Astronomers using the Gemini Multi-Object Spectrograph on the Fredrick C. Gillett Gemini North telescope on Mauna Kea, Hawai‘I, have photographed the stunning polar-ring galaxy NGC 660.
This image shows the polar-ring galaxy NGC 660 (Gemini Observatory / AURA)
Polar-ring galaxies are peculiar objects in which a polar or very inclined ring of gas and stars surrounds the host galaxy.
The galaxy NGC 660, which lies about 40 million light-years away in the constellation Pisces, is the only polar-ring galaxy known with what is called a late-type lenticular galaxy as its host.
According to the astronomers, the image shows not a single dynamic body but either the ‘bloody’ aftermath of one galaxy piercing the heart of another or the remains of a furious tidal struggle between two galaxies that shattered one galaxy’s lamp, scattered its dust and gas, and formed a colorful, 40,000-light-year-long ring of visual glory.
Dr Brian Svoboda of the University of Arizona, who studied the chemical and temperature environment of NGC 660, said: “unique morphology arises from a previous interaction with a gas-rich galaxy.”
NGC 660’s enormous edge-on polar ring contains more gas than its host, which strongly suggests a violent formation.
“One of the main characteristics of NGC 660 is that the ring is not truly polar, but is inclined about 45 degrees from the plane of the disk. The simulations for the piercing mergers cannot reproduce these low inclination polar rings; however, the tidal accretion scenario can,” Dr Svoboda said.
NGC 660’s polar ring resolves into hundreds of objects, a considerable part of which are blue and red supergiant stars. The youngest detected stars in the ring formed only about 7 million years ago, 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. None of the other images I’ve seen, including those from the Hubble Space Telescope, show the star forming regions with such clarity,” the astronomer said.
“A tidal accretion event will place gas in the polar ring without strongly interacting with the original gas rich host. Tidal interaction is consistent with an influx of gas into the nuclear region creating the starburst that we observe now. Further evidence is NGC 660’s lack of a double nucleus (i.e. two super-massive black holes), which one would expect from a merger.”
Astronomers have not detected any ‘tails’ extending from NGC 660, a key signature of many tidal interactions. Usually, when galaxy passes close to another, tidal forces eject stars, gas, and dust into a graceful tail of extragalactic debris and stretched them far into space. But both polar-ring models have produced systems without tidal tails while creating active star formation in the polar ring.
“While it can’t be proven with a great deal of confidence, in the case of NGC 660, I think that there is good evidence to suggest that the origin of the polar ring lies in the tidal accretion event scenario. The ring may be about 1 billion years old, so the stripped galaxy could have moved out of the field by the time of our observations now,” Dr Svoboda said.
According to a study published in 2004 in the journal Astronomy & Astrophysics (arXiv.org version), NGC 660’s polar ring resolves into hundreds of objects, a considerable part of which are blue and red supergiant stars. The youngest detected stars in the ring formed only about 7 million years ago, indicating a long, ongoing process.
The astronomers also detected a compact radio source less than 32 light-years in extent at the host’s core. They believe this powerful radio emitter is a super cluster of stars in a dense cloud of dust and gas containing perhaps a few thousand hot, blue youthful stars.
The ring in a polar-ring galaxy rotates at a speed comparable to that of its host galaxy. By determining how fast a polar ring rotates at different distances from the center of the system, astronomers can search for evidence of elusive and mysterious dark matter in NGC 660’s halo.
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Bibliographic information: Karataeva G.M. et al. 2004. The stellar content of the ring in NGC 660. A&A 421, 833-837; doi: 10.1051/0004-6361:20047080
