20,000 galaxies detected in this image. Red represents the infrared wavelength images (which is invisible to our eyes, but can be detected by the cameras on Hubble), while blue represents shorter wavelengths. More than 2,000 separate exposures were used to create these mosaics, requiring a total of more than 1 million seconds of Hubble observing time.
Credit: Anton Koekemoer

Many of the CANDELS team members help to visually inspect the exposures to make sure that there are no other issues - for example, other satellites in space sometimes move across the field while Hubble is taking the exposure, which will leave a bright streak across the image! These are then masked out and excluded later on. Bright stars can also sometimes cause reflection 'ghosts', which we need to be aware of. The images also need to be aligned properly - Hubble can point with fairly good accuracy (about 1 arcsecond), but we actually need about 100 times better accuracy to eventually be able to combine the images, since the pixels on the cameras have a typical size of 0.04 - 0.12 arcseconds. We have written computer programs that do this by first identifying all the objects in each exposure, then matching them up between different exposures to find and remove the shifts. This process also aligns the images to other catalogs of objects from other telescopes on the ground, which helps us later when comparing the new Hubble results to data from other telescopes.

After the images have been aligned, they are ready to be stitched together, or combined, into a set of "mosaics", using other computer programs that we have written. This step takes all the exposures of a CANDELS field in a given filter, and combines them in an optimally weighted way that resembles dropping small spots onto a single output image - this process is called "drizzling"! Also, since a multitude of images are combined together in this way in a single step, the software for doing this is called "Multidrizzle"! The end result from this is a large combined mosaic, for each different filter, of each CANDELS field, together with a few related mosaics that indicate how much exposure time went into each portion of the field, and how much background noise is present across each CANDELS field.

The mosaics are large! The current ones can span up to 30,000x70,000 pixels on a side, meaning that this single mosaic image would correspond to a 2,000-Megapixel camera! The resulting file size is 8 Gigabytes, which is actually too large for some of our software programs to cope with so we generally break them up into smaller more digestible chunks, or make "binned" versions at a coarser pixel scale, for comparison with data from other telescopes that have lower spatial resolution than Hubble. We use these mosaics to make the final color pictures, which are "approximate true-color", in the sense that we use different images from filters covering a range of different wavelengths. However, since the longest wavelengths are in the infrared (which would be invisible to our eyes), we generally use red for those images, and use blue and green for the images obtained at shorter wavelengths.

This is then where the image processing ends and the science begins - we use these mosaics to build catalogs of galaxies and measure their shapes and other properties, all of which are the subject of the various scientific results that we are pursuing in CANDELS. So, that's where the real fun begins!

The Search for the Most Distant Galaxies

If you've looked at the night sky on a clear night far from a city, you've felt it. Something about this sight, of endless stars upon a field of black, with the disk of our Galaxy painting a milky path through it all, leads to a pulling. Some sort of visceral urge to understand why we are here, where we come from, and how the Universe ended up the way it did. This is a trait unique to humans on our planet, and is one of many which sets us apart from other species on our world. It is this primal feeling that urges some of us to become astronomers, to use science to attempt to quantitatively answer some of these fundamental questions. Using modern telescope facilities to search for (and discover!) very distant galaxies satisfies this need to probe our origins.

Not that long ago, we did not yet even know that there existed galaxies outside our own Milky Way. Remarkably, less than a century later, we now know that there are many more galaxies, perhaps hundreds of billions in our visible universe, and we see these galaxies as they existed in the past. This funny trick of physics happens because light has a finite speed (meaning that it takes it a certain amount of time to get from here to there). For example, it takes the light from the Sun eight minutes to reach the Earth; thus, we see the Sun as it existed eight minutes ago. The closest large galaxy, Andromeda, is two million light years away - it takes light from Andromeda two million years to reach us, so we see this galaxy two million years in the past. By looking at more and more distant galaxies, we can in essence watch the Universe play in reverse, and learn how galaxies form and evolve into the gorgeous spirals and majestic ellipticals we see today.

The speed of light is not the only factor affecting our observations. We have known since the time of Edwin Hubble that the Universe is expanding, and that more distant galaxies are speeding away from us faster than those that are close by. This adds an additional effect known as "redshift". Just as a train whistle lowers in pitch as a train speeds away from you due to the Doppler effect stretching the sound waves, light waves also get stretched when observed from a receding galaxy. As the wave gets stretched, the light appears to become redder. The farther away a galaxy is, the faster it appears moving away from us due to the expansion of the universe, and the more its light gets reddened. We can measure this shift in the galaxy color, which we call its "redshift". The higher the redshift, the more distant a galaxy is (and thus the redder it appears).

Since the Hubble Space Telescope was upgraded with the sensitive optical (meaning light you can see with your eyes) camera known as the "Advanced Camera for Surveys" (or "ACS") in 2002, we have had the ability to find galaxies as far away as a redshift of six. However, more distant galaxies were impossible to see, as at higher redshifts, all of the galaxy's light had been shifted out of the optical, and into the near-infrared (just redder than both our eyes and ACS can see).

With the installation of the new Wide Field Camera 3 on Hubble in 2009, we had our first opportunity to image deeply into the near-infrared. The image below shows the deepest near-infrared image ever taken, in the Hubble Ultra Deep field. In 2009 and 2010, a number of teams studied this image, and found ~10's of galaxies at redshifts of seven and eight. Some of these galaxies are so distant, that we are seeing them only 500 million years after the Big Bang***. This might sound old, but the Big Bang happened ~13.8 billion years ago, thus we're peering 96% of the way back into the Universe.

