Spreading out and staying sharp - Creating diverse rotation curves via
  baryonic and self-interaction effects
  Galactic rotation curves are a fundamental constraint for any cosmological
model. We use controlled N-body simulations of galaxies to study the
gravitational effect of baryons in a scenario with collisionless cold dark
matter (CDM) versus one with a self-interacting dark matter (SIDM) component.
In particular, we examine the inner profiles of the rotation curves in the
velocity range Vmax = 30-250 km/s, whose diversity has been found to be greater
than predicted by the Lambda-CDM scenario. We find that the scatter in the
observed rotation curves exceeds that predicted by dark matter only
mass-concentration relations in either the CDM nor SIDM models. Allowing for
realistic baryonic content and spatial distributions, however, helps create a
large variety of rotation curve shapes; which is in better agreement with
observations in the case of self-interactions due to the characteristic cored
profiles being more accommodating to the slowly rising rotation curves than
CDM. We find individual fits to model two of the most remarkable outliers of
similar Vmax, UGC 5721 and IC 2574, the former a cusp-like rotation curve and
the latter a seemingly 8 kpc cored profile. This diversity in SIDM arises as
permutations of overly concentrated halos with compact baryonic distributions
versus underdense halos with extended baryonic disks. The SIDM solution is
promising and its feasibility ultimately depends on the sampling of the halo
mass-concentration relation and its interplay with the baryonic profiles,
emphasising the need for a better understanding of the frequency of extreme
outliers present in current observational samples.
