If black holes are cool, how much cooler are rogue black holes? That’s what scientists have discovered “roaming around the Milky Way” (press release).

Nature reporter Alex Witze was at the AAS meeting where the finding was announced by astronomer Kelly Holley-Bockelmann, and blogged about it over at In the Field:

Her research team discovered the errant black holes – each of which is 100 to 1,000 times the mass of our sun — by studying clumps of ancient stars known as globular clusters. These are rough environments, in which black holes are constantly sinking toward the center of the cluster, occasionally meeting in a violet merger that throws one or the other of them out of the cluster at speeds up to 9 million miles per hour. Holley-Bockelmann’s computer simulations show that scientists haven’t spotted nearly as many black holes getting kicked out of globular clusters as one might expect. And so, she says, there must be extra black holes lurking there, invisibly — some of the biggest rogues ever spotted in our galaxy.

There is evidence for huge ‘super-massive’ black holes sitting in the middle of galaxies and for very small holes produced from giant stars. Holley-Bockelmann’s work modelled “intermediate mass” black holes, whose prevalence and life stories are less clear.

She found that when two black holes rotating at different speeds or of different sizes combine the new, merged black hole gets a massive kick, speeding off at up to 4,000 kilometres per hour.

Other black hole news from AAS: the biggest black hole ever detected! It clocks in at 18 billion times more massive than our sun (Space.com).

Image top: black hole / NASA

Image lower: mid-sized black holes merge with smaller, star-sized black holes and receive a hefty velocity kick / Vanderbilt