The Sagittarius dwarf galaxy as a microlensing target

¹ DASGAL, Observatoire de Paris, 61 avenue de l'Observatoire, 75014 Paris, France
² Centre d'Analyse des Images - INSU, France
³ Institut d'Astrophysique de Paris, 98bis Boulevard Arago, 75014 Paris, France

Corresponding author: P. Cseresnjes, patrick.cseresnjes@obspm.fr

Received: 22 June 2000
Accepted: 23 January 2001

Abstract

We estimate the optical depth, time-scale distribution and fraction of microlensing events originating from sources in the Sagittarius dwarf galaxy (Sgr) lensed by deflectors in the Milky Way. These events have a time-scale longer by a factor ~1.3 than the MW/MW events and occur mainly on sources fainter than mag below Sgr's turn off. The fraction of events involving a source in Sgr depends on the location and extinction of the field and on the limiting magnitude of the survey. The contribution of the MW/Sgr events is negligible (1%) at very low latitudes (2°) but increases continuously towards higher and becomes dominant near the highest density region of the dwarf galaxy. Sgr is present within the fields of current microlensing surveys and any optical depth map inferred from observations will become biased by the presence of Sgr towards higher where the contribution of MW/Sgr events is significant. Systematic spectroscopic measurements on the sources of all the microlensing events may allow detection of this kind of event for which the degeneracy on the lens mass can be significantly reduced.