Medium-sized black holes lurk in quiet corners of the Universe.
Black holes used to come in two sizes - small and extra large. Astronomers have now found medium. These intermediate black holes could help us understand how galaxies form.
The new black holes lie at the centre of globular star clusters. These are miniature galaxies within galaxies, tightly packed groups of stars tens of light years across, containing between 10,000 and one million stars.
The black holes found in star clusters weigh a few thousand times as much as the Sun. This is squarely between the two previously known types of black hole. "They're the missing link," says astrophysicist Steinn Sigurdsson of Pennsylvania State University.
Small black holes, as massive as 5-10 Suns, form when individual stars explode and collapse in on themselves. In contrast, the immense black holes at the centre of galaxies have the mass of millions or billions of Suns.
Immense and medium-sized black holes have some intriguing similarities. Both, for instance, are about 200 times lighter than their attendant galaxies.
Some process must link star clusters and their black holes, so that their growth keeps pace, says astronomer Karl Gebhardt of the University of Texas, Austin. "It's telling us something about how galaxies and globular clusters formed," says Gebhardt. The galactic holes might form from the merging of black holes found in star clusters, he suggests.
Gebhardt's team used the Hubble Space Telescope to peer into a globular cluster called G1 in the Andromeda galaxy, our nearest galactic neighbour at 2.2 million light years away. A second team, led by Roeland van der Marel of the Space Telescope Science Institute in Baltimore, Maryland, studied one in the Milky Way called M15, a mere 32,000 light years away.
Black holes can't be seen - their extreme gravity drags in light. They give themselves away by accelerating the stars around them, like a cosmic slingshot.
M15 and G1 are the only two globular clusters examined for black holes so far. All clusters may contain them; there are 150 in the Milky Way alone.
First or last?
Astronomers are uncertain which came first: black holes, or galaxies and clusters. Some think that the black holes appeared at the centre of existing star groups. Others think that the black holes formed first and drew stars around them.
Globular clusters contain the oldest known stars in the Universe - M15 is 13 billion years old, G1 at least 10 billion. The presence of black holes at their centre supports the idea that the black holes came first, says astrophysicist Martin Ward of the University of Leicester, UK - "but it's still early days".