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

We report on the detection of main-sequence stars belonging to the recently identified Canis Major (CMa) galaxy in a field located at similar or equal to4.degrees2 from the centre 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-circle dot similar or equal to 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 CMa system in the range similar to4-10 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 on to the main body of CMa are also briefly analysed. The position, distance and stellar population of the old open clusters Arp-Madore 2 and Tombaugh 2 strongly suggest that they are physically associated with the CMa galaxy. Using our own photometry and data from the Two-Micron All-Sky Survey and the Guide Star Catalogue 2.2 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.