摘要：We present the detection, performed with the Westerbork Synthesis Radio Telescope (WSRT)
and the Karl Jansky Very Large Array (VLA), of a fast H I 21 cm outflow
in the ultra-luminous infrared galaxy Mrk 231. The outflow is observed as shallow H I
absorption blueshifted ~1300 km s-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 ×
1018Tspin cm-2. We derive the Tspin to lie in
the range 400−2000 K and the
corresponding H I densities are nHI ~ 10−100 cm-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 H I outflow is a wide-angle nuclear wind, as earlier proposed to
explain the neutral outflow traced by Na I and molecular gas in this source. Our results
suggest that an H I 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 H I outflow likely represents the inner part of the broad wind identified
on larger scales in atomic Na I. The mass outflow rate of the H I outflow (between 8 and
18 M⊙ yr-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 H I absorption. These characteristics are commonly seen
in other cases of outflows driven by the active galactic nucleus (AGN) suggesting that the
H I may represent a short intermediate phase in the rapid cooling of the gas. The results
further confirm H I 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 unprecedented depth of the low-resolution WSRT image
reveals radio emission extending 50′′ (43 kpc) to the south and 20′′ (17 kpc) to the north.
关键词：galaxies: active;galaxies: individual: Mrk 231;ISM: jets and outflows;radio lines: galaxies
