The End is Just the Beginning

Hubble's last observation for CANDELS is scheduled for August 10, the end of next week. Not counting the images taken next week, Hubble has taken three thousand four hundred and fifty eight pictures for CANDELS over the past three years. So now what?

The upcoming schedule for the last few CANDELS images.

Turning photons into numbers

Hubble's cameras have detectors that capture the minute amounts of light from distant stars and galaxies and convert that light into a tiny electrical signal. That electrical signal is recorded digitally as a set of ones and zeros. Hubble beams this data back to earth with a radio antenna every few hours. It will typically be several hours to a day before we have the data on our computers.

Turning the numbers into images

Even with fancy computers and software, it takes us about a month to take the raw images that are radioed back to earth from Hubble, and carefully assemble them into science-quality mosaics. This process involves identifying and masking artifacts left by charged particles in the solar wind, or trails of passing satelites or space junk, and subtracting off any scattered light that reflected into the camera from the earth's surface. We typically take several raw images of each patch of the sky, shifting the telescope a tiny bit between images. So we can almost always tell what is a real star or galaxy, and what is an artifact. We've been keeping up pretty well with the influx of data. If you are patient, you can download our processed images from the Mikulski Archive web site.

Turning the images back into numbers

In the end, we expect to detect about 250,000 galaxies in the CANDELS images. To use the images to try to make progress in understanding galaxy evolution, we need to measure the properties of the galaxies: their brightness, their colors, their sizes, and their shapes. We need to use those measurements to try to infer something more fundamental about the galaxies: their star-formation rates, their stellar masses, their ages, the amount of interstellar dust that they contain, and whether or not they harbor a central black hole.

In other words, we need to turn the images back into numbers.

We have a lot of specialized software to help us do this. To detect the galaxies, we use a customized version of a computer program called SExtractor, which identifies faint smudges of light and tries to make a semi-intelligent decision about whether two adjacent smudges are part of a single galaxy or two separate galaxies. That is, it segments the images into different regions, assigning most of the pixels to "background sky" and a few percent of them to separately-detected stars or galaxies.  It took quite a bit of work to get to the point where we were reasonably satisfied with how it was doing this. Nonetheless, there are still a few percent of the sources that have either been poorly "deblended" by the software, or are not real (the most common offender being scattered light from a bright star). At this stage, we can flag most of the artifacts. We're stuck with SExtractor's image segmentation for now, which forces us to worry about the issue for almost every CANDELS paper. If anyone would like a nice image-processing challenge, improving the segmentation step -- using all of the available color information from the multiple images -- would be a big step forward.

The image segmentation step. The picture to the left shows a small slice of the CANDELS image, with two bright galaxies and a lot of fainter ones. The image to the right illustrates how SExtractor segments the image into different galaxies, which sometimes overlap. Human judgment doesn't always agree with what this software does. Sometimes it becomes more obvious how to break up the objects if you look at full color images instead of black & white images. SExtractor only works in black & white.

SExtractor not only detects the galaxies, it measures their sizes, shapes and brightnesses. It does this very quickly and reasonably precisely. We have done thousands of experiments inserting artificial galaxies into the images to quantify the accuracy and precision of these measurements. These experiments allow us to determine some statistical corrections to SExtractor's measurements, so that we can infer more accurate brightnesses of the galaxies (for example).

Having measured the Hubble images, we next need to make use of data from other observatories. Primarily these are large ground-based telescopes like the VLT in Chile and the Spitzer infrared telescope. These images are not nearly as sharp as the Hubble images, so unfortunately many of the galaxies are blended together. However, the Hubble images tell us where almost all of the galaxies are.  So we can cut out the individual sources from the Hubble images, blur them to match the image quality of the other telescope, and then ask the computer to tell us what combination of brightnesses of the blended sources best reproduces the blended image. We use a program called TFIT to do this. As was the case for SExtractor, we have done extensive tests with artificial galaxies to convince ourselves that TFIT is doing the measurements correctly. By and large we are satisfied that it is doing an excellent job, but we have a relatively long wish-list of things that we would like to improve, both to remove systematic biases at the few percent level in the brightnesses of all the galaxies, and to deal with the very rare "problem cases" where the answers don't make sense.

This animation starts with an X-ray image from the Chandra telescope, 
showing two sources which probably harbor central black holes. It then
transitions to the Hubble images, where you can see that some of the
galaxies have spiral features and others don't. It then transitions to the
infrared images from the Spitzer, and finally the Herschel observatory.
You can see that the resolution of the infrared images is much poorer
than the Hubble images -- the galaxies are all blended together.
Nevertheless, you can make out that some galaxies are "bluer" than
others at infrared wavlengths. Looking at the last image, you can convince
yourself that the brightest source of far-infrared radiation in this image
(which comes primarily from heated dust) is probably the galaxy just
below the center, which is also an X-ray sources.

The output of this is a "Multi-wavelength Photometry Catalog," which is one of the most useful products for scientific study. We are striving to make this as reliable as possible because much of the science depends on that.

The multi-wavelength catalogs can be used to estimate "photometric redshifts," which tell us roughly how far away each galaxy is. The brightnesses and colors of each galaxy can be used to infer the stellar mass of each galaxy, the star-formation rates, dust content, and ages of each galaxy.  Our simulations tell us that the stellar mass estimates are pretty accurate (generally to within about a factor of two of the truth), but the estimates for the other quantities are pretty shaky. A lot of work is going into trying to quantify the biases and uncertainties, and perhaps find some way to improve the estimates.

