A revised view of the Canis Major stellar overdensity with DECam and Gaia: new evidence of a stellar warp of blue stars

TL;DR

This study combines DECam imaging and Gaia data to re-evaluate the Canis Major overdensity, revealing it as part of the Milky Way's warped disk rather than a separate dwarf galaxy.

Contribution

It provides new evidence that the Canis Major overdensity is due to the Milky Way's warp, not an accreted dwarf galaxy, using combined imaging and astrometric data.

Findings

Canis Major is part of the Milky Way's warped disk.

Blue Plume stars are widespread, not confined to the overdensity.

The structure's orbit suggests on-plane rotation around the Galaxy.

Abstract

We present DECam imaging combined with Gaia DR2 data to study the Canis Major overdensity. The presence of the so-called Blue Plume stars in a low-pollution area of the color-magnitude diagram allows us to derive the distance and proper motions of this stellar feature along the line of sight of its hypothetical core. The stellar overdensity extends on a large area of the sky at low Galactic latitudes, below the plane, and between 230$^\circ < \ell < 255^\circ$. According to the orbit derived for Canis Major, it presents an on-plane rotation around the Milky Way. Moreover, additional overdensities of Blue Plume stars are found around the plane and across the Galaxy, proving that these objects are not only associated with that structure. The spatial distribution of these stars, derived using Gaia astrometric data, confirms that the detection of the Canis Major overdensity results more from the warped structure of the Milky Way disk than from the accretion of a dwarf galaxy.