The Cluster of Blue Stars Surrounding the M31 Nuclear Black Hole

TL;DR

This study used high-resolution Hubble imaging to resolve a compact cluster of UV-bright, young stars around M31's nuclear black hole, suggesting recent star formation in the vicinity of the black hole.

Contribution

First detailed imaging of the P3 star cluster around M31's black hole, indicating it consists mainly of young stars formed in a recent starburst.

Findings

P3 contains young, early-type main sequence stars.

The stellar population is consistent with a 100-200 Myr old starburst.

Old population evolution cannot fully explain P3 stars.

Abstract

We obtained U_330 and B band images of the M31 nucleus using the High Resolution Camera of the Advanced Camera for Surveys on board the Hubble Space Telescope (HST). The spatial resolution in the U_330-band, 0.03" FWHM, or 0.1 pc at M31, is sufficient to resolve the outskirts of the compact cluster (P3) of UV-bright stars surrounding the M31 black hole. The center of the cluster is marked by an extended source that is both brighter and redder than the other point sources within P3; it is likely to be a blend of several bright stars. We hypothesize that it marks the location of the M31 black hole. Both stellar photometry and a surface brightness fluctuation analysis, show that the P3 stellar population is consistent with early-type main sequence stars formed in a ~100 - ~200 Myr old starburst population. Evolutionary tracks of post early asymptotic giant-branch stars, associated with late-stage evolution of an old population, also traverse the U and U-B domain occupied by the P3 stars; but we argue that only a few stars could be accounted for that way. PEAGB evolution is very rapid, and there is no progenitor population of red giants associated with P3. The result that P3 comprises young stars is consistent with inferences from earlier HST observations of the integrated light of the cluster. Like the Milky Way, M31 harbors a black hole closely surrounded by apparently young stars.