Microlensing by compact objects with masses between ~0.001 M_sun_ and 300 M_sun_ will amplify the continuum emission of a quasar, without significantly changing its line emission. Thus, compact objects with masses associated with stars, subdwarfs, and stellar remnants will reduce the apparent equivalent widths of quasar emission lines. It is possible to detect this population of lenses by searching for an increase in the number of small equivalent width quasars with redshift. This increase was looked for, but not found, in quasar samples taken from the Einstein Medium Sensitivity Survey and the Steidel & Sargent absorption-line studies. Thus, {OMEGA}_c_, the cosmological density of compact objects relative to the critical density, is <~0.1 in the mass range 0.01 M_sun_- 20 M_sun_ (for {OMEGA} < 0.6). For any value of {OMEGA}, {OMEGA}_c_ <~0.2 in the larger mass range 0.001 M_sun_-60 M_sun_, and {OMEGA}_c_ < 1 for 0.001 M_sun_-300 M_sun_. Subdwarfs, stellar objects, or their remnants (e.g., MACHOS) cannot close the universe.
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Dalcanton et al. (1994) studied this question.