Systematics in virial mass estimators for pressure-supported systems
  Mass estimators are a key tool to infer the dark matter content in
pressure-supported systems like dwarf spheroidal galaxies (dSphs). We construct
an estimator for enclosed masses based on the virial theorem which is
insensitive to anisotropy in the velocity dispersion and tailored to yield
masses with minimum uncertainty introduced by our ignorance on (i) the shape of
the inner halo profile, and (ii) how deeply the stellar component is embedded
within the halo: $M(<1.8\,R_\mathrm{h}) \approx 3.5 \times 1.8\,R_\mathrm{h}
\langle \sigma_\mathrm{los}^2 \rangle G^{-1}$, where by $R_\mathrm{h}$ we
denote the projected half-light radius and by $\langle \sigma_\mathrm{los}^2
\rangle$ the luminosity-averaged squared line-of-sight velocity dispersion.
Tests against controlled simulations show that this estimator provides unbiased
enclosed masses with an accuracy of $\sim 10$ per cent. This confirms the
robustness of similar previously proposed mass estimators. Application to
published kinematic data of Milky Way dSphs reveals a tight correlation between
enclosed mass and luminosity. Using $N$-body models we show that tidal
stripping has little effect on this relation. Comparison against cuspy and
cored dark matter haloes extracted from controlled re-simulations of the
Aquarius A2 merger tree shows that the high mass densities of ultrafaint
galaxies are not compatible with large dark matter cores, and that the (total)
halo masses of the classical Milky Way dSphs span a remarkably narrow range ($8
\lesssim \mathrm{log_{10}}\,(M/\mathrm{M_\odot}) \lesssim 10$) at present,
showing no clear trend with either galaxy size or luminosity.
