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

We investigated AGN activity in low-mass galaxies, an important regime that can shed light on to black hole (BH) formation and evolution, and their interaction with their host galaxies. We identified 336 AGN candidates from a parent sample of ∼48 000 nearby low-mass galaxies (M * ≤ 109.5 M⊙, z < 0.1) in the Sloan Digital Sky Survey. We selected the AGN using the classical BPT diagram, a similar optical emission line diagnostic based on the ${\mathrm{He}}\,\small {II}\:$ λ4686 line, and mid-IR colour cuts. Different criteria select host galaxies with different physical properties such as stellar mass and optical colour and only 3 out of 336 sources fulfil all three criteria. This could be in part due to selection biases. The resulting AGN fraction of ∼0.7 per cent is at least one order of magnitude below the one estimated for more massive galaxies. At optical wavelengths, the ${\mathrm{He}}\,\small {II}\:$ -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 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.