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. By the time the DXS commences, the fundamental goal of quantifying the contents of the nearby Universe, in terms of the spatial distribution of the output of electromagnetic radiation from galaxies, will have been completed by the 2dF, SDSS, and IRAS galaxy redshift surveys (optical and far-ir wavelengths dominate the global energy budget). The DXS in combination with other imaging surveys (using XMM, GALEX, MEGACAM, SIRTF, and SZ surveys), and including near-ir spectroscopy with FMOS (section 5.5), aims to produce a comparable map at high redshift. The purpose of the DXS, then, is to quantify the evolution of the properties of the Universe over a timespan greater than half the current age of the Universe. The four main goals of the DXS are the following.
1. To detect a large sample of high-redshift galaxy clusters 1.0<z<1.5 to measure the evolution of the cluster mass function N(M,z). This is the principal goal of the DXS. It sets the scope of the survey and the case forms a major part of the DXS proposal. The measurement of the evolution of the cluster mass function N(M,z) can be used to obtain constraints on cosmological parameters (e.g. Viana and Liddle, 1996, Eke et al., 1998). These constraints will be complementary to existing and future cosmic microwave background and supernova measures, and can remove degeneracies. Ultimately we hope to make an important contribution to reaching beyond the three-parameter Ho, omegam, lambda cosmology that describes the geometry and dynamics of the Universe, to obtain useful constraints on the dark energy equation of state parameter w=P/rho, and thereby to explore quintessence models.
In this proposal we emphasise the statistical study of the evolution of the abundance of clusters. Nevertheless the DXS sample of high-redshift clusters will be useful for a variety of interesting studies which will test the current paradigm for the growth of structure and the formation of galaxies, including measuring the evolution of the properties of the galaxies in clusters, and the variation with cluster mass (e.g. Burke, Collins, and Mann, 2001, Kodama and Bower, 2001), investigating the properties of individual clusters (e.g. gravitational lensing measurements of the dark matter mass profiles, Tyson et al, 1998), and exploiting the use of clusters as gravitational telescopes (e.g. Ellis et al, 2001).
2. The clustering of galaxies, by type and luminosity, at z=1. The 2dF and SDSS galaxy redshift surveys (respectively 2X105 redshifts, complete, and 1X106 redshifts, projected), are providing accurate measures of the clustering of galaxies in the nearby Universe out to the largest scales, >100 Mpc. These results provide constraints on cosmological parameters (from the shape of the galaxy power spectrum on large scales) and allow tests of the predictions of theories of galaxy formation, exploring the relation between light and mass through the measurement of clustering as a function of galaxy type, luminosity, and star-formation history. These surveys use catalogues of galaxies selected at optical wavelengths. As a consequence, the measurement of the evolution from a comparable galaxy sample at substantial lookback time, z>1, can only be achieved with deep wide-field imaging at near-ir wavelengths: the SDSS ugriz passbands correspond to the observed wavelength range 0.9 to 2.3 micron at z=1.5. WFCAM is the only instrument that can provide the deep and wide-field near-ir imaging required for this work, for the next few years.
To obtain a representative view of the properties of galaxies at z>1, near-ir surveys must cover a comparable luminosity range to that probed by current surveys of the local Universe (e.g. L~L*+2). At z=1 this corresponds to a depth of K=21. For the redshift range 1<z<1.5 a survey over 35sq. degs covers the same volume as the 2dF survey out to z=0.2.
3. Star formation at z>1 from ultraviolet to far-infrared wavelengths. The multiwavelength SED of normal galaxies is predominantly thermal, with a hot component at optical wavelengths, from the stellar photospheres seen directly in regions of low extinction, and a cool component at far-infrared wavelengths, from regions of obscured star formation where the dominant ultraviolet radiation from massive stars is absorbed by dust and reradiated at longer wavelengths. In the local universe about one-third of the energy emerges at far-infrared wavelengths, but at higher redshift this proportion probably increases, and may dominate, as indicated by observations of luminous sub-mm galaxies and of the cosmic optical and far-infrared background radiation. The DXS will observe fields with complementary deep observations at wavelengths covering the ultraviolet region (from the GALEX mission), the optical, and the mid to far-infrared (SWIRE fields). These multiwavelength fields will provide the data for a comprehensive study of the contribution from star formation at z>1 to the cosmic energy output over the electromagnetic spectrum. The near-ir data are vital for the measurement of photometric redshifts of galaxies, since these wavelengths sample the restframe optical at z>1. And the near-ir data are crucial for the identification of highly reddened objects, detected at longer wavelengths, in order to establish their redshift distribution.
4. The contribution of AGN to the cosmic energy budget. While the cosmic background radiation is dominated by star formation at optical and far-infrared to sub-mm wavelengths, in the mid-infrared AGN make an important contribution, while at X-ray wavelengths they dominate. An important question is what is the contribution, and its evolution, of AGN to the total cosmic energy budget? This is generally believed to be about 10%, but has proved difficult to quantify because the bulk of the hard X-ray background is attributed to AGN that are heavily obscured at optical wavelengths. A multiwavelength study covering X-ray (XMM), near-ir (DXS), and mid-ir (SWIRE) can answer this question. The mid-ir and X-ray data are complementary, as these are the wavelengths at which obscured AGN are most visible. The near-ir data are required for identification, to allow spectroscopic observations, and for quantifying the extinction.
To achieve the four goals listed above requires deep near-ir data over a large area. The DXS survey aims to cover three main areas each covering 10 to 15 sq. degs. With a transverse size of 200 Mpc X 200 Mpc comoving (3.5o\times3.5o at z=1)¹ and 1000 Mpc along the line of sight, this is the minimum volume needed to reliably establish the correlation function on large scales at z~1, and to identify statistically useful samples of galaxy clusters at z>1 (as we quantify below). This requirement for deep, panoramic near-infrared imaging means that these goals cannot be addressed before the advent of WFCAM.
¹ omegam=0.3, lambda=0.7, h=0.7