The Host-galaxy Properties of Type 1 versus Type 2 Active Galactic Nuclei

Abstract

The unified model of active galactic nuclei (AGNs) proposes that different AGN optical spectral types are caused by different viewing angles with respect to an obscuring "torus". Therefore, this model predicts that type 1 and type 2 AGNs should have similar host-galaxy properties. We investigate this prediction with 2463 X-ray selected AGNs in the COSMOS field. We divide our sample into type 1 and type 2 AGNs based on their spectra, morphologies, and variability. We derive their host-galaxy stellar masses (M ) through SED fitting, and find that the host M of type 1 AGNs tend to be slightly smaller than those of type 2 AGNs by ∆logM ≈ 0.2 dex (≈ 4σ significance). Besides deriving star-formation rates (SFRs) from SED fitting, we also utilize far-infrared (FIR) photometry and a stacking method to obtain FIR-based SFRs. We find that the SFRs of type 1 and type 2 sources are similar once their redshifts and X-ray luminosities are controlled. We also investigate cosmic environment, and find that the surface number densities (sub-Mpc) and cosmic-web environments (≈ 1-10 Mpc) are similar for both populations. In summary, our analyses show that the host galaxies of type 1 and type 2 AGNs have similar SFR and cosmic environment in general, but the former tend to have lower M than the latter. The difference in M indicates that the AGN unification model is not strictly correct and both host galaxy and torus may contribute to the optical obscuration of AGNs.

Physical properties of the CDFS X-ray sources through fitting spectral energy distributions

“…Figure 11 shows that type-I AGNs have smaller M * than type 2 AGNs and that their SFRs are similar. The different M * is also in agreement with Zou et al (2019) and indicates that dust in the host galaxy may contribute to the optical obscuration of AGNs. Since there are fewer type-I AGNs in our sample, such results may also be biased.…”

“…The simple AGN unified model considers that the different types of AGNs should be attributed to different viewing angles and thus predicts that their host galaxy properties should be similar. However, some studies have suggested that their host galaxy properties were not exactly similar (Bornancini & García Lambas 2018;Zou et al 2019). Zou et al (2019) The fraction of optically obscured AGNs is plotted versus the X-ray luminosity of 5 bins (log L X <42.5; 42.5<log L X <43; 43<log L X <43.5; 43.5<log L X <44; log L X >44 ).…”

“…However, some studies have suggested that their host galaxy properties were not exactly similar (Bornancini & García Lambas 2018;Zou et al 2019). Zou et al (2019) The fraction of optically obscured AGNs is plotted versus the X-ray luminosity of 5 bins (log L X <42.5; 42.5<log L X <43; 43<log L X <43.5; 43.5<log L X <44; log L X >44 ). And the number of AGNs in each bin is 34, 36, 60, 56, and 52. have slightly smaller M * than the type-II AGNs, considering that dust in the host galaxy might contribute to the optical obscuration of AGNs.…”

x-cigale: fitting AGN/galaxy SEDs from X-ray to infrared

“…The model assumes that the central engine is surrounded by a dusty torus (i.e. the AGN unified model; Antonucci 1993;Urry & Padovani 1995;Netzer 2015;Zou et al 2019). The torus absorbs a fraction of the UV and optical emission from the central engine and reemits the energy as IR photons.…”

The Environmental Dependence of the Age of Active Galaxies and the Dependence of the Clustering Properties of Active Galaxies on Age

“…In the last several decades, many works have focused on the study of active galactic nuclei (AGNs) (Dressler et al 1985;Miller et al 2003;Krumpe et al 2012;Enoki et al 2014;Kagaya et al 2017;Chen et al 2018;Liu et al 2019;Zou et al 2019). Dressler et al (1985) and Miller et al (2003) explored the correlation between the presence of AGNs and the local environment of AGN host galaxies.…”