Candidate AGNs in Late-Type Galaxies

Abstract

We have assembled a sample of 64 late-type spiral galaxies (T types 6.0-9.0, corresponding to Hubble types Scd-Sm) with archival Chandra data. At a signal-to-noise (S/N) threshold of 3, we find 12 objects with X-ray point-source detections in close proximity with the optical or near-infrared position of the nucleus (median offset δ= 1.6"), suggestive of possible low-luminosity active galactic nuclei (AGNs). Including measurements with 3 > S/N > 1.5, our detections increase to 18. These X-ray sources range in luminosity from L_X(2-10 keV) = 10^{37.1} to 10^{39.6} ergs s^-1. Considering possible contamination from low-mass X-ray binaries (LMXBs), we estimate that ~5 detections are possible LMXBs instead of true AGNs, based on the probability of observing a LMXB in a nuclear star cluster typically found in these late-type spiral galaxies. Given the typical ages of nuclear star clusters, contamination by high-mass X-ray binaries is unlikely. This AGN fraction is higher than that observed in optical surveys, indicating that active nuclei, and hence central black holes, are more common than previously suggested. The incidence of AGN activity in such late-type spiral galaxies also suggests that nuclear massive black holes can form and grow in galaxies with little or no evidence for bulges. Follow-up multiwavelength observations will be necessary to confirm the true nature of these sources.