[en] We apply The Tractor image modeling code to improve upon existing multi-band photometry for the Spitzer Extragalactic Representative Volume Survey (SERVS). SERVS consists of post-cryogenic Spitzer observations at 3.6 and 4.5 μ m over five well-studied deep fields spanning 18 deg². In concert with data from ground-based near-infrared (NIR) and optical surveys, SERVS aims to provide a census of the properties of massive galaxies out to z  ≈ 5. To accomplish this, we are using The Tractor to perform “forced photometry.” This technique employs prior measurements of source positions and surface brightness profiles from a high-resolution fiducial band from the VISTA Deep Extragalactic Observations survey to model and fit the fluxes at lower-resolution bands. We discuss our implementation of The Tractor over a square-degree test region within the XMM Large Scale Structure field with deep imaging in 12 NIR/optical bands. Our new multi-band source catalogs offer a number of advantages over traditional position-matched catalogs, including (1) consistent source cross-identification between bands, (2) de-blending of sources that are clearly resolved in the fiducial band but blended in the lower resolution SERVS data, (3) a higher source detection fraction in each band, (4) a larger number of candidate galaxies in the redshift range 5 <  z  < 6, and (5) a statistically significant improvement in the photometric redshift accuracy as evidenced by the significant decrease in the fraction of outliers compared to spectroscopic redshifts. Thus, forced photometry using The Tractor offers a means of improving the accuracy of multi-band extragalactic surveys designed for galaxy evolution studies. We will extend our application of this technique to the full SERVS footprint in the future.

[en] We use photometric redshifts derived from new u-band through 4.5 μm Spitzer IRAC photometry in the 4.8 deg² of the XMM-LSS field to construct surface density maps in the redshift range of 0.1–1.5. Our density maps show evidence for large-scale structure in the form of filaments spanning several tens of megaparsecs. Using these maps, we identify 339 overdensities that our simulated light-cone analysis suggests are likely associated with dark matter halos with masses, M _halo, log(M _halo/M _⊙) > 13.7. From this list of overdensities we recover 43 of 70 known X-ray-detected and spectroscopically confirmed clusters. The missing X-ray clusters are largely at lower redshifts and lower masses than our target log(M _halo/M _⊙) > 13.7. The bulk of the overdensities are compact, but a quarter show extended morphologies that include likely projection effects, clusters embedded in apparent filaments, and at least one potential cluster merger (at z ∼ 1.28). The strongest overdensity in our highest-redshift slice (at z ∼ 1.5) shows a compact red galaxy core, potentially implying a massive evolved cluster.

[en] We provide a coherent, uniform measurement of the evolution of the logarithmic star formation rate (SFR)–stellar mass (M _*) relation, called the main sequence (MS) of star-forming galaxies , for star-forming and all galaxies out to $z\sim <!--\; ???\; -->5$. We measure the MS using mean stacks of 3 GHz radio-continuum images to derive average SFRs for ∼ 200,000 mass-selected galaxies at z > 0.3 in the COSMOS field. We describe the MS relation by adopting a new model that incorporates a linear relation at low stellar mass (log(M _*/M _⊙) < 10) and a flattening at high stellar mass that becomes more prominent at low redshift (z < 1.5). We find that the SFR density peaks at 1.5 < z < 2, and at each epoch there is a characteristic stellar mass (M _* = 1–4 × 10¹⁰ M _⊙) that contributes the most to the overall SFR density. This characteristic mass increases with redshift, at least to z ∼ 2.5. We find no significant evidence for variations in the MS relation for galaxies in different environments traced by the galaxy number density at 0.3 < z < 3, nor for galaxies in X-ray groups at z ∼ 0.75. We confirm that massive bulge-dominated galaxies have lower SFRs than disk-dominated galaxies at a fixed stellar mass at z < 1.2. As a consequence, the increase in bulge-dominated galaxies in the local star-forming population leads to a flattening of the MS at high stellar masses. This indicates that “mass quenching” is linked with changes in the morphological composition of galaxies at a fixed stellar mass.