The galaxy-halo size relation of low-mass galaxies in FIRE

Rohr et al., available on arXiv

Abstract: Galaxy sizes correlate closely with the sizes of their parent dark matter haloes, suggesting a link between halo formation and galaxy growth. However, the precise nature of this relation and its scatter remains to be understood fully, especially for low-mass galaxies. We analyse the galaxy-halo size relation for low-mass (Mstar~10^(7−9) Msun) central galaxies over the past 12.5 billion years with the help of cosmological volume simulations (FIREbox) from the Feedback in Realistic Environments (FIRE) project. We find a nearly linear relationship between the half-stellar mass galaxy size R1/2 and the parent dark matter halo virial radius Rvir. This relation evolves only weakly since redshift z=5: R1/2kpc=(0.053+/-0.002)(Rvir/35kpc)^(0.934+/-0.054), with a nearly constant scatter =0.084 dex. Whilst this ratio is similar to what is expected from models where galaxy disc sizes are set by halo angular momentum, the low-mass galaxies in our sample are not angular momentum supported, with stellar rotational to circular velocity ratios vrot/vcirc~0.15. Introducing redshift as another parameter to the GHSR does not decrease the scatter. Furthermore, this scatter does not correlate with any of the halo properties we investigate — including spin and concentration — suggesting that baryonic processes and feedback physics are instead critical in setting the scatter in the galaxy-halo size relation. Given the relatively small scatter and the weak dependence of the galaxy-halo size relation on redshift and halo properties for these low-mass central galaxies, we propose using galaxy sizes as an independent method from stellar masses to infer halo masses.