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A. S. Wilson, Y. Yang (U. Md.), G. Cecil (U. NC)
This paper presents high resolution X-ray observations with Chandra of NGC 4258 and infers the nature of the so called ``anomalous arms'' in this galaxy. The anomalous arms dominate the X-ray image; diffuse X-ray emission from the ``plateaux'' regions, seen in radio and H\alpha imaging, is also found. X-ray spectra have been obtained at various locations along the anomalous arms and are well described by thermal (mekal) models with kT in the range 0.37 - 0.6 keV.
The radio jets propagate in a different direction to that of the anomalous arms; the jets are inclined by 60\circ (in 3-d) from the rotation axis of the galactic disk. These jets drive mass motions into the low density galactic halo. This high velocity halo gas impacts on the dense galactic gas disk and shock heats it along and around a ``line of damage'', which is the projection of the jets onto the galactic gas disk as viewed down the galaxy disk rotation axis. However, because NGC 4258 is highly inclined (i = 64\circ), the ``line of damage'' projects on the sky in a different direction to the jets themselves. We calculate the expected p.a. of the ``line of damage'' on the sky and find that it coincides with the anomalous arms. It is, therefore, proposed that the anomalous arms, which are known to lie in the galactic disk, represent disk gas which has been shocked by mass motions driven by the out-of-plane radio jets. This model accounts for: (1) the diffuse character of the anomalous arms, 2) the inferred ionization of the optical line-emitting gas by shock waves, (3) the angle between the anomalous arms and the radio jets, (4) the sharp brightness gradients along the outer edges of the anomalous arms (these edges represent the standing shock where the high velocity, jet-driven halo gas meets the disk), (5) the existence of the ``plateaux'', and (6) the wide range of radial velocities observed in the ``plateaux''.