***The Big Bang refers to our theory of the early development of the universe.  It is extremely well tested, and the theory makes a number of predictions which we have verified observationally, including the cosmic microwave background, the expansion of the universe, and the distribution of primordial chemical elements.

This large image shows the near-infrared view of the Hubble Ultra Deep Field.  The smaller boxes show the 31 galaxies we discovered with redshifts greater than 6.3 in this field ("z" stands for redshift; don't ask why, astronomers give weird names to things).  Credit:  Steven Finkelstein

As you might imagine, this is an incredibly exciting time for this brand of science. However, to understand these galaxies, we need much larger samples, and this is where CANDELS comes in. With the full CANDELS dataset, we should find hundreds of these extremely distant galaxies (indeed with the data we already have we have found ~150 galaxies at these high redshifts). With this excellent sample of early-Universe galaxies, we can study in detail how the earliest galaxies in the Universe formed, and infer how they evolved down to those at much lower redshifts, where we have a better grip on things.

This is a first post in a series by myself and Russell Ryan, where we will lead you through the field of distant galaxies. In our next post, we will discuss how we actually find these galaxies. For example, in the Hubble Ultra Deep Field, which is a single pointing of Hubble, there are over 3000 galaxies, while only a hundred or so are the distant galaxies we're looking for. As you can imagine, it can be difficult! In subsequent posts we will talk about the discoveries being made by our team, including the colors of these galaxies, how their light and mass evolve with time, and how much they impacted a major event in the Universe which we call "reionization", which marks the time when galaxies first turned on and illuminated the universe with their light. Stay tuned!

A tour of the five CANDELS fields. First stop: GOODS

Astronomers like surveys, and they like acronyms. CANDELS (one acronym) is a survey of five different fields on the sky, each of which has its own name (more acronyms - sometimes, confusingly, more than one name is used for the same field on the sky). But more interestingly, each has its own history. The fields were chosen for different reasons and observed as part of different surveys, by different teams, with different telescopes. However, in each case, these five fields have gradually accreted more observations, with other telescopes, at other wavelengths, and have become the premier locations on the sky for deep field studies of galaxy formation and evolution. And CANDELS is just the latest step in this process of accumulating valuable new data on these valuable pieces of celestial real estate.

In a series of posts on this blog, we will give short histories of the five CANDELS fields and explain why they were chosen for CANDELS. Today, we start with two of the five fields that, together, form the Great Observatories Origins Deep Survey, or GOODS. This article is loaded with acronyms - be warned!  We will try to define them as we go along.

GOODS was born, at least in part, as a successor and extension of the Hubble Deep Field (HDF), a very small, very deep, and very famous survey done with the Hubble Space Telescope (HST) Wide Field and Planetary Camera 2 (WFPC2) in 1995. This project was initiated by Dr. Robert Williams, then the director of the Space Telescope Science Institute (STScI) in Baltimore, as a service to the astronomical community, and was planned and executed by a team of astronomers from STScI, including Dr. Harry Ferguson (now one of the two co-leaders of the CANDELS team, along with Dr. Sandra Faber from UC Santa Cruz), this author, and many others who are now part of the CANDELS. At the time, it was the deepest optical image of the distant universe ever obtained, in four filters (i.e., observing at four different wavelengths) from the near ultraviolet (UV) to far-red optical light. The HDF was a tremendous success, both scientifically and in terms of public interest - the HDF image has become an icon of Hubble astronomy. This success can partially be attributed to the fact that the team at STScI made fully calibrated data products and released them very quickly to the whole astronomical community to use for research.  Hundreds of scientific papers were written using the HDF data, including important first attempts to map out the star formation history of the universe during the last 12 billion years of cosmic time.   Moreover, the HDF became a catalyst for more observations, with other telescopes and instruments, at other wavelengths. Because the Hubble data were available to everyone, many other astronomers made their best and deepest observations with other facilities on the same spot in the sky, and in turn made their data available to others. Previously, it had been surprisingly uncommon and difficult to get observers to coordinate the best observations of different types on the same fields - for example, imaging at optical, infrared, radio, and X-ray wavelengths, each of which reveal different physical processes at work in galaxies, as well as spectroscopy to measure galaxy distances. This would seem like an obvious thing to do, but astronomers in competing teams tended to carve out new surveys in their own chosen patches of the sky, and it was difficult to obtain all of the different data that one might want on a given spot on the sky. However, the HDF was somehow seen as the property of everyone, with its uniquely deep and high resolution Hubble multi-color data, and it became a magnet for multi-wavelength surveys. This was an important part of its success.

However, the HDF had limitations as well. It was a very tiny patch on the sky, covering only 5 square arcminutes, or less than 1% of the area that the full moon appears to cover on the sky. Initially, there was only one HDF, in the northern sky near Ursa Major. Therefore it was hard to check whether conclusions drawn from the galaxies in the HDF were really universal, or whether one might find statistical differences in galaxy properties from another, equally small patch elsewhere. Moreover, it could not be observed with new facilities coming on line in the southern hemisphere, like the Very Large Telescopes (VLTs) at the European Southern Observatory (ESO). (Several years later, a second "Hubble Deep Field South" was observed, but it was never studied as thoroughly as the first HDF, in part because it came too late to be "new and exciting" like the original HDF-North, and in part because its location on the sky proved to be less convenient for other observatories - see more on this below.)

