* Astronomy

Hungry Black Hole Spawns Bizarre Four-Armed Galaxy

Where most spiral galaxies have two twisting arms, a neighbour of the Milky Way is a four-armed monster. A new photo snapped by the Hubble Space Telescope, combined with observations by amateur astronomers, reveals these arms in stunning detail.
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Title: A Stellar Dynamical Measurement of the Black Hole Mass in the Maser Galaxy NGC 4258
Authors: Christos Siopis, Karl Gebhardt, Tod R. Lauer, John Kormendy, Jason Pinkney, Douglas Richstone, S. M. Faber, Scott Tremaine, M. C. Aller, Ralf Bender, Gary Bower, Alan Dressler, Alexei V. Filippenko, Richard Green, Luis C. Ho, John Magorrian

We determine the mass of the black hole at the centre of the spiral galaxy NGC 4258 by constructing axisymmetric dynamical models of the galaxy. These models are constrained by high spatial resolution imaging and long-slit spectroscopy of the nuclear region obtained with the Hubble Space Telescope, complemented by ground-based observations extending to larger radii. Our best mass estimate is MBH = (3.3 ± 0.2) x 10^7 \MSun for a distance of 7.28 Mpc (statistical errors only). This is within 15% of (3.82± 0.01) x 10^7 \MSun, the mass determined from the kinematics of water masers (rescaled to the same distance) assuming they are in Keplerian rotation in a warped disk. The construction of accurate dynamical models of NGC 4258 is somewhat compromised by an unresolved active nucleus and colour gradients, the latter caused by variations in the stellar population and/or obscuring dust. These problems are not present in the ~ 30 other black hole mass determinations from stellar dynamics that have been published by us and other groups; thus, the relatively close agreement between the stellar dynamical mass and the maser mass in NGC 4258 enhances our confidence in the black hole masses determined in other galaxies from stellar dynamics using similar methods and data of comparable quality.

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Chandra X-ray Image of NGC 4258
NGC 4258 is a spiral galaxy about 25 million light years from Earth, also known as M106. The Chandra image of NGC 4258 reveals the so-called anomalous arms. These mysterious structures are due to shock waves generated by the supermassive black hole in the nucleus of the galaxy.


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Credit: NASA/CXC/Univ. of Maryland/A.S. Wilson et al.

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Using 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 Agencys XMM-Newton X-ray observatory, NASAs Chandra X-ray Observatory, and NASAs 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" - 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. They have been a mystery since they were first discovered in the early 1960s" - Yuxuan Yang, lead author of the team.


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Credits: NASA/CXC/Univ. of Maryland/A.S. Wilson et al. Optical: Pal.Obs. DSS; IR: NASA/JPL-Caltech; VLA & NRAO/AUI/NSF

By analysing 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 M106s 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" - Yuxuan 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 M106s 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-Newtons 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" - Yuxuan 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 M106s 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 NASAs 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 galaxys disk. Moreover, Chandras 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 M106s central black hole.

Source ESA