Astronomers using ESO’s Very Large Telescope have detected a stellar-mass black hole much farther away than any other previously known in a spiral galaxy called NGC 300, six million light-years from Earth. With a mass above fifteen times that of the Sun and joined with a star that will soon become a supernova, then a black hole itself. This is also the second most massive stellar-mass black hole ever found.
"This is the most distant stellar-mass black hole ever weighed, and it's the first one we've seen outside our own galactic neighborhood, the Local Group," says Paul Crowther, Professor of Astrophysics at the University of Sheffield and lead author of the paper reporting the study. The black hole's odd partner is a Wolf-Rayet star, which also has a mass of about twenty times as much as the Sun. Wolf-Rayet stars begin life as cosmic titans, but they live fast and die hard, exploding as supernova and blasting vast amounts of heavy elements into space for use in later generations of stars and planets before their cores implode to form black holes.
Many astronomers believe that one of the most plausible reasons we have yet to detect intelligent life in the universe is due to the deadly effects of local supernova explosions that wipe out all life in a given region of a galaxy.
While there is, on average, only one supernova per galaxy per century, there is something on the order of 100 billion galaxies in the observable Universe. Taking 10 billion years for the age of the Universe (it's actually 13.7 billion, but stars didn't form for the first few hundred million), Dr. Richard Mushotzky of the NASA Goddard Space Flight Center, derived a figure of 1 billion supernovae per year, or 30 supernovae per second in the observable Universe!
Certain rare stars -real killers -type 11 stars, are core-collapse hypernova that generate deadly gamma ray bursts (GRBs). These long burst objects release 1000 times the non-neutrino energy release of an ordinary "core-collapse" supernova.
The stellar-mass black holes found in the Milky Way weigh up to ten times the mass of the Sun but, outside our own galaxy, they may just be "minor-league players," since astronomers have found another black hole with a mass over fifteen times the mass of the Sun. In 2007, an X-ray instrument aboard NASA's Swift observatory scrutinised the surroundings of the brightest X-ray source in NGC 300 discovered earlier with the European Space Agency's XMM-Newton X-ray observatory."We recorded periodic, extremely intense X-ray emission, a clue that a black hole might be lurking in the area," explains team member Stefania Carpano from ESA.
Thanks to new observations performed with the FORS2 instrument mounted on ESO's Very Large Telescope, astronomers have confirmed their earlier hunch. The new data show that the black hole and the Wolf-Rayet star dance around each other, with a period of about 32 hours. The astronomers found that the black hole is stripping matter away from the star as they orbit. It will take a few billion years, however, until the actual merger.
"This is indeed a very 'intimate' couple," notes collaborator Robin Barnard. "How such a tightly bound system has been formed is still a mystery."
Only one other system of this type has previously been seen, but other systems comprising a black hole and a companion star are not unknown to astronomers. Based on these systems, the astronomers see a connection between black hole mass and galactic chemistry. "We have noticed that the most massive black holes tend to be found in smaller galaxies that contain less 'heavy' chemical elements," says Crowther.
"Bigger galaxies that are richer in heavy elements, such as the Milky Way, only succeed in producing black holes with smaller masses." Astronomers believe that a higher concentration of heavy chemical elements influences how a massive star evolves, increasing how much matter it sheds, resulting in a smaller black hole when the remnant finally collapses.
In less than a million years, it will be the Wolf-Rayet star's turn to go supernova and become a black hole.
"If the system survives this second explosion, the two black holes will merge, emitting copious amounts of energy in the form of gravitational waves as they combine," concludes Crowther. "Our study does however show that such systems might exist, and those that have already evolved into a binary black hole might be detected by probes of gravitational waves, such as LIGO or Virgo."
Casey Kazan via ESO
Image courtesy of F. Bresolin (IfA/U. Hawaii): Most of the stars visible in this image do not belong to NGC 300, but are instead foreground Milky Way objects. The HST image shown in this release covers the very central part of the galaxy, devoid of large HII regions