New models for a triaxial Milky Way spheroid and effect on the microlensing optical depth to the Large Magellanic Cloud

Abstract

We obtain models for a triaxial Milky Way spheroid based on data by Newberg and Yanny. The best fits to the data occur for a spheroid centre that is shifted by 3 kpc from the Galactic Centre. We investigate effects of the triaxiality on the microlensing optical depth to the Large Magellanic Cloud (LMC). The optical depth can be used to ascertain the number of massive compact halo objects (MACHOs); a larger spheroid contribution would imply fewer halo MACHOs. On the one hand, the triaxiality gives rise to more spheroid mass along the line of sight between us and the LMC and thus a larger optical depth. However, shifting the spheroid centre leads to an effect that goes in the other direction: the best fit to the spheroid centre is away from the line of sight to the LMC. As a consequence, these two effects tend to cancel so that the change in optical depth due to the Newberg/Yanny triaxial halo is at most 50%. After subtracting the spheroid contribution in the four models that we consider, the MACHO contribution (central value) to the mass of the Galactic Halo varies from ∼(8–20)% if all excess lensing events observed by the MACHO Collaboration are assumed to be due to MACHOs. Here the maximum is due to the original MACHO Collaboration results and the minimum is consistent with 0% at the 1σ error level in the data.