MNELLS: The MUSE Nearby Early-Type Galaxy Lens Locator Survey

Abstract

Low-redshift strong-lensing galaxies can provide robust measurements of the stellar mass-to-light ratios in early-type galaxies (ETGs), 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 Multi-Unit Spectroscopic Explorer (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) that 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, α. Combining the new lens and new upper limits, with the previously discovered systems, we infer an average 〈α〉 = 1.06 ± 0.08 (marginalized over the intrinsic scatter), which is inconsistent with a Salpeter-like IMF (α = 1.55) at the 6σ 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 25 observations is expected to yield a new strong-lens system. Our observational results are consistent with this expected yield.