THE CHANDRA VIEW OF NGC 4178: THE LOWEST MASS BLACK HOLE IN A BULGELESS DISK GALAXY?
Using high-resolution Chandra data, we report the presence of a weak X-ray point source coincident with the nucleus of NGC 4178, a late-type bulgeless disk galaxy known to have high-ionization mid-infrared (mid-IR) lines typically associated with active galactic nuclei (AGNs). Although the faintness of this source precludes a direct spectral analysis, we are able to infer its basic spectral properties using hardness ratios. X-ray modeling, combined with the nuclear mid-IR characteristics, suggests that NGC 4178 may host a highly absorbed AGN accreting at a high rate with a bolometric luminosity on order of 1043 erg s–1. The black hole mass estimate, based on our Chandra data and archival Very Large Array data using the most recent fundamental plane relations, is ~104-105 M ☉, possibly the lowest mass nuclear black hole currently known. There are also three off-nuclear sources, two with a similar brightness to the nuclear source at 36'' and 32'' from the center. As with the nuclear source, hardness ratios are used to estimate spectra for these two sources, and both are consistent with a simple power-law (PL) model with absorption. These two sources have X-ray luminosities of the order of ~1038 erg s–1, which place them at the threshold between X-ray binaries and ultraluminous X-ray sources (ULXs). The third off-nuclear source, located 49'' from the center, is the brightest source detected, with an X-ray luminosity of ~1040 erg s–1. Its spectrum is well fit with an absorbed PL model, suggesting that it is a ULX. We also fit its spectrum with the Bulk Motion Comptonization model and suggest that this source is consistent with an intermediate-mass black hole of mass (6 ± 2) × 103 M ☉.