NuSTAR Observes Two Bulgeless Galaxies: No Hard X-Ray AGN Detected in NGC 4178 or J0851+3926

Abstract

The discovery over the last several decades of low- and moderate-luminosity active galactic nuclei (AGNs) in disk-dominated galaxies—which show no “classical” bulges—suggests that secular mechanisms represent an important growth pathway for supermassive black holes in these systems. We present new follow-up NuSTAR observations of the optically elusive AGNs in two bulgeless galaxies, NGC 4178 and J0851+3926. Galaxy NGC 4178 was originally reported as hosting an AGN based on the detection of [Ne v] mid-infrared emission detected by Spitzer, and based on Chandra X-ray imaging, it has since been argued to host either a heavily obscured AGN or a supernova remnant. Galaxy J0851+3926 was originally identified as an AGN based on its Wide-Field Infrared Survey Explorer mid-IR colors, and follow-up near-infrared spectroscopy previously revealed a hidden broad-line region, offering compelling evidence for an optically elusive AGN. Neither AGN is detected within the new NuSTAR imaging, and we derive upper limits on the hard X-ray 10–24 keV fluxes of <7.41 × 10−14 and <9.40 × 10−14 erg cm−2 s−1 for the AGNs in NGC 4178 and J0851+3926, respectively. If these nondetections are due to large absorbing columns along the line of sight, the nondetections in NGC 4178 and J0851+3926 could be explained with column densities of log(N H/cm2) > 24.2 and 24.1, respectively. The nature of the nuclear activity in NGC 4178 remains inconclusive; it is plausible that the [Ne v] traces a period of higher activity in the past, but that the AGN is relatively quiescent now. The nondetection in J0851+3926 and multiwavelength properties are consistent with the AGN being heavily obscured.