The other type of measurement is galaxy shapes and sizes. We are going at this in a whole variety of ways. We use a computer program called GALFIT to fit a smooth light profile to each galaxy and extract some basic numbers that characterize the size and shape of the galaxy and gives us a measurement of how centrally concentrated the light is in each galaxy.  We have some other programs that make estimates of concentration without assuming a smooth light profile, and that measure the asymmetry of the images. Teams of people both in CANDELS and in Galaxy Zoo are inspecting the images and classifying the the galaxies into various categories.  We also have some software that tries to identify separate clumps of light within each galaxy. We need to calibrate all those measurements by inserting artifical galaxies into the images. This has been done for GALFIT, but it is just getting started for the other measurements.

Finally, there are measurements of correlations between galaxies. Galaxies have a propensity to cluster together. We can measure this quantitatively, but we have to carefully account for the fact that the images don't all have the same exposure time. In some patches of sky we can detect fainter galaxies than in other patches of sky. Once again, we resort to inserting artificial galaxies into the images and recovering them to quantify how our detection limits vary across the full survey. The clustering estimates make use of the photometric redshifts, so we also need to do lots of simulations to understand the effect of photometric redshift errors on the clustering measurements.

So you have a bunch of numbers. Now what?

This is where the fun really starts. This is where we can try to give rigorous answers to the scientific questions posed in our original proposal.  We would like to know, for example, how galaxies build up their masses over time. To address this, we can collect the galaxies together in different redshift slices and we can estimate how their stellar masses change as the universe gets older. We can compare these evolving stellar masses to our estimates of the star formation rates to see whether they are consistent. If they aren't, then we must be doing something wrong. Either we are missing some galaxies (for example, because they are obscured by dust), or we are not measuring the mass or the star-formation rates correctly. This kind of consistency check is essential. It's how we gain confidence that we really understand what we are seeing.

We can ask whether galaxies that have active nuclei -- black holes that are surrounded by hot gas that emits x-rays -- look any different from galaxies with the same stellar mass that don't emit X-rays. So far,  somewhat suprisingly, the answer is that they look the same. Now that we have all of the data in hand, we can look more closely with better statistics. It's possible that some classes of galaxies with active nuclei (perhaps the brightest ones) look different, for example.

We can look at how galaxies grow in size. This has been a long-standing puzzle. We know that galaxies were smaller in the past. For galaxies that are continually forming stars, this is not really a problem -- the later stars presumably form in the outskirts, making the galaxies bigger over time. However, galaxies that have stopped forming stars (become quiescent) already when the universe was only 3-5 billion years old are much smaller than quiescent galaxies today. The suspicion is that such galaxies merge together, becoming bigger without forming many more stars. But that suspicion has been hard to verify because we simply haven't had surveys that are large enough to accurately measure the number of galaxies that are in pairs or in the process of merging.

There are lots and lots of other questions, outlined in our original science goals for the survey. Sometimes the work involves comparing directly to theoretical predictions -- for example there are some beautiful predictions for the evolution of star-forming clumps that we can begin to test by dissecting the images. Sometimes there are unexpected discoveries.

So even though the last images are going to be here next week, we still have about two full years of work to make all the measurements, produce the catalogs, and really dig in and try to make sense of what we are seeing.

In communicating our findings to fellow astronomers and to the general public, we often strive to find a very clear diagram or figure to make the statistical evidence readily apparent. In fact, these diagrams are often the route to fame for an astronomer. Every beginning astronomy student learns about the Hubble diagram or the Hertsprung-Russell diagram. So maybe in the end, having converted the photons into numbers, the numbers into images, and the images back into numbers, we need to very cleverly turn these numbers back into images. Yes it seems silly, but that's often where we get to exercise our scientific creativity, and it's also often how we gain the most insight into the workings of the universe.

Astro and the City

Many times, when I tell somebody that I'm an astrophysicist, they imagine that my job involves sleepless nights looking at the stars, worrying about inclement weather, city lights, an atmosphere eager to capture my beloved photons, and the occasional cosmic debris that could get in my way. How cool would that be? Sometimes, I wish I could I bask in that glory and I am tempted not to reveal that I belong to the much less glamorous subset of the "theorists" - my concerns tend to be about codes that don't run, take too long to run, or whose final outcome doesn't make any sense. Somehow taken aback by this news, my interlocutors (before checking that I don't know much about the multiverse either, and turning around to speak to someone more interesting) sometimes offer as a consolation: Well, at least you can do your job from anywhere. And it's true. In particular, I can do it from the best city in the world and not care about its crazy light pollution, or proclivity to snowy, rainy, or otherwise very humid days. Astronomy in NYC is very much alive - because even if you can't look up, you will look around, and that's what science is about. 

When I first started my job as an assistant professor at CityTech, a four-year college that is part of CUNY, I was thrilled, in order to: 1. Have a job; 2. Have a permanent job; 3. Have a permanent job in a place I like a lot. There were challenges; there still are. Others on this blog have described the time management problems of your first year in a tenure-track position better than I could do it myself, so I won't try. But what I found to be very unique about this place was an unanticipated support network of colleagues, and some incredible opportunities to connect with the public. 

