Hard X-ray selected AGNs in low-mass galaxies from the NuSTAR serendipitous survey

Abstract

We present a sample of 10 low-mass active galactic nuclei (AGNs) selected from the 40-month NuSTAR serendipitous survey. The sample is selected to have robust NuSTAR detections at $3-24$~keV, to be at $z<0.3$, and to have optical r-band magnitudes at least 0.5~mag fainter than an $L_{\star }$ galaxy at its redshift. The median values of absolute magnitude, stellar mass and 2--10 X-ray luminosity of our sample are $⟨M_r⟩=-20.03$, $⟨M_{\star }⟩=4.6\times 10^9{M}_{\odot }$, and $⟨L_{2-10\mathrm{keV}}⟩=3.1\times 10^{42}$ erg s${}^{-1}$, respectively. Five objects have detectable broad H$\hat{I}\pm$ emission in their optical spectra, indicating black-hole masses of $(1.1-10.4)\times 10^6{M}_{\odot }$. We find that $30_{-10}^{+17}\%$ of the galaxies in our sample do not show AGN-like optical narrow emission lines, and one of the ten galaxies in our sample, J115851+4243.2, shows evidence for heavy X-ray absorption. This result implies that a non-negligible fraction of low-mass galaxies might harbor accreting massive black holes that are missed by optical spectroscopic surveys and $<10$ keV X-ray surveys. The mid-IR colors of our sample also indicate these optically normal low-mass AGNs cannot be efficiently identified with typical AGN selection criteria based on WISE colors. While the hard ($>10$ keV) X-ray selected low-mass AGN sample size is still limited, our results show that sensitive NuSTAR observations are capable of probing faint hard X-ray emission originating from the nuclei of low-mass galaxies out to moderate redshift ($z<0.3$), thus providing a critical step in understanding AGN demographics in low-mass galaxies.

Accepted for publication in ApJ. 27 pages, 6 tables, 10 figures, including one online figure set available at https://ctjchen.github.io/ctc_nustardwarf_figset.pdf (v2: updated acknowledgement.)