The role of AGN and obscuration in the position of the host galaxy relative to the main sequence

Abstract

We use X-ray active galactic nuclei (AGN) observed by the Chandra X-ray Observatory within the 9.3 deg2 Boötes field of the NDWFS to study whether there is a correlation between X-ray luminosity (LX) and star formation rate (SFR) of the host galaxy, at 0.5 < z < 2.0, with respect to the position of the galaxy to the main sequence (SFRnorm). About half of the sources in the X-ray sample have spectroscopic redshifts. We also construct a reference galaxy catalogue. For both datasets we use photometric data from the optical to the far-infrared compiled by the HELP project, and apply spectral energy distribution fitting, using the X-CIGALE code. We exclude quiescent sources from both the X-ray and the reference samples. We also account for the mass completeness of our dataset, in different redshifts bins. Our analysis highlights the importance of studying the SFR–LX relation in a uniform manner, taking into account systematics and selection effects. Our results suggest, in less massive galaxies (log [M*(M⊙)] ∼ 11), that an AGN enhances the SFR of the host galaxy by ∼50% compared to non-AGN systems. A flat relation is observed for the most massive galaxies. The SFRnorm does not evolve with redshift. The results, although tentative, are consistent with a scenario where, in less massive systems, both AGN and star formation are fed by cold gas supplied by a merger event. In more massive galaxies the flat relation could be explained by a different supermasssive black hole fuelling mechanism that is decoupled from the star formation of the host galaxy (e.g., hot diffuse gas). Finally, we compare the host galaxy properties of X-ray absorbed and unabsorbed sources. Our results show no difference, which suggests that X-ray absorption is not linked with the properties of the galaxy.

Galaxy properties of type 1 and 2 X-ray selected AGN and comparison among different classification criteria

“…Some previous studies found a correlation between N H and M * (Buchner et al 2017;Lanzuisi et al 2017). However, other X-ray studies found that X-ray absorbed and unabsorbed AGN live in galaxies with similar stellar mass (Merloni et al 2014;Masoura et al 2021;Mountrichas et al 2021b). In our sample, most type 1 sources have lower N H values (median of log [N H (cm −2 ) = 20.60) compared to type 2 sources (median of log [N H (cm −2 )] = 21.49), but the distribution of the latter is very broad (Fig.…”

“…For the latter, we use expression (9) of Schreiber et al (2015). As mentioned in Mountrichas et al 2021b, using an expression from the literature to calculate the SFR of star-forming MS galaxies and compare it with the SFR of X-ray AGN to estimate SFR norm , hints at a number of systematics. For example, different methods and/or (SED fitting) algorithms may be used in the estimation of SFR and different definitions of MS may be applied in different studies.…”

“…The latter is used to weigh each source based on its redshift and X-ray luminosity (e.g. Mountrichas et al 2016;Masoura et al 2021;Mountrichas et al 2021b). In practice, this means that we restrict the redshift range of type 1 and 2 sources to 0.1 < z < 0.9 and the X-ray luminosity to 43.0 < log [L X,2−10keV (ergs −1 )] < 44.8.…”

“…Most studies that examined the host galaxy properties of X-ray selected AGN and classified them into obscured and unobscured using X-ray criteria, e.g. the value of the hydrogen column density, N H , agree that both X-ray absorbed and unabsorbed AGN live in galaxies with similar stellar mass, M * , and star-formation rate, SFR, (Merloni et al 2014;Masoura et al 2021;Mountrichas et al 2021b, , but see Lanzuisi et al 2017). Chen et al (2015) used IR selected AGN in the Boötes field and classified their sources using optical/mid-IR colours.…”

Star formation of X-ray AGN in COSMOS: The role of AGN activity and galaxy stellar mass

“…In the SED fitting analysis, we use the same grid used in Mountrichas et al (2021c). This allows us to avoid any systematic effects introduced by using different modules and parameter space during the SED fitting process and facilitates a better comparison with the results presented in the Mountrichas et al study.…”