Black Hole Count Keeps Growing For The Andromeda Galaxy
June 13, 2013

Andromeda Galaxy Is Full Of Holes, Black Holes That Is

April Flowers for - Your Universe Online

Astronomers have discovered an unprecedented number of black holes in the Andromeda Galaxy (M31), one of the Milky Way´s nearest cosmic neighbors, using data from NASA´s Chandra X-ray Observatory.

By analyzing more than 150 Chandra observations taken over 13 years, the research team has identified 26 black hole candidates. This is the largest number discovered to date in a galaxy outside our own. Many scientists consider Andromeda to be the sister galaxy to our own, and expect the two to collide several billion years from now.

"While we are excited to find so many black holes in Andromeda, we think it's just the tip of the iceberg," said Robin Barnard of Harvard-Smithsonian Center for Astrophysics (CfA). "Most black holes won't have close companions and will be invisible to us."

The black hole candidates are in the stellar mass category. This means they formed during the death throes of very massive stars, and generally have five to ten times the mass of our sun. These otherwise invisible objects become detectable as material is pulled from a companion star and heated up to produce radiation before it disappears into the black hole.

To begin identifying these black holes, the team had to make sure they were stellar mass systems in the Andromeda Galaxy and not supermassive black holes at the heart of more distant galaxies. The astronomers used a novel technique that analyzed information about the brightness and variability of the X-ray sources in the Chandra data because stellar mass systems change more quickly than supermassive black holes, making it possible to differentiate the two.

Then, to classify the stellar mass systems as black holes, the researchers observed that the X-ray sources had special characteristics. The Andromeda mass systems were brighter than a certain high level of X-rays and had a particular X-ray color. Cosmic sources that contain neutron stars — the dense cores of dead stars — do not show both of these characteristics at the same time. Black holes, on the other hand, do.

Further support for these findings was provided by the European Space Agency´s XMM-Newton X-ray Observatory in the form of X-ray spectra, the distribution of X-rays with energy, for some of the black hole candidates.

"By observing in snapshots covering more than a dozen years, we are able to build up a uniquely useful view of M31," said Michael Garcia, also of CfA. "The resulting very long exposure allows us to test if individual sources are black holes or neutron stars."

Previously, the team identified nine black hole candidates in the region covered by the Chandra data. The current study increases the total to 35, eight of which are associated with global clusters - the ancient concentrations of stars distributed in a spherical pattern about the center of the galaxy. This is one way that Andromeda differs from the Milky Way. Astronomers have yet to discover similar black holes in one of the Milky Way´s globular clusters.

Seven of the 35 candidates are within 1,000 light-years of the center of the Andromeda Galaxy, which is more than the total number of black hold candidates with similar properties located near the center of our own galaxy. Astronomers are not surprised by this, as the bulge of stars in the middle of Andromeda is bigger, allowing more black holes to form.

"When it comes to finding black holes in the central region of a galaxy, it is indeed the case where bigger is better," said Stephen Murray of Johns Hopkins University and CfA. "In the case of Andromeda we have a bigger bulge and a bigger supermassive black hole than in the Milky Way, so we expect more smaller black holes are made there as well."

Earlier predictions about the properties of X-ray sources near the center of M31 are confirmed by these new results. Rasmus Voss and Marat Gilfanov of the Max Planck Institute for Astrophysics previously used Chandra to reveal that there was an unusually large number of X-ray sources near the center of M31. Voss and Gilfanov predicted that a majority of these extra X-ray sources would contain black holes that had encountered and captured low mass stars, which is strongly supported by the detection of the seven black hole candidates close to the center of Andromeda.

"We are particularly excited to see so many black hole candidates this close to the center, because we expected to see them and have been searching for years," said Barnard.

The results of their study were recently published in The Astrophysical Journal.