Michel Spiro

Evidence for gravitational microlensing by dark objects in the Galactic halo

THE flat rotation curves of spiral galaxies, including our own, indicate that they are surrounded by unseen haloes of ‘dark matter’1,2. In the absence of a massive halo, stars and gas in the outer portions of a galaxy would orbit the centre more slowly, just as the outer planets in the Solar System circle the Sun more slowly than the inner ones. So far, however, there has been no direct observational evidence for the dark matter, or its characteristics. Paczyński3suggested that dark bodies in the halo of our Galaxy can be detected when they act as gravitational ‘microlenses’, amplifying the light from stars in nearby galaxies. The duration of such an event depends on the mass, distance and velocity of the dark object. We have been monitoring the brightness of three million stars in the Large Magellanic Cloud for over three years, and here report the detection of two possible microlensing events. The brightening of the stars was symmetrical in time, achromatic and not repeated during the monitoring period. The timescales of the two events are about thirty days and imply that the masses of the lensing objects lie between a few hundredths and one solar mass. The number of events observed is consistent with the number expected if the halo is dominated by objects with masses in this range.

The EROS2 search for microlensing events towards the spiral arms: the complete seven season results

the EROS-2 project has been designed to search for microlensing events towards any dense stellar field. The densest parts of the Galactic spiral arms have been monitored to maximize the microlensing signal expected from the stars of the Galactic disk and bulge. 12.9 million stars have been monitored during 7 seasons towards 4 directions in the Galactic plane, away from the Galactic center. A total of 27 microlensing event candidates have been found. Estimates of the optical depths from the 22 best events are provided. A first order interpretation shows that simple Galactic models with a standard disk and an elongated bulge without a thick disk or a spiral structure are in agreement with our observations. We find that the average microlensing optical depth towards the complete EROS-cataloged stars of the spiral arms is

Search for MACHO's in the halo of our galaxy: Status report (French collaboration)

\bar{\tau} =0.51\pm .13\times 10^{-6}

Are there MACHOs in our galactic halo

, a number that is stable when the selection criteria are moderately varied. As the EROS catalog is almost complete up to

Searches for Dark Matter

I_C=18.5

Observation of microlensing toward the galactic spiral arms. EROS II 3 year survey

, the optical depth estimated for the sub-sample of bright target stars with

Observation of periodic variable stars towards the Galactic spiral arms by EROS II

I_C<18.5

SEARCH FOR PLANETARY MASS OBJECTS IN THE GALACTIC HALO THROUGH MICROLENSING

(

Experimental limits on the contribution of sub-stellar and stellar objects to the galactic halo

\bar{\tau}=0.39\pm .11\times 10^{-6}

Microlensing optical depth of the Large Magellanic Cloud

) is easier to interpret. The set of microlensing events that we have observed is consistent with a simple Galactic model. A more precise interpretation would require either a better knowledge of the distance distribution of the target stars, or a simulation based on a Galactic model. For this purpose, we define and discuss the concept of optical depth for a given catalog or for a limiting magnitude.