Probing the Canis Major stellar over-density as due to the Galactic warp

Abstract

Abstract. Proper-motion, star counts and photometric catalog simulations are used to explain the detected stellar over-density in the region of Canis Major, claimed to be the core of a disrupted dwarf galaxy (CMa, Martin et al. 2004, MNRAS, 348, 12; Bellazzini et al. 2004, [arXiv:astro-ph/0311119]), as due to the Galactic warp and flare in the external disk. We compare the kinematics of CMa M-giant selected sample with surrounding Galactic disk stars in the UCAC2 catalog and find no peculiar proper motion signature: CMa stars mimic thick disk kinematics. Moreover, when taking into account the Galactic warp and flare of the disk, 2MASS star count profiles reproduce the CMa stellar over-density. This star count analysis is confirmed by direct comparison with synthetic color−magnitude diagrams simulated with the Besançon models (Robin et al. 2003, A&A, 409, 523) that include the warp and flare of the disk. The presented evidence casts doubt on the identification of the CMa overdensity as the core of a disrupted Milky Way satellite. This however does not make clear the origin of over-densities responsible for the ring structure in the anticenter direction of the Galactic halo (Newberg et al. 2002, ApJ, 569, 245; Yanny et al. 2003, ApJ, 588, 824).

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

“…An alternative scenario presents the Monoceros Ring as the result of a distortion of the Galactic plane (Momany et al 2004(Momany et al , 2006Hammersley & López-Corredoira 2011). These studies suggest that the observed star counts are reproducible considering a flared thick disk without a cut-off at ∼ R 14 kpc.…”

“…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). …”

The QUEST RR Lyrae Survey. II. The Halo Overdensities in the First Catalog

“…There is also strong evidence for another large stream, the Monoceros stream ( Newberg et al 2002;Yanny et al 2003), which forms a ringlike structure around the Milky Way (Ibata et al 2003). Several authors have argued that the parent galaxy of the Monoceros stream lies in the constellation Canis Major, but there is considerable controversy over this association and even the existence of the Canis Major galaxy (e.g., Martin et al 2004;Momany et al 2004;Martínez-Delgado et al 2005;Carraro et al 2005;Peñarrubia et al 2005). These ongoing merger events may be only the most recent examples of a long history of mergers that built the Milky Way from an ensemble of smaller systems, as proposed by the hierarchical picture of galaxy formation (e.g., Bullock & Johnston 2005).…”

Does the second caustic ring of dark matter cause the Monoceros Ring of stars?

“…An alternative proposal is that the Ring is a manifestation of the Galactic warp [10,11]. In this paper we explore a different proposal altogether, namely, that the Monoceros Ring of stars formed as a result of the gravitational forces exerted by the second caustic ring of dark matter in the Milky Way [12].…”