An evolutionary missing link? A modest-mass early-type galaxy hosting an over-sized nuclear black hole

TL;DR

This paper reports the discovery of a galaxy with an unusually large black hole relative to its modest host, challenging existing galaxy evolution models and highlighting the importance of separating AGN and host galaxy emissions.

Contribution

It presents detailed optical spectroscopy of a unique galaxy with an over-sized black hole, providing insights into galaxy-black hole co-evolution and challenging downsizing assumptions.

Findings

Black hole mass estimated at 3.5×10^8 M_sun

Host galaxy mass around 2.5×10^10 M_sun

AGN luminosity about 12% of Eddington limit

Abstract

SAGE1C\,J053634.78$-$722658.5 is a galaxy at redshift $z=0.14$, discovered behind the Large Magellanic Cloud in the {\it Spitzer} Space Telescope "Surveying the Agents of Galaxy Evolution" Spectroscopy survey (SAGE-Spec). It has very strong silicate emission at 10 $\mu$m but negligible far-IR and UV emission. This makes it a candidate for a bare AGN source in the IR, perhaps seen pole-on, without significant IR emission from the host galaxy. In this paper we present optical spectra taken with the Southern African Large Telescope (SALT) to investigate the nature of the underlying host galaxy and its AGN. We find broad H$\alpha$ emission characteristic of an AGN, plus absorption lines associated with a mature stellar population ($>9$ Gyr), and refine its redshift determination to $z=0.1428\pm0.0001$. There is no evidence for any emission lines associated with star formation. This remarkable object exemplifies the need for separating the emission from any AGN from that of the host galaxy when employing infrared diagnostic diagrams. We estimate the black hole mass, $M_{\rm BH}=3.5\pm0.8\times10^8$ M$_\odot$, host galaxy mass, $M_{\rm stars}=2.5^{2.5}_{1.2}\times10^{10}$ M$_\odot$, and accretion luminosity, $L_{\rm bol}({\rm AGN})=5.3\pm0.4\times10^{45}$ erg s$^{-1}$ ($\approx12$ per cent of the Eddington luminosity) and find the AGN to be more prominent than expected for a host galaxy of this modest size. The old age is in tension with the downsizing paradigm in which this galaxy would recently have transformed from a star-forming disc galaxy into an early-type, passively evolving galaxy.