A 45 Year Old Mystery Cracked?
30 Apr, 2007 03:24 pm
M106 (also known as NGC 4258) is a spiral galaxy 23.5 million light-years away in the constellation Canes Venatici. The galaxy has a two large spiral arms which are dominated by stars. In early 1960's, two French astronomers, Court`es and Cruvellier, noticed in their image of ionized gas in M 106 that in addition to the stellar arms, another pair of ghostly arms emanate from the bright nucleus and spiral out in a trailing sense in between the two stellar arms. These structures are smooth, bright in radio and X-ray emission but do not contain a lot of stars, suggesting these "anomalous arms" are gaseous. The nature of the anomalous arms in M 106 has remained a mystery in astronomy for the past 45 years.
Figure: The image clearly shows that the upper left part of the anomalous arms is blocked by one of the spiral arms traced by IR.
Green : Adaptively smoothed X-ray image using Chandra observatory.
Red: Spitzer 8 micro IR image .
Blue: Radio image using VLA.
This idea is bolstered by the discovery of activity in the nucleus of M 106. However, there is strong evidence suggesting that the inner 3000-6000 light years of the anomalous arms locate in the galactic disk, which is very thin (only about a hundred light years in thickness). It is hard to imagine how a pair of jets could travel long distances in a thin disk without leaving it! Another piece of evidence which argues against a jet nature of the anomalous arms is that a pair of jets which points in different directions from the anomalous arms have been discovered by the Very Large Array of radio telescopes. It is highly unlikely that an active galaxy could have more than one pair of jets.
In 2001, Andrew Wilson, Yuxuan Yang at University of Maryland, College Park, and Gerald Cecil at University of North Carolina, Chapel Hill obtained the first high resolution X-ray Image of M 106 using NASA's Chandra Observatory. They found an excellent agreement between the X-ray and radio emission. By solving the geometry of the radio jets and the anomalous arms, these researchers noted that the two active radio jets are tipped only 30 degrees with respect to the galactic disk of M 106. If one could vertically project the jets onto the disk, they would line up almost perfectly with the anomalous arms. Figuring that this alignment can hardly be a coincidence, the researchers suggest that the jets may heat the gas and form a expanding cocoon along the line they travel. Because the jets lie close to the galactic disk of M 106, the cocoon hits the cold 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, the particles been accelerated to relativistic energies gives the radio emission.
One of the predictions of this scenario is that the gas in the anomalous arms would be gradually be pushed out of the disk by the expanding cocoon. Given the orientation of the jets and the disk, the X-ray radiation from the gas that is pushed out of the disk in the northern anomalous arm, which is on the far side of the disk, would suffer absorption from the dense gas in the galactic disk. On the other hand, X-ray radiation from the gas in the southern arm , which is on the near side of the galactic disk, would suffer little absorption from the galaxy disk of M 106. Such an effect has clearly been seen recently in the X-ray spectra of the anomalous arms, obtained from ESA's XMM-Newton X-ray telescope. The results are reported in a paper to be published in the May 10th issue of Astrophysical Journal (Yang et al. 2007). This effect has also been demonstrated dramatically when combining the multi-wavelength images from Chandra, the Very Large Array in radio, and NASA’s Spitzer infrared space telescopes. The northern anomalous arm is clearly blocked by one of the spiral arms traced by the Spitzer image.
The establishment of the links between radio jets and the anomalous arms allows astronomers to estimate of the total power of these jets. Comparing the power in the jets with the accretion power in the central blackhole of M 106, astronomers could make progress in understanding a central problem in astronomy: how blackholes and accretion disks launch jets, a phenomenon commonly seen in the universe but still poorly understood.
et al , “Spatially resolved X-ray spectra of NGC 4258” to be published in Astrophysical Journal , 2007 May 10th issue.