The launch of the new Spitzer Space Telescope provided an opportunity for a new, bigger survey: GOODS. Over the years, NASA created four "Great Observatories" operating at different wavelengths: Hubble for ultraviolet and optical (and later, near-infrared) observations, the Chandra X-ray Observatory, the Compton Gamma Ray Observatory, and Spitzer for mid- to far-infrared observations.   Since the era of the original HDF observations, research had begun to show that optical data alone missed much of the "action" in the distant universe. The very faint, distant galaxies of greatest interest for studying the early history of the universe are observed to have very large redshifts: the expansion of the universe has stretched the waves of light emitted from these galaxies, shifting them to redder wavelengths. The farther away and farther back in time we observe galaxies, the more their light is stretched or "redshifted". For very distant galaxies, optical Hubble images see light that was emitted in the ultraviolet rest frame. Ultraviolet light is important, because it is produced by hot, young, newly-formed stars.  But it is also easily absorbed by dust, and dust is extremely common in young galaxies forming new stars....so, optical observations can miss most of the energy that is produced in newly formed galaxies. The absorbing dust re-radiates this energy in the mid- and far-infrared. Also, the young, hot, UV-emitting stars only make up a small part of the mass of a galaxy. The optical light emitted by older stars, which provide the bulk of the stellar mass in a galaxy, is redshifted into the near- and mid-infrared.

Spitzer was a new space telescope that hugely improved the sensitivity and angular resolution of infrared observations. For the first time, it became possible to detect the older stars from normal galaxies out to the furthest reaches and earliest epochs probed in the HDF, as well as the dust emission which actually represents the bulk of the energetic output from many young, distant galaxies. The Spitzer Science Center in Pasadena planned most of the first year of Spitzer observations along a model similar to that used for the Hubble Deep Field, with very large survey programs whose data would quickly be made available to everyone, to stimulate widespread use for research.

An international team of astronomers, including many who had played important roles in the original Hubble Deep Field, formed a team to propose a coordinated "Great Observatories" survey, i.e., GOODS. They designed the survey to cover a total sky area 64 times larger than that of the original HDF, in order to get much better statistics for studying galaxy evolution.  They planned for two fields from the start, one in the north and one in the south, to provide independent cross-checks on the statistical distributions of galaxy properties, and to enable telescopes in both hemispheres to survey these fields as well. And the survey was designed from the start to coordinate data at many different wavelengths, including new campaigns of observations from ground-based telescopes at the ESO Southern Observatory and the US National Optical Astronomy Observatory.

The location of the HDF-North on the sky (near Ursa Major, as mentioned above) was chosen largely to optimize the efficiency of the original Hubble observations. HST is in a low-earth orbit, and at any time the earth blocks nearly half of the sky from Hubble's view. However, the HDF was placed in Hubble's "continuous viewing zone", near the pole of its orbit, where it could stare at the same patch of the sky for nearly the whole 10-day period over which the original observations were carried out. Otherwise, the field was chosen to point far out of the plane of our own galaxy, to minimize the number of bright Milky Way stars whose glare would affect observations of the faint, distant galaxies, and also to reduce the amount of gas and dust from our own Milky Way through which we would have to peer in order to see the distant universe. That made it an excellent place to observe from other facilities as well, and indeed another NASA Great Observatory, Chandra, soon centered its deepest X-ray observations on the position of the HDF-North. (That X-ray survey soon came to be known as the Chandra Deep Field North, or CDF-N). That, along with a wealth of other ground-based data, made the HDF an obvious place to center the Spitzer observations for one of the two GOODS fields.

Dr. Riccardo Giacconi was the first director of STScI, and a pioneer of X-ray astronomy, for which he won the Nobel Prize in Physics in 2002. Giacconi later went on to become the director of the European Southern Observatory. As an important team member of the Chandra X-ray Observatory, Giacconi planned a new Chandra Deep Field in the South to study the distant X-ray universe, and wanted to located it optimally for observations from ESO telescopes like the VLT. Although Giacconi originally considered the HDF-South (mentioned above), this turned out not to be ideal either for observations with the VLT, or for X-ray astronomy because there is more Milky Way gas along the line of sight to the HDF-South than toward the HDF-North. Giacconi and his colleagues therefore chose a new field, soon known as the Chandra Deep Field South (or CDF-S), that is ideally placed for observations with both Chandra and the VLTs. The GOODS team then chose this as the site for their second field, GOODS-South, so that both GOODS fields would have well-matched data from Spitzer and Chandra. A strong bond was forged with astronomers at ESO, and the international GOODS team and other members of the European astronomical community have planned and executed several large programs of imaging and spectroscopic observations in GOODS-South with the Very Large Telescopes and its rich suite of instruments.

GOODS for Spitzer was approved in 2000, but the launch of Spitzer was delayed for several years, until 2003. In the meanwhile, the GOODS team successfully proposed for matching Hubble observations from a new optical instrument, the Advanced Camera for Surveys (or ACS), that was installed in 2002. ACS observes a wider field and is much more efficient than the old WFPC2 camera used for the original Hubble Deep Field, making it possible to survey the much-larger GOODS regions to very faint limits through four filters. The GOODS ACS observations were carried out in 2002 and 2003, and the Spitzer observations followed in 2004. Following the HDF tradition, all data were quickly made available to the whole astronomical community, and like the HDF, the GOODS data have been used by thousands of astronomers both in and out of the original team, leading to hundreds of publications.

