Star formation inside a galactic outflow

Star formation at a rate of more than 15 solar masses a year has been observed inside a massive outflow of gas from a nearby galaxy; this could also be happening inside other galactic outflows. Star birth in gas flows Massive, galactic-scale outflows of molecular gas with the physical conditions necessary to form stars have recently been observed and several models predict that star formation could ignite within the outflow itself. Roberto Maiolino et al . report spectroscopic observations that unambiguously reveal star formation occurring in a galactic outflow at a redshift of 0.0448 and at an inferred rate exceeding 15 times the mass of the Sun per year. This new mode of star formation might be occurring in other galactic outflows and could have implications for the morphological evolution of galaxies, while contributing to the population of high-velocity stars. Recent observations have revealed massive galactic molecular outflows 1 , 2 , 3 that may have the physical conditions (high gas densities 4 , 5 , 6 ) required to form stars. Indeed, several recent models predict that such massive outflows may ignite star formation within the outflow itself 7 , 8 , 9 , 10 , 11 . This star-formation mode, in which stars form with high radial velocities, could contribute to the morphological evolution of galaxies 12 , to the evolution in size and velocity dispersion of the spheroidal component of galaxies 11 , 13 , and would contribute to the population of high-velocity stars, which could even escape the galaxy 13 . Such star formation could provide in situ chemical enrichment of the circumgalactic and intergalactic medium (through supernova explosions of young stars on large orbits), and some models also predict it to contribute substantially to the star-formation rate observed in distant galaxies 9 . Although there exists observational evidence for star formation triggered by outflows or jets into their host galaxy, as a consequence of gas compression, evidence for star formation occurring within galactic outflows is still missing. Here we report spectroscopic observations that unambiguously reveal star formation occurring in a galactic outflow at a redshift of 0.0448. The inferred star-formation rate in the outflow is larger than 15 solar masses per year. Star formation may also be occurring in other galactic outflows, but may have been missed by previous observations owing to the lack of adequate diagnostics 14 , 15 .