The Chandra View of NGC 4178: The Lowest Mass Black Hole in a Bulgeless Disk Galaxy?

TL;DR

This study uses Chandra X-ray observations to identify a potential low-mass black hole in a bulgeless galaxy, revealing a highly absorbed AGN and several off-nuclear X-ray sources, including a candidate intermediate-mass black hole.

Contribution

First detection of a low-mass, highly absorbed AGN in a bulgeless galaxy using X-ray and mid-IR data, with estimates of black hole mass and characterization of off-nuclear sources.

Findings

Black hole mass estimated at 10^4 - 10^5 solar masses.

Detection of off-nuclear sources with luminosities near X-ray binary and ULX thresholds.

Identification of a candidate intermediate-mass black hole at 6×2×10^3 solar masses.

Abstract

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 10^43 ergs/s. The black hole mass estimate, based on our Chandra data and archival VLA data using the most recent fundamental plane relations is \sim 10^4 - 10^5 M\odot, 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 model with absorption. These two sources have X-ray luminosities of the order of \sim 10^38 ergs/s, which place them at the threshold between X-ray binaries and ultra-luminous 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 \sim 10^40 ergs/s. Its spectrum is well-fit with an absorbed power law model, suggesting that it is a ULX. We also fit its spectrum with the Bulk Motion Comptonization (BMC) model and suggest that this source is consistent with an intermediate-mass black hole (IMBH) of mass (6\times2)\times10^3 M\odot.