October 25, 2012
Monster Galaxy Found With Largest Core Ever Seen
Lee Rannals for redOrbit.com — Your Universe Online
A team of astronomers has grabbed an image of a huge elliptical galaxy with a core that is bigger than any seen before.
The galaxy is a member of a class of galaxies with an unusually diffuse core filled without a concentrated peak of light around a central black hole. Viewing the core would be like seeing a city with no center lights.
The intriguing explanation for the huge core is that it is related to the action of one or more black holes.
The team measured the amount of starlight across the galaxy using Hubble's Advanced Camera for Surveys and Wide Field Camera 3.
Observations by the team revealed the galaxy's puffy core is the largest so far, at 10,000 light years. A galaxy's core size is typically correlated with the dimensions of its host galaxy, and in this case the central region is much larger than normal.
The team said the core is more than three times larger than the center of other, very luminous galaxies.
Astronomers believe one scenario for the puffy core is that a pair of merging black holes gravitationally stirred up and scattered the stars. Another idea is the merging black holes were ejected from the core.
Previous observations revealed that supermassive black holes, with masses millions or billions times more than the Sun, reside at the centers of nearly all galaxies and may play a role in shaping those regions.
"Expecting to find a black hole in every galaxy is sort of like expecting to find a pit inside a peach," astronomer Tod Lauer of the National Optical Astronomy Observatory in Tucson, Arizona, a co-author of the Hubble study, said in a statement. "With this Hubble observation, we cut into the biggest peach and we can't find the pit. We don't know for sure that the black hole is not there, but Hubble shows that there's no concentration of stars in the core."
During their observations, the astronomers expected to see a slight cusp of light in the galaxy's center, marking the location of the black hole and attendant stars. However, they instead found the starlight's intensity remained daily even across the galaxy.
"When I first saw the image of this galaxy, I knew right away that it was unusual," Team leader Marc Postman of the Space Telescope Science Institute in Baltimore, Maryland, said in a statement. "The core was very diffuse and very large. The challenge was then to make sense of all the data, given what we knew from previous Hubble observations, and come up with a plausible explanation for the intriguing nature of this particular galaxy."
One reason for the gigantic core could be due to two central black holes orbiting each other. These black holes collectively could have been as massive as several billion suns.
Another possibility is the black hole merger created gravity waves, which are ripples in the fabric of space.
"The black hole is the anchor for the stars," Lauer said in the statement. "If you take it out, all of a sudden you have a lot less mass. The stars aren't held together very well and they move outwards, enlarging the core even more."
The team said the ejected black hole scenario may be far fetched, but it's what makes observing the Universe intriguing, Postman said.
"This is a system that's interesting enough that it pushes against a lot of questions," Lauer said. "We have thought an awful lot about what black holes do. But we haven't been able to test our theories."
He said this is an interesting place where a lot of ideas can come together and can be tested.
The team is now conducting follow-up observations with the Very Large Array (VLA) radio telescope in New Mexico. They expect material falling onto a black hole to emit radio waves, among other types of radiation.
The astronomers will compare VLA data with the Hubble images to more precisely pin down the location of the black hole, assuming it exists.