Probing the mass fraction of machos in extragalactic halos

Current microlensing searches calibrate the mass fraction of the Milky Way halo that is in the form of massive astrophysical compact halo objects (MACHOs). We show that surveys like the Sloan Digital Sky Survey (SDSS) can probe the same quantity in halos of distant galaxies. Microlensing of background quasars by MACHOs in intervening galaxies would distort the equivalent width distribution of the quasar emission lines by an amplitude that depends on the projected quasar-galaxy separation. For a statistical sample of ∼10⁵ quasars as expected in the SDSS, this distortion is detectable at the ≳2 σ level out to a quasar-galaxy impact parameter of several tens of kpc, as long as extragalactic halos are made of MACHOs. Detection of this signal would test whether the MACHO fraction inferred for the Milky Way halo is typical of other galaxies.