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Volume 430, Number 3, February II 2005
Page(s) 853 - 864
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20041566

A&A 430, 853-864 (2005)
DOI: 10.1051/0004-6361:20041566

Detecting cold H $\mathsf{_2}$ globules in the outer Galactic disc by microlensing towards the Maffei 1 elliptical

R. Fux

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

(Received 30 June 2004 / Accepted 20 September 2004 )

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 $b=-0.6^{\circ}$, 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 $\tau\sim 0.7\times 10^{-6}$ if most of the dark mass in the Milky Way resides in such cores, and the typical event timescale to $\la $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 $5\sigma$ 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.

Key words: cosmology: dark matter -- Galaxy: disk -- ISM: molecules -- gravitational lensing -- methods: observational

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© ESO 2005