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

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 (𝑀 ★ ∼ 10 7−9 M ) 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 𝑅 1/2 and the parent dark matter halo virial radius 𝑅 vir . This relation evolves only weakly since redshift 𝑧 = 5: 𝑅 1/2 [kpc] = (0.053 ± 0.002) (𝑅 vir /35 kpc) 0.934±0.054 , with a nearly constant scatter 𝜎 = 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 𝑣 rot /𝑣 circ ∼ 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 concentrationsuggesting 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.