Many different surveys have been done to study distant galaxies at high redshifts. Some cover wider regions of the sky than that of GOODS, but are not as sensitive to very, very faint, distant galaxies. A few surveys, like the original Hubble Deep Field (which is surrounded by GOODS-North) and the later Hubble Ultra-Deep Field (which itself was centered within GOODS-South), reach fainter than GOODS, but cover much smaller regions of the sky. This "wedding cake", with different levels of area and depth, is important to survey both brighter and fainter galaxies throughout cosmic history.

Relative sizes of the regions on the sky observed in several important surveys of the distant universe. The two GOODS fields are shown at left, with the full moon in the upper left for comparison. Very deep surveys like the Hubble Deep Field (HDF) and the Hubble Ultradeep Field (HUDF), seen at lower left, can detect fainter galaxies, but cover only very tiny regions on the sky. Other surveys like COSMOS and the NOAO Deep Wide Field Survey cover much wider regions of the sky, usually to shallower depths, i.e., with less sensitivity to very faint galaxies. However, they encompass larger and perhaps more statistically representative volumes of the universe. The image in the background shows a computer simulation of the clustering of galaxies, viewed at a redshift z = 1, nearly 8 billion years ago. The colors represent the density of galaxies (individual galaxies would be small dots on this scale, barely visible here), with regions of higher and lower density shown in pink and blue, respectively. Small surveys may sample under- or over-dense regions, while larger surveys can average over density variations, but may not be sensitive to the ordinary, relatively faint galaxies that are most numerous in the universe.

Each of the two GOODS fields is roughly rectangular, with dimensions 10 arcminutes by 16 arcminutes on a side, about 20% of the apparent area of the full moon. The exact sizes and orientations of the fields were designed to optimize the original Spitzer observations, and programs from other telescopes and instruments, including the new CANDELS Hubble observations with the near-infrared Wide Field Camera 3 (WFC3), have covered more or less the original footprint. In particular, CANDELS observations of the GOODS fields themselves form a small "wedding cake" with two layers: very deep WFC3 images covering the central region of each GOODS field, and shallower images covering the wider, remaining parts of GOODS, and matching the sensitivity of the WFC3 images that are being obtained in the other three CANDELS "wide" fields.

The GOODS fields have been targeted for extremely deep observations at radio, millimeter, far-, mid- and near-infrared, optical, ultraviolet, and X-ray wavelengths. Besides CANDELS, some of the newest data on GOODS comes from yet another space telescope, the Herschel far-infrared observatory, which has obtained the most sensitive and direct measurement yet of the emission from dust heated by young stars and active nuclei in galaxies at high redshift.  Each wavelength reveals important new information about the stars in galaxies and super-massive black holes that often reside in their centers. GOODS was a pioneer in coordinated, multi-wavelength deep surveys, and an important foundation on which CANDELS is continuing to build today.

Meet Janine

Heya, I'm Janine, one of the blog writers for the CANDELS blog. We thought it's a nice idea to tell you a little bit more about ourselves in this blog, too, instead of just writing about science results. After all, we're all just humans and there is nothing scary or intimidating about scientists (well, most of them anyway).

So there's really 2 sides to me that I can tell you about. Dr. Janine Pforr, the scientist, and Janine, the private person. Let me start telling you a bit about my scientist side to give you an idea how I ended up where I am right now.

I grew up in Germany (and yes, I'm a German) where I went to school for 12 years and finished with the so-called Abitur (you'd call it A-levels in England or high-school diploma in the US) which allows you to go to study at Universities. During the last 2 years of schooling you have to pick out 2 major subjects in which you will receive more training than in the other remaining subjects. For me these were Maths and Physics. I always liked them and was good at it and had shortly before deciding found out that this is what I'd need to become an astronomer. So really there wasn't that much choice involved. This was what I had to do to and I did it (I know it sounds simple but that's just how I felt). I finished school in 2001 and went on to study Physics at the University of Heidelberg (you can't study astronomy as an undergrad in Germany).

View of Heidelberg from the Castle
credit & copyright: Janine Pforr

Sunset over Portsmouth, UK
credit & copyright: Janine Pforr

It took me 5 and a half years before getting my diploma in Physics. During this time I tried to attend as many astronomy lectures as I could, volunteered for mini research projects and met many like-minded people and made great friends with whom I'm still in contact, some of them are astronomers like me. The last year of the diploma studies involves a research project and writing up a dipoma thesis (you can compare it to a masters). Not surprisingly, I chose an astronomy topic for this and carried out my diploma thesis research at the State Observatory in Heidelberg, the Landessternwarte Heidelberg-Koenigstuhl, under the supervision of Dr. Jochen Heidt. My thesis consisted of studying the radio sources in the FORS Deep Field, a very deep survey of a small region in the sky. I really enjoyed my time there, not only scientifically but also personally by getting involved in the local outreach activities such as giving guided tours.

