Infrared signature of active massive black holes in nearby dwarf galaxies^⋆

¹ Institute of Astro and Particle Physics, University of Innsbruck, Technikerstraße 25, 6020, Innsbruck, Austria
e-mail: Francine.Marleau@uibk.ac.at; Dominic.Clancy@uibk.ac.at; Rebecca.Habas@uibk.ac.at
² Astrophysics and Space Research Group, School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

Received: 3 October 2016
Accepted: 28 March 2017

Abstract

Context. We investigate the possible presence of active galactic nuclei (AGN) in dwarf galaxies and other nearby galaxies to identify candidates for follow-up confirmation and dynamical mass measurements.

Aims. We identify candidate active central massive black holes (CMBH) using their mid-infrared emission, verify their nature using existing catalogues and optical line emission diagnostics, and study the relationship between their mass and the mass of their host galaxy.

Methods. We use the Wide-field Infrared Survey Explorer (WISE) All-Sky Release Source Catalog and examine the infrared colours of a sample of dwarf galaxies and other nearby galaxies in order to identify both unobscured and obscured candidate AGN by applying the infrared colour diagnostic. Stellar masses of galaxies are obtained using a combination of three independent methods. Black hole masses are estimated using the bolometric luminosity of the AGN candidates and computed for three cases of the bolometric-to-Eddington luminosity ratio.

Results. We identify 303 candidate AGN, of which 276 were subsequently found to have been independently identified as AGN via other methods. The remaining 9% require follow-up observations for confirmation. The activity is detected in galaxies with stellar masses from ~ 10⁶ to 10⁹ M_⊙; assuming the candidates are AGN, the black hole masses are estimated to be ~ 10³ − 10⁶ M_⊙, adopting L_bol = 0.1 L_Edd. The black hole masses probed are several orders of magnitude smaller than previously reported for centrally located massive black holes. We examine the stellar mass versus black hole mass relationship in this low galaxy mass regime. We find that it is consistent with the existing relation extending linearly (in log-log space) into the lower mass regime.

Conclusions. These findings suggest that CMBH are present in low-mass galaxies and in the Local Universe, and provide new impetus for follow-up dynamical studies of quiescent black holes in local dwarf galaxies.