We discussed in detail microlensing by globular clusters. 47 Tuc was taken as an example for which we performed the calculation of the optical depth, the microlensing event rate and the average lensing duration for all possible geometries of the system SMC-47 Tuc-Milky Way. In addition, we studied the dependence of these parameters on the mass function.
We have seen that for the case, where the source is a star in the SMC and the lens is a MACHO in 47 Tuc, one can expect an observable eventrate of per year. However, this result depends crucially on the total amount of dark matter and its distribution in the cluster, which both are not well known at present.
We then applied these results to study microlensing by globular clusters towards the galactic center, where locally the optical depth can be dominated by dark matter inside clusters. However, since globular clusters are very localized objects, the expected number of events as obtained by these scaling arguments is small. A larger event rate is expected, if the average MACHO mass inside the cluster is well below one solar mass, the total amount of dark matter is larger than or the density of observable stars behind the cluster is significantly higher than the assumed value of 50 stars per . Indeed, analysing the event distribution around the three clusters inside the observation fields of MACHO and OGLE, we find an increase of the microlensing event rate by at least a factor of about 2, as compared to that expected for MACHOs located in the disk and the bulge. This increase suggests the presence of a substantial amount of dark matter in form of light objects such as brown dwarfs.
Given this promising preliminary results it is important to systematically analyse future lensing data as a function of the position around the mentioned globular clusters. In fact, having few more events at disposal will already be very helpful to draw more firm conclusions and get better limits on the content of dark matter in globular clusters.
In addition, we propose to favour observation fields around globular clusters for future campaigns. In particular NGC 6553 is a very promising candidate for lensing by dark objects in globular clusters, since its luminosity is high and also its distance is such that the tidal radius of the cluster corresponds to a relatively large angular size. Moreover, it would be important to have more precise knowledge of , which is at present one of the main sources of uncertainty.
© European Southern Observatory (ESO) 1998
Online publication: July 20, 1998