Rogue stars ejected from the galaxy are found in intergalactic spaceMay 01, 2012
In fact, the primary mechanism that astronomers have come up with that can give a star the two-million-plus mile-per-hour kick it takes requires a close encounter with the supermassive black hole at the galaxy's core.
So far astronomers have found 16 of these "hypervelocity" stars. Although they are traveling fast enough to eventually escape the galaxy's gravitational grasp, they have been discovered while they are still inside the galaxy.
Now, Vanderbilt astronomers report in the May issue of the Astronomical Journal that they have identified a group of more than 675 stars on the outskirts of the Milky Way that they argue are hypervelocity stars that have been ejected from the galactic core. They selected these stars based on their location in intergalactic space between the Milky Way and the nearby Andromeda galaxy and by their peculiar red coloration.
"These stars really stand out. They are red giant stars with high metallicity which gives them an unusual color," says Assistant Professor Kelly Holley-Bockelmann, who conducted the study with graduate student Lauren Palladino.
In astronomy and cosmology, "metallicity" is a measure of the proportion of chemical elements other than hydrogen and helium that a star contains. In this case, high metallicity is a signature that indicates an inner galactic origin: Older stars and stars from the galactic fringes tend to have lower metallicities.
The researchers identified these candidates by analyzing the millions of stars catalogued in the Sloan Digital Sky Survey.
"We figured that these rogue stars must be there, outside the galaxy, but no one had ever looked for them. So we decided to give it a try," said Holley-Bockelmann, who is studying the behavior of the black hole at the center of the Milky Way galaxy.
Astronomers have now found evidence for giant black holes at the centers of many galaxies. They estimate that the Milky Way's central black hole has a mass of four million solar masses. They calculate that the gravitational field surrounding such a supermassive black hole is strong enough to accelerate stars to hypervelocities.
The typical scenario involves a binary pair of stars that get caught in the black hole's grip. As one of the stars spirals in towards the black hole, its companion is flung outward at a tremendous velocity.
A second scenario takes place during periods when the central black hole is in the process of ingesting a smaller black hole. Any star that ventures too close to the circling pair can also get a hypervelocity kick.
Red giant stars are the end stage in the evolution of small, yellow stars like the Sun. So, the stars in Holley-Bockelmann's rogues' gallery should have been small stars like the Sun when they tangled with the central black hole. As they traveled outward, they continued to age until they reached the red giant stage. Even traveling at hypervelocities, it would take a star about 10 million years to travel from the central hub to the spiral's edge, 50,000 light years away.
"Studying these rogue stars can provide us with new insights into the history and evolution of our home galaxy," said Holley-Bockelmann. The researchers' next step is determine if any of their candidates are unusually red brown dwarfs instead of red giants. Because brown dwarfs produce a lot less light than red giants, they would have to be much closer to appear equally bright.
Heather Morrison at Case Western Reserve University, Patrick Durrell and John Feldmeier at Youngstown State University, Robin Ciardullo and Richard Wade at Pennsylvania State University, and J. Davy Kirkpatrick and Patrick Lowrance at the California Institute of California also contributed to the research, which was funded by grants from the National Science Foundation and the Department of Education's Graduate Assistance in Areas of National Need fellowship.
Related Black Hole Current Events and Black Hole News Articles
Out of An Hours-long Explosion, A Stand-In For The First Stars
Astronomers analyzing a long-lasting blast of high-energy light observed in 2013 report finding features strikingly similar to those expected from an explosion from the universe's earliest stars.
Supermassive black hole blows molecular gas out of galaxy at 1 million kilometers per hour
New research by academics at the University of Sheffield has solved a long-standing mystery surrounding the evolution of galaxies, deepening our understanding of the future of the Milky Way.
A hotspot for powerful cosmic rays
An observatory run by the University of Utah found a "hotspot" beneath the Big Dipper emitting a disproportionate number of the highest-energy cosmic rays. The discovery moves physics another step toward identifying the mysterious sources of the most energetic particles in the universe.
Swiftly moving gas streamer eclipses supermassive black hole
An international team of astronomers has discovered that the supermassive black hole at the heart of the galaxy NGC 5548 has recently undergone strange, unexpected behavior rarely seen in the heart of active galaxies.
Astronomers pierce galactic clouds to shine light on black hole development
An international team of scientists including a Virginia Tech physicist have discovered that winds blowing from a supermassive black hole in a nearby galaxy work to obscure observations and x-rays.
Swiftly moving gas streamer eclipses supermassive black hole
Astronomers have discovered strange and unexpected behaviour around the supermassive black hole at the heart of the galaxy NGC 5548.
Swiftly Moving Gas Streamer Eclipses Supermassive Black Hole
An international team of astronomers, using data from several NASA and European Space Agency (ESA) space observatories, has discovered unexpected behavior from the supermassive black hole at the heart of the galaxy NGC 5548, located 244.6 million light-years from Earth.
Surprisingly strong magnetic fields challenge black holes' pull
A new study of supermassive black holes at the centers of galaxies has found magnetic fields play an impressive role in the systems' dynamics.
Black Hole 'Batteries' Keep Blazars Going and Going
Astronomers studying two classes of black-hole-powered galaxies monitored by NASA's Fermi Gamma-ray Space Telescope have found evidence that they represent different sides of the same cosmic coin.
Image release: A violent, complex scene of colliding galaxy clusters
Astronomers using the Karl G. Jansky Very Large Array (VLA) and the Chandra X-Ray Observatory have produced a spectacular image revealing new details of violent collisions involving at least four clusters of galaxies.
More Black Hole Current Events and Black Hole News Articles