A Canis Major Overdensity Imaging Survey. I. Stellar Content and Star-Count Maps: A Distinctly Elongated Body of Main-Sequence Stars

[Abridged] We present first results from a large-area (~80degx20deg),sparsely sampled two-filter (B,R) imaging survey towards the Canis Majorstellar over-density, claimed to be a disrupting Milky Way satellite galaxy.Utilizing stellar colour-magnitude diagrams reaching to B ~ 22 mag, we providea first delineation of its surface density distribution using main sequencestars. Its projected shape is highly elongated, nearly parallel to the Galacticplane, with an axis ratio of >~ 5:1, substantially more so than what Martin etal. originally found. We also provide a first map of a prominent over-densityof blue, presumably younger main sequence stars, which appears to have amaximum near [l,b = 240,-7 deg]. The young population is markedly morelocalized. We estimate an upper limit on the line-of-sight (l.o.s.) depth ofthe old population based on the main sequence width, obtaining sigma_los < 1.8+/- 0.3 kpc, at an adopted D_helio = 7.5 +/- 1 kpc. For the young stellarpopulation, we find sigma_los < 1.5 kpc. There are different explanations forthe CMa over-density: (a) a partially disrupting dwarf galaxy on a low-latitudeorbit, (b) a projection of the warped outer Galactic disk, and (c) a projectionof an out-of-plane spiral arm. While the data provide no firm arguments againstthe less well-defined third scenario, they have clear implications for each ofthe others: (a) We infer from the strong elongation in longitude, andsimulations in the literature, that the over-density is unlikely to be agravitationally bound system at the present epoch, but may well be just arecently disrupted satellite remnant. (b) Based on modeling, the line-of-sightdepth of the over-density in old stars is clearly inconsistent with publishedlocally axi-symmetric descriptions of the warped Galactic disk.Comment: 27 pages, 15 figures, Submitted to AJ. Several figures are given as JPEGs. High res. version: www.mpia-hd.mpg.de/homes/butler/cma.pdf.g