Star-formation-driven outflows in local dwarf galaxies as revealed from [CII] observations by Herschel

Abstract

We characterize the physical properties of star-formation-driven outflows in a sample of 29 local dwarf galaxies drawn from the Dwarf Galaxy Survey. We made use of Herschel/PACS archival data to search for atomic outflow signatures in the wings of individual [CII] 158 μm spectra and in their stacked line profile. We find a clear excess of emission in the high-velocity tails of 11 sources, which can be explained with an additional broad component (tracing the outflowing gas) in the modeling of their spectra. The remaining objects are likely hosts of weaker outflows that can still be detected in the average stacked spectrum. In both cases, we estimate the atomic mass outflow rates which result to be comparable with the star-formation rates of the galaxies, implying mass-loading factors (i.e., outflow efficiencies) of the order of unity. Outflow velocities in all the 11 galaxies with individual detections are larger than (or compatible with) the escape velocities of their dark matter halos, with an average fraction of 40% of gas escaping into the intergalactic medium (IGM). Depletion timescales due to outflows are lower than those due to gas consumption by star formation in most of our sources, ranging from one hundred million to a few billion years. From the energetic point of view, our outflows are mostly consistent with momentum-driven winds generated by the radiation pressure of young stellar populations on dust grains, although the energy-driven scenario is not excluded if considering a coupling efficiency up to 20% between the energy injected by supernovae and the interstellar medium. Overall, our results suggest that, despite their low efficiencies, galactic outflows can regulate the star-formation history of dwarf galaxies. Specifically, they are able to enrich with metals the circumgalactic medium of these sources, bringing on average a non-negligible amount of gas into the IGM, where it will no longer be available for new star formation. Our findings are suitable for tuning chemical evolution models attempting to describe the physical processes shaping the evolution of dwarf galaxies.