On the Evolution of Star‐forming Galaxies

Abstract

The evolution in the comoving space density of the global average galaxy star formation rate (SFR) out to a redshift around unity is well established. Beyond z ≈ 1 there is growing evidence that this evolution is flat or even increasing, contrary to early indications of a turnover. Some recent analyses of z ≈ 6 photometric dropouts are suggestive of a decline from z = 3 to z ≈ 6, but there is still very little constraint on the extent of dust obscuration at such high redshifts. In less than a decade, numerous measurements of galaxy SFR density spanning z = 0 to as high as z ≈ 6 have rapidly broadened our understanding of galaxy evolution, and a summary of existing SFR density measurements is presented here. This global star formation history compilation is found to be consistent to within factors of about 3 over essentially the entire range 0 < z 6, and it can be used to constrain the evolution of the luminosity function (LF) for star-forming galaxies. The LF evolution for star-forming galaxies has been previously explored using optical source counts, as well as radio source counts at 1.4 GHz, and a well-known degeneracy between luminosity evolution [L (1 + z)Q] and density evolution [ (1 + z)P] is found. Combining the constraints from the global SFR density evolution with those from the 1.4 GHz radio source counts at submillijansky levels allows this degeneracy to be broken and a best-fitting evolutionary form to be established. The preferred evolution in a H0 = 70, ΩM = 0.3, ΩΛ = 0.7 cosmology from these combined constraints is Q = 2.70 ± 0.60, P = 0.15 ± 0.60.