The VIMOS UltraDeep Survey – a spectroscopic survey of high redshift galaxies

The VIMOS UltraDeep Survey (short: VUDS) is an observational program to gain spectroscopic measurements for ~10,000 galaxies at high redshift, when the Universe was only between 1- 3 billion years old (today, the Universe is 13.8 billion years old). This is a particularly interesting era to study in terms of galaxy evolution since astronomers expect galaxies at that epoch to look very different from today. For example, at that early time we observe that galaxies have a much more disturbed morphology compared to the beautiful structured spiral galaxies or smooth elliptical galaxies that we see in the local Universe. We expect that galaxies formed many more stars at that time partly triggered by disturbances from the merging of galaxies but also because more gas was still available to form stars in those galaxies. The time between redshift 2 to 6 (i.e. the first 1-3 billion years of the Universe’s age) is thus a major epoch of galaxy assembly.

Figure 1: Very Large Telescope in Chile, photo credit: R. Thomas.

With CANDELS, galaxies in that epoch are studied mostly based on photometry, meaning images taken at different wavelengths. We described in earlier blog posts how with photometry at many different wavelengths astronomers are able to study the properties of galaxies through comparing the observed data to model galaxy spectra.

With VUDS galaxy evolution is approached from the spectroscopic side. A spectrum of an object is created by dispersing all its emitted light by directing it through a disperser like a prism, meaning the light is split up according to its wavelength. An easy example is the creation of a rainbow where the light from the sun hits raindrops in the air which act as dispersers and split the originally "white" sunlight up by wavelength, creating the typical coloured stripes. Such spectra allow us to study the properties of galaxies in much more detail compared to the study of images alone. 

The VUDS survey covers about 1 square degree in the sky. As a comparison the diameter of the full moon is about 0.5 degrees and its area is ~0.2 square degrees, which means it’s a fifth of the area covered by the VUDS survey. However, this 1 square degree of area of the VUDS survey is split over 3 separate fields in the sky that have been observed with a lot of different instruments and at many wavelengths already, creating a unique and precious data set for astronomers to carry out their studies. The three fields are the COSMOS field (which overlaps with the CANDELS-COSMOS field), the Extended-Chandra Deep Field South (which overlaps with the CANDELS-GOODS-South field) and the VVDS-2h field. Within those 3 fields spectra of ~10,000 galaxies were taken with the VIMOS multi-object spectrograph at the Very Large Telescope (VLT) in Chile (Figure 1). We described how multi-object spectroscopy works in more detail in this recent post. In short, suffice it to say that with that instrument, astronomers are able to take a spectrum of many galaxies at the same time. VUDS is the largest spectroscopic survey of galaxies at these early cosmic times.

Two of the 3 fields covered by VUDS overlap with the CANDELS area. The spectra and spectroscopic redshifts in that overlap area (~ 700 galaxies) were just publicly released by the VUDS team.

Figure 2: Stacked spectrum of galaxies between redshift 3 to 4 with the most reliable spectroscopic redshifts in VUDS. Vertical dashed lines indicate known spectral lines which are used to determine spectroscopic redshift and galaxy properties.

For the VUDS survey, the objects which were targeted for the spectroscopy, were selected primarily based on their redshift as derived purely from photometry (again, see this blog post here). Additionally, some sources were added based on their photometric colours (i.e. the difference in brightness between two wavelength bands) which indicate a high redshift. These objects were then observed with two different grisms -- one for the blue wavelength end and one for the red wavelength end – for about 14 hours each. The resulting spectra cover a wavelength range from the blue optical to the very red optical. This means that for these high-redshift galaxies, we really observed their ultra-violet to blue optical wavelength range which are shifted due to the redshift into the optical wavelength range covered by the VIMOS instrument. This wavelength range reveals many properties of galaxies, especially with regard to their star formation. In Figure 2 we show you a stacked spectrum of some VUDS sources in which also the spectral lines are indicated. In Figure 3 you can see all the spectra of the VUDS survey compiled into a picture and sorted by redshift, where each line represents one spectrum. Emission and absorption lines in this image are nicely visible in this as bright and dark lines that stretch across the image from left bottom to top right. This also illustrates how spectral features are redshifted towards redder wavelengths. The most common spectral lines and features in these spectra are the Hydrogen Lyman-alpha, Lyman-beta and Lyman-gamma lines, the Lyman limit (below which almost all emission is absorbed by neutral Hydrogen around newly formed stars), the Carbon lines (CII, CIII and CIV, where the Roman numbers behind the letters indicate the ionization level of the element) and lines from Helium (He), Oxygen (O), Silicon (Si) and Aluminium (Al). These lines are used not only to determine the spectroscopic redshift of these galaxies (i.e., through their known rest-frame wavelength), but also other galaxy properties such as star formation and chemical composition of the galaxies. Overall in VUDS we were able to determine reliable spectroscopic redshifts for ~6000 galaxies which cover a large range of brightnesses and stellar masses. Some of the galaxies in this survey form up to 1000 solar masses per year!

