M31* and its circumnuclear environment

TL;DR

This study investigates the circumnuclear environment of M31 using multiwavelength data, revealing the properties of its supermassive black hole, hot gas, and interactions with the nuclear spiral, highlighting the role of supernovae in galactic regulation.

Contribution

It provides the first detailed characterization of M31's circumnuclear hot gas and the interaction mechanisms with the nuclear spiral, emphasizing the influence of Type Ia supernovae.

Findings

M31* has an extremely low X-ray luminosity, about 10^{-10} of Eddington.

The circumnuclear hot gas has a temperature of ~0.3 keV and density of ~0.1 cm^{-3}.

Evidence suggests thermal evaporation between the nuclear spiral and hot gas.

Abstract

We present a multiwavelength investigation of the circumnuclear environment of M31. Based on Chandra/ACIS data, we tightly constrain the X-ray luminosity of M31*, the central supermassive black hole of the galaxy, to be L (0.3-7 keV)<= 1.2x10^{36}erg/s, approximately 10^{-10} of the Eddington luminosity. From the diffuse X-ray emission, we characterize the circumnuclear hot gas with a temperature of ~0.3 keV and a density of ~0.1 cm^{-3}. In the absence of an active SMBH and recent star formation, the most likely heating source for the hot gas is Type Ia SNe. The presence of cooler, dusty gas residing in a nuclear spiral has long been known in terms of optical line emission and extinction. We further reveal the infrared emission of the nuclear spiral and evaluate the relative importance of various possible ionizing sources. We show evidence for interaction between the nuclear spiral and the hot gas, probably via thermal evaporation. This mechanism lends natural understandings to 1) the inactivity of M31*, in spite of a probably continuous supply of gas from outer disk regions, and 2) the launch of a bulge outflow of hot gas, primarily mass-loaded from the circumnuclear regions. One particular prediction of such a scenario is the presence of gas with intermediate temperatures arising from the conductive interfaces. The FUSE observations do show strong OVI$\lambda$1032 and 1038 absorption lines against the bulge starlight, but the effective OVI column density (~4x10^{14} cm^{-2}), may be attributed to foreground gas located in the bulge and/or the highly inclined disk of M31. Our study strongly argues that stellar feedback, particularly in the form of energy release from SNe Ia, may play an important role in regulating the evolution of SMBHs and the interstellar medium in galactic bulges.