X-Ray Bright Active Galactic Nuclei in Local Dwarf Galaxies: Insights from eROSITA

TL;DR

This study uses eROSITA data to identify and analyze active galactic nuclei in local dwarf galaxies, providing insights into black hole growth, occupation, and seeding mechanisms in low-mass galaxies.

Contribution

First large uniform catalog of SMBH-hosting dwarf galaxies from eROSITA, revealing their luminosity function, off-nuclear activity, and growth history.

Findings

Approximately 50% of AGN are off-nuclear.

Dwarf galaxy AGN follow local black hole-stellar mass relations.

Data favor models with moderate SMBH activity in dwarf galaxies.

Abstract

Although supermassive black holes (SMBHs) reside in the heart of virtually every massive galaxy, it remains debated whether dwarf galaxies commonly host SMBHs. Because low-mass galaxies may retain memory of the assembly history of their black holes (BHs), probing the BH occupation fraction of local dwarf galaxies might offer insights into the growth and seeding mechanisms of the first BHs. In this work, we exploit the Western half of the eROSITA all-sky survey (covering 20,000 deg$^2$) and compile a catalog of accreting SMBHs in local ($D<200$ Mpc) dwarf galaxies. Cleaning our sample from X-ray background sources, X-ray binaries, and ultraluminous X-ray sources, we identify 74 AGN-dwarf galaxy pairs. Using this large and uniform sample, we derive a luminosity function of dwarf galaxy AGN, fitting it with a power law function and obtaining dN/d$L_{\rm X} = (15.9\pm2.2)\times L_{\rm X}^{-1.63\pm0.05}$. Measuring the offset between the dwarf galaxies centroids and the X-ray sources, we find that $\approx50\%$ of the AGN are likely off-nuclear, in agreement with theoretical predictions. We compare the BH-to-stellar mass relation of our sample with the local and high-redshift relations, finding that our sources better adhere to the former, suggesting that local AGN across different mass scales underwent a similar growth history. Finally, we compare our sources with semi-analytical models: while our sample's shallowness prevents distinguishing between different seeding models, we find that the data favor models which keep SMBH in dwarf galaxies active at a moderate rate, motivating model improvement by comparison to AGN in the dwarf galaxy regime.