X-rays from Low-mass Active Galaxies in the Galaxy And Mass Assembly (GAMA) Survey

Abstract

We present new Chandra observations of seven broad-line AGNs in low-mass galaxies ($M_{\star }<5\times 10^9{M}_{\odot }$) selected from the GAMA sample of Salehirad et al. (2022). X-ray emission is detected in six galaxies, with luminosities spanning $\log (L_{0.5-8\mathrm{keV}}/\mathrm{erg}{\mathrm{s}}^{-1})\approx 40.5$--43.8. Their X-ray luminosities generally exceed predictions for X-ray binary populations, and their proximity to the optical galaxy nuclei favors accreting massive black holes as the dominant emission source. Spectral fits for four sources yield photon indices $\Gamma \approx 1.7$--2.2, with little evidence for intrinsic absorption in most cases. Virial black hole masses and hard X-ray bolometric corrections imply nominal Eddington ratios ${\lambda }_{\mathrm{Edd}}\approx 0.004$--1.1, although uncertainties are substantial. Optical tracers generally imply higher bolometric luminosities and Eddington ratios, particularly for the X-ray-undetected galaxy. The X-ray-bright sources are broadly consistent with optical AGN scaling relations involving H$\alpha$ and [O III], whereas most lie below the established X-ray--mid-infrared relation, potentially reflecting host-galaxy contamination. Four sources are consistent with the expected UV-to-X-ray spectral slope relation, while three show evidence for relative X-ray weakness. These results reveal diverse X-ray properties among broad-line AGNs in low-mass galaxies and highlight the importance of multiwavelength diagnostics for understanding black hole accretion and interpreting broad-line AGN candidates at high redshift.

arXiv comment19 pages, 8 figures, 7 tables. Submitted to ApJ