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

Geneva Observatory, Ch. des Maillettes 51, 1290 Sauverny, Switzerland e-mail: Roger.Fux@obs.unige.ch

Received: 30 June 2004
Accepted: 20 September 2004

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 H₂ 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 , offers an ideal target for such an experiment. The microlensing optical depth of H₂ cores along the line of sight to this galaxy is estimated to 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 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 amounts to about and respectively for these observing alternatives.