H0 Revisited
  I reanalyse the Riess et al. (2011, hereafter R11) Cepheid data using the
revised geometric maser distance to NGC 4258 of Humphreys et al. (2013). I
explore different outlier rejection criteria designed to give a reduced
chi-squared of unity and compare the results with the R11 rejection algorithm,
which produces a reduced chi-squared that is substantially less than unity and,
in some cases, to underestimates of the errors on parameters. I show that there
are sub-luminous low metallicity Cepheids in the R11 sample that skew the
global fits of the period-luminosity relation. This has a small but
non-negligible impact on the global fits using NGC 4258 as a distance scale
anchor, but adds a poorly constrained source of systematic error when using the
Large Magellanic Cloud (LMC) as an anchor. I also show that the small Milky Way
(MW) Cepheid sample with accurate parallax measurements leads to a distance to
NGC 4258 that is in tension with the maser distance. I conclude that H0 based
on the NGC 4258 maser distance is H0 = 70.6 +/- 3.3 km/s/Mpc compatible within
1 sigma with the recent determination from Planck for the base six-parameter
LCDM cosmology. If the H-band period-luminosity relation is assumed to be
independent of metallicity and the three distance anchors are combined, I find
H0 = 72.5 +/- 2.5 km/s/Mpc, which differs by 1.9 sigma from the Planck value.
The differences between the Planck results and these estimates of H0 are not
large enough to provide compelling evidence for new physics at this stage.
