MACHOs in M 31? Absence of evidence but not evidence of absence

Abstract

We present results of a microlensing survey toward the Andromeda Galaxy (M 31) carried out during four observing seasons at the Isaac Newton Telescope (INT). This survey is part of the larger microlensing survey toward M 31 performed by the Microlensing Exploration of the Galaxy and Andromeda (MEGA) collaboration. Using a fully automated search algorithm, we identify 14 candidate microlensing events, three of which are reported here for the first time. Observations obtained at the Mayall telescope are combined with the INT data to produce composite lightcurves for these candidates. The results from the survey are compared with theoretical predictions for the number and distribution of events. These predictions are based on a Monte Carlo calculation of the detection efficiency and disk-bulge-halo models for M 31. The models provide the full phase-space distribution functions (DFs) for the lens and source populations and are motivated by dynamical and observational considerations. They include differential extinction and span a wide range of parameter space characterised primarily by the mass-to-light ratios for the disk and bulge. For most models, the observed event rate is consistent with the rate predicted for self-lensing -a MACHO halo fraction of 30% or higher can be ruled at the 95% confidence level. The event distribution does show a large near-far asymmetry hinting at a halo contribution to the microlensing signal. Two candidate events are located at particularly large projected radii on the far side of the disk. These events are difficult to explain by self lensing and only somewhat easier to explain by MACHO lensing. A possibility is that one of these is due to a lens in a giant stellar stream.

An analysis of the M31 microlensing events

“…Indeed, to get a more meaningful comparison for the selflensing contribution, we normalised the values for the mass of the luminous components to those of the MEGA models. We found that our estimate of the (total) number of the self-lensing events agrees with the [18] prediction only when considering more extreme (maximal) parameters for the disk component.…”

“…1). In contrast, the MEGA collaboration [18] finds that their results, although not conclusive, agree with a no MA-CHO hypothesis. Although the issues involved in the microlensing observations towards the LMC or the M31 are indeed rather different, the results for the halo fraction in the form of MACHOs depend crucially on the prediction of the expected signal due to known luminous populations, this being dominated by the "self-lensing" signal where both source and lens belong to the same star population residing respectively either in the LMC or in M31.…”

“…2. We also adopt the observational parameters of the MEGA collaboration: we consider T obs = 2 yr and we account for the detection efficiency and maximum impact parameter as given by [18]. In particular, in Fig.…”

“…An analysis of the M31 microlensing events Philippe Jetzer Table 5 of [18] shows that there is fairly good agreement. Indeed, to get a more meaningful comparison for the selflensing contribution, we normalised the values for the mass of the luminous components to those of the MEGA models.…”

“…We explored these issues in detail [23,24] taking into account the latest MEGA results. In particular, we computed the predicted number of events, which we then compared with the observational results and addressed also the question of whether the expected self-lensing signal due to stars belonging either to the bulge or the disc of M31 is able, as claimed by [18], to fully explain the MEGA results.…”