Mystery spiral arms explained? (Forwarded)

ESA News
http://www.esa.int

10 April 2007

Mystery spiral arms explained?

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 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.

Note for editors

The results will appear in the 10 May 2007 issue of the Astrophysical
Journal, in an article titled: "Spatially resolved X-ray spectra of NGC
4258," by Y. Yang, B. Li, A. S. Wilson, C. S. Reynolds
(http://xxx.lanl.gov/abs/astro-ph/0701569)

For more information

Yuxuan Yang, lead author, University of Maryland
Email: yyang @ astro.umd.edu

Norbert Schartel, ESA XMM-Newton Project Scientist
Email: norbert.schartel @ sciops.esa.int

[NOTE: An image supporting this release are available at
http://www.esa.int/esaCP/SEMLVET4LZE_index_1.html ]