Determining the Motion of the Local Group Using SN Ia Light Curve Shapes

Abstract

We have measured our Galaxy's motion relative to distant galaxies in which type Ia supernovae (SN Ia) have been observed. The effective recession velocity of this sample is 7000 km s$^{-1}$, which approaches the depth of the survey of brightest cluster galaxies by Lauer and Postman (1994). We use the Light Curve Shape (LCS) method for deriving distances to SN Ia, providing relative distance estimates to individual supernovae with a precision of $\sim$ 5\% (Riess, Press, \& Kirshner 1995). Analyzing the distribution on the sky of velocity residuals from a pure Hubble flow for 13 recent SN Ia drawn primarily from the Cal\'{a}n/Tololo survey (Hamuy 1993a, 1994, 1995a, 1995b, Maza et al. 1994), we find the best solution for the motion of the Local Group in this frame is 600 $\pm 350$ km s$^{-1}$ in the direction b=260$\deg$ {\it l}=+54$\deg$ with a 1 $\sigma$ error ellipse that measures 90$\deg$ by 25$\deg$. This solution is consistent with the rest frame of the cosmic microwave background (CMB) as determined by the Cosmic Background Explorer (COBE) measurement of the dipole temperature anisotropy (Smoot et al. 1992). It is inconsistent with the velocity observed by Lauer and Postman.