CUNY, the City University of New York, is one of the largest public universities in the country; more than a quarter million people attend its 23 colleges. Still, none of these colleges has an Astronomy department; each of the 10 full time Astronomy faculty belongs to Physics, and despite that being my first love, I have now moved on with my life and want to do the fun stuff. So what we do is to gather in the Astrophysics Department at the Museum of Natural History, a place that is incredibly inspiring with its views of the Hayden Planetarium. Office space is tight and one could expect that some sort of seniority may determine whether or not you deserve a cube on the sixth floor. Instead, my colleagues made me feel welcome from day one, from escorting me to the common lunches when I was still too shy to go alone, to encouraging me to come more often and conquer my seat that way. They had successfully applied for a grant to support undergraduate research in the NYC area; I am now benefiting from it by supervising a great student that they had recruited. In no department of Astronomy I found such a knit-tight sense of community, and I think it comes from being somewhat outsiders ourselves. So, CUNYastro, thanks! I want people to know who you are. 

Image credit: Brooklyn Superhero

So I'm all set for the work part. What about reaching out to the public? Well, doing that in NYC is the most fun I've had in years. Last year I was asked to set up an activity for 6-8 grade kids at the New York Hall of Science. Sounds just normal so far… but the context was a little different: the singer Bjork produced an album called "Biophilia" and inspired by her love for science, and was promoting the album by organizing week long camps for children where they would learn about music, biology, physics, and Astronomy. The song I was illustrating was "Dark Matter" - it might be tricky to relate this topic to children, but it gets easier if, five blocks from where you live, there is a shop called "Brooklyn Superhero" where Dark Matter is sold by the gallon (and of course, they are not just a store, they organize all sorts of educational activities). I actually saw one of them at the last AAS, so they are becoming famous! 

Recently, an email was sent around to the Museum internal mailing list about an initiative called "The Intergalactic Travel Bureau", and I thought it would be fun to participate. We set up a travel agency in a temporary space close to Penn station, and proposed to our clientele destinations like Pluto (great for skiing and debates!), Jupiter (great hurricane watching and moon-hopping), Titan (swimming in frosty methane lakes), or Venus (gotta lose those stubborn 10 pounds before the holidays?). Since most of our clients couldn't afford the hefty fees or long leaves of absence from work associated with interplanetary travel, we would also give them the option of just sending postcards from these exotic destinations to friends and families.

Mark Rosin, Jana Grcevich, me, and Olivia Koski.
Image credit: Olivia Koski

It was a great way to connect to the public, especially because most of the people who came in weren't expecting us to be there, and/or had no previous interest in Astronomy - yet, everyone was a good sport and kept a straight face while we were describing the dangers of sailing the solar winds or diving into Europa's oceans. On my side, while happily riding back to Brooklyn on my scooter and enjoying a gorgeous "Manhattanhenge" looking west on 34th street, I found myself thinking of what very special mix of stories and people made this initiative possible. The original project was developed in the UK by an organization called Guerilla Science, whose US president, Mark Rosin, will be joining the faculty at Pratt in the Fall. The main organizer, Olivia Koski, is a former engineer with a passion for Physics and Astronomy and a graduate degree in Journalism. Someone she knew from graduate school (a professional astronomer turned graphic designer) saw these works of art depicting interplanetary travel. The artist, Steve Thomas, agreed to donate them to the project. An art organization, Chashama, donated the space for a week. With so many of institutions in and around the city, a bunch of astronomers, captained by Jana Grcevich, volunteered to staff the agency. And of course, many of the people who walk the streets of New York City every day were curious and open enough to come into our strange booth. 

Every day, I realized how lucky I am in being part of this amazing and diverse community. We might not have dark skies here in NYC, but we sure have bright people.

The Dangers of Swimming While Consuming Ice Cream

July has been a hectic month in Europe for some CANDELS astronomers. In the last post you heard from Fernando, who did a great job of organising a conference on "Declining Star Formation" at the UK National Astronomy Meeting in St Andrews, before jetting off to Finland for the European Week of Astronomy to discuss how to "Build Elliptical Galaxies" and the "Co-evolution of Galaxies and Black Holes". 

Taking a much needed break from the conference season

After a brief pause for some sea kayaking in the north-west highlands of Scotland, it's back to the summer conference season for me: this week I'm in Leiden in the Netherlands, where we're still talking about supermassive black holes [see the conference website here]. 

And it seems there's still a lot to talk about. To summarise the first two days in a nutshell, astronomers don't agree about how the supermassive black holes (SMBHs) that live in the centres of massive galaxies got to be as "supermassive" (i.e. heavy) as they are. The story starts with observations made over 10 years ago, which showed that the mass (i.e. weight) of a SMBH is correlated with the mass of the galaxy in which it lives. [For more info, read about the M-sigma relation here]. So, the more massive the galaxy, the more massive the supermassive black hole in its centre.

The problem is that a correlation between two observables does not necessarily mean they are causally connected. Take a more down to Earth example: it can be shown that as ice cream sales increase, the rate of deaths by drowning increases. If you take this statement at face value and don't stop to think about it, you would naturally conclude that ice cream consumption causes drowning. But of course there is a "hidden variable" that needs to be taken into account: the weather. This is a very appropriate example for the unusually warm temperatures we are enjoying here in the Netherlands this week! Obviously hot weather causes more people to eat ice cream and more people to take a swim, unfortunately sometimes fatally, but probably not while eating their ice cream.

