Comparison of star formation histories of AGN and non-AGN galaxies

Abstract

We used active galactic nuclei (AGNs) with X-ray luminosities, LX, 2 − 10 keV ∼ 1042.5 − 44 erg s−1, from the COSMOS-Legacy survey that lie within the UltraVISTA region and cross-matched them with the LEGA-C catalogue. The latter provides measurements of the calcium break, Dn4000, and Hδ Balmer line that allow us to study the stellar populations of AGNs and compare them with a galaxy reference catalogue. Our samples consist of 69 AGNs and 2176 non-AGN systems, within 0.6 < z < 1.3, that satisfy the same photometric selection criteria. We constructed the spectral energy distributions (SEDs) of both population and used the CIGALE code to investigate the effect of the two indices in the SED fitting process. Our analysis shows that the inclusion of Dn4000 and Hδ allows CIGALE to better constrain the ages of the stellar populations. Furthermore, we find an increase of the estimated stellar masses by ∼0.2 dex, in particular for systems with young stars (Dn4000 < 1.5), when the two indices are included in the SED fitting. We then compare the Dn4000 and Hδ of AGNs with sources in the reference catalogue, accounting for the different stellar mass of the two populations. Our analysis reveals that low-to-moderate LX AGNs tend to reside in galaxies with older stellar populations and are less likely to have experienced a recent star formation burst compared to galaxies in the control sample. Finally, we compared the two populations as a function of their morphology (bulge-dominated, BD, versus non-BD) and compactness (mass-to-size ratio). A similar fraction of AGN and non-AGN systems are classified as non-BD (∼70%). Our analysis shows that BD AGN tend to have younger stellar populations compared to BD non-AGN systems. On the other hand, non-BD AGNs have, on average, older stellar populations and are less likely to have experienced a burst compared to non-BD sources in the reference sample. Furthermore, AGNs tend to prefer more compact systems compared to non-AGNs.