Lowest-mass X-ray selected AGNs in the Bo\"otes Field

TL;DR

This study identifies and analyzes low-mass galaxy AGNs in the Boötes field using multi-wavelength data, revealing obscured active black holes with specific accretion rates and masses, enhancing understanding of low-mass black hole occupation.

Contribution

It provides new evidence for the presence of obscured low-mass AGNs in low-mass galaxies and refines estimates of their black hole masses and accretion rates using multi-wavelength analysis.

Findings

Three low-mass galaxies host X-ray luminous AGNs.

One source shows X-ray variability, supporting AGN activity.

Estimated black hole masses are around 10^5 to 10^6 solar masses.

Abstract

We present a multi-wavelength analysis of three candidate active galactic nuclei (AGNs) in low-mass galaxies in the Bo\"otes field with the aim of improving constraints on the occupation fraction of low-mass black holes (BHs). Galaxies with low stellar masses ($M_{\star} < 10^{9.5} M_{\odot}$) are particularly interesting hosts for AGNs as they may contain BHs that have not grown significantly since the epoch of their formation in the early Universe. Using archival data from the Chandra X-ray Observatory, we find three X-ray luminous low-mass galaxies and assess whether they host AGNs. We find one of these sources to be variable in the X-ray and compute its X-ray light curve and spectrum. We compute the X-ray, mid-infrared, and [O III] luminosities and compare them to established AGN luminosity relationships in the literature. We then fit various star-forming, dust emission, and AGN templates to the spectral energy distributions (SEDs). The star formation rates estimated from the SED fits are unable to explain the observed X-ray luminosities of the candidates, providing more support for the presence of AGNs. By analysing the deviation from linear relationships between X-ray and mid-infrared luminosities, we find these systems to be obscured (with $\log N_{\rm H}[{\rm {cm^{-2}}}] \sim 22.7, > 25.0$, and $24.4$, respectively). We employ the scaling relationship between BH mass and stellar velocity dispersion to estimate the BH masses as $\sim 10^5 - 10^6 M_{\odot}$ and accreting at Eddington ratios $10^{-2} < \lambda_{\rm Edd} <10^{-1}$.