Hidden AGNs in Early-Type Galaxies

TL;DR

This study uses stacking analysis of early-type galaxies in the Chandra COSMOS survey to identify hidden AGNs and characterize their X-ray emission, revealing two regimes of AGN activity at different redshifts and stellar masses.

Contribution

It provides new evidence for hidden, absorbed AGNs in ETGs at high redshift and low stellar mass, using spectral and luminosity analysis to distinguish AGN from hot gas emission.

Findings

Detection of hard, absorbed X-ray emission in high-z ETGs indicating hidden AGNs.

Enhanced X-ray luminosity in low-mass ETGs suggests inefficient accretion onto black holes.

X-ray spectra indicate emission consistent with low-rate accretion onto SMBHs.

Abstract

We present a stacking analysis of the complete sample of Early Type Galaxies (ETGs) in the \textit{Chandra} COSMOS (C-COSMOS) survey, to explore the nature of the X-ray luminosity in the redshift and stellar luminosity ranges \(0<z<1.5\) and \({10}^{9}<L_K/L_{\astrosun}<{10}^{13}\). Using established scaling relations, we subtract the contribution of X-ray binary populations, to estimate the combined emission of hot ISM and AGN. To discriminate between the relative importance of these two components, we (1) compare our results with the relation observed in the local universe \(L_{X,gas}\propto L_K^{4.5}\) for hot gaseous halos emission in ETGs, and (2) evaluate the spectral signature of each stacked bin. We find two regimes where the non-stellar X-ray emission is hard, consisten t with AGN emission. First, there is evidence of hard, absorbed X-ray emission in stacked bins including relatively high z (\(\sim 1.2\)) ETGs with average high X-ray luminosity (\(L_{X-LMXB}\gtrsim 6\times{10}^{42}\mbox{ erg}/\mbox{s}\)). These luminosities are consistent with the presence ofhighly absorbed "hidden" AGNs in these ETGs, which are not visible in their optical-IR spectra and spectral energy distributions. Second, confirming the early indication from our C-COSMOS study of X-ray detected ETGs, we find significantly enhanced X-ray luminoaity in lower stellar mass ETGs (\(L_K\lesssim{10}^{11}L_{\astrosun}\)), relative to the local \(L_{X,gas}\propto L_K^{4.5}\) relation. The stacked spectra of these ETGs also suggest X-ray emission harder than expected from gaseous hot halos. This emission is consistent with inefficient accretion \({10}^{-5}-{10}^{-4}\dot{M}_{Edd}\) onto \(M_{BH}\sim {10}^{6}-{10}^{8}\,M_{\astrosun}\).