A canis major overdensity imaging survey.: I.: Stellar content and star-count maps:: A distinctly elongated body of main-sequence stars

We present the first results from a large-area (similar to 80 degrees x 20 degrees), sparsely sampled, two-filter (B and R) imaging survey toward the Canis Major stellar overdensity, which is claimed to be a disruptingMilkyWay satellite galaxy. Using stellar color-magnitude diagrams reaching to B similar to 22 mag, we provide a first delineation of its surface density distribution using main-sequence stars. It is located below the Galactic midplane, and can be discerned to at least b = - 15 degrees. Its projected shape is highly elongated, nearly parallel to the Galactic plane, with an axis ratio of at least 5.1, substantially more so than what Martin and coworkers originally found. We also provide a first map of a prominent overdensity of blue, presumably younger main-sequence stars, which extends in latitude to b similar to -10 degrees. We estimate an upper limit on the line-of-sight depth sigma(los) of the old population based on the main-sequence width, obtaining sigma(los) < 1.8 +/- 0.3 kpc at an adopted D circle dot = 7.5 +/- 1 kpc. For the young stellar population, we find sigma(los) < 1: 5 kpc. The overall picture presented is one of a young stellar population that is less extended, both in terms of its line-of-sight depth and angular size, than the older population. While the data provide no firm arguments against an out-of-plane spiral arm interpretation, the data provide clear implications for others: (1) We infer from the strong elongation of the overdensity in longitude, and simulations in the literature, that the CMa overdensity is unlikely to be a gravitationally bound system at the present epoch, but may well be just a recently disrupted satellite remnant. The possible '' flattening '' of the youngmain-sequence population may, however, be a complexity for the satellite origin. (2) Based on modeling, the line-of-sight depth of the main-sequence overdensity in old stars is clearly inconsistent with published locally axisymmetric descriptions of the warped Galactic disk, such as those considered by Momany and coworkers. Without detailed modeling, the data set itself does not allow a distinction between interpretations as substructure in the warped outer Galactic disk or a disrupted satellite.