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Description
We present the detection, performed with the Westerbork Synthesis Radio Telescope (WSRT) and the Karl Jansky Very Large Array (VLA), of a fast HI 21-cm outflow in the ultra-luminous infrared galaxy Mrk 231. The outflow is observed as shallow HI absorption blueshifted ~1300 km/s with respect to the systemic velocity and located against the inner kpc of the radio source. The outflowing gas has an estimated column density between 5 and 15x10^18^Tspin.cm^-2^. We derive the Tspin to lie in the range 400-2000 K and the corresponding HI densities are n_HI_~10-100cm^-3^. We also obtained deeper continuum images than previously available. They confirm the complex structure of the radio continuum originating both from the AGN and star formation. At the resolution obtained with the VLA (~1-arcsec) we do not see a kpc-scale jet. Instead, we detect a plateau of emission, likely due to star formation, surrounding the bright nuclear region. We also detect a poorly collimated bridge which may represent the channel feeding the southern lobe. The unprecedented depth of the low-resolution WSRT image reveals radio emission extending 50-arcsec (43kpc) to the south and 20arcsec (17kpc) to the north.
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