Low-redshift strong-lensing galaxies can provide robust measurements of the stellar mass-to-light ratios in early-type galaxies ( ETG ) , and hence constrain variations in the stellar initial mass function ( IMF ) .
At present , only a few such systems are known .
Here , we report the first results from a blind search for gravitationally-lensed emission line sources behind 52 massive z < 0.07 ETGs with MUSE integral field spectroscopy .
For 16 galaxies , new observations were acquired , whilst the other 36 were analysed from archival data .
This project has previously yielded one confirmed galaxy-scale strong lens ( J0403-0239 ) which we report in an earlier paper .
J0403-0239 has since received follow-up observations , presented here , which indicate support for our earlier IMF results .
Three cluster-scale , and hence dark-matter-dominated , lensing systems were also discovered ( central galaxies of A4059 , A2052 and AS555 ) .
For nine further galaxies , we detect a singly-imaged but closely-projected source within 6 arcsec ( including one candidate with sources at three different redshifts ) ; such cases can be exploited to derive upper limits on the IMF mass-excess factor , \alpha .
Combining the new lens and new upper limits , with the previously-discovered systems , we infer an average \langle \alpha \rangle = 1.06 \pm 0.08 ( marginalised over the intrinsic scatter ) , which is inconsistent with a Salpeter-like IMF ( \alpha = 1.55 ) at the 6 \sigma level .
We test the detection threshold in these short-exposure MUSE observations with the injection and recovery of simulated sources , and predict that one in twenty-five observations is expected to yield a new strong-lens system .
Our observational results are consistent with this expected yield .