The kinematic identification of a thick stellar disc in M31

We present the first characterization of a thick-disc component in the Andromeda galaxy (M31) using kinematic data from the DEIMOS instrument on Keck II. Using 21 fields in the south-west of the galaxy, we measure the lag of this component with respect to the thin disc, as well as the dispersion, metallicity and scalelength of the component. We find an average lag between the two components of <> = 46.0 +/- 3.9 km s-1. The velocity dispersion of the thick disc is Sigma(thick) = 50.8 +/- 1.9 km s-1, greater than the value of dispersion we determine for the thin disc, Sigma(thin) = 35.7 +/- 1.0 km s-1. The thick disc is more metal poor than the thin disc, with [Fe/H](spec) = -1.0 +/- 0.1 compared with [Fe/H](spec) = -0.7 +/- 0.05 for the thin disc. We measure a radial scalelength of the thin and thick discs of h(r) = 7.3 +/- 1.0 and 8.0 +/- 1.2 kpc, respectively. From this, we infer scaleheights for both discs of 1.1 +/- 0.2 and 2.8 +/- 0.6 kpc, both of which are approximately two to three times larger than those observed in the Milky Way. We estimate a mass range for the thick-disc component of 2.4 x 1010 < M-*,M- thick < 4.1 x 1010 M-circle dot. This value provides a useful constraint on possible formation mechanisms, as any proposed method for forming a thick disc must be able to heat (or deposit) at least this amount of material.