Free Astronomy Magazine November-December 2014

NOVEMBER-DECEMBER 2014 GALAXIES A wide angle shot of the region that includes M60-UCD1. The dwarf is shown here en- gulfed by the nearby M60, that with the spiral NGC 4647 forms the Arp 116 system, visible just above the centre of the image. [NASA, ESA, DSS2] In the video below, a “flyover” of the two galaxies of that system set towards a fu- ture interaction. [NASA, ESA, Akira Fujii, DSS2] version of globular clusters and who instead consider them a product of the interaction between galaxies. What is though certain is that UCDs are among the more dense stellar systems in the universe. Of all UCDs known, the most striking is M60- UCD1. Located nearly 54 million light-years from Earth, it is just 21,500 light-years in projection away from the centre of M60, one of the largest elliptical galaxies in the local universe, with a mass equivalent to 1000 billion suns and at least 120,000 light- years in diameter, belonging to the Virgo cluster of galaxies, of which it is the third brightest galaxy. Given that 21,500 light- years is less than the distance separating the Sun from the centre of the Milky Way, it can be easily guessed that M60 and M60-UCD1, besides the name, must have something else in common. This pair of galaxy represents an ideal laboratory in which to validate the hy- pothesis according to which UCDs are the remnants of galaxies stripped of all the re- gions outside the nucleus, due to very close encounters with other galaxies. M60-UCD1 is not only interesting because of its close proximity to M60, but also because it is the most massive of all ultra-compact dwarfs, with a mass equivalent to some 140 million suns, concentrated in a spheroid of