The Diverse Morphologies and Structures of Dwarf Galaxies Hosting Optically-Selected Active Massive Black Holes

TL;DR

This study reveals the diverse morphologies of dwarf galaxies hosting active black holes, highlighting their structural variety and implications for understanding black hole seed origins in low-mass galaxies.

Contribution

It provides the first detailed morphological analysis of 41 dwarf galaxies with optically-selected active black holes using HST imaging.

Findings

Majority are disk-dominated with pseudobulges

Presence of nuclear star clusters or AGN continuum in all regular galaxies

Diverse morphologies including irregulars and signs of interactions

Abstract

We present a study of 41 dwarf galaxies hosting active massive black holes (BHs) using Hubble Space Telescope observations. The host galaxies have stellar masses in the range of $M_\star \sim 10^{8.5}-10^{9.5}~M_\odot$ and were selected to host active galactic nuclei (AGNs) based on narrow emission line ratios derived from Sloan Digital Sky Survey spectroscopy. We find a wide range of morphologies in our sample including both regular and irregular dwarf galaxies. We fit the HST images of the regular galaxies using GALFIT and find that the majority are disk-dominated with small pseudobulges, although we do find a handful of bulge-like/elliptical dwarf galaxies. We also find an unresolved source of light in all of the regular galaxies, which may indicate the presence of a nuclear star cluster and/or the detection of AGN continuum. Three of the galaxies in our sample appear to be Magellanic-type dwarf irregulars and two galaxies exhibit clear signatures of interactions/mergers. This work demonstrates the diverse nature of dwarf galaxies hosting optically-selected AGNs. It also has implications for constraining the origin of the first BH seeds using the local BH occupation fraction at low masses -- we must account for the various types of dwarf galaxies that may host BHs.