So the problem we are trying to solve is: is there a "causal connection" between galaxy mass and supermassive black hole mass? Or is there a "hidden variable" that causes them both to grow together? Let's look at the two main contenders:

Hidden variable: gravity

 

The Mice merger: a collision of massive galaxies with plenty of
gas in the local Universe. The tails of the Mice are caused
by strong gravitational forces acting on the gas and the stars in the
galaxies as they passed one another for the first time about
two hundred million years ago. The two galaxies will collide in around
700 million years time as gravity finally manages to pull them together.
If both galaxies have supermassive black holes in their centres,
they will merge at the same time. Image Credit:
http://hubblesite.org/newscenter/archive/releases/2002/11/image/d/

There are several important forces in physics that attract or repel objects towards or away from each another (I'm sure you can all recall these from your school physics lessons!). Luckily for astronomers interested in how galaxies form and evolve, the only dominant force that acts over large enough distances to be important to galaxies is the force of gravity (ok, this is not quite true, ask a cosmologist about dark energy, but it's not so relevant here). In the early Universe gas was distributed almost, but not completely, homogeneously. Over time, the regions with the highest densities (i.e. amount of mass in a given volume) had the largest gravity force and therefore attracted more gas, making them denser. Stars formed from this gas, and we think that the first black holes must have formed from these first stars. Time keeps progressing, and the regions with the highest densities always have the largest gravity force and always attract more gas, more stars and more black holes. So this looks trivial right? The biggest black holes form in the biggest galaxies, because of gravity.

The problem is, if you get your hands on a large supercomputer and try to recreate this process, you find that you very rapidly build galaxies that have orders of magnitude more stars than those that we observe in the real Universe. And the SMBHs in the centres of the simulated galaxies have a rather bad habit of swallowing everything they can get their hands on -- because of the force of gravity. So no nice tight correlation between the mass of the galaxies and the mass of the SMBH in the centre. Well, observational astronomers always tend to think the theoretical astronomers don't know what they are doing, and that might still be the case (yes, I'm mostly an observational astronomer). But when they *all* tell us it's impossible, we have to at least question whether we've missed something in our observations.

Causal connection: energy

 

In "simulation land" the problem is easily solved -- the SMBH uses some spare energy to regulate both its growth and the growth of its host galaxy. Effectively, when galaxies collide under the force of gravity, the black hole gets so much to eat that it gets indigestion, throws a tantrum and expels all the gas from around itself and from the whole galaxy. This stops the galaxy from forming more stars, starves the SMBH, and with a little time and a few mergers you can get a nice self-regulation set up so that galaxies build up mass at the same rate as the SMBHs in their centres - Hey Presto, tight correlation between black hole mass and galaxy mass. Unfortunately, as you can tell from my description,  the theorists have very little clue how this might actually work in the real world (and if you meet one who tells you they know, go and ask another one - there's almost as many ideas as there are theoretical astronomers when you dig down to the details). 

Now the observers have a problem: how can we find new observations that help the theorists to explain what's happening? We know that stuff falling into a black hole releases a lot of energy -- we see some of this energy being released in "Active Galactic Nuclei" (AGN - see the blog post by Carolin here about how we are  looking for links between AGN and merging galaxies). But lots of energy doesn't help on its own: this energy needs to affect the gas in the galaxies, and we just don't see any evidence of this happening in the majority of galaxies in the Universe, as would be needed to satisfy the theorists.

And that's where I'm going to stop - we've still got a lot of work to do to fully understand how SMBHs are formed and why their mass correlates so strongly with their host galaxy. Evolutionary biologists spend a lot of time wondering about the difference between correlation, co-evolution and causation. Both biologists and astronomers work with enormous datasets, in which many different correlations can easily be found. Astronomers have a unique advantage, with surveys like CANDELS we can look back in time nearly to when the first galaxies were forming, we use this (and other clever techniques) to see the co-evolution of galaxies and SMBHs directly. But understanding the cause of the correlations and co-evolution is a more difficult challenge. We'll certainly make some good progress over the next few days here in Leiden, and we'll be going home with new ideas to try out. And of course new collaborations started this week will bring together complementary skills to tackle the problem from new angles in the future. 

Dinner with two galaxy formation simulators, Peter Johansson and Kelly Holly-Bockelmann

CANDELS Conquers the Vikings' Land: a Galaxy Evolution Tour in Northern Europe

Dear CANDELS followers,

Best greetings from Helsinki. I am waiting at the moment to take my flight back home to Scotland. Before that, I would like to to recap here for you the events that happened during the last couple of weeks involving our CANDELS collaboration in the land of northern Europe. These last words are important: northern Europe. Why? Because sometimes it is necessary to halt for a moment in order to realise the huge impact CANDELS is having all around the world. Our survey has become the "de facto" standard extragalactic observations of the distant Universe. Actually, I began my scientific career with Chris Conselice working on another galaxy survey utilizing the previous near infrared HST camera NICMOS. Just to give a flavour of how much things have improved let me remember those times when I was tired with my PhD work. At that time, I would open on my personal computer the images from the old HST camera and their beauty was such that everything made sense again. So now just imagine what a professional astronomer feels when contemplating the brand-new CANDELS images. I could give you here numbers about their superb resolution, or its large area and depth, but I do think recalling my previous experiences is more personal, more touching. Perhaps the best comparison might be a person watching the world from glasses that does not fit him or her any longer. When changing them, new unexpected details and features appear everywhere. I can tell I really feel privileged working on these breathtaking data.

