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Session 50 - Active Galaxies.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center

[50.18] Reading the Twisted Tail of NGC 4258

G. Cecil, C. De Pree (UNC-Chapel Hill)

We have used the VLA A-configuration at \lambda20 cm to obtain new high-resolution (1\farcs5) flux and polarization images of the large-scale, twisted jets in NGC 4258, a galaxy of particular interest because its nucleus houses the best observed case of an accretion disk around a supermassive black hole. We have also analyzed recently archived long-exposure ROSAT PSPC spectra of the jets. We have excluded the AGN itself, which is a highly obscured, hard-spectrum X-ray source. The jets are prominent in radio synchrotron, emission-lines, and X-rays because of their strong interaction with the dense galaxy ISM. Previously, we showed from the visible emission-line spectrum (Cecil, Morse, amp; Veilleux 1995, ApJ, 452, 613) that the jets are convincing examples of photoionizing shocks. The 5\times deeper X-ray spectrum confirms our findings (Cecil, Wilson, amp; De Pree 1995, ApJ, 440, 181) that the SE jet has a thermal spectrum (kT = 0.3 keV) consistent with a \approx400 km s^-1 shock. However, the NW jet exhibits a harder spectrum that suggests a higher shock velocity (kT \approx 0.5 keV) and non-thermal contribution. In both cases, the extragalactic photoelectric-absorption is small. We will discuss models of the X-ray spectrum based on the MAPPINGS II code described by Dopita amp; Sutherland (1995, ApJ, in press). Our radio image shows striking inversion symmetry across the nucleus, with several abrupt changes in ouflow direction. It is very unlikely that these arise from localized jet/ISM deflections; they may instead reflect past feasts of the currently starved nucleus. The jets within 25\arcsec\ (800 pc) radius are projected along the spin axis of the accretion disk. The new images and spectra will be combined with existing datasets, including Fabry-Perot H\alpha and [O III] datacubes, to better constrain the history and driving mechanism of the outflow.

Program listing for Tuesday