The search for active black holes in nearby low-mass galaxies using optical and mid-IR data

TL;DR

This study identifies and analyzes active galactic nuclei in nearby low-mass galaxies using optical and mid-infrared data, revealing a lower AGN fraction compared to massive galaxies and highlighting selection biases.

Contribution

It introduces a multi-criteria approach to detect AGN in low-mass galaxies and compares the effectiveness of optical and mid-IR diagnostics in this regime.

Findings

AGN fraction in low-mass galaxies is about 0.7%.

HeII-based optical selection is more sensitive in star-forming hosts.

Archival X-ray and radio data support AGN identification.

Abstract

We investigated AGN activity in low-mass galaxies, an important regime that can shed light onto BH formation and evolution, and their interaction with their host galaxies. We identified 336 AGN candidates from a parent sample of $\sim 48,000$ nearby low-mass galaxies ($M_{\rm \star} \leq 10^{9.5}M_\odot$, $z < 0.1$) in the SDSS. We selected the AGN using the classical BPT diagram, a similar optical emission line diagnostic based on the HeII$\lambda$4686 line, and mid-IR color cuts. Different criteria select host galaxies with different physical properties such as stellar mass and optical color, and only 3 out of 336 sources fulfill all three criteria. This could be in part due to selection biases. The resulting AGN fraction of $\sim 0.7 \%$ is at least one order of magnitude below the one estimated for more massive galaxies. At optical wavelengths, the HeII-based AGN selection appears to be more sensitive to AGN hosted in star-forming galaxies than the classical BPT diagram, at least in the low-mass regime. The archival X-ray and radio data available for some of the optically selected AGN candidates seem to confirm their AGN nature, but follow-up observations are needed to confirm the AGN nature of the rest of the sample, especially in the case of mid-IR selection. Our sample will be important for future follow-up studies aiming to understand the relation between BHs and host galaxies in the low-mass regime.