The Closest View of a Dwarf Galaxy: New Evidence on the Nature of the Canis Major Overdensity

Abstract

We present a first deep colour-magnitude diagram of the putative central\nregion (0.5 deg x 0.5 deg) of the Canis Major stellar over-density\n(l,b)=(240,-8) found by Martin et al. (2004), which has been proposed as the\nremnant of a dwarf satellite accreted onto the Milky Way on a near-equatorial\norbit. We find a narrow (in apparent magnitude) main-sequence, extending 6\nmagnitudes below the turn-off to our limiting magnitude of B ~ 24.5 mag. This\nmain sequence has very high constrast (>3) with respect to the thin/thick\ndisk/halo background; its narrowness at brighter magnitudes clearly implies the\npresence of a distinct and possibly still bound stellar system. We derived the\nline-of-sight size (r_{1/2}) of this system based on the B-band width of the\nlower main sequence, obtaining 0.94 +/- 0.18 (random) +/- 0.18 (systematic)\nkpc. That size matches a model prediction for the main body of the parent\ngalaxy of the Monoceros tidal stream. The high density contrast and limited\nspatial extent in the radial direction are very hard to reconcile with the\nalternative explanation put forward to explain the Canis Major\nstellar-overdensity: a flared or warped Galactic disk viewed in projection\n(Momamy et al. 2004). We also derived a central surface brightness of mu_{V,0}=\n23.3 +/- 0.1 mag arcsec^{-2} and an absolute magnitude of M_{V}=-14.5 +/- 0.1\nmag. These values place the Canis Major object in the category of dwarf galaxy\nin the the L_{V}--size and M_V - mu_{V} planes for such objects. However, like\nthe Sagittarius dwarf, it is an outlier in the[Fe/H] -M_V plane in the sense\nthat it is too metal rich for its estimated absolute magnitude. This suggests\nthat the main mechanism driving its recent and current star formation history\n(possibly tidal stripping) is different to that of isolated dwarfs.\n