The AGN fuelling/feedback cycle in nearby radio galaxies -- IV.
  Molecular gas conditions and jet-ISM interaction in NGC3100
  This is the fourth paper of a series investigating the AGN fuelling/feedback
processes in a sample of eleven nearby low-excitation radio galaxies (LERGs).
In this paper we present follow-up Atacama Large Millimeter/submillimeter Array
(ALMA) observations of one source, NGC3100, targeting the $^{12}$CO(1-0),
$^{12}$CO(3-2), HCO$^{+}$(4-3), SiO(3-2) and HNCO(6-5) molecular transitions.
$^{12}$CO(1-0) and $^{12}$CO(3-2) lines are nicely detected and complement our
previous $^{12}$CO(2-1) data. By comparing the relative strength of these three
CO transitions, we find extreme gas excitation conditions (i.e. $T_{\rm
ex}\gtrsim50$ K) in regions that are spatially correlated with the radio lobes,
supporting the case for a jet-ISM interaction. An accurate study of the CO
kinematics demonstrates that, although the bulk of the gas is regularly
rotating, two distinct non-rotational kinematic components can be identified in
the inner gas regions: one can be associated to inflow/outflow streaming
motions induced by a two-armed spiral perturbation; the second one is
consistent with a jet-induced outflow with $v_{\rm max}\approx 200$ km s$^{-1}$
and $\dot{M}\lesssim 0.12$ M$_{\odot}$ yr$^{-1}$. These values indicate that
the jet-CO coupling ongoing in NGC3100 is only mildly affecting the gas
kinematics, as opposed to what expected from existing simulations and other
observational studies of (sub-)kpc scale jet-cold gas interactions.
HCO$^{+}$(4-3) emission is tentatively detected in a small area adjacent to the
base of the northern radio lobe, possibly tracing a region of jet-induced gas
compression. The SiO(3-2) and HNCO(6-5) shock tracers are undetected: this -
along with the tentative HCO$^{+}$(4-3) detection - may be consistent with a
deficiency of very dense (i.e. $n_{\rm crit} > 10^{6}$ cm$^{-3}$) cold gas in
the central regions of NGC3100.
