paper

GOODS-Herschel: radio-excess signature of hidden AGN activity in distant star-forming galaxies

arXiv:1210.2521 · doi:10.1051/0004-6361/201219880

Abstract

We present here a new spectral energy distribution (SED) fitting approach that we adopt to select radio-excess sources amongst distant star-forming galaxies in the GOODS-Herschel (North) field and to reveal the presence of hidden, highly obscured AGN. Through extensive SED analysis of 458 galaxies with radio 1.4 GHz and mid-IR 24 um detections using some of the deepest Chandra X-ray, Spitzer and Herschel infrared, and VLA radio data available to date, we have robustly identified a sample of 51 radio-excess AGN (~1300 deg^-2) out to redshift z~3. These radio-excess AGN have a significantly lower far-IR/radio ratio (q<1.68) than the typical relation observed for star-forming galaxies (q~2.2). We find that ~45% of these radio-excess sources have a dominant AGN component in the mid-IR band, while for the remainders the excess radio emission is the only indicator of AGN activity. The fraction of radio-excess AGN increases with X-ray luminosity reaching ~60% at Lx~10^44-10^45 erg/s, making these sources an important part of the total AGN population. However, almost half (24/51) of these radio-excess AGN are not detected in the deep Chandra X-ray data, suggesting that some of these sources might be heavily obscured. We also find that the specific star formation rates (sSFRs) of the radio-excess AGN are on average lower that those observed for X-ray selected AGN hosts, indicating that our sources are forming stars more slowly than typical AGN hosts, and possibly their star formation is progressively quenching.

28 pages, 13 figures, (abstract abridged). Accepted for publication in A&A

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