Radio jets as driving mechanism of fast outflows: the HI view

The complex and multi-phase nature of gas outflows is one of the properties
highlighted by the work in recent years on AGN-driven outflows.  In
particular, the cold gas is found to play a more important role than
previously expected.  Surprisingly, HI has been shown to be a good tracer of
fast outflows.  In radio AGN, we can trace these outflows by looking at HI in
absorption.  I will present the results from a survey (done with the
Westerbork Synthesis Radio Telescope) of radio sources up to z = 0.2, which
has allowed us to explore, for the first time in a systematic way, the
occurrence of HI outflows.  A number of new cases have been found, showing
outflow velocity up to about 1000 km/s.  We have followed up some interesting
cases with HI observations at high spatial resolution (reaching the pc scale
with VLBI).  In this way, we have located the region(s) where the HI outflows
occur and investigated the role of the radio jets as driving mechanism.
This relatively shallow survey was done in preparation of the upcoming
“blind” surveys about to start with SKA pathfinders (Apertif, ASKAP,
MeerKat).  These surveys will allow two important steps forward: build up much
better statistics of the HI outflows, and extend this search to higher
redshifts to investigate whether any evolution is seen in their occurrence and
properties.  I will summarise the status of these projects.