A search for stellar aggregates in dwarf spheroidal galaxies

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Abstract
We use the Path-Linkage-Criterion (PLC) technique to identify aggregates of stars in the dwarf spheroidal galaxies Leo I, Leo II, Fornax, Draco and Ursa Minor. The PLC technique is calibrated by applying it to random distributions of points matching the observed global two-dimensional surface density distributions of the five galaxies. Comparison of these results shows that stars are locally distributed at random in four of these dwarf spheroidals under study, and no aggregates of stars, above the noise, are identified (excluding the obvious globular clusters). Special attention is given to the putative cluster 6 of Fornax, which we can explain as a random fluctuation of the stellar surface density. Deep CCD data of the central regions of Ursa Minor and Draco are used to place limits on possible massive black holes in these systems. No excess of stars is found in the centre of Draco, giving a limit on any central black hole of |$\lt3\times10^5\enspace \text M_\odot$|⁠. For Ursa Minor, a non-random clump of stars is identified near the centre of the galaxy. If this clump were the signature of a black hole, as postulated by Strobel & Lake, the black hole mass would be less than |$\sim10^6\enspace \text M_\odot$|⁠. This is insufficient to explain the apparently high mass-to-light ratio of Ursa Minor, but significant enough to warrant further investigation.

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