Figure 3: Compilation of each spectrum taken in the VIMOS UltraDeep Survey and sorted by redshift. Redshift increases from the bottom to the top, meaning the further up in the image we go, the further into the past we look and the younger the Universe is. Marked are spectral emission (bright spots in the spectrum) and absorption lines (faint spots in the spectrum) at each redshift. This figure illustrates nicely how certain spectral features seem to be present in galaxies in this survey at the various redshifts and thus across cosmic time. Figure from Le Fevre et al. 2015, A&A 576, A79

Since the completion of the observations, many researchers in the international VUDS team work on all aspects of galaxy formation and evolution, from morphology to identifying proto-galaxy-clusters and groups, from studying the ultra-violet spectroscopic properties of very young galaxies to the merging history of the Universe, in alignment with the science goals of the overall survey. If you are interested in following the results from the VUDS survey, you can find our Facebook page here and our Twitter account here.

WFC3 Early Release Science (ERS) Observations

The GOODS (North and South) fields are two major fields observed by the CANDELS program. The GOODS-N field will be fully covered by CANDELS near-IR observations, while only ~70% of the GOODS-S field area will be observed by CANDELS near-IR observations. The main reason is that the remaining ~30% of the area in the GOODS-S field was already observed by the Wide Field Camera 3 (WFC3) Early Release Science (ERS) observations. Today, I will summarize these ERS observations.

The multiwavelength 10-band color image of the entire ERS mosaic in the GOODS-South field combines a broad range of colors, from the ultraviolet, through visible light, and into the near infrared. Such a detailed multi-color view of the universe has never before been assembled in such a combination of color, clarity, accuracy, and depth. Credit: NASA, ESA, R. Windhorst, S. Cohen, M. Mechtley, and M. Rutkowski, R. O'Connell, P. McCarthy, N. Hathi, R. Ryan, H. Yan, and A. Koekemoer.

The Scientific Oversight Committee (SOC) was constituted in 1998 to oversee the science definition and the technical development of the WFC3. In recognition of their effort during 11 years of WFC3 development, the STScI director allocated ~210 orbits (~300 hours) of the HST time to the SOC to do challenging science programs designed to test key capabilities of the WFC3. To this effect, the SOC distributed these orbits equally between two major programs, to investigate (1) nearby galaxies and (2) distant galaxies. The combination of these two programs is referred to as the 'WFC3 Early Release Science (ERS) Program." I will only talk about the 'distant galaxies' program, as it is most relevant to the CANDELS program. The 'distant galaxies' program was allocated 104 orbits (~150 hours).

I got involved in this program through my PhD advisor, Rogier Windhorst, who is a member of the SOC. Myself, Seth Cohen, Russell Ryan, and few SOC members were involved in planning this 'distant galaxies' program. The GOODS-S field was chosen because of the availability of extensive ancillary data in this field. The primary goal of the ERS program was to showcase capabilities of the WFC3, so we planned 3 sets of observations: ultraviolet imaging (total 40 orbits), near-infrared imaging (total 60 orbits), and near-infrared grism (low resolution spectroscopy, total 4 orbits). The ultraviolet and near infrared imaging covers the full ERS field (~45 arcmin²), while the grism observations cover only ~1/10th of the ERS field.

