A radial velocity survey of low Galactic latitude structures - I. Kinematics of the Canis Major dwarf galaxy

As part of a radial velocity survey of low Galactic latitude structures that we undertook with the 2dF spectrograph on the Anglo-Australian Telescope, we present the radial velocities of more than 1500 red giant branch and red clump stars towards the centre of the Canis Major dwarf galaxy. With a mean velocity of 72 +/- 7 km s(-1) at a heliocentric distance of 5.5 kpc and 114 +/- 2 km s(-1) at 8.5 kpc, these stars present a peculiar distance - radial velocity relation that is unlike that expected from thin or thick disc stars. Moreover, they belong to a kinematically cold population with an intrinsic dispersion that may be as low as 11(-1)(+3) km s(-1). A comparison of the velocity distribution obtained in this work with previous studies shows the importance of using our new reduction pipeline and averaging the velocities obtained from different templates. The radial velocity distribution is used to select Canis Major stars in the UCAC2.0 proper motion catalogue and derive proper motions in Galactic coordinates of (mu(l), mu(b)) = (-3.6 +/- 0.8 mas yr(-1), 1.5 +/- 0.4 mas yr(-1)) for the dwarf galaxy, which after correcting for the reflex solar motion along this line of sight gives (mu'(l), mu'(b)) = (-6.8 +/- 0.8 mas yr(-1), 0.8 +/- 0.4 mas yr(-1)), corresponding to a prograde orbit with a tangential velocity of similar to 235 km s(-1) at the average distance of similar to 7.2 kpc. All these kinematic constraints can be reproduced in simulations of the accretion of a dwarf on to the Galactic disc. Such a process could also be responsible for the Monoceros Ring that has recently been shown to encompass the Galactic disc. However, without constraints on the kinematics of the tidal anus emerging from the Canis Major dwarf, it is not yet possible to definitively prove a link between the two structures.