The postcollapse core of M15 imaged with the HST planetary camera

Abstract

We have obtained U-band images of the M15 core with the Planetary Camera of the Hubble Space Telescope. We are able to resolve stars down to the main-sequence turnoff (mu ~ 19.4) into the cluster center. We use crowded field photometry techniques to decompose M15 into bright resolved stars and a residual component consisting of stars at turnoff brightness or fainter. The residual component comprises 59% of the cluster light and follows a y =-0.71 power-law distribution for r > 1". The residual component flattens off interior to this radius and has a large core with rc = 2':2 = 0.13 pc. The core size may reflect postcollapse core expansion. The resolved stars have a slightly shallower distribution (y =-0.53) but have an abrupt overdensity for r < 1':5, which accounts for the unresolved surface brightness cusp at ground resolution. The bright stars do not become more highly concentrated at still smaller radii, however; neither the bright stars nor the residual component form a cusp at subarcsecond resolution. The total central density of light in all components is 8 x 10 5 L 0 pc-3 (U-band). The Peterson, Seitzer, and Cudworth central velocity dispersion implies a high core M/L ~ 8 (U-band). The existence of a core rather than a cusp at the 0.1 pc scale may imply that the centrally deduced dark matter is in a diffuse form rather than a massive black hole.

Internal dynamics of globular clusters

“…High-resolution imaging of the centre of M15 has resolved the luminosity cusp into essentially three bright stars. Postrefurbishment HST star-count data confirm that the 2.2 ′′ core radius observed by Lauer et al (1991), and questioned by Yanny et al (1994), is observed neither by Guhathakurta et al (1996) with WFPC2 data nor by Sosin & King (1996) with FOC data. This surface-density profile clearly continues to climb steadily within 2 ′′ .…”

The Dynamical Mass‐to‐Light Ratio Profile and Distance of the Globular Cluster M15

“…The surface brightness distribution of M15 has been studied in detail by a number of authors ( Lauer et al 1991;Trager et al 1995;Guhathakurta et al 1996;Sosin & King 1997). Noyola & Gebhardt (2005) reanalyzed archival Wide Field Planetary Camera 2 ( WFPC2) images with a new technique that measures the integrated light to determine the surface brightness distribution in the dense central regions of the cluster.…”

The X-ray source population of the globular cluster M15: Chandra high-resolution imaging

“… Our summed HRC‐I image of the centre of M15, showing the two bright sources, AC211 and M15 X‐2, and the positions of seven of the eight known millisecond pulsars (from SIMBAD). The circle represents the location and extent of the cluster core (which has a radius of 2.2 arcsec, see Lauer et al 1991). …”