October 19, 2012
Gemini Observatory Catches Rare Glimpse Of Polar Ring Galaxy
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Many images that we see from the space observatories are beautiful. Swirling colors and bright galaxies make for amazing images. But an image just released from the ground-based Gemini Observatory of the polar-ring galaxy NGC 660 might be the most hauntingly beautiful image ever. Add in the back story of a colorful and dramatic tale of two galaxies locked in a life-and-death struggle, and the picture becomes poetry in motion.All of this amazing action takes place in a single frame, with the polar-ring galaxy as the focus of attention.
Astronomers have only found a handful of polar-ring galaxies, which are peculiar objects. Because of this, not much is known about their origins. Most of them have an early-type spiral system as the central showpiece. This system is called a lenticular galaxy.
Located about 40 million light-years away, towards Pisces the Fishes, NGC 660 is the only polar-ring galaxy known to have a late-type lenticular galaxy as it's host. All polar-ring galaxies, however, display a ring of stars, dust and gas. These rings extend tens of thousands of light-years across, along an orbit nearly perpendicular to the main disk.
Scientists have created models of how polar-ring galaxies form and they offer two formation scenarios. The first is a piercing merger of two galaxies aligned at right angles. The second scenario is that the host galaxy tidally strips material from a passing gas-rich spiral and forms it into a ring.
Either way, what you see in this Gemini Legacy image is not a single dynamic body. Rather, it is either the "bloody" aftermath of one galaxy striking through the heart of another, or the remains of a tidal struggle between two galaxies where one's dust and gas are scattered. Whichever scenario is right, the result is a colorful, 40,000 light-year-long ring of visual glory.
University of Arizona researcher Brian Svoboda has been studying the chemical and temperature environment of NGC 660 and he believes the unique morphology is created from a previous interaction with a gas-rich galaxy. The fact the geometry of NGC 660 contains more gas and associated star formation than its host suggests a violent formation.
"One of the main characteristics of NGC 660 is that the ring is not truly polar, but is inclined ~45 degrees from the plane of the disk,” Svoboda points out. “The simulations for the piercing mergers cannot reproduce these low inclination polar rings; however, the tidal accretion scenario can."
A considerable part of the hundreds of objects that make up NGC 660's polar ring are blue and red supergiant stars. The youngest stars detected in the ring are only about 7 million years old, indicating a long, ongoing process.
"Gemini's incredible definition of the active star forming regions strewn through the polar ring in NGC 660, juxtaposed against the exquisite crossing dust lanes, is simply beautiful. It really is the most incredible picture I've seen of the galaxy," Svoboda exclaims. "None of the other images I've seen, including those from the Hubble Space Telescope, show the star forming regions with such clarity."
Astronomers would expect to find a collapsed core and a burst of star formation if NGC represents a merging of two galaxies, and they do see that. However, the high gas content of both NGC 660's disk and its polar ring throw a monkey wrench in the mix.
"A tidal accretion event will place gas in the polar ring without strongly interacting with the original gas rich host," Svoboda explains. "Tidal interaction is consistent with an influx of gas into the nuclear region creating the starburst that we observe now." Svoboda says further evidence is the lack of a double nucleus — two super-massive black holes — which he would expect from a merger.
"Tails" are a key signature of many tidal interactions, but astronomers have not detected any extending from NGC 660. These tails are usually created when galaxies pass close to one another. The tidal force ejects stars, gas and dust into a graceful tail of extragalactic debris, which stretch far into space. Both models, however, have produced systems without tidal tails while still creating active star formation.
Although it can't be proven with any degree of confidence, Svoboda believes there is good evidence that the origin of NGC 660's polar ring is the result of a tidal accretion event. He points to the fact the ring itself may be about 1 million years old, meaning the stripped galaxy could have moved out of the field of observation by now.
At the host's core is a compact source of less than 32 light-years in extent that is unseen to the eye, but bright at radio wavelengths.
Most galaxies have a majority of old red stars at their cores. NGC 660, however, has a furious burst of star formation at its core. Shock waves slammed into giant clouds of gas, which collapsed into behemoth blue stars during the gravitational interaction between the two galaxies. Many of these stars likely contain more than 100 times the mass of our Sun. The blue stars exploded in supernova, creating more shockwaves and perpetuating the process which continues still, forming new stars at the core of NGC 660. This makes NGC 660 not only a polar-ring galaxy, but a starburst galaxy as well. A starburst system is among the most dense and intense star-forming environments known.
A polar-ring rotates at a speed comparable to that of its host galaxy. Knowing how fast the polar-ring rotates at different distances from the center of the system allows astronomers to search for evidence of dark matter in the halo. The velocity of NGC 660's ring close to the core is normal according to radio observations. However, the velocity in the ring's outer edge remains consistent. Theoretically, the velocity should have dropped off due to the region's gas poor environment, pointing to the existence of huge amounts of dark matter in NGC 660.
Dark matter influences the dynamics of all galaxies, astronomers believe. Understanding dark matter, however, remains one of astronomy's greatest mysteries. Perhaps further observations of the enigmatic NGC 660 will shed more light on this mystery.