Letter to the Editor

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

¹ Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 2, 35122 Padova, Italy e-mail: [momany;piotto;carraro;deangeli;bedin]@pd.astro.it
² INAF - Osservatrio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy e-mail: [zaggia;bonifacio]@ts.astro.it

Received: 20 April 2004
Accepted: 25 May 2004

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. [CITE], MNRAS, 348, 12; Bellazzini et al. [CITE], [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.  [CITE], A&A, 409, 523) that include the warp and flare of the disk. The presented evidence casts doubt on the identification of the CMa over-density 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.  [CITE], ApJ, 569, 245; Yanny et al. 2003, ApJ, 588, 824).