Recent observations have revealed massive galactic molecular outflows1,2,3 that may have the physical conditions (high gas densities4,5,6) required to form stars. Indeed, several recent models predict that such massive outflows may ignite star formation within the outflow itself7,8,9,10,11. This star-formation mode, in which stars form with high radial velocities, could contribute to the morphological evolution of galaxies12, to the evolution in size and velocity dispersion of the spheroidal component of galaxies11,13, and would contribute to the population of high-velocity stars, which could even escape the galaxy13. 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 galaxies9. 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 diagnostics14,15.
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R.M., S.Car. and F.B. acknowledge support by the Science and Technology Facilities Council (STFC). R.M. acknowledges ERC Advanced Grant 695671 “QUENCH”. H.R.R. and A.C.F. acknowledge ERC Advanced Grant 340442. S.A., S.Caz., E.B. and L.C. acknowledge support from the Spanish Ministry of Economy, under grants AYA2012-32295 and ESP2015-68964-P.
The authors declare no competing financial interests.
Reviewer Information Nature thanks M. Sarzi, K. Zubovas and the other anonymous reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
a, Spectrum around the [P ii] λ = 1.188 μm line. The fitting components have the same colour coding as in Fig. 1b. b, Spectrum around the expected wavelength of [Si vi] at λ = 1.96 μm. The expected locations of the broad and narrow components of the [Si vi] line are marked. A feature corresponding to the expected location of the broadest component is observed, but is much narrower than the width of the same component observed in other nebular lines, and is at the location of an atmospheric absorption dip.
Although the quality of the MUSE data are not adequate to extract a very reliable stellar velocity field, a and b show the distribution of the stellar continuum around λ = 9,000 Å and the velocity field inferred from the reddest line of the Ca ii triplet, by applying Voronoi binning to the MUSE cube. b also shows the seeing difference between the X-shooter observation and the MUSE observation. c shows a rotation velocity fit to the southern galaxy, by masking the region to the Northeast (around the X-shooter slit), which is probably affected by outflowing stars.
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Maiolino, R., Russell, H., Fabian, A. et al. Star formation inside a galactic outflow. Nature 544, 202–206 (2017). https://doi.org/10.1038/nature21677
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