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 \sim 300 pc , with a width of \sim 50 pc and a velocity dispersion of \sim 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 \sim 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 ( \sim 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 \sim 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 .