Free Astronomy Magazine November-December 2014

GALAXIES A n example of how the su- permassive black hole of M60- UCD1, located in the nucleus of the very dense galaxy, might ap- pear to an ob- server. [NASA, ESA, STScI] In the video be- low, a simulation showing how over a period of about 500 million years M60 has reduced M60- UCD1 to the state of ultra- compact dwarf. [Holger Baum- gardt, University of Queensland] UCD1 that ratio rises to 15%. Of the 75 gal- axies of which we have reliable measure- ments of their central black hole mass, only one has an equally high ratio, but the black hole in M60-UCD1 is, in absolute terms, more massive. Seth and colleagues also confirm- ed that the brightness of the dwarf galaxy under consideration is in line with what ex- pected from its stellar mass, implying that many other dwarf galaxies could host super- massive blacks holes, which in turn suggests the existence of an as-yet-undetected popu- lation of blacks holes. As it is almost certain that such a small gal- axy (compact as it may be) cannot have generated a “monster” of that size, it is clear that M60-UCD1 can only be the rem- nant of a much larger stellar system. Sure enough, Seth’s team has showed through dynamic simulations that the present dwarf was in origin an elliptical galaxy with a mass of at least 10 billion solar masses, which had been almost completely engulfed by M60 during a very close passage between the two galaxies, presumably occurred about ten billion years ago. Not even dark matter seems to have escaped the “robbery”, given that in M60-UCD1 there is today no trace of it. According to the researchers, a far away day in the future, the small ultra-compact galaxy is fated to fall on its bulky neigh- bour, allowing its disproportionate black hole to merge with the even more massive one (4.5 billion solar masses) that domi- nates the centre of M60. n