Jumping back to the real world, two weeks ago I organized a parallel session in the National Astronomical Society meeting -- "national" in this context means "British" ;) -- in St. Andrews about the declining star formation of the Universe over cosmic time. Nowadays, it is fairly well established that the Universe peaked in its efficiency of creating stars some 10 billion years ago. Since then, it is slowly fading away, as it is running out of the gas that fuels its stars. The point is that when trying to explain how the Universe changes between its early stages and now... bang! CANDELS is one of the best tools available for these kind of studies. Afterwards, I flew this week to Finland to participate in the European Week of Astronomy and Space Science, where we discussed in a number of special sessions the consequences of galaxy evolution. I would like to highlight the symposia titled "The mystery of ellipticals" and "The co-evolution of black holes and galaxies", where several CANDELS members presented some awesome work and led the scientific discussion. And please, let me emphasize one more time that the quest of understanding the Universe does not only belong to a single group of people or nation; the CANDELS survey is a joint effort from a multinational group of people that has repercussions all around the world.

I guess you heard St. Andrews is not only about its destroyed cathedral
or high class university (where the Prince & Princess of Wales met)
but it is the so-called "home of golf", with multiple golf courses all
around surrounded by beautiful landscapes by the sea. Here I show
a photo from the  Royal and Ancient Golf Club and its famous
stone bridge. Image Credit

Diving a little bit deeper into what we have presented in the conferences, I will start by mentioning that you can find in this blog excellent previous posts about similar topics by friends like Romeel Davé, Tao Wang, Victoria Bruce and Guillermo Barro. However, our projects, although related to theirs, are slightly different. In the Scottish conference, we commenced by showing how the state-of-the-art astronomical simulations in the biggest computers in the world still struggle to produce realistic galaxies. Especially challenging is knowing how and when to switch on the Super Massive Black Holes (do not forget this link either) that are usually found in the centers of galaxies. Reproducing their behaviour accurately is mandatory in order to understand the most massive galaxies in the Universe, because without their energy and jets, we cannot explain how these galaxies stopped forming stars. The rationale is very easy: the bigger the galaxy is, the larger the amount of gas it contains. Unless we remove this gas, the galaxy would continue producing stars crazily, finishing up as a monster object which does not exist in the Universe. In fact, this was the second part of the meeting: the local Universe. And of course we concluded by connecting all the previous topics with the high redshift Universe. Several CANDELSiers (such Vivienne Wild, Caterina Lani and Victoria Bruce) presented very interesting results. The upshot of all this was that we have tested convincingly how galaxies (both the biggest and the big-ish, as the dwarf ones are elusive even in our galactic neighbourhood) change their morphologies, sizes, colours, and star formation over cosmic time/cosmic distance/redshift (choose your favourite term from the jargon) but thus far we have not identified how the mechanisms at play (the aforementioned black holes and galaxy mergers) contribute to this process.

M87 is, in many ways, the typical massive galaxy. Located at
the core of the Virgo galaxy cluster, it is red, devoid of young stars,
featureless and probably the most massive galaxy nearby in the
Universe. It hosts a huge supermassive black hole in its center,
which produces the jet of matter we see in the image. Many questions
arise from this picture: do massive galaxies always look the same
even in the primeval Universe? Is it indispensable to have a big black
hole in order to suppress the star formation of galaxies? Does
CANDELS show dead galaxies in the early Universe and what
can we learn from them? Image Credit

For the following week, I changed scenery. Now I am in Scandinavia, in a charming town (nice cathedral and castle, plus a river full of boats) called Turku, which is a communication point in between Sweden, the Baltic countries and St. Petersburg. This European meeting is, in many ways, similar to those of the American Astronomical Society (such as the one described in this post): crowded with people and with many special sessions.

Turku at night :) Astronomers just want to have fun
Image Credit: http://demoshane.deviantart.com/art/

I cannot give you a full account of what happened, but I will try to summarize the most relevant contributions related to our studies.  My presentation was focused on when is the crucial moment for the majority of the most massive galaxies of the Universe to transition into spheroidal, big and red elliptical galaxies, and whether the way stars move in these systems could shed some light into this problem. This second idea breaks many degeneracies, and it is based in a novel observational technique called 3D spectroscopy. One of the longstanding astronomical problems is how to infer properties of a 3D Universe which is projected in images of only two dimensions. The solution is based on the Doppler effect observed in the galaxy light. The parts of the galaxy which move in our line of sight appear bluer for the same physical principle as the sound of the ambulances have a higher pitch when it approaches closes to us; I am sure all of you are thinking of this. Combining the light wobbling with the images, my data suggested massive galaxies acquire their present appearance seven billion years ago approximately. Vivienne Wild, who was also in Scotland with me, explained her galaxy evolution ideas stressing the influence of the black holes in killing the galaxies' star formation. Elizabeth McGrath told us about a missing piece of evidence in this puzzle, which is the fact that CANDELS unveils many disk galaxies like our Milky Way in the distant Universe that seem to be "killed" as well, but without developing a spheroidal shape.

