Comparing the host galaxy ages of X-ray selected AGN in COSMOS

Obscured AGN are associated with older galaxies

¹ Institute for Astronomy Astrophysics Space Applications and Remote Sensing (IAASARS), National Observatory of Athens, I. Metaxa & V. Pavlou, Penteli 15236, Greece
e-mail: ig@noa.gr
² Instituto de Fisica de Cantabria (CSIC-Universidad de Cantabria), Avenida de los Castros, 39005 Santander, Spain
³ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, 9000 Gent, Belgium
⁴ Max-Planck Institut für Astronomie Königstuhl, 69117 Heidelberg, Germany
⁵ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti, 93/3, 40129 Bologna, Italy
⁶ Department of Physics and Astronomy, Clemson University, Kinard Lab of Physics, Clemson, SC 29634, USA

Received: 3 September 2022
Accepted: 25 January 2023

Abstract

We explore the properties of the host galaxies of X-ray selected AGN in the COSMOS field using the Chandra Legacy sample and the LEGA-C survey VLT optical spectra. Our main goal is to compare the relative ages of the host galaxies of the obscured and unobscured AGN by means of the calcium break D_n(4000) and the H_δ Balmer line. The host galaxy ages are examined in conjunction with other properties such as the galaxy stellar mass, and star-formation rate as well as the AGN Eddington ratio. Our sample consists of 50 unobscured or mildly obscured (N_H < 10²³ cm⁻²) and 23 heavily obscured AGN (N_H > 10²³ cm⁻²) in the redshift range z = 0.6 − 1. We take specific caution to create control samples in order to match the exact luminosity and redshift distributions for the obscured and unobscured AGN. The majority of unobscured AGN appear to live in young galaxies in contrast to the obscured AGN which appear to live in galaxies located between the young and old galaxy populations. This finding may be in contrast to those evolutionary AGN unification models which postulate that the AGN begin their life in a heavy obscuration phase. The host galaxies of the obscured AGN have significantly lower levels of specific star-formation. At the same time the obscured AGN have lower Eddington ratios indicating a link between the star-formation and the black hole accretion. We find that the distribution of the stellar masses of the host galaxies of obscured AGN is skewed towards higher stellar masses in agreement with previous findings. Our results on the relative age of obscured AGN are valid when we match our obscured and unobscured AGN samples according to the stellar mass of their host galaxies. All the above results become less conspicuous when a lower column density (log N_H(cm⁻²) = 21.5 or 22) is used to separate the obscured and unobscured AGN populations.