Widespread star formation inside galactic outflows

TL;DR

This study provides observational evidence that star formation occurs inside galactic outflows in a significant fraction of galaxies, potentially impacting galaxy evolution, chemical enrichment, and cosmic reionization.

Contribution

It is the first large-scale investigation confirming widespread star formation inside galactic outflows using integral field spectroscopy data.

Findings

Star formation occurs inside at least half of the galactic outflows in the sample.

Star formation in outflows could reach several hundred solar masses per year at high redshift.

Star formation in outflows may influence galaxy scaling relations and cosmic reionization.

Abstract

Several models have predicted that stars could form inside galactic outflows and that this would be a new major mode of galaxy evolution. Observations of galactic outflows have revealed that they host large amounts of dense and clumpy molecular gas, which provide conditions suitable for star formation. We have investigated the properties of the outflows in a large sample of galaxies by exploiting the integral field spectroscopic data of the large MaNGA-SDSS4 galaxy survey. We find that star formation occurs inside at least half of the galactic outflows in our sample. We also show that even if star formation is prominent inside many other galactic outflows, this may have not been revealed as the diagnostics are easily dominated by the presence of even faint AGN and shocks. If very massive outflows typical of distant galaxies and quasars follow the same scaling relations observed locally, then the star formation inside high-z outflows can be up to several 100 Msun/yr and could contribute substantially to the early formation of the spheroidal component of galaxies. Star formation in outflows can also potentially contribute to establishing the scaling relations between black holes and their host spheroids. Moreover, supernovae exploding on large orbits can chemically enrich in-situ and heat the circumgalactic and intergalactic medium. Finally, young stars ejected on large orbits may also contribute to the reionization of the Universe.