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Hubble Sees a Galaxy Hit a Bull's-Eye
Normal Galaxies & Starburst Galaxies
RA 02h 25m 04.40s Dec -24° 47' 17.00"
About 157 million light years
Image is 1.6 arcmin across (about 73,000 light years)
March 5 and October 2, 2009
11 hours 11 min
X-ray (Red); Optical (Red, Green, Blue)
Optical (NASA/STScI); X-ray (NASA/CXC/SAO/A.Prestwich et al)
December 6, 2012
Click the image to buy a print
ABOUT THIS IMAGE:
Bright pink nebulae almost completely encircle a spiral galaxy in this NASA/ESA Hubble Space Telescope image of NGC 922. The ring structure and the galaxys distorted spiral shape result from a smaller galaxy scoring a cosmic bullseye, hitting the center of NGC 922 some 330 million years ago.
In Hubbles image, NGC 922 clearly reveals itself not to be a normal spiral galaxy. The spiral arms are disrupted, a stream of stars extends out towards the top of the image, and a bright ring of nebulae encircles the core. Observing with NASAs Chandra X-ray Observatory (see rollover image) reveals more chaos in the form of ultraluminous X-ray sources dotted around the galaxy.
NGC 922s current unusual form is a result of a cosmic collision millions of years ago. A smaller galaxy, catalogued as 2MASXI J0224301-244443, plunged right through the heart of NGC 922 and shot out the other side.
As the small galaxy passed through the middle of NGC 922, it set up gravitational ripples that disrupted the clouds of gas, and triggered the formation of new stars whose radiation then lit up the remaining gas. The bright pink color of the resulting nebulae is a characteristic sign of this process, and it is caused by excited hydrogen gas (the dominant element in interstellar gas clouds). This process of excitation and emission of light by gases is similar to that in neon signs.
In theory, if two galaxies are aligned just right, with the small one passing through the center of the larger one, the ring of nebulae should form a perfect circle, but more often the two galaxies are slightly off kilter, leading to a circle that, like this one, is noticeably brighter on one side than the other.
These objects, called collisional ring galaxies, are relatively rare in our cosmic neighborhood. Although galaxy collisions and mergers are commonplace, the precise alignment and ratio of sizes necessary to form a ring like this is not, and the ring-like phenomenon is also thought to be relatively short-lived.
The chances of seeing one of these galaxies nearby is therefore quite low. Despite the immense number of galaxies in the Universe, this is one of only a handful known in our cosmic neighborhood (the Cartwheel Galaxy, see g0601cx, being the most famous example). Observations of the more distant Universe (where we see further into the past) show that these rings were more common in the past, however.
Hubbles image of NGC 922 consists of a series of exposures taken in visible light with Hubbles Wide Field Camera 3, and in visible and near-infrared light with the Wide Field and Planetary Camera 2.
Chandra X-ray Observatory press release:
In this holiday season of home cooking and carefully-honed recipes, some astronomers are asking: what is the best mix of ingredients for stars to make the largest number of plump black holes?
They are tackling this problem by studying the number of black holes in galaxies with different compositions. One of these galaxies, the ring galaxy NGC 922, is seen in this composite image containing X-rays from NASA's Chandra X-ray Observatory (blue) and optical data from the Hubble Space Telescope (appearing as pink, yellow and blue).
NGC 922 was formed by the collision between two galaxies - one seen in this image and another located outside the field of view. This collision triggered the formation of new stars in the shape of a ring. Some of these were massive stars that evolved and collapsed to form black holes.
Most of the bright X-ray sources in Chandra's image of NGC 922 are black holes pulling material in from the winds of massive companion stars. Seven of these are what astronomers classify as "ultraluminous X-ray sources" (ULXs). These are thought to contain stellar-mass black holes that are at least ten times more massive than the sun, which places them in the upper range for this class of black hole. They are a different class from the supermassive black holes found at the centers of galaxies, which are millions to billions of times the mass of the sun.
Theoretical work suggests that the most massive stellar-mass black holes should form in environments containing a relatively small fraction of elements heavier than hydrogen and helium, called â€metalsâ€ by astronomers. In massive stars, the processes that drive matter away from the stars in stellar winds work less efficiently if the fraction of metals is smaller. Thus, stars with fewer of these metals among their ingredients should lose less of their mass through winds as they evolve. A consequence of this reduced mass loss is that a larger proportion of massive stars will collapse to form black holes when their nuclear fuel is exhausted. This theory appeared to be supported by the detection of a large number (12) of ULXs in the Cartwheel galaxy (see g0601cx), where stars typically contain only about 30% of the metals found in the sun.
To test this theory, scientists studied NGC 922, which contains about the same fraction of metals as the sun, meaning that this galaxy is about three times richer in metals than the Cartwheel galaxy. Perhaps surprisingly, the number of ULXs found in NGC 922 is comparable to the number seen in the Cartwheel galaxy. Rather, the ULX tally appears to depend only on the rate at which stars are forming in the two galaxies, not on the fraction of metals they contain.
explanation for these results is that the theory predicting the most massive
stellar-mass black holes should form in metal poor conditions is incorrect.
Another explanation is that the metal fraction in the Cartwheel galaxy
is not low enough to have a clear effect on the production of unusually
massive stellar-mass black holes, and therefore will not cause an enhancement
in the number of ULXs. Recent models incorporating the evolution of stars
suggest that a clear enhancement in the number of ULXs might only be seen
when the metal fraction falls below about 15% of the Sun's value. Astronomers
are investigating this possibility by observing galaxies with extremely
low metal fractions using Chandra. The number of ULXs is being compared
with the number found in galaxies with higher metal content.