Saguaro cacti in the Sonoran Desert near Tucson, AZ
credit & copyright: Janine Pforr

I received my diploma in 2007 and then went on to do my PhD at the Institute of Cosmology and Gravitation under the supervision of Dr. Claudia Maraston. For this I moved to Portsmouth in England. In the 4 years of my PhD I looked at the stellar population properties of about 1.5 million galaxies in the SDSS and BOSS surveys (Sloan Digital Sky Survey and Baryon Oscillation Spectroscopic Survey) and with the help of simulated galaxies tried to figure out how the parameter choices one makes while determining the galaxy properties effects the outcome. 

End of September 2011 I successfully defended my PhD thesis and boarded a plane to America 2 days later to start my first Postdoctoral Researcher job (short post-doc) at the National Optical Astronomy Observatory in Tucson, Arizona. As you might have guessed, I'm working on CANDELS as a post-doc. Some of my tasks involve techniques and methods I learned during my diploma and PhD, some of it will be new. Basically, I am an observational astronomer interested in the properties of galaxies and galaxy evolution. Since I'm also very interested in public outreach I am now also leader of the education and public outreach working group within CANDELS. Part of the efforts other CANDELS scientists and me are carrying out in that area is the blog you're reading just now. We hope you like it and for any suggestions and ideas on how to improve are always welcome. Feel free to use the comment option below this post!

Park in Nottingham, UK
credit & copyright: Janine Pforr

Small Waterfall in Reid Park, Tucson, AZ
credit & copyright: Janine Pforr

Now, enough of all the resume things. I've told you how I came to be an astronomer, but who am I as a person? Yeah, I was a geek at school, but that's not a bad thing in my eyes. All my life I have loved the night sky with all the twinkling stars, the thought of vastness behind them and lots of non-girly things like tools and helping my dad with the crafty things around the house. I've built a cardboard castle when I was little and spent many of my weekends exploring the surrounding castles, fortresses and palaces with my dad. Everything medieval would do. I'm a fan of films and books, especially the kind that draw you into their world and let you read all through the night because you can't possibly manage to wait a couple of hours to see what happens next. One of my favourites is Harry Potter. But there are many others, too. I absolutely love photography because I love the idea of capturing moments in time and the memories with them. I'm fascinated by Asia, especially China because of its rich culture, long history and diverse nature, so much so that I started learning Mandarin during my PhD. Unfortunately, I have never been to China. Not yet, anyway. But I will definitely go some day. 

I really like nature, everything green and tree-y, and water, as ocean, lake, flowing river or just a puddle or rain drop. I know, living in the desert now, it's rare and what they call river over here is really just a dried-out riverbed that hardly ever sees moisture but I'm looking forward to the monsoons and the spectacular thunderstorms that come with it. Not surprisingly, I also like swimming, and at least pools they have over here. 

I'm a big fan of the "Ampelmann" (East German traffic light man). Yes, I'm from East Germany and I think ours is much cuter than the West-German version! Every now and then I do indulge in a bit of "Ostalgie", after all I was only 6 years old when the Wall came down. 

East-German traffic light man at pedestrian crossing in
Heidelberg, Germany, credit & copyright: Janine Pforr

I collect elephants and love yellow which I completely blame on my parents. My favourite toy as a baby was a yellow rubber elephant which I managed to protect from being thrown out over the years.

I love and dearly miss my family and friends, which are spread all over the world by now, a "side-effect" (it's good and bad) from being an early career astronomer: lots of travel and living in lots of different places for relatively short amounts of time. But you also meet lots of new people and make new friends around the world.

Everything else about me I think I keep to myself, a little mystery has to remain. Just know that despite the distinction I made above between scientist and private person, I really couldn't just be one or the other.

Galaxies with growing Black Holes: What makes them special?

Active Galaxies in the CANDELS survey with bright blue nuclear emission
from a growing Supermassive Black Hole (courtesy: David Rosario)

Supermassive Black Holes (we like to call them SMBHs for short) lie lurking in the heart of every large galaxy in the Universe, and possibly in many little galaxies too. A popular misconception is that black holes like these are continuously devouring stuff: stars, planets, the thin plasma that fills up most of space. In actual fact, one of the big mysteries in galaxy physics is why such monstrous black holes - some as massive as a billion suns - remain, for most of their existence, on a severe diet, not growing at all. In the area of space within a few hundred million light years of our own Milky Way galaxy, a region known as the Local Universe, there are hundreds of thousands of large galaxies like our own. Remarkably, only a few percent of them show the tell-tale signs of SMBH growth, manifested as Active Galactic Nuclei (or AGNs - see the blog post by Dale Kocevski for an introduction to these rare and remarkable objects). Nineteen out of any twenty SMBHs, while entirely capable of huge outbursts of energy as they feed on in-falling gas and stars, seem instead to be quietly biding their time in the nucleus of their 'host' galaxy. These black holes are being starved - not enough fuel comes their way to keep them active. Why exactly these black holes remain this way is not understood, but may have to do with the way that gas settles down into the centers of galaxies, and how black holes themselves influence these flows of gas.

One interesting clue to this conundrum is the observation that in the distant cosmos, far beyond the Local Universe, AGNs are both brighter and more common. In fact, much of the mass in Local SMBHs fell in many billions of years ago, during a period in the history of the Universe that we can only probe by looking deep into space (and, by extension, far back into time). Something about galaxies in the early Universe allowed black holes in their centers to grow faster than they do today. By comparing these distant galaxies to the ones we see today, we can figure out what affects black hole growth and shed some light on the question of why nearby SMBHs stay anorexic.

