Host galaxy properties of X-ray active galactic nuclei in the local Universe

Abstract

We study the host galaxy properties of active galactic nuclei (AGN) that have been detected in X-rays in the nearby Universe (z < 0.2). For that purpose, we use the catalogue provided by the ROSAT-2RXS in the 0.1-2.4 keV energy band, one of the largest X-ray datasets with spectroscopic observations. Our sample consists of ∼900 X-ray AGN. The catalogue provides classification of the sources into type 1 and 2 based on optical spectra. Approximately 25% of the AGN are type 2. We use the available optical, near-IR, and mid-IR photometry to construct spectral energy distributions (SEDs). We measure the stellar mass (M∗) and star formation rate (SFR) of the AGN by fitting these SEDs with the X-CIGALE code. We compare the M∗ and SFR of the two AGN populations, taking their different redshift and luminosity distributions into account. Based on our results, type 2 AGN tend to live in more massive galaxies compared to their type 1 counterparts (log [M (M S)] = 10.49-0.10+0.16 vs. 10.23-0.08+0.05), in agreement with previous studies at higher redshifts. In terms of SFRs, our analysis shows that, in the nearby Universe, the number of X-ray AGN that live in quiescent systems is higher compared to at higher redshifts, in accordance with previous studies in the local Universe. However, the majority of AGN (∼75%) live inside or above the main sequence.