“The Unexpected”–At the Heart of a Monster Elliptical Galaxy (Weekend Feature)

 

Abell Galaxy Cluster

 

“When I first saw the Hubble image of this galaxy, I knew right away that it was unusual,” Marc Postman of the Space Telescope Science Institute said of the monster elliptical galaxy, with a central core bigger than any seen before embracing a mystery never seen before. “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.”

Seven years before this week’s Event Horizon Telescope release of the gargantuan black hole image at the heart of Galaxy M87, astronomers using the NASA/ESA Hubble Space Telescope in fall of 2012 obtained a remarkable view of a monster elliptical galaxy, located three billion light-years away, the galaxy is the most massive and brightest galaxy in the Abell 2261 cluster with a core bigger than any seen before. Spanning a little over one million light-years, the galaxy is about ten times the diameter of the Milky Way galaxy. The bloated galaxy is a member of an unusual class of galaxies filled with a fog of starlight where there would normally be a concentrated peak of light around a central black hole.

A galaxy’s core size typically is correlated to the dimensions of its host galaxy, but in this case, the central region is much larger than astronomers would expect for the galaxy’s size. In fact, the bloated core is more than three times larger than the center of other very luminous galaxies.

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“Expecting to find a black hole in every galaxy is sort of like expecting to find a pit inside a peach,” explains astronomer 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.”

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Viewing the core is like seeing a city with no center, just houses sprinkled across a vast landscape. An international team of astronomers used Hubble’s Advanced Camera for Surveys and Wide Field Camera 3 to measure the amount of starlight across the galaxy, cataloged as 2MASX J17222717+3207571 but more commonly called A2261-BCG (shown below short for Abell 2261 Brightest Cluster Galaxy).

 

Abell 2261 Brightest Cluster Galaxy

 

The Hubble observations revealed that the galaxy’s puffy core, measuring about 10,000 light-years, is the largest yet seen. Astronomers using NASA’s Hubble Space Telescope obtained a remarkable new view of this whopper of an elliptical galaxy that may have been puffed up by the actions of one or more black holes in its core. Spanning a little more than one million light-years, the galaxy is about 10 times the diameter of our Milky Way galaxy.

Viewing the core is like seeing a city with no downtown, just houses sprinkled across a vast landscape.

Astronomers have proposed two possibilities for the puffy core. One scenario is that a pair of merging black holes gravitationally stirred up and scattered the stars. Another idea is that the merging black holes were ejected from the core. Left without an anchor, the stars began spreading out even more, creating the puffy-looking core.

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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.

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. Instead, the starlight’s intensity remained fairly even across the galaxy. One possibility for the puffy core may be due to two central black holes orbiting each other.

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.

In this scenario, stars circling in the giant galaxy’s center came close to the twin black holes. The stars were then given a gravitational boot out of the core. Each gravitational slingshot robbed the black holes of momentum, moving the pair ever closer together, until finally they merged, forming one supermassive black hole that still resides in the galaxy’s center.

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Another related possibility 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.

“The black hole is the anchor for the stars,” explains Laurer, a co-author of the Hubble study and a member of The Nuker Team: Hubble Space Telescope investigations of centers of galaxies. “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 admits that the ejected black-hole scenario may sound far-fetched, “but that’s what makes observing the Universe so intriguing—sometimes you find the unexpected,” Postman says.

“This is a system that’s interesting enough that it pushes against a lot of questions,” Lauer added.

“We have thought an awful lot about what black holes do. But we haven’t been able to test our theories. This is an interesting place where a lot of the ideas we’ve had can come together and can be tested, fairly exotic ideas about how black holes may interact with each other dynamically and how they would affect the surrounding stellar population.”

The Abell 2261 cluster is part of a multi-wavelength survey, led by Postman, called the Cluster Lensing And Supernova survey with Hubble (CLASH). The survey probes the distribution of dark matter in 25 massive galaxy clusters.

The massive galaxy cluster Abell 2744, nicknamed Pandora’s Cluster, takes on a ghostly look in this NASA/ESA Hubble Space Telescope view shown at the top of the page. In this image the total starlight from the cluster has been artificially colored blue. This reveals that not all the starlight is contained within the cities of stars — the galaxies — which appear as bright blue-white blobs. A fraction of the starlight is also dispersed throughout the cluster, as seen in the darker blue regions.

This light comes from dead galaxies. The galaxies were torn apart long ago by the cluster’s gravitational forces, and their stars were scattered into what is known as intracluster space — the space between the galaxies.

These orphaned stars roam the cluster, without being gravitationally tethered to any single galaxy. Because these extremely faint stars are brightest at near-infrared wavelengths of light, this type of observation could only be accomplished with Hubble’s infrared sensitivity to extraordinarily dim light. The galaxies that are not colored blue are either in the foreground or background and are not part of the cluster

The Daily Galaxy via Provided by NASA/Hubble Space Telescope Science Institute

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