Astron. Astrophys. 329, 522-537 (1998)

1. Introduction

The presence of large quantities of "dark matter" in spiral galaxies like our own is indicated by their flat rotation curves (e.g. Primack et al, 1988). In our Galaxy, the dynamic mass is thought to be 3 to 8 times larger than the visible mass. According to primordial nucleosynthesis, baryonic dark matter can be up to 10 times more important than visible matter (depending on the value of ): all the Galactic dark matter could thus be baryonic. An excellent candidate would be objects too light to burn hydrogen (m M ) (Carr, 1990, Saumon et al, 1994, Baraffe et al, 1995) and massive enough to have survived during a few billion years (m M ) (de Rújula et al, 1992).

We report here results from a search for unseen compact objects in the Galactic Halo performed by our collaboration "EROS" (Expérience de Recherche d'Objets Sombres) at the European Southern Observatory at La Silla, Chile. Such objects can be detected via the gravitational microlensing effect (Paczynski, 1986) which would lead to an apparent temporary brightening of background stars as the unseen object passes near the line of sight. The time scale for the magnification is proportional to the square root of the deflector mass.

To fully cover the range of possible microlensing time scales, from a few minutes to a few months, EROS has carried out two observing programmes, one using a 16 CCD camera to search for short time scale microlensing phenomena, and the other using Schmidt photographic plates. Here, we focus on the analysis and interpretation of results obtained with the CCD camera. Results obtained with the Schmidt telescope are detailed in (Cavalier, 1994), (Laurent, 1995), (Ansari et al, 1996), and limits obtained by combining both EROS programmes are presented in (Renault et al, 1997).

We first summarize the characteristics of the microlensing signal, in particular the finite source size effect. Then we describe the observational device and the different steps from the CCD images to the search for a signal on the light curves. After computing the number of expected events for a range of models of the Galaxy, we put some constraints on the nature of the dark Halo.

© European Southern Observatory (ESO) 1998

Online publication: December 8, 1997
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