A multinational team of astronomers using the Advanced Camera for Surveys and the Wide Field Camera 3 aboard NASA’s Hubble Space Telescope has captured a remarkable image of an elliptical galaxy known as 2MASX J17222717+3207571.
This image shows the giant elliptical galaxy 2MASX J17222717+3207571, also known as A2261-BCG (NASA / ESA / M. Postman / Space Telescope Science Institute / T. Lauer / National Optical Astronomy Observatory / the CLASH team)
The galaxy 2MASX J17222717+3207571, also known as A2261-BCG (short for Abell 2261 Brightest Cluster Galaxy), is about ten times the diameter of the Milky Way Galaxy, and located 3 billion light-years away
The new observations reveal that the galaxy’s core, measuring about 10 000 light-years, is the largest yet seen.
The team has proposed two scenarios for the galaxy’s puffy core. One idea is that a pair of merging black holes gravitationally stirred up and scattered the stars. These black holes collectively could have been as massive as several billion suns. One of the black holes would be native to the galaxy, while the second could have been added from a smaller galaxy that was gobbled up by the massive elliptical.
Another idea is that the black hole merger created gravity waves, which are ripples in the fabric of space. According to the theory of general relativity, a pair of merging black holes produces ripples of gravity that radiate away. If the black holes are of unequal mass, then some of the energy may radiate more strongly in one direction, providing the equivalent of a rocket thrust. The imbalance of forces would have ejected the merged black hole from the center at speeds of millions of kilometers per hour, resulting in the rarity of a galaxy without a central black hole.
“When I first saw the image of this galaxy, I knew right away that it was unusual,” said Dr Marc Postman of the Space Telescope Science Institute in Baltimore, lead author of the study published in the Astrophysical Journal (arXiv.org version).
This image shows the core of the giant elliptical galaxy A2261-BCG (Marc Postman et al)
“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,” he said.
Previous Hubble observations have 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 central regions.
“Expecting to find a black hole in every galaxy is sort of like expecting to find a pit inside a peach,” explained co-author Dr Tod Lauer of the National Optical Astronomy Observatory in Tucson. “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.”
“The ejected black-hole scenario may sound far-fetched, but that’s what makes observing the Universe so intriguing – sometimes you find the unexpected,” Dr Postman said.
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Bibliographic information: Marc Postman et al. 2012. A Brightest Cluster Galaxy with an Extremely Large Flat Core. ApJ vol. 756, no. 2, 159; doi: 10.1088/0004-637X/756/2/159
