SPECIAL REPORT: UKIDSS - the successor to 2MASS

Steve Warren

Imperial College, Univ. London, UK.

Introduction

After commissioning of the Wide-Field Camera (WFCAM) at the end of 2003, about half of all UKIRT time will be devoted to a set of ambitious near-IR public surveys - the UKIRT Infrared Deep Sky Survey (UKIDSS). The large solid angle of the WFCAM field of view, 0.21 sq. degs per exposure, and the photon gathering power of UKIRT, mean that it is now possible to consider a survey in the near-IR that matches the scope and depth of the Sloan Digital Sky Survey (SDSS) at optical wavelengths. Recognising this opportunity, in 2000 Andy Lawrence (IfA, Edinburgh) suggested that a consortium of interested UK astronomers be set up to design and undertake a set of surveys with WFCAM, on behalf of the UK community. UKIDSS is the outcome of this initiative and consists of a set of five complementary surveys ranging in depth and area from K=18.4 over 4000 sq. degs to K=23 over 0.77 sq. degs. Details of the surveys are provided in Table 1. These depths are between 3 and 7 mags deeper than 2MASS, and the volume surveyed is an order of magnitude greater. The surveys will take 6.5 years to complete. The UKIRT Board have approved the surveys, subject to renewal of approval on a rolling basis at two year intervals. The URL of the UKIDSS web page is http://www.ukidss.org.

Instrument

The WFCAM instrument is being designed and built at the UK Astronomy Technology Centre in Edinburgh ( www.roe.ac.uk/atc/projects/wfcam), and is due for delivery to JAC in September 2003. The instrument will sit at a forward Cassegrain position, and uses four 2048x2048 Rockwell PACE HgCdTe arrays, with pixel scale 0.4 arcsec.

The large pixel scale was chosen to provide fast survey speed. To improve the sampling the observations will be microstepped 2x2, with a step N+0.5 pixels, and the images will be interlaced. The result is that the survey speed is maximised while still ensuring adequate sampling of the good image quality achieved at UKIRT.

Surveys and science goals

Details of the five surveys are provided in Table 1, which lists the filters, depths, and number of nights required (which includes an allowance for weather). A brief summary of the science goals of each of the surveys follows.

Large Area Survey (4000 sq. degs, K=18.4)

The LAS, covering high Galactic latitudes, is the near-IR counterpart of the SDSS. For typical galaxy colours the JHK depths of the LAS are a good match to the SDSS u'g'r'i'z' depths, and the combined database will provide accurate photometry of galaxies in the local universe over the wavelength range 0.35-2.3 micron. In fact the LAS explores the entire observable Universe, aiming to detect both the nearest astronomical object outside the solar system (very cool T dwarfs in the solar neighbourhood, at sub-parsec distances) as well as the most distant (quasars of redshift z=7). The Y band covers 0.97-1.07 micron, and fills the wavelength gap between SDSS z' and the J band.

Galactic Plane Survey (1800 sq. degs, K=19.0)

The GPS aims to map half the Galactic plane within -5 < b < +5 to K=19.0. The depth will be built up in three passes, providing variability information. The GPS will produce the highest-resolution clear atlas of the Milky Way enabling a proper census of the contents of the disk and providing a catalogue for identification of sources detected at other wavelengths, such as X-ray binaries. It will be possible to map star formation regions throughout the Milky Way, measuring star formation efficiency versus Galactic radius. The survey will increase the number of known Young Stellar Objects (YSOs) by an order of magnitude, and rare brief-duration YSO variables such as FU Orionis stars will be found in significant numbers for the first time.

Galactic Clusters Survey (1600 sq. degs, K=18.7)

The purpose of the GCS is the study of the sub-stellar mass function. Eleven large open star clusters and star formation associations will be targeted. By reaching a mass limit of typically 30 Jupiter masses the GCS will provide an accurate measure of the IMF below the hydrogen burning limit for clusters with a range of ages and metallicities. This will allow the definitive study of the question of the universality of the IMF.

Deep Extragalactic Survey (35 sq. degs, K=21.0)

The theme of the DXS is a comparison of the properties of the Universe at 1.0 < z < 1.5 against the properties of the Universe today. The two main goals are i) to measure the abundance of rich galaxy clusters at 1< z< 1.5, in order to obtain constraints on cosmological parameters, and ii) to measure galaxy clustering at z > 1, and more specifically the evolution of bias. The near-IR is the most sensitive wavelength region for detecting high-redshift elliptical galaxies and for identifying obscured X-ray and far-IR sources. The chosen DXS fields are targets of intensive multi-wavelength campaigns, and will allow a detailed balance sheet to be drawn up of the multi-wavelength contribution of star formation and AGN to the cosmic energy budget.

Ultra Deep Survey (0.77 sq. degs, K=23.0)

The UDS aims to map 0.77 sq. degs of sky to a depth of K=23, sufficiently deep to reach L* elliptical galaxies at z=3. The survey will map a region 100 Mpc comoving across and 2 Gpc deep (2 < z < 4), giving the first detailed picture of large scale structure at z=3. The combination of depth and area will make this the most important existing archive of near-IR data for statistical studies of the early stages of galaxy formation. The three primary aims are i) the abundance of high-redshift ellipticals, ii) the clustering of galaxies at z=3, and iii) the relationship between EROs, ULIRGs, and AGN.

Consortium membership

The UKIDSS consortium is a group of some 60 UK and Japanese astronomers who put together the science case for the surveys, who made the proposal to the UKIRT Board for the observing time, and who will execute the surveys together with JAC staff astronomers. Membership of the consortium is non-exclusive, and confers no data rights. The survey data are part of the contribution in kind of the UK to the ESO membership joining fee. The data will be available to all ESO member state astronomers - which of course now includes the UK astronomers! - immediately the data enter the archive. A small number of Japanese astronomers have been invited to join the UKIDSS consortium as part of a process of promoting scientific collaboration between Japanese and UK astronomers in joint exploitation of UKIRT and Subaru.

Andy Lawrence is the UKIDSS Principal Investigator, and Steve Warren is the Survey Scientist, who coordinates the effort of consortium members. Any ESO astronomers who would like to be actively involved in the preparatory work and the observing are invited to contact Steve Warren (sjw4@ic.ac.uk).