Dense Molecular Gas Tracers in the Outflow of the Starburst Galaxy NGC
  253
  We present a detailed study of a molecular outflow feature in the nearby
starburst galaxy NGC 253 using ALMA. We find that this feature is clearly
associated with the edge of NGC 253's prominent ionized outflow, has a
projected length of ~300 pc, with a width of ~50 pc and a velocity dispersion
of ~40 km s^-1, consistent with an ejection from the disk about 1 Myr ago. The
kinematics of the molecular gas in this feature can be interpreted (albeit not
uniquely) as accelerating at a rate of 1 km s^-1 pc^-1. In this scenario, the
gas is approaching escape velocity at the last measured point. Strikingly,
bright tracers of dense molecular gas (HCN, CN, HCO+, CS) are also detected in
the molecular outflow: We measure an HCN(1-0)/CO(1-0) line ratio of ~1/10 in
the outflow, similar to that in the central starburst region of NGC 253 and
other starburst galaxies. By contrast, the HCN/CO line ratio in the NGC 253
disk is significantly lower (~1/30), similar to other nearby galaxy disks. This
strongly suggests that the streamer gas originates from the starburst, and that
its physical state does not change significantly over timescales of ~1 Myr
during its entrainment in the outflow. Simple calculations indicate that
radiation pressure is not the main mechanism for driving the outflow. The
presence of such dense material in molecular outflows needs to be accounted for
in simulations of galactic outflows.
