The evolution of the baryon fraction in halos as a cause of scatter in
  the galaxy stellar mass in the EAGLE simulation
  The EAGLE simulation suite has previously been used to investigate the
relationship between the stellar mass of galaxies, $M_{*}$, and the properties
of dark matter halos, using the hydrodynamical reference simulation combined
with a dark matter only (DMO) simulation having identical initial conditions.
The stellar masses of central galaxies in halos with $M_{\mathrm{200c}} >
10^{11} \mathrm{M_{\odot}}$ were shown to correlate with the DMO halo maximum
circular velocity, with $\approx 0.2$ dex of scatter that is uncorrelated with
other DMO halo properties. Here we revisit the origin of the scatter in the
$M_{*}-V_{\mathrm{max, DMO}}$ relation in EAGLE at $z = 0.1$. We find that the
scatter in $M_{*}$ correlates with the mean age of the galaxy stellar
population such that more massive galaxies at fixed $V_{\mathrm{max, DMO}}$ are
younger. The scatter in the stellar mass and mean stellar population age
results from variation in the baryonic mass, $M_{\mathrm{bary}} =
M_{\mathrm{gas}} + M_{*}$, of the galaxies' progenitors at fixed halo mass and
concentration. At the redshift of peak correlation ($z \approx 1$), the
progenitor baryonic mass accounts for $75\%$ of the variance in the $z=0.1$
$M_{*}-V_{\mathrm{max, DMO}}$ relation. The scatter in the baryonic mass, in
turn, is primarily set by differences in feedback strength and gas accretion
over the course of the evolution of each halo.
