Geometry of star-forming galaxies from SDSS, 3D-HST, and CANDELS

We determine the intrinsic, three-dimensional shape distribution of star-forming galaxies at 0 10(10) M-circle dot) disks are the most common geometric shape at all z less than or similar to 2. Lower-mass galaxies at z > 1 possess a broad range of geometric shapes: the fraction of elongated (prolate) galaxies increases toward higher redshifts and lower masses. Galaxies with stellar mass 10(9) M-circle dot (10(10) M-circle dot) are a mix of roughly equal numbers of elongated and disk galaxies at z similar to 1 (z similar to 2). This suggests that galaxies in this mass range do not yet have disks that are sustained over many orbital periods, implying that galaxies with present-day stellar mass comparable to that of the Milky Way typically first formed such sustained stellar disks at redshift z similar to 1.5-2. Combined with constraints on the evolution of the star formation rate density and the distribution of star formation over galaxies with different masses, our findings imply that, averaged over cosmic time, the majority of stars formed in disks.We determine the intrinsic, three-dimensional shape distribution of star-forming galaxies at 0 10(10) M-circle dot) disks are the most common geometric shape at all z less than or similar to 2. Lower-mass galaxies at z > 1 possess a broad range of geometric shapes: the fraction of elongated (prolate) galaxies increases toward higher redshifts and lower masses. Galaxies with stellar mass 10(9) M-circle dot (10(10) M-circle dot) are a mix of roughly equal numbers of elongated and disk galaxies at z similar to 1 (z similar to 2). This suggests that galaxies in this mass range do not yet have disks that are sustained over many orbital periods, implying that galaxies with present-day stellar mass comparable to that of the Milky Way typically first formed such sustained stellar disks at redshift z similar to 1.5-2. Combined with constraints on the evolution of the star formation rate density and the distribution of star formation over galaxies with different masses, our findings imply that, averaged over cosmic time, the majority of stars formed in disks.A