COSMOS: Technical Details
The aim of COSMOS is to throughly map the morphology of galaxies as a function of local environment (density) and epoch, all the way from high redshift (z > 3) to the nearby (z < 0.5) Universe. Substantial Large Scale Structure (LSS) exists on scales up to 100 Mpc (co-moving), influencing galaxy evolution and morphological mix. Therefore, the field size is adopted to encompass comoving areas of 50 x 50 Mpc at z=0.5, 137 x 137 Mpc at z=2 and 170 x 170 Mpc at z=3, fully sampling all scales currently envisaged for LSS. The comoving volume to z=4, covered by the COSMOS, is 9 x 107 Mpc3, comparable to those sampled in the local Universe by the SLOAN and 2dF surveys. Imaging in F814W will provide sufficiently deep data to fully characterize morphology, multiplicity and interaction of L* galaxies to z ~ 2 (I ~ 26 mag), measure structural parameters of galaxies and perform bulge/disk decomposition.
Combined with the follow-up observations from space- and ground-based facilities, COSMOS addresses fundamental issues in observational cosmology, including:
- the evolution of LSS, galaxies, clusters and CDM on scales up to > 1014 Msun, well sampled as a function of redshift
- the formation, assembly and evolution of galaxies and star formation as a function of LSS environment, morphology and redshift
- the dark matter distributions and properties out to z ~ 1 using gravitational lensing shear maps
- evolution of AGNs and the relationship between black hole growth and galaxy evolution
Large Scale Structure
The need to sample very large scales arises from the fact that demonstrated structure exists up to a total mass of 1014 Msun, with existing smaller surveys having a lower probability of enclosing the very large masses at z ~ 1 (Figure 1). The existing and on-going projects such as the GOODS and GEMS adequately sample masses up to 3 x 1013 Msun, whereas COSMOS is expected to sample the largest known structures ~ 2 x 1014 Msun out to z ~ 1, corresponding to Coma-size clusters. An example of a large structure found in COSMOS at redshift 0.8 is shown in Figure 2.
Assembly and Evolution of Galaxies
Galaxies in the early Universe are built up by two major processes: dissipational collapse and merging of lower mass protogalactic and galactic components. Their intrinsic evolution is then driven by the conversion of primordial and interstellar gas into stars, with galactic merging and interactions triggering for star formation and starbursts.
The wide-area COSMOS-ACS survey will yield 105 galaxies with multi-waveband color information. Combined with photometric and spectroscopic redshifts, COSMOS allows measurement of both the luminosity and 2D-3D correlation functions of galaxies of different types and their evolution with redshift and environment. This is of fundamental importance in constraining galaxy formation scenarios. Moreover, combined with multi-waveband ground-based data, we investigate the SFR and AGN activity as a function of morphology, size, redshift and LSS environment.
Comparison with other HST Surveys
The relative sensitivities and areas of major HST surveys are compared in Figure 3. COSMOS has 9 times the area of GEMS (the next largest survey) with sensitivity just 1.25 times less than GOODS (in 20% the time due to the higher throughput of F814W vs F850LP -- GOODS). With one-orbit exposures, I-F814W yields SNR = 10 for a point source with IAB = 27 mag (and 26 mag for a moderate redshift, extended galaxy).
Redshifts
In the COSMOS survey redshift (corresponding to depth along the line of sight or lookback time) is obtained to ~ 5% accuracy from Subaru imaging photometry and photometric redshifts for approximately a million galaxies. More accurate spectroscopic redshifts will be determined from the VLT and Magellan spectroscopy surveys encompassing over 50,000 objects.