Detecting cold H₂globules in the outer Galactic disc by microlensing towards the Maffei 1 elliptical

Abstract

Abstract.A candidate source of dark matter in spiral galaxies is 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 • , 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 τ ∼ 0.7× 10 −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-h observing runs, separated in time by at least several days, should yield of the order of 10 positive detections at the 5σ level using ground-based 8 m-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.

Simulations of galactic disks including a dark baryonic component

“…Grenier et al (2005) showed that much gas in the solar neighborhood is revealed only by γ-rays. Other indirect detections could be possible using micro-lensing events (Draine 1998;Fux 2005). However, this method has not been exploited up to now.…”