NGC1277 has been amply debated in both of our meetings. Seemingly, this galaxy hosts the largest observed supermassive black hole in comparison with its mass, cointaining half of it! Moreover, its size is tiny, roughly a third of our Milky Way. What are the secrets of its galaxy? Or is it just the astronomers are missing something? More questions yet to be answered, this galaxy is really mysterious. Credit: Hubble Space Telescope

As you can see, there are still many amazing discoveries for which we do not have yet a convincing explanation. The CANDELS project and astronomers are committed to investigating how our ever-changing Universe grew and behaved during its infancy. Surrounded by this awe-inspiring cosmos I can only refer you to words by the Spanish (like me) Roman philosopher Seneca "Nature does not reveal all her secrets at once" or more recently to our admired colleague Carl Sagan who said "Somewhere, something incredible is awaiting to be known".

(As this is the first time I am posting in English for an internet blog, I asked for help to two good friends of mine, Andrew Davis and Jeyhan Kartaltepe. A big thanks and a smile to both of them).

Rapid Response Astronomy

Nothing in astronomy ever changes.  

That is to say, nearly every thing we study in astronomy is effectively unchanging. The stars and galaxies we look at through our telescopes are just about as constant and eternal as you can get. Even a massive star with a "fast" life cycle takes millions of years to exhibit any visible change, far longer than the time available to human observers. So for most astronomical observations we can really take our time; there's never any hurry to catch a galaxy or a cluster of stars before it disappears.

For an observatory like the Hubble Space Telescope, this means that most of the observations being done are fully designed and scheduled well in advance. The typical process for observing with HST is spread over many months. First, astronomers prepare a proposal describing the science they want to do, and how they'll use HST to do it.  These are submitted each year around the first week of March. Then in May a panel of volunteer astronomers is gathered in Baltimore at the Space Telescope Science Institute (STScI) to review all the proposals and select the ones that will be awarded time on HST. The successful proposers then go through another round of preparation, where they pin down the details of exactly how the observations will be done. The observations can happen anytime over the next year or so, and special large programs like CANDELS get spread out over multiple years. 

The Hubble Space Telescope.
 Image Credit:  NASA, Z. Levay

The specifics of when HST actually collects the data are hammered out by a dedicated team of STScI research support staff. These program coordinators and calendar builders have the job of piecing together the puzzle of many hundreds of different HST observations. Each observation has a unique set of constraints to consider: When is the target visible? What does the rotation of the telescope need to be? Are there bright stars near the target that HST can use to lock its position? Each week the calendar builders balance these competing requirements and put together a very detailed schedule for exactly what HST will do two weeks in the future. Efficiently packing and organizing those observations is a big task, and one with real significance. Astronomers and telescope operators know that every observation with HST is a precious resource, and represents a substantial investment in this science. The total cost of HST divided by its lifetime works out to about $15 per second, or $54,000 per hour. All the careful advance planning is really critical for maximizing the science return from that investment. 

For CANDELS, however, we can't plan out all of our observations many months in advance. One of our primary science goals is to detect and analyze distant supernovae: stars reaching the end of their life-cycle with a violent explosion. The explosion itself occurs without warning in a fraction of a second, and we can observe the after-glow for weeks and months afterward. There is no way to predict when and where these explosions may appear, but when we do spot one, we often need to quickly mobilize HST for follow-up observations, while the supernova is still bright enough to see. For this type of object, the HST operators allow a special mode for submitting observation plans, called the Target Of Opportunity (ToO) mode.

Here's how it works:

When we discover a new supernova of interest, like the record-setting SN Wilson, we sift through all the available data and decide that we want to get a quick follow-up observation, maybe as soon as next week. We quickly contact our program coordinators at STScI and tell them that we're going to trigger a ToO observation. Then we plan out the observations and submit them for review. To make room for our new ToO supernova, the calendar builders then pull out some of the pre-planned observations from other programs (they'll get put back in sometime later in the year). 

The bright star in the lower left is SN 1994D
in the galaxy NGC 4526.
Image Credit: High-z SN Search Team, HST, NASA

With experience and good organization, the detailed observing plan for a new SN can be arranged in a few hours - but sometimes we only have a few hours to spare. For most ground-based observatories, normal ToO observations can be slotted in on the same night that a supernova is discovered. For HST, however, it is much more complex and risky to make sudden changes, so each week the HST schedule gets locked in place on Wednesday morning. We need to give the program coordinators and calendar builders at least 4-5 hours to process a new observation, so that means that we have a weekly deadline of 12 noon each Tuesday for any new ToO interruptions.   

This brings us to the peculiar situation that if we happen to discover a new supernova on a Wednesday, then we have almost a week to leisurely examine the data and decide whether it warrants a ToO trigger. If that same supernova is found on a Monday, though, we are scrambling to get all our decisions made and plans in place before the Tuesday noon deadline. This can lead to some long hours on Monday nights when CANDELS observations are coming down from HST - but its exciting and rare to get any kind of astronomy in rapid action. We supernova hunters really appreciate what a unique privilege it is to get to push around an orbiting space telescope at the last minute when our science requires it. 

How to Feed a Black Hole

What are AGN?

