1999ApJ...516..127U

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 10⁹-10¹⁰ K and 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 H I 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)x10³ 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 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 3 orders of magnitude. The large-scale radio emission extends beyond the bulk of the optical galaxy but has an initial axis similar to a series of optical star-forming knots several kiloparsecs from the nucleus. This diffuse radio source is probably generated by energy deposition from a slow-moving nuclear jet, which conceivably could help energize the off-nuclear starburst as well.