Multiwavelength postage stamps of objects with interesting morphological structure
in the 10-band ERS color images of the GOODS-South field: from left
to right, high signal-to-noise detections of ERS galaxies resembling the main
cosmological parameters w, λ and ρ respectively. These images illustrate the rich and
unique morphological information available in the 10-band panchromatic ERS data set.
Credit: Rogier Windhorst and WFC3 SOC

These observations are being used for various science goals, including identifying and understanding passive galaxies, star-forming galaxies at intermediate redshifts, high redshift galaxies, emission-line galaxies, and galaxy morphology/mergers at various redshifts. These data are also useful to identify faint red stars in our own galaxy. Details of ERS observations are in Windhorst et al. 2011. The intermediate depth ERS observations are excellent complementary data to the CANDELS wide shallow and the CANDELS deep observations. CANDELS team is generating a large near-infrared mosaic in the GOODS-S field, which will include ERS as well as CANDELS datasets in a single image. Keep coming back for more results from these datasets.

Ultraviolet Observations in CANDELS

The CANDELS project is primarily focused on observations at optical and near-infrared wavelengths, that is light that is received at the wavelengths that the human eye can see and a little bit longer. However, a fortunate trick of the Hubble Space Telescope (HST) orbit also allows us to observe one of the five CANDELS fields at ultraviolet (UV) wavelengths, that is slightly shorter wavelengths than the blue end of the rainbow. UV observations are especially interesting, because very massive, very hot stars emit light strongly at those wavelengths.

How We Get UV Observations

Optical HST image of the Hubble Deep Field (in gray scale) with
Far-ultraviolet observations over layed (in purple). The Far-ultraviolet
are wavelengths even further in the blue direction than the UV
observations planned for CANDELS. Most galaxies are not detected
in the far-UV, despite the image being very deep, because their light is
shifted redward by the Doppler shift ("redshift"). Image Credit: Harry Teplitz

HST orbits the Earth, which means that most of the time when it wants to point at an interesting position on the sky, it can only do so during a fraction of the orbit during which the Earth isn't in the way. There are a few places on the sky, though, for which HST can point continuously during the entire orbit. The GOODS-North field in CANDELS is one of these special places. This means that we have about twice as much time available to observe that field than usual. CANDELS uses some of the extra time to observe GOODS-North in the UV.

As with all CANDELS fields, GOODS-North is observed many times, building up the signal through repeated observations. The schedule of the repetitions is designed for the supernova search. In practice, this means that even though we got the first UV observations in the spring of 2012, it will be spring of 2013 before we have enough data to see most of the UV objects in the images.

Major Science Goals

The most massive, young, and hot stars emit light strongly in the UV. This makes UV observations particularly effective for studying galaxies that are forming many stars.  These data allow us to find these galaxies and to study how they formed.  

In a little more detail, we have three major goals for the UV observations:

1.  Finding and studying strongly star-forming galaxies:  

The most popular way to find distant star-forming galaxies is to look for a strong feature in the distribution of their light across the spectrum.  This technique, referred to as looking for the "Lyman break" or "dropouts", will be familiar to people who have read about the rest of the goals for CANDELS, because it is also used to find galaxies in the very distant Universe. The advantage of adding UV data to CANDELS is that it allows us to use the same method to find galaxies when the Universe was about 25% of its current age. And, by using the same technique to find them, we can compare those galaxies directly to the much more distant ones.
 

In practice, the "dropout" technique means that we look for galaxies that are bright in most of the CANDELS bands, but are much fainter in the UV.  That is, they "drop out" of the UV. This is an indication that the UV light that they emit has been absorbed by neutral hydrogen before it reaches HST.

Once we find these galaxies, we can ask many intersting questions about them: Do they tend to be big or small? Do they tend to have a lot of dust? Do they tend to be in groups together or are they isolated?  

2.  The build up of galaxy structure from sub-galactic clumps

Galaxies grow and develop structure over time. Through gravitational effects, possibly including merging with other galaxies, they become like the galaxies we see today. We know that they undergo periods of intense star formation, but we are still learning about how they form their distinctive structures like spiral arms.  

There is evidence that as many galaxies grow, they form small clusters of hot stars, which are often called "clumps", which then migrate together into larger structures. Measuring the number, size, and brightness of these clumps can help us understand how galaxies form their structure. It can also help us distinguish which kinds of galaxies form through the mergers of smaller galaxies, and which kinds form primarily by themselves.  

