No Excess of RR Lyrae Stars in the Canis Major Overdensity

TL;DR

A survey of the Canis Major overdensity found no excess RR Lyrae stars, suggesting the overdensity is likely due to Milky Way disk and spiral arm populations rather than a dwarf galaxy.

Contribution

This study provides the first extensive survey of RR Lyrae stars in Canis Major, challenging the dwarf galaxy hypothesis for its origin.

Findings

Number of RR Lyrae stars detected is consistent with Galactic halo and disk populations.

No excess RR Lyrae stars found to support the dwarf galaxy hypothesis.

Supports the idea that the overdensity is due to Milky Way structures.

Abstract

Our multi-epoch survey of ~20 sq. deg. of the Canis Major overdensity has detected only 10 RR Lyrae stars (RRLS). We show that this number is consistent with the number expected from the Galactic halo and thick disk populations alone, leaving no excess that can be attributed to the dwarf spheroidal (dSph) galaxy that some authors have proposed as the origin of the CMa overdensity. If this galaxy resembles the dSph satellites of the Milky Way and of M31 and has the putative Mv~-14.5, our survey should have detected several tens of RRLS. Even if Mv<-12, the expected excess is >10, which is not observed. Either the old stellar population of this galaxy has unique properties or, as others have argued before, the CMa overdensity is produced by the thin and thick disk and spiral arm populations of the Milky Way and not by a collision with a dSph satellite galaxy.