This is where CANDELS comes in. With the superb new capabilities of our survey, we can actually get a good look at the very galaxies that contain growing SMBHs across a vast swath of cosmic history and compare them to galaxies that don't host AGNs, to see if these galaxies are special in some way. Since CANDELS provides pictures of galaxies across a wide range of wavelengths (or colors), we can separate out the light that comes from the powerful shining active nucleus and home in on the galaxy itself.

In a recent paper from the CANDELS collaboration, we carefully looked at galaxies that shine powerfully in the X-rays, a sure sign of gas heated to millions of degrees as it falls into a growing black hole. We found that such 'active galaxies' only really distinguish themselves in one way - they contain a lot of stars. This is not too surprising; we know that such massive galaxies harbor massive black holes, and massive black holes are exactly what is needed to produce AGNs. After accounting for this difference between AGNs and other galaxies, AGN hosts have the same basic range in shape and color as normal galaxies of similar mass. Another thing we can do with the superb CANDELS images is place good constraints on the past history of star-formation in these distant galaxies. We found that active galaxies form stars in much the same way as normal galaxies, both at the time we observe them and in their past history.

What does this mean? From our study, we can tell that galaxies that host growing black holes are not remarkable in any way. In other words, the mechanisms that carry gas into the black hole mostly work on small scales, probably within a few tens of light years of the nucleus itself, and don't immediately affect the rest of the host galaxy. As the project progresses, we will be able to build on this early work by looking at more and brighter AGNs. In time, CANDELS will be a powerful tool as we uncover the complete story of how black holes and galaxies evolve in the early Universe. Stay tuned to this blog as the picture develops.

Astronomy in the Last Frontier

What better place to hear about the current frontiers of astronomy and astrophysics than the Last Frontier? That's what hundreds of astronomers from all over the world were thinking as they arrived in Anchorage, Alaska for the 220th meeting of the American Astronomical Society (AAS). Alaska truly is a land of astronomical marvels. It is known for the Aurora Borealis, caused by ejections of charged particles from the sun, as they follow the magnetic field lines of the Earth and crash down on the polar regions. And today, one week away from the summer solstice, Alaska earns its name as the land of the midnight sun. The sun circles overhead for 20 hours before briefly dipping below the horizon in four hour long twilight.

CANDELS scientists Tommy Wiklind, Tomas Dahlen,
and Ray Lucas discuss redshift measurements
in Anchorage, Credit: Christina Williams

The frontiers of astronomy are vast indeed. There have been presentations all week about cutting edge research, from the formation of the first galaxies in the universe to exoplanets in our own galaxy, from star-formation to cosmology. The CANDELS collaboration has been making waves here as well. Many members are here presenting their work using CANDELS data. Some highlights: Norman Grogin is studying distant galaxies whose central black holes are actively accreting matter, mysteriously varying the luminosity of their nuclei. New breakthroughs are being made in astronomical image construction by Ray Lucas. Tomas Dahlen showed how CANDELS data are helping us learn better ways to estimate the redshifts of galaxies, and how massive they are. And Viviana Aquaviva is showing what the colors of galaxies can tell us, for example, about the age of their stars. Among the CANDELS results being presented at the AAS meeting were mine. I'm a graduate student studying galaxy evolution, and this is an amazing opportunity to share my dissertation research on high-redshift galaxies, and why some of them stop forming stars only three billion years after the big bang. One of my favorite parts of science is finding out what other scientists think about new results, so the poster sessions at the AAS meetings are the perfect venue. People come to see the presentation, which is displayed in a large hall with other presentations organized by astronomical topic, and there are many hours for great discussions, and meeting new scientists.

Sandy Faber receives the Henry Russell Norris Lectureship
Credit: Christina Williams

Here in Anchorage, CANDELS Principle Investigator Sandy Faber was awarded the Henry Norris Russell Lectureship of the American Astronomical Society, for her lifetime of contributions to the field of Astronomy. These include breakthroughs in galaxy evolution and the distribution of dark matter, astronomical instrumentation and her dedication to mentoring the world's future leaders in Astronomy. Her Russell Lecture Tuesday morning was a status report of the cold dark matter theory of galaxy evolution. Generally, we think that the universe is made of up of mostly cold dark matter and dark energy. All we can see and interact with is the icing on the cake: ordinary matter and the light it emits. Where the ordinary matter ends up (in stars and galaxies) and how the galaxies cluster together in a cosmic web of structure is largely dependent on the properties of the dark matter and its gravitational attraction.

So, what was Sandy's conclusion about galaxy evolution and our understanding of it? While we appear to have some things remarkably right in terms of the theory of how dark matter behaves, even though we can't directly observe it, there is still much work to do to understand how the universe makes real galaxies. For example, the theory of cold dark matter predicts a large number of tiny galaxies, "satellites", which orbit larger more massive galaxies, and we only find a fraction of them. Another example, in simulations of galaxy evolution, we don't understand how galaxies receive new gas from the intergalactic medium. As we look farther back in time, galaxies form stars at remarkably higher rates than today, and we don't understand why the star formation in galaxies suddenly stopped. This is an area where CANDELS data will be indispensable to fill in the blanks, and help us solve some of the still outstanding mysteries of how galaxies evolve through cosmic time. Stay tuned for new results, the next AAS meeting is in January 2013 in Long Beach, California. What new breakthroughs will we be presenting there?

