UDS: The Ultra Deep Survey

This post is the third in a series of posts describing the five fields being targeted by CANDELS and is focused on the Ultra Deep Survey. Our previous posts described the GOODS-North and -South fields and COSMOS.

It's probably fair to say that the Ultra Deep Survey (UDS) is the least well known of the five survey fields that are now being targeted by CANDELS. However, even before the start of the CANDELS program, the UDS featured the deepest near-infrared (near-IR) imaging of any wide-area survey and was a premier field for studying the evolution of massive galaxies in the distant Universe. In fact, the UDS field has a rich history, extending back more than a decade and, as is often the case in astronomy, tends to be known by different names, depending on what wavelength of light you happen to be working with. When we refer to the field as the UDS, we are specifically referring to a near-IR imaging program (Principal Investigator: Omar Almaini, University of Nottingham) which has been pursued by the United Kingdom Infrared Telescope (UKIRT) in Hawaii for the last seven years. Given that CANDELS itself is fundamentally a near-IR imaging survey, before discussing the UDS in detail, it is perhaps worthwhile considering why observing galaxies in the near-IR part of the electromagnetic spectrum is such a powerful technique.

Left: The UK Infrared Telescope (UKIRT) on Mauna Kea, Hawaii. Right: The Wide-Field Camera (WFCAM) which is being used to observe the Ultra Deep Survey (along with four other major surveys). Image credit: Joint Astronomy Center, Hawaii

One of the key goals of extra-galactic astronomy is to try to obtain a full understanding of the formation and evolution of the most massive galaxies in the Universe. This goal is fundamental because most stars in the Universe eventually end up in massive galaxies and the evolution of the most massive galaxies has traditionally been the most difficult to accurately describe within the framework of theoretical models. Consequently, the optical properties of massive galaxies in the relatively nearby Universe have been the subject of intense study for many decades.

However, in order to gain a better understanding of galaxy evolution it is clearly necessary to investigate how the galaxies evolve as a function of cosmic time. It is within this context that observations in the near-IR (wavelengths of 1.0-2.5 microns, just long-ward of where the human eye loses sensitivity at about 0.7 microns) are key. In fact, observations in the near-IR have numerous advantages (and disadvantages), but two strengths of near-IR imaging are particularly relevant here.

The first key advantage of near-IR observations is that they allow us to study the optical properties of very distant galaxies.  As we observe galaxies at greater and greater distances, the light emitted by these distant galaxies is stretched, or "redshifted" in the jargon, during its journey towards us due to the continued expansion of the Universe. In fact, because redshift (denoted by the letter "z")  is directly related to distance, astronomers invariably refer to galaxies as being "at redshift z=1", rather than at a distance of so many billion light years. Due to the redshifting effect, the ultra-violet (UV) and optical light emitted by distant galaxies is shifted towards the near-IR part of the electromagnetic spectrum. Consequently, by observing distant galaxies in the near-IR we can actually study their optical properties, and therefore compare them on an equal basis with galaxies observed in the local Universe. The second key advantage of near-IR observations is that they allow astronomers to get a much more accurate measurement of the stellar mass of a galaxy, because near-IR luminosity is a much better tracer of stellar mass than optical/UV light.  The straightforward reason for this is that near-IR light is dominated by the low-mass, long-lived, stars that actually account for the bulk of a galaxy's mass. In contrast, if a galaxy is undergoing star-formation, optical (and particularly UV) light can be dominated by very massive/luminous short-lived stars which, although spectacular, only account for a small percentage of the total stellar mass.

These two key properties of near-IR observations were the primary drivers behind the first generation of near-IR surveys which were undertaken about a decade ago. Perhaps the key result from these early near-IR surveys was that, unexpectedly at the time, at a redshift of z=1 (at which point the Universe was basically half its current age) a large fraction of the most massive galaxies (galaxies containing the same stellar mass as 100 billion stars like our Sun) where already in existence. However, if you push your observations further back in time, out to perhaps a redshift of z=3 (when the Universe was only 15% of its current age), you find that only a small fraction of the massive galaxies we observe locally already existed at this earlier epoch. Consequently, it became clear that the 3.5 billion years of history contained within the redshift interval 1<z<3 constitutes the "epoch of massive galaxy assembly", during which the vast majority of the massive galaxies we observe in the local Universe were formed. The primary motivation for the Ultra Deep Survey was to study this key epoch in the evolution of the Universe.

