Obscured AGNs in Bulgeless Hosts discovered by WISE: The Case Study of SDSS J1224+5555

There is mounting evidence that supermassive black holes (SMBHs) form and grow in bulgeless galaxies. However, a robust determination of the fraction of active galactic nuclei (AGNs) in bulgeless galaxies, an important constraint to models of SMBH seed formation and merger-free models of AGN fueling, is unknown, since optical studies have been shown to be incomplete for AGNs in low-mass galaxies. In a recent study using the Wide-field Infrared Survey Explorer, we discovered hundreds of bulgeless galaxies that display mid-infrared signatures of extremely hot dust suggestive of powerful accreting massive black holes, despite having no signatures of black hole activity at optical wavelengths. Here we report X-ray follow-up observations of J122434.66+555522.3, a nearby (z = 0.052) isolated bulgeless galaxy that contains an unresolved X-ray source detected at the 3σ level by XMM-Newton with an observed luminosity uncorrected for intrinsic absorption of {L}_2-10{keV}=(1.1+/- 0.4)× {10}⁴⁰ erg s^-1. Ground-based near-infrared spectroscopy with the Large Binocular Telescope and multiwavelength observations from ultraviolet to millimeter wavelengths together suggest that J1224+5555 harbors a highly absorbed AGN with an intrinsic absorption of {N}_{{H}}\gt {10}²⁴ cm^-2. The hard X-ray luminosity of the putative AGN corrected for absorption is {L}_2-10{keV}∼ 3× {10}⁴² erg s^-1, which, depending on the bolometric correction factor, corresponds to a bolometric luminosity of the AGN of {L}_{bol}.∼ 6× {10}⁴³-3 × 10⁴⁴ {erg} {{{s}}}^-1 and a lower mass limit for the black hole of {M}_{BH}≃ 2× {10}⁶ {M}_⊙ , based on the Eddington limit. While enhanced X-ray emission and hot dust can be produced by star formation in extremely low metallicity environments typical in dwarf galaxies, J1224+5555 has a stellar mass of ∼ 2.0× {10}¹⁰ {M}_⊙ and an above solar metallicity (12 + {logO}/{{H}} = 9.11), typical of our WISE-selected bulgeless galaxy sample. While collectively these observations suggest the presence of an AGN, we caution that identifying obscured AGNs in the low-luminosity regime is challenging and often requires multiwavelength observations. These observations suggest that low-luminosity AGNs can be heavily obscured and reside in optically quiescent galaxies, adding to the growing body of evidence that the fraction of bulgeless galaxies with accreting black holes may be significantly underestimated based on optical studies.