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 ($M_{\star }\sim 10^{7-9}{\mathrm{M}}_{\odot }$) 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 $R_{\mathrm{vir}}$. This relation evolves only weakly since redshift $z=5$: $R_{1/2}\mathrm{kpc}=(0.053\pm 0.002)(R_{\mathrm{vir}}/35\mathrm{kpc}{)}^{0.934\pm 0.054}$, with a nearly constant scatter $⟨\sigma ⟩=0.084[\mathrm{dex}]$. 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 $v_{\mathrm{rot}}/v_{\mathrm{circ}}\sim 0.15$. 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.

14 pages, 9 figures, 3 tables + 3 appendices (5 pages, 5 figures, 2 tables). Accepted for publication in MNRAS. Reuploaded after proofing stage

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