A cluster of massive galaxies discovered using a combination 
of the X-ray, optical, near-IR and mid-IR data in the UDS. The 
galaxy cluster (concentration of red objects in the center of the 
image) is at redshift z=1.6, an epoch when the Universe was only 
30% of its present age. Image credit: NASA/JPL-Caltech/
C. Papovich (Texas A&M University).

The Ultra-Deep Survey (UDS) is actually just one of five surveys being pursued by the United Kingdom Infrared Telescope (UKIRT), using a dedicated, wide-field, near-IR survey camera (WFCAM).  As is nearly always the case these days (and is certainly true for CANDELS), the near-IR surveys conducted by UKIRT form a so-called "wedding cake" structure, which describes the arrangement of increasing sky coverage, but decreasing sensitivity, as you proceed from top-to-bottom of the wedding cake.  Covering a sky area of 0.8 square degrees, the UDS forms the top layer of the wedding cake, covering the smallest area of sky, but with the necessary sensitivity to detect and study galaxies in the very distant Universe.

As a result of the huge investment of observing time necessary to build-up any large-scale multi-wavelength survey field, the observations are always obtained over a number of years. Consequently, the same area of sky tends to end-up being referred to by a confusing number of different names and acronyms. This is certainly the case for the UDS.  The original dataset which initiated everything else consists of deep X-ray observations by the XMM-Newton X-ray satellite, taken as part of the so-called XMM Large Scale Survey (XMM-LSS). Subsequently, very sensitive optical imaging was taken within the same area by the 8m-class Subaru telescope in Hawaii which, together with the existing X-ray imaging, formed the Subaru XMM Deep Survey (SXDS). The near-IR observations with WFCAM that constitute the UDS were then located in the middle of the SXDS in order to take full advantage of the existing optical and X-ray imaging.

In addition to the existing data, another crucial aspect of the location of the UDS is the fact that it is an equatorial survey field. This simply means that it is located close to the celestial equator (the projection of the Earth's equator onto the celestial sphere) and as such, is within the overlap region which can be readily observed by telescopes situated in both the northern and southern hemispheres. This is a key advantage because it means that the world's most powerful telescopes, which are mostly either located on Hawaii in the north or in Chile in the south, can all contribute to building-up the necessary database of multi-wavelength imaging and spectroscopy.

Indeed, over the last decade the archive of multi-wavelength data in the UDS has become enormous, spanning wavelengths from high energy X-rays at one extreme, to ultra-sensitive radio data at the other (more details of multi-wavelength data available in the UDS can be found on the UDS website). In addition to the imaging and spectroscopy contributed by some of the world's most powerful ground-based telescopes (i.e., Subaru, VLT, UKIRT), many of the key datasets in the UDS have been delivered by space-based observatories. In addition to the original X-ray data from the XMM-Newton satellite, the UDS has been imaged in the UV with the GALEX satellite, in the mid-IR with the Spitzer satellite and now, with the advent of CANDELS, at high-spatial resolution in the near-IR with the Hubble Space Telescope (HST).

A true-colour image of the Ultra Deep Survey field. The zoom-in shows the location of a galaxy at redshift z=6 (red arrow), identified at an epoch when the Universe was only 5% of its present age. Image credit: Omar Almaini, Nottingham University (P.I. of UDS)

Armed with this impressive archive of multi-wavelength data, many astronomy groups around the world are now exploiting the UDS to tackle the outstanding issues of galaxy evolution. In the past few years, much work has been done to address the key science goals which originally motivated the UDS, exploring how the number densities, luminosities, stellar masses, sizes and morphologies of massive galaxies evolve as a function of cosmic time. Moreover, the UDS dataset has been used extensively to study how massive galaxies cluster together, both on small and large scales, and how their clustering is linked to the properties of the dark matter halos in which they reside. Interestingly, out-with the original science goals, the UDS dataset has also been extensively used (including by this author) to push the study of massive galaxies back into the early history of the Universe at redshifts of z>6, when the Universe was only 5% of its current age.

In this context, the availability of HST imaging from CANDELS offers the prospect of hugely enhancing the power of the UDS dataset for pursuing  detailed studies of galaxy evolution at high redshift. The unrivalled sensitivity and sharpness of the near-IR imaging provided by HST will allow the properties of galaxies to be studied in detail at much larger distances than was previously possible. As a consequence, in combination with the vast array of data already assembled, the HST imaging provided by CANDELS promises to secure the position of the UDS as a leading resource for the study of galaxy evolution for many years to come.