Absolute Proper Motion of the Canis Major Dwarf Galaxy Candidate

Abstract

We have measured the absolute proper motion of the candidate Canis Major dwarf galaxy (CMa) at $(l,b) = (240\arcdeg, -8\arcdeg)$. Likely main-sequence stars in CMa have been selected from a region in the color-magnitude diagram that has very little contamination from known Milky Way components. We obtain ${\mu }_{l}cosb=-1.47\pm 0.37$ and ${\mu }_b=-1.07\pm 0.38$ mas yr${}^{-1}$, on the ICRS system via Hipparcos stars. Together with the radial velocity of 109 km s${}^{-1}$, and the assumed distance of 8 kpc, these results imply a space motion of $(\Pi ,\Theta ,W)=(-5\pm 12,188\pm 10,-49\pm 15)$ km s${}^{-1}$. While CMa has in-plane rotation similar to the mean of thick disk stars, it shows significant $(3\sigma )$ motion perpendicular to the disk, and differs even more $(7\sigma )$ from that expected for the Galactic warp. The $W$ velocity lends support to the argument that the CMa overdensity is part of a satellite galaxy remnant.

arXiv comment12 pages, 3 figures, accepted for publication in ApJ Letters

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