The Morphology of Dwarf Galaxies Hosting Variable Active Galactic Nuclei

TL;DR

This study uses HST imaging to analyze the structures of dwarf galaxies hosting low-mass AGN, revealing diverse morphologies and demonstrating that variability selection can identify lower mass and luminosity AGN missed by spectroscopy.

Contribution

It provides the first detailed morphological analysis of dwarf galaxies with AGN identified via variability, highlighting the potential of this method to find lower mass black holes.

Findings

Diverse galaxy morphologies, mostly pseudo-bulges and disks.

Black hole masses estimated between 10^3.7 and 10^6.6 solar masses.

Variability-selected AGN are lower luminosity and potentially missed by spectroscopic surveys.

Abstract

We analyze Hubble Space Telescope (HST) optical imaging of eight low-mass galaxies hosting active galactic nuclei (AGN) identified via their photometric variability in \cite{baldassare_search_2020}. We use GALFIT to model the 2D galaxy light profiles, and find a diversity of morphologies. The galaxies with regular morphologies are best fit with pseudo-bulges and disks, rather than classical bulges. We estimate black hole masses using scaling relations and find black hole masses of 10$^{3.7-6.6}$ M$_\odot$. We compare this sample to dwarf galaxies with AGN selected via optical spectroscopy. On average, the variable host galaxies have lower mass black holes. We analyze the brightest point source in each galaxy and find their properties are not entirely consistent with star clusters, indicating that they are likely AGN. These point sources are found to have lower luminosities than spectroscopically selected dwarf AGN, but brighter than the point sources in dwarf galaxies not identified as AGN. Our detailed imaging analysis shows that variability selection has the potential to find lower mass black holes and lower luminosity AGN than optical spectroscopy. These active dwarfs may have been missed by spectroscopic searches due to star formation dilution or low gas content.