Star formation and dust obscuration at z~2: galaxies at the dawn of downsizing

TL;DR

This study uses radio and UV data to analyze star formation rates and dust obscuration in galaxies at z~2, revealing mass-dependent dust effects and a decline in star formation over cosmic time.

Contribution

It provides new insights into the relationship between stellar mass, dust attenuation, and star formation rates in high-redshift galaxies using stacking analysis.

Findings

Massive galaxies are more dust-obscured than lower mass ones.

UV-based SFRs underestimate true rates without dust correction.

Star formation rates decline significantly from z~2.3 to the present.

Abstract

We present first results of a study aimed to constrain the star formation rate and dust content of galaxies at z~2. We use a sample of BzK-selected star-forming galaxies, drawn from the COSMOS survey, to perform a stacking analysis of their 1.4 GHz radio continuum as a function of different stellar population properties, after removing AGN contaminants from the sample. Dust unbiased star formation rates are derived from radio fluxes assuming the local radio-IR correlation. The main results of this work are: i) specific star formation rates are constant over about 1 dex in stellar mass and up to the highest stellar mass probed; ii) the dust attenuation is a strong function of galaxy stellar mass with more massive galaxies being more obscured than lower mass objects; iii) a single value of the UV extinction applied to all galaxies would lead to grossly underestimate the SFR in massive galaxies; iv) correcting the observed UV luminosities for dust attenuation based on the Calzetti recipe provide results in very good agreement with the radio derived ones; v) the mean specific star formation rate of our sample steadily decreases by a factor of ~4 with decreasing redshift from z=2.3 to 1.4 and a factor of ~40 down the local Universe. These empirical SFRs would cause galaxies to dramatically overgrow in mass if maintained all the way to low redshifts, we suggest that this does not happen because star formation is progressively quenched, likely starting from the most massive galaxies.