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The VLA-COSMOS 3 GHz Large Project : AGN and host-galaxy properties out to z ≲ 6.

Delvecchio, I. and Smolčić, V. and Zamorani, G. and Lagos, C. Del P. and Berta, S. and Delhaize, J. and Baran, N. and Alexander, D. M. and Rosario, D. J. and Gonzalez-Perez, V. and Ilbert, O. and Lacey, C. G. and Le Fèvre, O. and Miettinen, O. and Aravena, M. and Bondi, M. and Carilli, C. and Ciliegi, P. and Mooley, K. and Novak, M. and Schinnerer, E. and Capak, P. and Civano, F. and Fanidakis, N. and Herrera Ruiz, N. and Karim, A. and Laigle, C. and Marchesi, S. and McCracken, H. J. and Middleberg, E. and Salvato, M. and Tasca, L. (2017) 'The VLA-COSMOS 3 GHz Large Project : AGN and host-galaxy properties out to z ≲ 6.', Astronomy & astrophysics., 602 . A3.


We explore the multiwavelength properties of AGN host galaxies for different classes of radio-selected AGN out to z ≲ 6 via a multiwavelength analysis of about 7700 radio sources in the COSMOS field. The sources were selected with the Very Large Array (VLA) at 3 GHz (10 cm) within the VLA–COSMOS 3 GHz Large Project, and cross-matched with multiwavelength ancillary data. This is the largest sample of high-redshift (z ≲ 6) radio sources with exquisite photometric coverage and redshift measurements available. We constructed a sample of moderate-to-high radiative luminosity AGN (HLAGN) via spectral energy distribution decomposition combined with standard X-ray and mid-infrared diagnostics. Within the remainder of the sample we further identified low-to-moderate radiative luminosity AGN (MLAGN) via excess in radio emission relative to the star formation rates in their host galaxies. We show that at each redshift our HLAGN havesystematically higher radiative luminosities than MLAGN and that their AGN power occurs predominantly in radiative form, while MLAGN display a substantial mechanical AGN luminosity component. We found significant differences in the host properties of the two AGN classes, as a function of redshift. At z< 1.5, MLAGN appear to reside in significantly more massive and less star-forming galaxies compared to HLAGN. At z> 1.5, we observed a reversal in the behaviour of the stellar mass distributions with the HLAGN populating the higher stellar mass tail. We interpret this finding as a possible hint of the downsizing of galaxies hosting HLAGN, with the most massive galaxies triggering AGN activity earlier than less massive galaxies, and then fading to MLAGN at lower redshifts. Our conclusion is that HLAGN and MLAGN samples trace two distinct galaxy and AGN populations in a wide range of redshifts, possibly resembling the radio AGN types often referred to as radiative- and jet-mode (or high- and low-excitation), respectively, whose properties might depend on the different availability of cold gas supplies.

Item Type:Article
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Publisher statement:Reproduced with permission from Astronomy & Astrophysics, © ESO 2017
Date accepted:21 October 2016
Date deposited:30 June 2017
Date of first online publication:13 June 2017
Date first made open access:30 June 2017

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