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 (m_v≈ 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 r_c = 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_☉ 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.

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