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

Del Moro, A., Alexander, D M, Mullaney, J R, Daddi, E, Pannella, M, Bauer, F E, Pope, A, Dickinson, M, Elbaz, D, Barthel, P D, Garrett, M A, Brandt, W N, Charmandaris, V, Chary, R R, Dasyra, K, Gilli, R, Hickox, R C, Hwang, H S, Ivison, R J, Juneau, S, Le Floc'h, E, Luo, B, Morrison, G E, Rovilos, E, Sargent, M T and Xue, Y Q (2013) GOODS-Herschel: radio-excess signature of hidden AGN activity in distant star-forming galaxies. Astronomy and Astrophysics, 549 (A59). pp. 1-28. ISSN 0004-6361

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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.

Item Type: Article
Keywords: galaxies: active, quasars: general, infrared: galaxies, galaxies: star formation, X-rays: galaxies
Schools and Departments: School of Mathematical and Physical Sciences > Physics and Astronomy
Subjects: Q Science > QB Astronomy
Depositing User: Mark Sargent
Date Deposited: 21 May 2014 12:01
Last Modified: 02 Jul 2019 22:48

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