Detection of the Canis Major galaxy at (l;b)=(244;-8) and in the background of Galactic open clusters

Abstract

We report on the detection of Main Sequence stars belonging to the recently identified Canis Major galaxy in a field located at ~ 4.2 deg from the center of the stellar system. With Main Sequence fitting we obtain a distance modulus (m-M)_0=14.5 +/- 0.3 to the dwarf, corresponding to a distance of D_{sun} = 8.0 +/- 1.2 kpc, in full agreement with previous estimates based on the photometric parallax of M-giants. From the comparison with theoretical isochrones we constrain the age of the main population of the Canis Major system in the range \~ 4-10 Gyr. A blue plume of likely younger stars (age < 1-2 Gyr) is also identified. The available Colour Magnitude Diagrams of open clusters that may be projected onto the main body of Canis Major are also briefly analyzed. The position, distance and stellar population of the old open clusters AM-2 and Tombaugh 2 strongly suggest that they are physically associated with the Canis Major galaxy. Using our own photometry and data from 2MASS and the GSC2.2 surveys we demonstrate that the claim by Momany et al. that the CMa overdensity is entirely due to the Galactic warp is not supported by the existing observations, once all the available pieces of information are taken into account. It is shown that the CMa overdensity clearly emerges at a heliocentric distance of ~ 8 kpc above any overdensity possibly produced by the Galactic warp.