Radio Continuum Evidence for Outflow and Absorption in the Seyfert 1 Galaxy Markarian 231

The Very Long Baseline Array (VLBA) and the Very Large Array (VLA) have been used to image the continuum radio emission from Mrk 231, a Seyfert 1 galaxy and the brightest infrared galaxy in the local universe. The smallest VLBA scales reveal a double, or possibly triple, source less than 2 pc in extent. The components of this central source have minimum brightness temperatures of 10 9 –10 10 K, spectral turnovers between 2 and 10 GHz, and appear to define the galaxy nucleus plus the inner regions of a jet. The strongest component is probably synchrotron self-absorbed, while the weaker component to the northeast may be either free-free absorbed or synchrotron self-absorbed. On larger VLBA scales, the images confirm a previously known north-south triple source extending 40 pc and elongated perpendicular to a 350-pc starburst disk traced by Hi and CO. Both lobes of the triple show evidence for free-free absorption near 2 GHz, probably due to ionized gas with a density of 1–2×10 3 cm −3 in the innermost regions of the starburst disk. This free-free absorption resembles that toward the counterjet of 3C 84, but requires ionized gas at lower density located considerably farther from the central source. The absorbing gas may be ionized by the active nucleus or by local regions of enhanced star formation, possibly in the inner part of the starburst disk. The elongation position angle of the 40-pc triple differs by 65 • from that of the 2-pc source. Unless the radio source is seen nearly end-on, the different symmetry axes on different scales in Mrk 231 imply a dramatic curvature in the inner part of the Mrk 231 radio jet. A comparison of VLBA and VLA flux densities indicates that the radio continuum from the 350-pc disk has a spectral index near −0.4 at frequencies above 1.4 GHz and is plausibly energized by a massive burst of star formation, with the overall 1 The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. – 2 – spectrum flattened somewhat by a contribution from free-free absorption. On VLA scales, asymmetric and diffuse emission extends for more than 25 kpc. This emission has a steep spectrum, exhibits linear polarization exceeding 50% at some locations, and shares the symmetry axis of the 40-pc triple, but on a scale larger by three orders of magnitude. …