We present the first characterization of a thick disc component in the Andromeda galaxy ( M31 ) using kinematic data from the DEIMOS multi-object spectrograph 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 scale length of the component .
We find an average lag between the two components of \langle \Delta v \rangle = 46.0 \pm 3.9 { km s ^ { -1 } } .
The velocity dispersion of the thick disc is \sigma _ { thick } = 50.8 \pm 1.9 { km s ^ { -1 } } , greater than the value of dispersion we determine for the thin disc , \sigma _ { thin } = 35.7 \pm 1.0 { km s ^ { -1 } } .
The thick disc is more metal poor than the thin disc , with [ Fe/H ] _ { spec } = -1.0 \pm 0.1 compared to [ Fe/H ] _ { spec } = -0.7 \pm 0.05 for the thin disc .
We measure a radial scale length of the thin and thick discs of h _ { r } = 7.3 \pm 1.0 kpc and h _ { r } = 8.0 \pm 1.2 kpc .
From this , we infer scale heights for both discs of 1.1 \pm 0.2 kpc and 2.8 \pm 0.6 kpc , both of which are \sim 2–3 times larger than those observed in the Milky Way .
We estimate a mass range for the thick disc component of 2.4 \times 10 ^ { 10 } { M _ { \odot } } < M _ { *,thick } < 4.1 \times 10 ^ { 10 } { M _ { \odot } } .
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 .