Cosmic Collisions: Galaxy Mergers and Interactions

One of the most spectacular events in the universe occurs when two galaxies collide with one another. The gravitational pull between two companion galaxies results in a sort of cosmic dance as the pair of galaxies orbit each other. Over time gravitational interactions can cause the two galaxies to get closer until they eventually merge into a single galaxy. This is illustrated in the video below, which shows a simulation by Josh Barnes, from the University of Hawaii, of two galaxies merging. At each close pass, galaxies can strip material (including stars, gas, and dust) from each other, often producing long and beautiful tidal tails and morphological disturbances (as seen in the Hubble image of 'The Mice' shown above). This process is a slow one and can take millions or billions of years, so our view of the merger timeline is based on snap shots of the many objects we observe at each of the various stages. In today’s universe, this is a rare event. Studies of objects in the nearby universe have found that only 1-3% of objects appear to be involved in such a merger event.

 Simulation of two galaxies merging

Credit: Josh Barnes (University of Hawaii)

So why do galaxy mergers, though beautiful, matter to astronomers? Even though these events are rare now, we know that they were much more common in the past. As we look at galaxies farther away from us, and thus as they were at an earlier time in the universe, we find such interactions and mergers happened more frequently. Mergers between galaxies appear to be quite important for the evolution of galaxies over the history of the universe. These events can affect many aspects of a galaxy: they can change its overall morphology, for example, two spiral galaxies can merge together and form an elliptical galaxy; they cause galaxies to grow in mass; and they change the overall number of galaxies in the universe (i.e., where once there were several small galaxies, now there might be just one large galaxy). Indeed, we now know that our very own Milky Way Galaxy will one day merge with our large spiral neighbor, the Andromeda Galaxy. But don’t worry – this won’t happen for another 4 billion years so we won’t be around to see the effects!

The Antennae Galaxies, Credit: NASA, B. Whitmore, F. Schweizer

There are other ways that galaxy mergers can be important. When two galaxies collide with one another, the chances of two stars colliding is very rare because stars themselves are tiny compared to the vast distances of space between them. However, galaxies have large clouds of gas and dust within them, and these can and do collide. When this happens, the collision can induce huge bursts of star formation and many new stars are born from the gas clouds. An example of this has been seen for the famous pair of interacting galaxies called the Antennae (see image to the right). Over 1000 star clusters have been identified in the Antennae (blue in image) as a result of this collision. In addition, much of this gas can be funneled toward the center of the galaxy and provide fuel for feeding a central black hole. One of the current open questions in galaxy evolution is how many stars are produced because of such collisions and how important are these mergers for feeding black holes and creating active galaxies.

Identifying Galaxy Mergers

Credit: ASA, ESA, the Hubble Heritage 
(STScI/AURA)-ESA/Hubble Collaboration,
and W. Keel (University of Alabama)

In order to study the effects of galaxy mergers, the first step is to try to identify as many of them as we can. We use deep images of galaxies in order to do this. From the pictures you see in this post, you might think this is easy to do! Well, even for nearby galaxies, it requires looking at a lot of individual objects to identify the few percent that might have tidal tails or surrounding debris. Such an effort has been undertaken by the Galaxy Zoo project using images from the Sloan Digital Sky Survey and the help of many citizen scientists - ordinary people who have volunteered to look at galaxy images.. We can also identify galaxies in pairs in an automatic way, but this requires knowing the distance to both galaxies very precisely, as well as how fast they are moving relative to one another, in order to ensure that they are gravitationally bound and will one day merge. Many of the galaxies that appear to be next to each other are really chance projections along the line of sight, such as in the image to the left. These two galaxies are not interacting with each other. In fact, they are not at the same distance at all! They just happen to be aligned so that they look like they are.


Challenges at High Redshift

CANDELS images of possible high redshift galaxy mergers
Credit: Jeyhan Kartaltepe

While identifying merging galaxies might seem to be straightforward when looking at the nearby universe, this becomes much more complicated as we look at more distant galaxies at high redshift. The farther away a galaxy is, the fainter it is and the smaller it is. This makes it much more difficult to see features such as tidal tails and debris, which are often much fainter than the nuclei of the galaxies. If two galaxies are very close to each other they can blend together in images and look like one galaxy. This is where the Hubble Space Telescope comes in. Since Hubble is in space, we can get around the blurring effects of the Earth’s atmosphere. Another difficulty is that at high redshift, the light we see from galaxies is actually shifted toward longer wavelengths, or redder colors, from the original bluer light that was emitted. This means we might actually be sensitive to light from different processes and parts of galaxies than we are for nearby galaxies. CANDELS is ideal for studying the properties of galaxies at high redshift, and for identifying galaxy mergers, because it is the first large survey to obtain high-resolution data in the near-infrared. This allows us to study distant galaxies in the same way that we study nearby galaxies - though it still isn’t easy! Take a look at the sample of candidate high redshift mergers and interactions in the above panel. This is made even more complicated because galaxies at high redshift can be strange and irregular for other reasons, besides galaxy mergers. In a future post, we will discuss some of these alternate possibilities.

The story of galaxy mergers is just beginning! Stay tuned to hear more about what we learn about galaxy mergers in the distant universe from the CANDELS survey.