We now believe that in the centers of most, if not all, massive galaxies, there resides a supermassive black hole. In some cases, weighing over a billion times the mass of the sun. In most galaxies, these black holes lie dormant and can only be found through their gravitational influence. However, in a small fraction of galaxies, the supermassive black holes are seen to be 'active', astronomers call these 'Active Galactic Nuclei' or AGN. During these short phases, gas is accreting onto the black hole in an accretion disk. Gas in an accretion disk is heated to high temperatures and emits radiation through a wide range of wavelengths, most prominently in the X-ray, UV and optical part of the spectrum. AGN can often outshine the entire galaxy they reside in, but they span a very wide range in luminosities. 

How are black holes fed?

One question has puzzled astronomers since we first learned of the nature of AGN: how does a dormant supermassive black hole turn into an AGN? How is it triggered? Feeding even a very bright quasar requires a surprisingly sparse supply of gas: about the mass of the sun per year is required for bright AGN, while fainter AGN require considerably less than that. This might not seem like a lot, but AGN are known to be active for ten or even a hundred million years. If they are to be fed during that entire time, even a very small amount per year adds up to an impressive total mass. A bright AGN can swallow the entire gas supply of the galaxy it resides in during a single active phase.

There is another problem with feeding AGN: it is actually surprisingly difficult to funnel gas that is available in galaxies into their central black holes. The gas in galaxies is generally settled in a disk-like structure. Moving gas towards the center - where the black hole is located - requires stripping the gas of an overwhelming part of its angular momentum.  This requires some kind of a disturbance. There are different ways to achieve feeding the gas into the black hole, and in particular one process has become very popular amongst astronomers: mergers of galaxies. We will not touch on other possibilities in this post, but look at how mergers might trigger AGN and what the data tell us.

Galaxy mergers and black hole feeding

Simulation showing how gas in a merger is moved
towards the  supermassive black hole.
Image Credit: Phil Hopkins

When galaxies collide, both of them can carry considerable amounts of gas, and during the collision, the normal motions of the gas are disturbed and it can move to the center of the newly formed system to feed the black hole and start an active phase. In mergers, we therefore find good conditions to trigger luminous AGN since large amounts of gas become available and are funneled to the black hole within a short amount of time. Therefore, mergers are believed to be closely connected to AGN.

When AGN were first studied, it was also found that many were located in galaxies that looked very disturbed. In fact, many of the AGN in the vicinity of the Milky Way are located in galaxies that appear to have undergone mergers very recently. However, just looking at the incidence of merger features in galaxies is not sufficient, we must take into account what percentage of non-active galaxies show signs of merging. We need a so-called control sample. And while many galaxies showing AGN activity do show merger features, CANDELS researchers have shown that this just reflects the fact that galaxies in general often undergo interaction. So, what is happening? What is the real connection between mergers and AGN?

Does the luminosity of the AGN matter?

HST images of nearby luminous AGN showing clear signs of
interaction in their host galaxies
Image Credit: HubbleSite

One possibility I am interested in studying is that mergers are only responsible for some AGN. As mentioned earlier, depending on the luminosity of the AGN, the amount of gas required to feed it changes dramatically. Faint AGN can rather easily be triggered by smaller events, so they do not necessarily need to be connected to mergers. Also, when looking at the whole population of AGN, the brightest AGN form a minority in the overall AGN population. Could it be that mergers only trigger the most luminous of AGN?

To answer this question, we look at AGN at a low redshift (z=0.5-0.8) over a wide range of luminosities -- the brightest AGN in our sample are about a thousand times more luminous than the faintest ones. This also means that the brightest ones require about a thousand times more gas to shine as bright as they do. For all these AGN, we then look at a sample of control galaxies that are about equally massive and compare how asymmetric they appear. When galaxies undergo interaction, they appear asymmetric and disturbed, the more they settle, the more symmetric they will become. Comparing the levels of asymmetry in AGN hosting galaxies and normal control galaxies therefore lets us compare how likely they are to be connected to a recent galaxy interaction. 

It turns out that similar to previous studies, we find that host galaxies of AGN look no more disturbed than normal galaxies. However, because we choose AGN that are more nearby, we can also study these differences as a function of luminosity. This has not been studied previously. Dividing the AGN into different bins according to their luminosities, we can also determine if there are differences between AGN host galaxies and control galaxies only for certain AGN luminosities. We do not find any differences, even for the more luminous AGN where we would expect a stronger connection to mergers.

If the host galaxies of even luminous AGN are no more disturbed than normal galaxies, what does this mean? One possibility is that there is a very long delay between the collision of galaxies and the phase during which the AGN gets triggered. While this is possible, the delay would have to be very long for all merger features to fade. The other possibility is that the AGN we study are still not quite bright enough to see merger triggering in effect. The most luminous AGN are extremely rare, and even large fields cover only a few of them, the very brightest AGN are so rare that they are not found in CANDELS fields. Studying more extreme AGN might therefore lead us to understand how mergers and AGN are connected. 

Astronomer of the Month: Dale Kocevski

Tell us a little about yourself! 

