Geometry of Star-Forming Galaxies from SDSS, 3D-HST and CANDELS

TL;DR

This study investigates the 3D shapes of star-forming galaxies from redshift 0 to 2.5, revealing that massive galaxies are predominantly disk-shaped across this range, while lower-mass galaxies show increasing elongation at higher redshifts.

Contribution

It extends the understanding of galaxy shapes to higher redshifts and lower masses, showing the evolution from elongated to disk-dominated structures over cosmic time.

Findings

Massive galaxies are mainly oblate disks at all redshifts studied.

Lower-mass galaxies become more elongated at higher redshifts.

Most stars formed in disk galaxies over cosmic history.

Abstract

We determine the intrinsic, 3-dimensional shape distribution of star-forming galaxies at 0<z<2.5, as inferred from their observed projected axis ratios. In the present-day universe star-forming galaxies of all masses 1e9 - 1e11 Msol are predominantly thin, nearly oblate disks, in line with previous studies. We now extend this to higher redshifts, and find that among massive galaxies (M* > 1e10 Msol) disks are the most common geometric shape at all z < 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 1e9 Msol (1e10 Msol) are a mix of roughly equal numbers of elongated and disk galaxies at z~1 (z~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~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.