Another piece of the puzzle: The fast HI outflow in Mrk 231

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 super(-1) 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 15 x 10 super(18)T sub(spin) cm super(-2). We derive the T sub(spin) to lie in the range 400-2000 K and the corresponding HI densities are n sub(HI)~ 10-100 cm super(-3). Our results complement previous findings and confirm the multiphase nature of the outflow in Mrk 231. Although effects of the interaction between the radio plasma and the surrounding medium cannot be ruled out, the energetics and the lack of a clear kpc-scale jet suggest that the most likely origin of the HI outflow is a wide-angle nuclear wind, as earlier proposed to explain the neutral outflow traced by NaI and molecular gas in this source. Our results suggest that an HI component is present in fast outflows regardless of the acceleration mechanism (wind vs. jet driven) and that it must be connected with common properties of the pre-interaction gas involved. Considering the observed similarity of their column densities, the HI outflow likely represents the inner part of the broad wind identified on larger scales in atomic NaI. The mass outflow rate of the HI outflow (between 8 and 18 M sub([middot in circle]) yr super(-1)) does not appear to be as large as that observed in molecular gas, partly owing to the smaller sizes of the outflowing region sampled by the HI absorption. These characteristics are commonly seen in other cases of outflows driven by the active galactic nucleus (AGN) suggesting that the HI may represent a short intermediate phase in the rapid cooling of the gas. The results further confirm HI as a good tracer for AGN-driven outflows not only in powerful radio sources. 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") 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 unp