2009ApJ...700.1759G

We report the detection of a weak X-ray point-source coincident with the nucleus of the bulgeless disk galaxy NGC 3621, recently discovered by Spitzer to display high-ionization mid-infrared lines typically associated with active galactic nucleus (AGN). These Chandra observations provide confirmation for the presence of an AGN in this galaxy, adding to the growing evidence that black holes do form and grow in isolated bulgeless disk galaxies. Although the low signal-to-noise ratio of the X-ray spectrum prevents us from carrying out a detailed spectral analysis of the nuclear source, the X-ray results, combined with the IR and optical spectroscopic results, suggests that NGC 3621 harbors a heavily absorbed AGN, with a supermassive black hole of relatively small mass accreting at a high rate. Chandra also reveals the presence of two bright sources straddling the nucleus located almost symmetrically at 20'' from the center. Both sources have 0.5-8 keV spectra that are well fitted by an absorbed power-law model. Assuming they are at the distance of NGC 3621, these two sources have luminosities of the order of 10³⁹ erg/s, which make them ultraluminous X-ray sources and suggest that they are black hole systems. Estimates of the black hole mass based on the X-ray spectral analysis and scaling laws of black hole systems suggest that the two bright sources might be intermediate mass black holes with M_BH of the order of a few thousand solar masses. However, higher quality X-ray data combined with multiwavelength observations are necessary to confirm these conclusions.