|
Mystery spiral arms explained?
April 11, 2007Using a trio of space observatories, astronomers may have cracked a 45-year old mystery surrounding two ghostly spiral arms in the galaxy M106 (NGC 4258).
The results, obtained by a team from the University of Maryland (USA), took advantage of the unique capabilities of the European Space Agency's XMM-Newton X-ray observatory, NASA's Chandra X-ray Observatory, and NASA's Spitzer Space Telescope.
M106 (also known as NGC 4258) is a spiral galaxy 23.5 million light-years away, in the constellation Canes Venatici. In visible-light images, two prominent arms emanate from the bright nucleus and spiral outward. These arms are dominated by young, bright stars, which light up the gas within the arms. "But in radio and X-ray images, two additional spiral arms dominate the picture, appearing as ghostly apparitions between the main arms," says team member Andrew Wilson of the University of Maryland. These so-called "anomalous arms" consist mostly of gas.
"The nature of these anomalous arms is a long-standing puzzle in astronomy," says Yuxuan Yang, lead author of the team. "They have been a mystery since they were first discovered in the early 1960s."
By analyzing data from XMM-Newton, Spitzer, and Chandra, the team in Maryland have confirmed earlier suspicions that the ghostly arms represent regions of gas that are being violently heated by shock waves.
It has been previously suggested that the anomalous arms are jets of particles being ejected by a supermassive black hole in M106's nucleus. But radio observations at the Very Large Array in New Mexico later identified another pair of jets originating in the core. "It is highly unlikely that an active galactic nucleus could have more than one pair of jets," says Yang.
In 2001, another team of astronomers at the University of North Carolina (USA), noted that the two jets are tipped 30 degrees with respect to the disk. But if one could vertically project the jets onto the disk, they would line up almost perfectly with the anomalous arms. Figuring that this alignment was not strictly a matter of chance, the team proposed that the jets heat the gas in their line of travel, forming an expanding cocoon. Because the jets lie close to M106's disk, the cocoon heats gas in the disk and generates shock waves, heating the gas to millions of degrees and causing it to radiate brightly in X-rays and other wavelengths.
To test this idea, Yang and his colleagues looked at archival spectral observations from XMM-Newton. With XMM-Newton's superb sensitivity, the team could measure the gas temperature in the anomalous arms and also see how X-rays from the gas are absorbed en route by intervening material.
"One of the predictions of this scenario is that the anomalous arms will gradually be pushed out of the galactic disk plane by jet-heated gas," says Yang. The XMM-Newton spectra show that X-rays are absorbed more strongly in the direction of the northwest arm than in the southeast arm. The results strongly suggest that the southeast arm is partly on the near side of M106's disk, and the northwest arm is partly on the far side.
The scientists noted that these observations show clear consistency with their scenario. Confirmation of this interpretation has recently come from archival observations from NASA's Spitzer Space Telescope, whose infrared view shows clear signs that X-ray emission from the northwest arm is being absorbed by warm gas and dust in the galaxy's disk. Moreover, Chandra's superior imaging resolution gives clear indications of gas shocked by interactions with the two jets.
Besides addressing the mystery of the anomalous arms, these observations allowed the team to estimate the energy in the jets and gauge their relationship to M106's central black hole.
European Space Agency
By analyzing data from XMM-Newton, Spitzer, and Chandra, the team in Maryland have confirmed earlier suspicions that the ghostly arms represent regions of gas that are being violently heated by shock waves.
It has been previously suggested that the anomalous arms are jets of particles being ejected by a supermassive black hole in M106's nucleus. But radio observations at the Very Large Array in New Mexico later identified another pair of jets originating in the core. "It is highly unlikely that an active galactic nucleus could have more than one pair of jets," says Yang.
