Little galaxies can pack a mighty punch. Astronomers detected and carefully weighed the black holes residing at the centres of a pair of extremely compact galaxies, finding to their surprise that they make up a large fraction of the mass of the host galaxies themselves.
These “ultra-compact dwarf galaxies” are now the second and third ones known to house such gargantuan black holes, confirming that the first one — discovered by the same group in 2014 — was not an anomaly.
“We’re three for three so far,” says Anil Seth at the University of Utah in Salt Lake City. He and his colleagues argue that these uniquely dense galaxies were once much larger galaxies, but have been tidally stretched and disrupted by neighbouring galaxies in their crowded environment.
The two newly studied galaxies reside in the Virgo cluster, which is a dangerous neighborhood. Thousands of other galaxies live there too, which means there are many opportunities for them to whiz close by and strip off each other’s stars.