My name is Dale Kocevski and I’m an assistant professor in the Physics and Astronomy Department at the University of Kentucky in Lexington. As is the case with most academics, I’ve moved around a lot during the course of my education and professional career. Before moving to Lexington in the fall of 2012, I lived in beautiful Santa Cruz, California, where I held a postdoctoral research position at the University of California. Prior to that, I carried out my graduate work at the University of Hawaii in Honolulu and I received my Bachelor’s degree from the University of Michigan in Ann Arbor. I am originally from Michigan and grew up in the greater Detroit area. Although I definitely miss California, it’s nice to be back within driving distance of family, after spending 14 years living out west. I enjoy hiking and mountaineering, traveling, watching hockey (Go Wings!), and surfing (at least when I lived near the ocean). While in California, my wife and I spent many weekends exploring the Sierra Nevada Mountains, especially in and around Yosemite National Park. The Sierras are probably the single thing I miss most about California.

Snowshoeing in Yosemite National Park

What is your specific area of research?  What is your role within the CANDELS team? 

My research focuses on the study of distant galaxies that host actively accreting supermassive black holes and the active galactic nuclei (AGN) that they power. There is mounting evidence that the evolution of galaxies is closely linked to the growth of their central black holes, but how this connection is established remains one of the key unanswered questions in astrophysics today. Current theories propose that this link is forged, in part, but the energy released during an AGN phase. In fact, without the energy input from AGN, most galaxy evolution models fail to reproduce many key properties of present-day galaxies.  Despite this, several major questions remain about AGN and their potential impact on galaxy evolution.

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)

To answer these questions, I use multi-wavelength observations ranging from the X-ray to the Infrared to study the morphologies, stellar populations, and environments of galaxies that host AGN.  I currently lead the AGN working group within CANDELS, which is meant to foster collaboration between team members that share a mutual interest in supermassive black holes and active galaxies. I also played a role in the early design and implementation of the CANDELS survey. This involved helping to develop an observing strategy that made efficient use of our allotment of Hubble Space Telescope time, while also providing the data necessary to answer the wide range of science questions that are at the heart of the CANDELS survey.

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

I became interested in science, and space in particular, at an early age. I was always curious about things that seemed unknown and TV shows about time travel and starships helped to fuel my fascination with science. Picking up on this, my uncle gave my brother and me our first telescope when I was 12.  Observing Jupiter and Saturn with that little backyard refractor proved to be a very formative experience. I soon began reading popular science books about astronomy and astrophysics. Not long after this, I attended NASA’s Space Camp in Huntsville, Alabama, which only solidified my interest in science and technology.  In fact, much of my early fascination with space was the direct result of NASA’s fantastic outreach efforts.  This is one reason I find it particularly troubling that NASA’s education and outreach funding is now on the chopping block due to federal budget cuts.

Later, my decision to become a research astronomer was very much influenced by my time as an undergraduate at the University of Michigan. I took my first trip to a research observatory while at UofM, when I visited the MDM observatory on Kitt Peak in Arizona as part of an undergraduate research project with Prof. Patrick Seitzer. I also spent two summers participating in the National Science Foundation’s Research Experience for Undergraduates (REU) internship program. I spent the two summers working at the Arecibo Observatory in Puerto Rico and the Space Telescope Science Institute in Baltimore, Maryland. After being immersed in astronomy for four years, I decided to continue onto graduate school and pursue a career in the field.

REU students at Arecibo Observatory - Summer 1998

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

Although the academic lifestyle can have its downsides, it does also have its perks. Being able to set your own schedule and focus on scientific puzzles that interest you most is definitely one of them. Another is our travel-oriented lifestyle. Between attending scientific conferences, visiting collaborators, and collecting data during observing runs, astronomers get to travel quite a bit.  I personally love to travel, so I always take these opportunities to explore a region I haven’t previously been to.

What is your favorite astronomical facility?

Overlooking the summit of Mauna Kea from the UH 2.2 meter telescope

My favorite facility is probably the Hubble Space Telescope. The images HST is able to capture still manage to amaze me to this day. That said, I have a soft spot for the University of Hawaii 2.2 meter telescope on Mauna Kea. I spent a great many nights using that telescope to carry out my thesis work. It wasn’t the most state-of-the-art facility, by any means. For example, the telescope control room lacked heat, which made for some long and cold nights observing at 14,000 ft. Everything at the UH 2.2m seemed to be on the verge of breaking down (yet rarely did!), but the data it provided was fantastic and I couldn’t have completed my thesis without it. 


Who has been your biggest scientific role model and why? 

Catching a Wings vs Sharks game with Sandra Faber

I’ve had the opportunity to work with a number of great scientists in graduate school and during my postdoctoral positions.  If I had to single out one role model, it would be Sandra Faber, PI of the CANDELS survey. Working with Sandy, I am always amazed by her ability to identify open questions in astronomy, especially in the subfield of galaxy evolution, size-up the problem at hand, and relatively quickly identify pathways to solving that problem. Recently Sandy was awarded the National Medal of Science by President Obama, so I’m definitely not the first person to appreciate Sandy’s skill as a scientist. 


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

The sheer size of the Universe has always been mind-boggling to me. The patches of sky imaged by the CANDELS survey with the Hubble Space Telescope are relatively small compared to the size of the entire sky, yet they contain tens of thousands of galaxies.  There are literally more galaxies in the sky than any astronomer could hope to count in a lifetime. On top of this, the portion of the Universe that is observable from Earth is likely only a tiny fraction of the entire Universe. Sometimes astronomy can be as humbling as it is exhilarating.