Microlensing by gas filaments

Gas in the interstellar medium is generally organized in filamentary structures, which may also be relevant for a complementary explanation of the dark matter in the Galactic halo. We examine the possibility that such structures may act as gravitational microlenses on background sources. To this end, we derive the general properties of a cylindrical lens and compare the light curves produced by such microlensing events with those generated by spherically symmetric clouds. We find that establishing the symmetry of the lens through just the analysis of the light curve may be problematic, whereas analysis of the astrometric shift of the centroid of the image can discriminate between the two classes of clouds. On the basis of our analysis, we find that only gas filaments with a very high density could be detectable. Such clouds are unlikely to exist in a long-lived state. Therefore, microlensing is not the right tool to decide in a conclusive way about the existence and relevance of gas filaments in the halo, which could thus well be present and escape detection by ordinary microlensing surveys.