Excitation and acceleration of molecular outflows in LIRGs: The extended
  ESO 320-G030 outflow on 200-pc scales
  We used high-spatial resolution (70 pc; 0.3") CO multi-transition (1-0, 2-1,
4-3, and 6-5) ALMA data to study the physical conditions and kinematics of the
cold molecular outflow in the local LIRG ESO320-G030 (d=48 Mpc, log
LIR/Lsun=11.3). ESO320-G030 is a double-barred isolated spiral, but its compact
and obscured nuclear starburst (SFR~15 Msun/yr; Av~40 mag) resembles those of
more luminous ULIRGs. In the outflow, the 1-0/2-1 ratio is enhanced with
respect to the rest of the galaxy and the CO(4-3) transition is undetected.
This indicates that the outflowing molecular gas is less excited than the gas
in the nuclear starburst (launching site) and the galaxy disk. Non-LTE
radiative transfer modeling reveals that the properties of the outflow
molecular clouds differ from those of the nuclear and disk clouds: The kinetic
temperature is lower (~9 K) in the outflow, and the outflowing clouds have
lower column densities. Assuming a 10^-4 CO abundance, the large internal
velocity gradients, 60^+250_-45 km/s/pc, imply that the outflowing molecular
clouds are not bound by self-gravity. All this suggests that the life-cycle
(formation, collapse, dissipation) of the disk clouds might differ from that of
the outflowing clouds which might not be able to form stars. The low Tkin of
the molecular outflow remains constant up to 1.7 kpc. This indicates that the
heating by the hotter ionized outflow phase is not efficient and may favor the
survival of the outflow molecular phase. The velocity structure of the outflow
shows a 0.8 km/s/pc velocity gradient between 190-560 pc and then a constant
maximum velocity (~750 km/s) up to 1.7 kpc. This is compatible with a pure
gravitational evolution of the outflow under certain mass outflow rate and
launching velocity variations. Alternatively, ram pressure acceleration and
cloud evaporation could explain the observed kinematics and size of the
molecular phase.
