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arXiv · 2211.17271

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

Ryan W. Pfeifle·Shobita Satyapal·Claudio Ricci·Nathan J. Secrest·Mario Gliozzi·Thomas Bohn·Gabriela Canalizo·Michael A. Reefe
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Abstract

The discovery over the last several decades of moderate luminosity 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. 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. J0851+3926 was originally identified as an AGN based on its WISE 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 $<7.41\times10^{-14}$ erg cm$^{-2}$ s$^{-1}$ and $<9.40\times10^{-14}$ erg cm$^{-2}$ s$^{-1}$ for the AGNs in NGC 4178 and J0851+3926, respectively. If these non-detections are due to large absorbing columns along the line of sight, the non-detections in NGC 4178 and J0851+3926 could be explained with column densities of log($N_{\rm{H}}/\rm{cm}^2)>24.2$ and log($N_{\rm{H}}/\rm{cm}^2)>24.1$, 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 non-detection in J0851+3926 and multiwavelength properties are consistent with the AGN being heavily obscured.

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Ryan W. Pfeifle, Shobita Satyapal, Claudio Ricci, Nathan J. Secrest, Mario Gliozzi, Thomas Bohn, Gabriela Canalizo, Michael A. Reefe. 2022-11-30. NuSTAR Observes Two Bulgeless Galaxies: No Hard X-ray AGN Detected in NGC 4178 or J0851+3926. https://doi.org/10.3847/1538-4357/aca7ff

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