The Relation Between Ejecta Velocity, Intrinsic Color, and Host-Galaxy
  Mass for High-Redshift Type Ia Supernovae
  Recently, using a large low-redshift sample of Type Ia supernovae (SNe Ia),
we discovered a relation between SN Ia ejecta velocity and intrinsic color that
improves the distance precision of SNe Ia and reduces potential systematic
biases related to dust reddening. No SN Ia cosmological results have yet made a
correction for the "velocity-color" relation. To test the existence of such a
relation and constrain its properties at high redshift, we examine a sample of
75 SNe Ia discovered and observed by the Sloan Digital Sky Survey-II (SDSS-II)
Supernova Survey and Supernova Legacy Survey (SNLS). From each spectrum, we
measure ejecta velocities at maximum brightness for the Ca H&K and Si II 6355
features, v_Ca^0 and v_Si^0, respectively. Using SN light-curve parameters, we
determine the intrinsic B_max - V_max for each SN. Similar to what was found at
low-redshift, we find that SNe Ia with higher ejecta velocity tend to be
intrinsically redder than SNe Ia with lower ejecta velocity. The distributions
of ejecta velocities for SNe Ia at low and high redshift are similar,
indicating that current cosmological results should have little bias related to
the velocity-color relation. Additionally, we find a slight (2.4-sigma
significant) trend between SN Ia ejecta velocity and host-galaxy mass such that
SNe Ia in high-mass host galaxies tend to have lower ejecta velocities as
probed by v_Ca^0. These results emphasize the importance of spectroscopy for SN
Ia cosmology.
