Confirmation of a Star Formation Bias in Type Ia Supernova Distances and
  its Effect on Measurement of the Hubble Constant
  Previously we used the Nearby Supernova Factory sample to show that SNe~Ia
having locally star-forming environments are dimmer than SNe~Ia having locally
passive environments.Here we use the \constitution\ sample together with host
galaxy data from \GALEX\ to independently confirm that result. The effect is
seen using both the SALT2 and MLCS2k2 lightcurve fitting and standardization
methods, with brightness differences of $0.094 \pm 0.037\ \mathrm{mag}$ for
SALT2 and $0.155 \pm 0.041\ \mathrm{mag}$ for MLCS2k2 with $R_V=2.5$. When
combined with our previous measurement the effect is $0.094 \pm 0.025\
\mathrm{mag}$ for SALT2. If the ratio of these local SN~Ia environments changes
with redshift or sample selection, this can lead to a bias in cosmological
measurements. We explore this issue further, using as an example the direct
measurement of $H_0$. \GALEX{} observations show that the SNe~Ia having
standardized absolute magnitudes calibrated via the Cepheid period--luminosity
relation using {\textit{HST}} originate in predominately star-forming
environments, whereas only ~50% of the Hubble-flow comparison sample have
locally star-forming environments. As a consequence, the $H_0$ measurement
using SNe~Ia is currently overestimated. Correcting for this bias, we find a
value of $H_0^{corr}=70.6\pm 2.6\ \mathrm{km\ s^{-1}\ Mpc^{-1}}$ when using the
LMC distance, Milky Way parallaxes and the NGC~4258 megamaser as the Cepheid
zeropoint, and $68.8\pm 3.3\ \mathrm{km\ s^{-1}\ Mpc^{-1}}$ when only using
NGC~4258. Our correction brings the direct measurement of $H_0$ within $\sim
1\,\sigma$ of recent indirect measurements based on the CMB power spectrum.
