Detecting cold H2 globules in the outer Galactic disc by microlensing towards the Maffei 1 elliptical

Abstract

A serious candidate for dark matter in spiral galaxies are cold molecular hydrogen globules with a condensed central core and a disc-like space distribution probably similar to that of neutral hydrogen. This paper shows that the H_2 cores are sufficiently compact and massive to be detected by microlensing in the outer Galactic disc and that the Maffei 1 elliptical galaxy, at a distance of 3 Mpc and Galactic latitude b=-0.6 deg, offers an ideal target for such an experiment. The microlensing optical depth of H_2 cores along the line of sight to this galaxy is estimated to tau~0.7x10^(-6) if most of the dark mass in the Milky Way resides in such cores, and the typical event timescale to ~< 1 day. Detection rates are computed both in the classical and pixel lensing approaches in the I- and K-bands, and for a representative selection of existing observing facilities. In the more efficient pixel lensing case, two 10-hour observing runs, separated in time by at least several days, should yield of order 10 positive detections at the 5-sigma level using ground-based 8m-class telescopes in the K-band or the Hubble Space Telescope ACS camera in the I-band, and the corresponding fraction of events with timescale measurable to an accuracy better than 50% amounts to about 9% and 4% respectively for these observing alternatives.