Is cold gas fuelling the radio galaxy NGC 315?
  We present WSRT, VLA and VLBI observations of the HI absorption in the radio
galaxy NGC 315. The main result is that two HI absorbing systems are detected
against the central region. In addition to the known highly redshifted, very
narrow component, we detect relatively broad (FWZI 150 km/s) absorption. This
broad component is redshifted by ~80 km/s compared to the systemic velocity,
while the narrow absorption is redshifted ~ 490 km/s. Both HI absorption
components are spatially resolved at the pc-scale of the VLBI observations. The
broad component shows strong gradients in density (or excitation) and velocity
along the jet. We conclude that this gas is physically close to the AGN,
although the nature of the gas resulting in the broad absorption is not
completely clear. The possibility that it is entrained by the radio jet appears
unlikely. Gas located in a thick circum-nuclear toroidal structure cannot be
completely ruled out although it appears difficult to reconcile with the
observed morphology and kinematics of the HI. A perhaps more likely scenario is
that the gas producing the broad absorption could be (directly or indirectly)
connected with the fuelling of the AGN, i.e. gas that is falling into the
nucleus. If this is the case, the accretion rate derived is similar to that
found for other X-ray luminous elliptical galaxies, although lower than that
derived from the radio core luminosity for NGC 315. The density distribution of
the narrow component is, featureless. Moreover, we detect a small amount of HI
in emission a few kpc SW of the AGN, coincident with faint optical absorption
features and at velocities very similar to the narrow absorption. This suggests
that the gas causing the narrow absorption is not close to the AGN and is more
likely caused by clouds falling into NGC 315.
