A Chandra view of NGC 3621: a bulgeless galaxy hosting an AGN in its early phase?

TL;DR

This study confirms the presence of an active galactic nucleus (AGN) in the bulgeless galaxy NGC 3621 through Chandra X-ray observations, revealing a heavily absorbed supermassive black hole and potential intermediate-mass black hole candidates.

Contribution

First X-ray detection of an AGN in a bulgeless galaxy, supporting black hole formation in isolated disk galaxies and identifying candidate intermediate-mass black holes.

Findings

Confirmed AGN presence in NGC 3621 via X-ray detection.

Identified two bright X-ray sources as potential intermediate-mass black holes.

Suggests black hole growth occurs in bulgeless, isolated galaxies.

Abstract

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 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 X-ray 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 1.e39 erg/s, which make them ULXs 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 2 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 multi-wavelength observations are necessary to confirm these conclusions.