In 2001, another team of astronomers at the University of North Carolina (USA), noted that the two jets are tipped 30 degrees with respect to the disk. But if one could vertically project the jets onto the disk, they would line up almost perfectly with the anomalous arms. Figuring that this alignment was not strictly a matter of chance, the team proposed that the jets heat the gas in their line of travel, forming an expanding cocoon. Because the jets lie close to M106's disk, the cocoon heats gas in the disk and generates shock waves, heating the gas to millions of degrees and causing it to radiate brightly in X-rays and other wavelengths.
To test this idea, Yang and his colleagues looked at archival spectral observations from XMM-Newton. With XMM-Newton's superb sensitivity, the team could measure the gas temperature in the anomalous arms and also see how X-rays from the gas are absorbed en route by intervening material.
"One of the predictions of this scenario is that the anomalous arms will gradually be pushed out of the galactic disk plane by jet-heated gas," says Yang. The XMM-Newton spectra show that X-rays are absorbed more strongly in the direction of the northwest arm than in the southeast arm. The results strongly suggest that the southeast arm is partly on the near side of M106's disk, and the northwest arm is partly on the far side.
The scientists noted that these observations show clear consistency with their scenario. Confirmation of this interpretation has recently come from archival observations from NASA's Spitzer Space Telescope, whose infrared view shows clear signs that X-ray emission from the northwest arm is being absorbed by warm gas and dust in the galaxy's disk. Moreover, Chandra's superior imaging resolution gives clear indications of gas shocked by interactions with the two jets.
Besides addressing the mystery of the anomalous arms, these observations allowed the team to estimate the energy in the jets and gauge their relationship to M106's central black hole.
European Space Agency
Spiral Arms News and Current Spiral Arms Events RSSHubble's sweeping view of the Coma Galaxy Cluster
The NASA/ESA Hubble Space Telescope captures the magnificent starry population of the Coma Cluster of Galaxies, one of the densest known galaxy collections in the Universe.
Large binocular telescope achieves first binocular light
The Large Binocular Telescope on Mount Graham, Ariz., has taken celestial images using its twin side-by-side, 8.4-meter (27.6 foot) primary mirrors together, achieving first "binocular" light.
UO plays key role in LIGO's new view of a cosmic event
An international team of physicists, including University of Oregon scientists, has concluded that last February's intense burst of gamma rays possibly coming from the Andromeda Galaxy lacked a gravitational wave. That absence, they say, rules out an initial interpretation that the burst came from merging neutron stars or black holes within Andromeda.
Hubble's 17th anniversary -- extreme star birth in the Carina Nebula
Hubble's new view of the Carina Nebula shows the process of star birth at a new level of detail.
The Purple Rose of Virgo
ntil now NGC 5584 was just one galaxy among many others, located to the West of the Virgo Cluster. Known only as a number in galaxy surveys, its sheer beauty is now revealed in all its glory in a new VLT image. Since 1 March, this purple cosmic rose also holds the brightest stellar explosion of the year, known as SN 2007af.
NASA sees into the eye of a monster storm on Saturn
NASA's Cassini spacecraft has seen something never before seen on another planet - a hurricane-like storm at Saturn's south pole with a well-developed eye, ringed by towering clouds.
Forming super-Earths by ultraviolet stripping
A new explanation for forming "super-Earths" suggests that they are more likely to be found orbiting red dwarf stars—the most abundant type of star—than gas giant planets like Jupiter and Saturn.
New Cassini image shows "A" ring contains more debris than once thought
Views of Saturn's stunning ring system from above by the Cassini-Huygens spacecraft now orbiting the planet indicate the prominent A ring contains more debris than once thought, according to a new University of Colorado at Boulder study.
Scientists Probe Black Hole's Inner Sanctum
How does matter spiral its way to the center of a galaxy and into the mouth of a supermassive black hole?
New observations show dynamic particle clumps in Saturn's A ring
New observations from the Cassini spacecraft now at Saturn indicate the particles comprising one of its most prominent rings are trapped in ever-changing clusters of debris that are regularly torn apart and reassembled by gravitational forces from the planet.
More Spiral Arms News Articles
| © 2008 BrightSurf.com |