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Chandra X-ray Image of NGC 4258
(Credit: NASA/CXC/Univ. of Maryland/A.S. Wilson et al.)

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NGC 4258:
Mysterious Arms Revealed

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

A combination of space and ground-based observations, including X-ray data from Chandra, has helped reveal the nature of the so-called anomalous arms in the spiral galaxy NGC 4258 (also known as M106). These arms have been known for decades, but their origin remained mysterious to astronomers.

In visible (shown in gold) and infrared (red) light, 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 (purple) and Chandra's X-ray (blue) images, two additional spiral arms are seen.

By analyzing data from XMM-Newton, Spitzer, and Chandra, scientists have confirmed earlier suspicions that the ghostly arms represent regions of gas that are being violently heated by shock waves. Previously, some astronomers had suggested that the anomalous arms are jets of particles being ejected by a supermassive black hole in nucleus of NGC 4258. But radio observations at the Very Large Array later identified another pair of jets originating in the core.

However, the jets do heat the gas in their line of travel, forming an expanding cocoon. Because the jets lie close to M106's disk, the cocoon generates shock waves and heat the gas in the disk to millions of degrees, causing it to radiate brightly in X-rays and other wavelengths.

Fast Facts for NGC 4258:
Credit  X-ray: NASA/CXC/Univ. of Maryland/A.S. Wilson et al.; Optical: Pal.Obs. DSS; IR: NASA/JPL-Caltech; VLA: NRAO/AUI/NSF
Scale  Image is 9.2 x 7.5 arcmin.
Category  Normal Galaxies & Starburst Galaxies, Quasars & Active Galaxies
Coordinates (J2000)  RA 2h 19m 00s | Dec +47° 18´ 00"
Constellation  Canes Venatici
Observation Date  Three pointings from April 17, 2000 to May 29, 2001
Observation Time  4 hours
Obs. ID  350, 1618, 2340
Color Code  X-ray (blue); Optical (gold); IR (red); Radio (purple)
Instrument  ACIS
Also Known As M106
References Yang, Li, Wilson, Reynolds, ApJ, May 10 issue; Chandra Observations and the Nature of the Anomalous Arms of NGC 4258 (M106). Wilson, A.S., Yang,Y., Cecil, G., 2001, ApJ, 560, 689W
Distance Estimate  About 25 million light years
Release Date  April 10, 2007