Clumps have been studied extensively in galaxies when the Universe was about 25% of its current age. UV observations, which will see the hot stars that make up the clumps, will allow us to study them in galaxies later in the histroy of the Universe, when it was around 50% of its current age.

3.  How does ionizing radiation escape from galaxies?

There was a period of time in the relatively early Universe, known as the "dark ages", when most of the electrons and protons in the Universe were together in Hydrogen atoms. When galaxies began to form, they emitted a lot of energetic photons (referred to as "ionizing radiation") which broke apart the Hydrogen atoms, an event called "Reionization." Exactly how this happened is one of the great mysteries of cosmology. In particular, we don't know how the photons got out of the galaxies that contained the hot stars that emitted them.

 
In order to figure out how this ionization radiation escapes, we need to be able to observe it. The best way to do this is to look at galaxies that are similar to those which caused Reionization, but are a little closer so that we can study them. 

The CANDELS UV observations will allow us to make some of the best measurements ever taken of ionizing radiation escaping from galaxies.

The Next Steps

CANDELS is currently taking UV images, with a single pass of the GOODS-North field once every couple of months. It will take about a year to build up enough of these images to reach the sensitivity needed for the science goals described above.  We are eagerly waiting for next year, when we can begin to look at these exciting data and see what we can learn about strongly star-forming galaxies, the build up of galaxy structure, and how ionizing radiation escapes from galaxies. Stay tuned!

Baby Galaxies in the Distant Universe

A newborn baby will typically double its weight in about 4 months. Imagine an adult growing at an equally monstrous rate! Galaxies are like people in that they also grow fast when they are young, and slowly when they are old. This post is about baby galaxies discovered by CANDELS that we found to be forming stars at a ferocious rate when the universe was 4 billion years old.

Let us do a little thought experiment and scale the life time of galaxies -- essentially, the age of the universe (13.7 billion years) -- to the life span of a human (say, 80 years). The CANDELS baby galaxies are, on this scale, about a month old and will double their mass in stars by the end of the second month. In comparison, our Milky Way Galaxy would, at its current rate, double its mass in about 80 years, not too different from the current growth rate of the average weight of people in a rich country, I'm afraid.

Like their human counterparts, these baby galaxies consume vast amounts of fuel relative to their weight. This fuel consists of gas that is very simple in its composition, with a very low level of contamination in the form of elements heavier than Helium. Out of this pristine gas the galaxies form new stars continuously, quickly building up their stellar bodies.

The largest and hottest among the millions of newly formed stars dominate the energy budget of an entire galaxy. It is their energy output that we observed with CANDELS, in particular because they ionize gas which subsequently recombines and emits an enormous amount of light at a single wavelength. This line radiation makes these unexpected objects stand out among the thousands of older, more slowly growing 'kid' galaxies seen by CANDELS in the distant, that is, younger, universe, much like a crying infant is easily recognized in a crowd of gossiping teenagers.

Especially when the large, hot stars reach the end of their lives, when they explode as supernovae -- on our human time scale, these stars do not grow older than a couple of weeks, the price they pay for their fast-paced life styles -- they generate a lot of mayhem. Much of the present gas is blown out of the galaxy, making its immediate environment acutely aware of its digestive prodigiousness.

The number of baby galaxies we observed suggests that the age of the universe when it was giving birth to the observed baby galaxies is a healthy 23 years on our human time scale. (How such a young universe had already given birth to thousands of teenager galaxies is food for a discussion that we will have at some other time.) Interestingly, it was around the same time that the universe as a whole was producing new stars at the highest rate it ever would. 

At the present day, that rate is about a factor 10 smaller. The baby galaxies, in particular, are not produced in any significant number today. Even though rare examples are known, the incidence of such strongly star forming baby galaxies in the younger universe was at least 100 times larger than it is today. This commonness at earlier times is precisely one of the most interesting features. We see galaxies form their first significant batch of stars, in a way that was not anticipated or predicted by our galaxy formation models. Much work is still ahead of us to find out what causes the high star formation rate and what role these objects play in the big picture of galaxy formation. 

CANDELS has allowed us to find this new class of distant galaxies, which has already instigated much discussion and will certainly motivate further investigations. Perhaps the most pressing question is, as always, what these baby galaxies will grow up to become. Will their unhealthy growth result in their own demise, or will they settle for a more sustainable growth level and lead steady lives for many decades?

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.