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

Abstract

We present the first deep color-magnitude diagram of the putative central region (0N5 ; 0N5) of the Canis Major stellar overdensity (l; b) ¼ (240; À8) found recently by Martin and coworkers, which has been proposed as the remnant of a dwarf satellite accreted onto the Milky Way on a near-equatorial orbit. We find a narrow (in apparent magnitude) main sequence extending 6 mag below the turnoff to our limiting magnitude of B $ 24:5 mag. This main sequence has very high contrast (>3) with respect to the thin/thick disk/halo background; its narrowness at brighter magnitudes clearly implies the presence of a distinct and possibly still bound stellar system. We derived the line-of-sight size (r 1/2 ) of this system based on the B-band width of the lower main sequence, obtaining 0:94 AE 0:18 (random) AE 0:18 (systematic) kpc. That size matches a model prediction for the main body of the parent galaxy of the Monoceros tidal stream. The high-density contrast and limited spatial extent in the radial direction are very hard to reconcile with the alternative explanation put forward to explain the Canis Major stellar overdensity: a flared or warped Galactic disk viewed in projection, as found in the recent work of Momany and coworkers. We also derived a central surface brightness of V ;0 ¼ 23:3 AE 0:1 mag arcsec À2 and an absolute magnitude of M V ¼ À14:5 AE 0:1 mag. These values place the Canis Major object in the category of dwarf galaxy, considering the L V -size and M V À V planes for such objects. However, like the Sagittarius dwarf, it is an outlier in the ½Fe/ H À M V plane in the sense that it is too metal-rich for its estimated absolute magnitude. This suggests that the main mechanism driving its recent and current star formation history (possibly tidal stripping) is different from that of isolated dwarfs.

A Helium Spread among the Main‐Sequence Stars in NGC 2808

“…This is particularly relevant in the present case, since the cluster may be physically associated with the newly discovered Canis Major galaxy , which in any case is projected in the same area of sky as this stellar system, whose optical CMD displays a narrow and well-defined MS with a TO around V ' 19:0 Martinez-Delgado et al 2005). The only way to assess this point with our data is to compare the radial distribution of blue MS stars with that of ordinary MS cluster stars.…”

A Search for a Dark Matter Annihilation Signal Toward the Canis Major Overdensity With H.E.S.S.

“…It is a very extended object (Δl = 12 • , Δb = 10 • ) with a roundish core approximately centered at l = 240 • and b = −8 • according to various star surveys in this region (Martin et al 2004;Martinez-Delgado et al 2005). In contrast to the Sgr dSph, neither dispersion velocity measurements nor luminosity profiles are available, so an accurate modeling of the CMa DM halo profile is not possible.…”

A Megacam Survey of Outer Halo Satellites. Iv. Two Foreground Populations Possibly Associated With the Monoceros Substructure in the Direction of NGC 2419 and Koposov 2

“…One of the leading interpretations is that the Monoceros Ring is the remnant of a past accretion event (e.g., Conn et al 2005;Jurić et al 2008;Chou et al 2010;Sollima et al 2011), similar to that generated by the disruption of the Sagittarius (Sgr) dwarf galaxy, which is orbiting around our Galaxy in an almost polar orbit (e.g., Ibata et al 1994;Majewski et al 2003;Bonifacio et al 2004;Martínez-Delgado et al 2004;Martin et al 2004;Bellazzini et al 2006;Belokurov et al 2006;Siegel et al 2007;Koposov et al 2012). In contrast to Sgr, Monoceros lacks a known progenitor system, though it has been proposed and later discarded that the Canis Major overdensity is the accreted system that formed Monoceros (Martin et al 2004;Momany et al 2004Momany et al , 2006Martínez-Delgado et al 2005;Bellazzini et al 2006;Moitinho et al 2006;Butler et al 2007;Mateu et al 2009). …”