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tokens: own reradiated IR photons ( for the most favorable geometry), so we assume that this boosting is unimportant. NGC 253 has a total infrared luminosity of erg s −1 , about half of which originates within a diameter of ∼200 pc from its center (Leroy et al. 2015b ), i.e., R src = 100 pc. The SW streamer has a “minimum mass” , a projected size on the sky of pc 2 , and a typical distance to the center of the order of pc. If we assume that the geometry corresponds to a cylinder 60 pc in diameter () illuminated by erg s −1 , the force due to radiation pressure applied at the base of the streamer would be dyn, resulting in an estimated acceleration of cm s −2 . In the absence of gravity, the resulting velocity at pc would be km s −1 , lower than the observed velocity before projection correction (Figure 4 ). If we instead assume that the radiation is exerting pressure on the long side of the cylinder (the geometry with the maximum area), f rad would be five times larger, with a corresponding km s −1 . Note, however, that the ratio of gravity to radiation pressure for stars is , where erg s −1 g for a fully sampled IMF containing young stars, and is the surface density of the cloud (Ψ is the light-to-mass ratio of the stellar population, e.g., Equation (33) in Kim et al. 2016 ). For the end-on geometry g cm −2 , while for the side-on geometry g cm −2 , resulting in . So, for our assumed geometry and mass, the net acceleration is between 6% and 81% of a rad , which would result in km s −1 . From the above estimates, it appears that radiation can contribute maybe up to a few tens of percent of the momentum, but it is extremely unlikely to completely explain the observations. Doing so would require using the most favorable geometric assumption (maximal cloud area, which maximizes f rad and minimizes ) and also assuming that there is almost no projection correction to the measured velocity (which requires a contrived geometry). This suggests that radiation pressure is not the dominant mechanism for accelerating the streamer, though it is non-negligible. Note also that any increase in the mass of the streamer over its minimum mass (see discussion in Section 3.6.1 ), or accounting for the old stars in the galaxy disk outside the starburst (which would increase gravity), would also reduce the importance of radiation pressure. Much higher resolution observations of the SW streamer may help further elucidate these questions, in particular, which mechanisms are driving the molecular gas in the outflow. 4. Summary Our new ALMA band 3 observations (CO and dense gas tracers) of the central starburst region of NGC 253 give new insights on the properties of the molecular outflowing gas originally discussed in Bolatto et al. ( 2013 ). The most prominent outflow feature toward the south, which is the main subject of this paper, possesses a large intrinsic velocity dispersion. Its extent and dispersion are consistent with an ejection from the disk starting about ∼1 Myr