Monday, June 25, 2012

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, 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.

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