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Session 16 - Low Luminosity AGN.
Oral session, Monday, June 10
Union Theater,
Water masers in the nucleus of the weakly active galaxy NGC4258 are known to lie in an extremely thin disk in Keplerian rotation around a central mass of 3.5\times10^7 M_\sun. A group of masers at the galaxy systemic velocity are believed to lie along the front edge of the disk, which is tipped down about 8^\circ from the line-of-site (LOS). Two additional groups of features are offset \pm1000 kms^-1 from the systemic features. The blue-shifted features are persistently an order of magnitude weaker than the red-shifted features. Unlike the systemic features, these features show no radial velocity accelerations and are thought to lie along the diameter defined by the intersection of the disk with the plane of the sky. Systematic declination offsets in the distribution of these features indicate that the disk is warped; the position angle of the disk changes smoothly by \sim8^\circ over the 0.15 pc of maser emission. It is thought that the energy needed to collisionally pump the masers is provided by direct exposure to the central x-ray source as a result of the warp in the disk. Such x-ray illumination will create a cool molecular layer below a warm partially ionized atomic layer and a narrow, fully ionized layer. Microwave emission traveling through these atomic layers will encounter optical depths of about 2 due to free-free thermal absorption. Because of the disk inclination, both the red- and blue-shifted maser emission travel over the upper surface of the disk. However, because of the warp, only the blue-shifted side of the disk receives direct x-ray illumination on its upper surface. Thus, we postulate that the blue-shifted microwave emission is persistently weak in comparison to the red-shifted emission because it must propagate through an opaque atomic layer, while the red-shifted emission does not.
