Four distant starburst galaxies observed by ALMA

This image shows the four distant starburst galaxies observed by ALMA. The top images depict the 13CO emission from each galaxy, while the bottom ones show their C18O emission. The ratio of these two isotopologues allowed astronomers to determine that these starburst galaxies have an excess of massive stars. Credit: ESO/Zhang et al.; ALMA (ESO/NAOJ/NRAO).