Photo:Chandra's image of the elliptical galaxy NGC 4697 reveals diffuse hot gas dotted with many point-like sources. As in the elliptical galaxies, NGC 4649 and NGC 1553, the point-like sources are due to black holes and neutron stars in binary star systems. Material pulled off a normal star is heated and emits X-radiation as it falls toward its black hole or neutron star companion.
photo:Chandra's image of the lenticular galaxy NGC 1553 reveals diffuse hot gas dotted with many point-like sources. As in the elliptical galaxies, NGC 4649 and NGC 4697, the point-like sources are due to black holes and neutron stars in binary star systems where material pulled off a normal star is heated and emits X-radiation as it falls toward its black hole or neutron star companion.
A newfound population of black holes prowls the cosmic equivalent of singles establishments where they troll for stars with hopes of finding companions. As could be expected, the massive objects then proceed to suck the life out of their newfound partners. Curiously, all of this takes place in what can be described as galactic senior citizens communities.
The action, in Craig Sarazin's view, is at a chain of places called the Globular Cluster Bar.
Sarazin, a University of Virginia professor, used NASA's Chandra X-ray Observatory to study three distant galaxies. His team detected several new X-ray sources that represent black holes that have partnered with normal stars in a form of mutual attraction that feeds one and destroys the other, activity that generates X-rays.
When Sarazin and his colleagues studied the same galaxies in optical light, they found the X-ray sources congregated in so-called globular clusters, highly popular places where stars hang out and where, the thinking goes, encounters of this sort are more likely. "Globular clusters are the singles bars of the stellar world," Sarazin said Tuesday at the 200th meeting of the American Astronomical Society. "They're the place where a lonely black hole can go to find a companion."
Black holes are remnants of giant stars that have exploded. They cannot be seen, but astronomers detect one when it siphons gas off a companion star that is normal, more like our Sun. The gas spirals inward, heats up, and emits X-rays. [A small number of the X-ray sources seen in the new images involve neutron stars, not black holes.]
It is not surprising that black holes would gain companions more frequently when in a globular cluster, which are common sub-units of galaxies that can contain a million stars in a region of space that is normally host to just one or two stars.
But it is somewhat surprising, Sarazin said, for all this activity to be going on in elliptical galaxies, which are the variety his team studied.
photo:Chandra's image of the elliptical galaxy NGC 4649 reveals a large, bright cloud of hot gas and 165 point-like sources. As in the elliptical galaxies, NGC 4697 and NGC 1553, most of the point-like sources are due to black holes and neutron stars in binary star systems.
Elliptical galaxies are mature, on the order of 10 billion years old. They are rather bland and inactive compared to the more dramatic spiral-shaped galaxies like our own Milky Way, which are younger and undergoing higher rates of star formation.
"These elliptical galaxies are sort of geriatric stellar communities," Sarazin said. "They contain elderly stars which are very inactive at this point in their lives."
At least they're alive. Black holes, on the other hand, are nothing more than stellar corpses. The several dozen black holes Sarazin found, as it turns out, points to a time long ago when these elliptical galaxies were younger and wilder places.
"They went through a sort of wild adolescent period shortly after they formed," he explained. "They had lots of star formation going on. They produced lots of massive stars, very bright stars. Lots of supernovae explosions. And the deaths of those massive stars produced the black holes and neutron stars we see today."
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