Continuing formation of the central star cluster in M87

Abstract

The rising velocity dispersion of the stars in the neighbourhood of the central luminosity spike observed1–3 in the elliptical radio galaxy M87 was originally considered to be evidence for the presence of a massive black hole4 (M ∼ 5×109M⊙). A later spectrum3 taken in good seeing, however, shows that most of the light in the spike is due to stars, which restricts any black hole model; the luminosity spike may be a star cluster formed from gas infalling from the outer regions of M875,6. Our analysis7 of Einstein Observatory X-ray data on M87 revealed a cooling inflow of gas occurring at the rate required to build such a cluster. We show here that the back-reaction on the flow of the nuclear luminosity is small unless M87 was much brighter in the past and we suggest that massive central star clusters may be common in the central galaxies in many clusters and, on a smaller scale, in many elliptical galaxies.