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

Del Moro, A. and Chary, R. R. (2013) GOODS-Herschel: radio-excess signature of hidden AGN activity in distant star-forming galaxies. Astronomy and Astrophysics, 549 . Art. No. A59. ISSN 0004-6361. doi:10.1051/0004-6361/201219880.

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Context. A tight correlation exists between far-infrared and radio emission for star-forming galaxies (SFGs), which seems to hold out to high redshifts (z ≈ 2). Any excess of radio emission over that expected from star formation processes is most likely produced by an active galactic nucleus (AGN), often hidden by large amounts of dust and gas. Identifying these radio-excess sources will allow us to study a population of AGN unbiased by obscuration and thus find some of the most obscured, Compton-thick AGN, which are in large part unidentified even in the deepest X-ray and infrared (IR) surveys. Aims. 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. Methods. Through extensive SED analysis of 458 galaxies with radio 1.4 GHz and mid-IR 24 μm 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, 3σ) than the typical relation observed for star-forming galaxies (q ≈ 2.2). Results. 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 presence of an AGN is also confirmed by the detection of a compact radio core in deep VLBI 1.4 GHz observations for eight of our radio-excess sources (≈16%; ≈66% of the VLBI detected sources in this field), with the excess radio flux measured from our SED analysis agreeing, to within a factor of two, with the radio core emission measured by VLBI. We find that the fraction of radio-excess AGN increases with X-ray luminosity reaching ~60% at L_X ≈ 10^(44)−10^(45) erg s^(-1), 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. Amongst the radio-excess AGN we can distinguish three groups of objects: i) AGN clearly identified in infrared (and often in X-rays), a fraction of which are likely to be distant Compton-thick AGN; ii) moderate luminosity AGN (L_X ≲ 10^(43) erg s^(-1)) hosted in strong star-forming galaxies; and iii) a small fraction of low accretion-rate AGN hosted in passive (i.e. weak or no star-forming) galaxies. 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.

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Chary, R. R.0000-0001-7583-0621
Additional Information:© 2013 ESO. Received: 25 June 2012. Accepted: 28 September 2012. Published online 18 December 2012. We thank the anonymous referee for the useful comments on our paper. We gratefully acknowledge support from the STFC Rolling Grant (A.D.M.; D.M.A.). This work is based on observations made with Herschel, a European Space Agency Cornerstone Mission with significant participation by NASA. F.E.B. acknowledges support from Programa de Financiamiento Basal, CONICYT-Chile (under grants FONDECYT 1101024 and FONDAP-CATA 15010003), and Chandra X-ray Center grant SAO SP1-12007B. W.N.B., B.L., Y.Q.X. acknowledge the Chandra X-ray Center grant SP1-12007A and NASA ADP grant NNX10AC99G. R.G. acknowledges support from the Italian Space Agency (ASI) under the contract ASI-INAF I/009/10/0. Y.Q.X. acknowledges support of the Youth 1000 Plan (QingNianQianRen) program and the USTC startup funding. We are grateful to N. Drory for sharing the SED fitting code used to estimate galaxy stellar masses.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Science and Technology Facilities Council (STFC)UNSPECIFIED
Comisión Nacional de Investigación Científica y Tecnológica (CONICYT)UNSPECIFIED
Fondo Nacional de Desarrollo Científico y Tecnológico (FONDECYT)1101024
Fondo de Financiamiento de Centros de Investigación en Áreas Prioritarias (FONDAP)15010003
Smithsonian Astronomical ObservatorySAO SP1-12007B
Chandra X-ray Center grantSP1-12007A
Agenzia Spaziale Italiana (ASI)I/009/10/0
Youth 1000 Plan QingNianQianRen ProgramUNSPECIFIED
Subject Keywords:galaxies: active – quasars: general – infrared: galaxies – galaxies: star formation – X-rays: galaxies
Record Number:CaltechAUTHORS:20130226-084618652
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Official Citation:GOODS-Herschel: radio-excess signature of hidden AGN activity in distant star-forming galaxies A59 A. Del Moro, D. M. Alexander, J. R. Mullaney, E. Daddi, M. Pannella, F. E. Bauer, A. Pope, M. Dickinson, D. Elbaz, P. D. Barthel, et al. (16 more) Published online: 18 December 2012 DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:37130
Deposited By: Ruth Sustaita
Deposited On:26 Feb 2013 17:18
Last Modified:09 Nov 2021 23:27

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