Baryon effects on the internal structure of LCDM halos in the EAGLE
  simulations
  We investigate the internal structure and density profiles of halos of mass
$10^{10}-10^{14}~M_\odot$ in the Evolution and Assembly of Galaxies and their
Environment (EAGLE) simulations. These follow the formation of galaxies in a
$\Lambda$CDM Universe and include a treatment of the baryon physics thought to
be relevant. The EAGLE simulations reproduce the observed present-day galaxy
stellar mass function, as well as many other properties of the galaxy
population as a function of time. We find significant differences between the
masses of halos in the EAGLE simulations and in simulations that follow only
the dark matter component. Nevertheless, halos are well described by the
Navarro-Frenk-White (NFW) density profile at radii larger than ~5% of the
virial radius but, closer to the centre, the presence of stars can produce
cuspier profiles. Central enhancements in the total mass profile are most
important in halos of mass $10^{12}-10^{13}M_\odot$, where the stellar fraction
peaks. Over the radial range where they are well resolved, the resulting galaxy
rotation curves are in very good agreement with observational data for galaxies
with stellar mass $M_*<5\times10^{10}M_\odot$. We present an empirical fitting
function that describes the total mass profiles and show that its parameters
are strongly correlated with halo mass.
