Much like stars and planets, black holes also fall into different categories. Astronomers have now spotted a record-breaking heavyweight black hole at the center of a galaxy known as Holm 15A. The black hole weighs 40 billion times the mass of our sun.
Holm 15A is a central elliptical galaxy within the Abell 85 cluster, which contains more than 500 galaxies. The cluster is 700 million light-years from Earth —- that’s twice as far as the distance for other direct measurements of black hole mass.
But it was the center of Holm 15A that interested scientists. The large galaxy is highly visible due to its wealth of stars, but the center of it was very faint and diffuse. And the center was nearly the size of the Large Magellanic Cloud, a neighboring satellite galaxy of our own Milky Way.
Intrigued by this hint of a potentially large black hole, astronomers used the Very Large Telescope and the Wendelstein Observatory to obtain data and new observations.
“There are only a few dozen direct mass measurements of super-massive black holes, and never before has it been attempted at such a distance,” said Jens Thomas, scientist at the Max Planck Institute for Extraterrestrial Physics. “But we already had some idea of the size of the black hole in this particular galaxy, so we tried it.”
The data revealed that the center was even more faint than expected.
“The light profile in the inner core is also very flat,” said Kianusch Mehregan, a doctoral student at Max Planck who performed the data analysis. “This means that most of the stars in the center must have been expelled due to interactions in previous mergers.”
Galaxies collide and merge all the time. When two galaxies merge, the black hole acting like the engine at the center of the galaxy uses its gravity to toss out stars from the center of the new galactic partner. And if there is a lack of cold gas at the center of the galaxy, no new stars can form.
They were able to establish a relationship between the mass of the black hole and the muted surface brightness of the galaxy’s star-depleted core.
“The newest generation of computer simulations of galaxy mergers gave us predictions that do indeed match the observed properties rather well,” Thomas said. “This means that the shape of the light profile and the trajectories of the stars contain valuable archaeological information about the specific circumstances of core formation in this galaxy – as well as other very massive galaxies.”
Each time the galaxy merges with another one, stars are lost and the black hole gains mass – which accounts for the incredibly high mass of this particular black hole.
Because they were able to measure and weigh the black hole in correlation to the galactic core brightness, researchers believe they could use this to determine black hole mass estimates for even more distant galaxies far from the reach of direct measurements.