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Session 5 - High Luminosity AGN.
Display session, Monday, June 10
Tripp Commons,
Mrk 231 is a dusty Seyfert 1/starburst galaxy at a redshift of 0.0422, having an extremely high infrared luminosity of about 2\times 10^12 L_ødot. Optical images reveal a variety of extranuclear features suggesting that Mrk 231 underwent a galaxy merger about 10^8 years ago. We have now imaged the nucleus of Mrk 231, using the VLBA at wavelengths of 6, 13, and 22 cm to achieve resolutions ranging from 1.5 to 8 milliarcsec. The galaxy shows a two-sided radio source approximately 50 milliarcsec in extent, corresponding to a linear diameter of 30 h^-1 pc. The triple radio source is aligned nearly North-South, in good agreement with the position angle of the radio structure on scales of tens of arcseconds, but in contrast with the nearly East-West alignment of the core at 1-milliarcsec resolution. The strong, nearly unresolved central component (40--120 mJy) has an inverted spectrum, implying that it is the active nucleus of the galaxy. However, images tapered to the same resolution at each of the three observing frequencies reveal that the northern component (1--2 mJy) also has an inverted spectrum. This component, located 12 h^-1 pc from the core, has a spectral index of \alpha\approx +1.8 (S_\nu\propto \nu^+\alpha) between 22 and 13 cm; the spectrum flattens out between 13 and 6 cm. This implies that the northern component is probably free-free absorbed in a manner similar to the northern component in the radio galaxy 3C 84. Assuming that the absorption takes place in narrow-line clouds with a temperature near 10,000 K, the inferred emission measure, at a distance of 12 h^-1 pc from the Mrk 231 nucleus, is approximately 10^7 cm^-6pc.
