POINT-AGAPE Pixel Lensing Survey of M31 : Evidence for a MACHO contribution to Galactic Halos

TL;DR

The POINT-AGAPE survey of M31 detected microlensing events suggesting that at least 20% of the galactic halo mass could be in the form of MACHOs, indicating a significant dark matter component.

Contribution

First evidence from M31 microlensing data indicating a substantial MACHO contribution to galactic halos, supporting dark matter hypotheses.

Findings

Detected 6 microlensing events over 3 years

At least 20% of halo mass in MACHOs if their mass is 0.5-1 M_sun

Supports MACHO contribution through a binary microlensing candidate

Abstract

The POINT-AGAPE collaboration is carrying out a search for gravitational microlensing toward M31 to reveal galactic dark matter in the form of MACHOs (Massive Astrophysical Compact Halo Objects) in the halos of the Milky Way and M31. A high-threshold analysis of 3 years of data yields 6 bright, short--duration microlensing events, which are confronted to a simulation of the observations and the analysis. The observed signal is much larger than expected from self lensing alone and we conclude, at the 95% confidence level, that at least 20% of the halo mass in the direction of M31 must be in the form of MACHOs if their average mass lies in the range 0.5-1 M$_\odot$. This lower bound drops to 8% for MACHOs with masses $\sim 0.01$ M$_\odot$. In addition, we discuss a likely binary microlensing candidate with caustic crossing. Its location, some 32' away from the centre of M31, supports our conclusion that we are detecting a MACHO signal in the direction of M31.