Supermassive Black Holes and ‘Active Galaxies’

Artist impression of a supermassive black hole
surrounded by an accretion disk of infalling gas
and twin, highly-collimated plasma jets.
Credit: Mark Garlick (University of Warwick)

It has recently become clear that at the center of most, if not all, galaxies lies a black hole of epic proportions. These so-called supermassive black holes are typically a billion times more massive than the stellar-mass black holes formed when stars die and collapse in on themselves.  Their event horizons are also significantly larger, with radii on the order of billions of kilometers (roughly the radius of our solar system). Once inside this horizon, nothing in the Universe, not even light, can escape the gravitational pull of the singularity at the center of the black hole.

In most galaxies, including the Milky Way, supermassive black holes lie dormant and are not readily detectable. Astronomers have inferred the presence of a black hole at the heart of our Galaxy due to the orbital motion of stars as they circle a massive, yet invisible object near the Galactic center (see the movie below). In about 10% of galaxies, though, supermassive black holes are actively growing through the accretion of gas and sometimes stars. When gas spirals into a black hole, it forms an accretion disk and rapidly heats up.  As it does so, it emits immense amounts of energy and the gas becomes visible at a variety of wavelengths. When this happens, astronomers refer to the black hole and its surrounding gas as an active galactic nucleus, or AGN. These accretion events can be so energetic that even moderate amounts of gas infall (like the mass of our Sun accreted over the course of a year) will result in an AGN that outshines the starlight of its entire host galaxy.  

The animation above shows the motion of stars orbiting the black hole at the center of the Milky Way. One of these stars has been observed over its complete 15.8-year-long orbit. The star approached the black hole to within one light day, which is only about five times the distance between the planet Neptune and the Sun. Credit: ESO

Finding Active Galactic Nuclei

Illustration showing the structure of an active galactic
nucleus (AGN). At its center lies a supermassive black
hole which is surrounded by an accretion disk of orbiting
gas and dust and two powerful jets of outflowing plasma.
Credit: Meg Urry (Yale) & Paolo Padovani (ESO) 

There are several observational signatures that astronomers use to detect AGN out to cosmological distances. Primary among these is the radiation produced by the gas in or near the accretion disk surrounding the central black hole. As this gas heats up, it emits radiation at optical, ultraviolet and X-ray wavelengths. If the accretion disk is obscured by interstellar gas and dust, some of this radiation will be absorbed and re-radiated at infrared wavelengths. The net result is excess emission near the center of a galaxy with a unique spectral energy distribution that can be detected. X-rays observations are a particularly powerful way to find AGN as galaxies themselves do not produce strong X-ray emission. In addition, the radiation from the accretion disk can excite cold atomic gas close to the black hole which then produces unique emission lines that are visible in the spectrum of the galaxy. On occasion, AGN will also produce powerful plasma jets, twin highly-collimated outflows that emerge on opposite sides of the accretion disk, which are detectable at radio wavelengths. Many of these signatures, especially those at optical and radio wavelengths, can be extremely luminous. As a result, many of the most distant objects ever detected are galaxies with a central AGN. The most distant AGN found thus far is at a redshift of z=7.1, an era when the Universe was only 760 million years old (5% of its current age). 

The Role of AGN in Galaxy Evolution

Although AGN have been studied for more than half a century, their potential importance to the evolution of galaxies has only recently become evident. Observations over the past decade have revealed that a tight correlation exists between the mass of a galaxy’s spheroid component, or central ‘bulge’, and the black hole at its center. This correlation suggests that the formation and growth of supermassive black holes is intimately linked to the growth of their host galaxies. In fact, when astronomers measure the rate at which black holes are growing as a function of cosmic time, we find this to be directly proportional to the rate at which galaxies grow as measured by their star formation rates. Understanding how this connection between galaxies and their central black holes is established and maintained is one of the key goals of modern astronomy and the CANDELS survey.

Current theories propose that this link is forged, in part, by the energy released during an AGN phase. Computer simulations have shown that a sufficiently energetic AGN can drive outflows that halt the accretion of gas onto the central black hole, while simultaneously acting to suppress the surrounding galaxy's star formation activity. In this way, AGN can self-regulate the growth of both supermassive black holes and their host galaxies. This scenario has been widely adopted such that most cosmological models of galaxy evolution now invoke feedback from an AGN as the primary mechanism to terminate the star formation activity of massive galaxies. In fact, without the energy input from AGN, many models fail to reproduce fundamental properties of galaxies, such as their color bimodality and space densities. The dramatic effects of AGN feedback can be seen in the movie below, which shows the simulated collision of two spiral galaxies with supermassive black holes at their centers. As the merger progresses, outflows driven by the central black holes eventually sweep away the cold gas within the two galaxies, effectively terminating their star formation activity.   

The Future with CANDELS 

Despite this emerging picture, several major questions remain about AGN and their potential impact on galaxy evolution. What mechanisms fuel black hole growth and turn a dormant black hole into an AGN? What is the precise nature of AGN feedback?  What role do AGN play in giving rise to the first generation of quenched galaxies. Using the latest imaging from the Hubble Space Telescope, the CANDELS survey is now providing insight into many of these questions by allowing us to characterize the stellar populations and structural properties of AGN host galaxies over a redshift range than encompasses 2/3 of cosmic time. In the coming years, CANDELS will help us construct an integrated model for the triggering of AGN activity, the quenching of star formation, and the structural evolution of galaxies in the early Universe.