The postcollapse core of M15 imaged with the HST Planetary Camera

Tod R. Lauer, Jon A. Holtzman, S. M. Faber, William A. Baum, Douglas G. Currie, S. P. Ewald, Edward J. Groth, J. Jeff Hester, T. Kelsall, Robert M. Light, C. Roger Lynds, Earl J. O'Neil, Donald P. Schneider, Edward J. Shaya, James A. Westphal

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46 Scopus citations


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 γ = -0.71 power-law distribution for r > 1″. The residual component flattens off interior to this radius and has a large core with r = 2″.2 = 0.13 pc. The core size may reflect postcollapse core expansion. The resolved stars have a slightly shallower distribution (γ = -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 × 105 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.

Original languageEnglish (US)
Pages (from-to)L45-L49
JournalAstrophysical Journal
Issue number2 PART 2
StatePublished - Mar 10 1991

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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