Free Astronomy Magazine November-December 2014

36 NOVEMBER-DECEMBER 2014 GALAXIES just 160 light-years in diameter. Moreover, it is also a source of X-rays, which as recent- ly as last year were attributed by a group of researchers to a supermassive black hole that would be nesting at the centre of the very compact stellar system. Asserting that a dwarf galaxy, massive as it may be, can harbour a black hole such as those typically hosted by galaxies thousands of times larg- er and more massive, seems paradoxical. This especially for the fact that the central black hole of a galaxy grows with the same galaxy and characterizes its physical-chemi- cal properties. For example, there is a true and actual interdependence between the black hole’s mass and the properties of the bulge (central swelling of spiral galaxies), such that just by examining the latter we can have an idea of the type of “monster” nesting inside it. Put simply, supermassive blacks holes, such as those with masses greater than 1 million solar masses, are undoubtedly expected in large galaxies, but not in dwarf ones and even less in globular clusters. Consequently, if one of them were to be found in a UCD, this would be a concrete proof that in re- mote times the dwarf was a normal galaxy, and that only subsequently it lost all matter outside the nucleus. The possible discovery would also lead to exclude its presumed glob- ular nature and thus shed new light on the entire class of ultra-compact dwarf galaxies. Another crucial issue concerning these star systems, including M60-UCD1, is that by (rightly) showing a stellar mass consistent with the brightness, the moment in which we determine −through kinetic studies− a total mass much greater than that visible, we are faced with two problematic options: either admit the existence of a supermas- sive black hole, or assume that the central region is occupied by a large number of very massive (but not very bright) stars. This second option is even less convincing than the first, since it would entail an anom- aly in what astronomers call “stellar initial mass function”, which in summary is an em- pirical description of the distribution of stel- lar masses according to the mass available at the time of their formation. Plainly put, the stars within M60-UCD1, for unknown T he large elliptical galaxy M60 surrounds with its peripheral regions the dwarf galaxy M60- UCD1 (enlarged with scale and isophotes in the right panel). M60-UCD1 was a normal elliptical before being stripped of all its matter outside the nucleus by M60. The same fate could befall the nearby spiral galaxy NGC 4647, visible at the top. Left, a close-up of M60-UCD1. [NASA, ESA, A. Seth (University of Utah, USA)]