A Test For An Intervening Stellar Population As The Origin of Microlensing Events Toward the LMC

Abstract

Short version: Zaritsky and Lin (1997, ZL97) have recently presented observations which can be interpreted as evidence for an intervening stellar population along the line-of-sight to the LMC, at a distance of ~34 kpc. They suggest that it may belong to a low surface-brightness dwarf galaxy, or to the tidal debris of a disrupted galaxy, and derive an optical depth for gravitational lensing through the intervening system which is consistent with the observed value. Such interpretation of the microlensing events toward the LMC has major implications for the nature of the dark matter, and thus should be tested in every conceivable way. We suggest a method utilizing the microlensing data themselves to discriminate between the various hypotheses. We find that the first eight MACHO events toward the LMC are as consistent with a standard halo distribution of 0.5 solar-mass MACHOs (best fit), as they are with an intervening population having a transverse speed of ~240 km/s and a Miller-Scalo stellar IMF. Monte Carlo simulations show that in order to completely rule out either hypothesis, about 40 events will be required if the IMF is rising below the H-burning mass limit, and about 140 if it is falling. If the ZL97 intervening population hypothesis is correct then: (a) the currently observed range of event durations directly implies a lens mass range of ~0.06-0.8 solar-mass, which brackets the mass of ~0.2 solar mass where many investigations claim that the stellar IMF peaks, and (b) the event distribution provides valuable information on the low-mass end of the stellar IMF.