Globular cluster photometry with the Hubble Space Telescope. II. U, V, and I measurements of M15

The projected density distribution of resolved stars near the center of M15 is shown to be consistent with either a power-law cusp N(r)∼r^α, with α∼-0.85±0.2, or with a King model with a core of radius ≲2″ (best fit r_core=1″.3). The inferred slope is in agreement with the theoretical value, α = -0.75, calculated by Bahcall and Wolf for the distribution of equal-mass stars surrounding a massive black hole and is also consistent with the radial profile expected from core collapse without a central black hole. The object AC 214 is a candidate for the central density cusp. Analysis of Monte Carlo simulations of the diffuse light indicates that, using current analysis techniques and available data, the residual light is not a reliable indicator of the true density distribution. This is contrary to earlier work. Photometric measurements in V and I of more than 5X10³ stars (and in U, V, and I of ≳ 1500 stars) are used to construct color-magnitude diagrams in the central 1′ of M15. Fourteen blue straggler candidates are identified in the inner 20″. The central color gradient noticed by previous researchers is caused by a central depletion of bright red giant stars rather than an excess of blue stragglers or blue horizontal branch stars.