Low‐Level Nuclear Activity in Nearby Spiral Galaxies

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Abstract
We are conducting a search for supermassive black holes (SMBHs) with masses below ~107 M☉ by looking for signs of extremely low-level nuclear activity in nearby galaxies that are not known to be AGNs. Our survey has the following characteristics: (1) X-ray selection using the Chandra X-Ray Observatory, since X-rays are a ubiquitous feature of AGNs; (2) emphasis on late-type spiral and dwarf galaxies as the galaxies most likely to have low-mass SMBHs; (3) use of multiwavelength data to verify that the source is an AGN; and (4) use of the highest angular resolution available for observations in X-rays and other bands, in order to separate nuclear from off-nuclear sources and to minimize contamination by host galaxy light. Here we show the feasibility of this technique of finding AGNs by applying it to six nearby, face-on spiral galaxies (NGC 3169, NGC 3184, NGC 4102, NGC 4647, NGC 4713, and NGC 5457) for which data already exist in the Chandra archive. All six show nuclear X-ray sources. The data as they exist at present are ambiguous regarding the nature of the nuclear X-ray sources in NGC 4713 and NGC 4647. We conclude, in accord with previous studies, that NGC 3169 and NGC 4102 are almost certainly AGNs. Most interestingly, a strong argument can be made that NGC 3184 and NGC 5457, both of type Scd, host AGNs.