We report the discovery of a new low-redshift galaxy-scale gravitational lens, identified from a systematic search of publicly available MUSE observations. The lens galaxy, 2MASXJ04035024-0239275, is a giant elliptical at z = 0.06604 with a velocity dispersion of σ = 314 km s −1 . The lensed source has a redshift of 0.19165 and forms a pair of bright images either side of the lens centre. The Einstein radius is 1.5 arcsec, projecting to 1.8 kpc, which is just one quarter of the galaxy effective radius. After correcting for an estimated 19 per cent dark matter contribution, we find that the stellar mass-to-light ratio from lensing is consistent with that expected for a Milky Way initial mass function (IMF). Combining the new system with three previouslystudied low-redshift lenses of similar σ, the derived mean mass excess factor (relative to a Kroupa IMF) is α = 1.09±0.08. With all four systems, the intrinsic scatter in α for massive elliptical galaxies can be limited to < 0.32, at 90 per cent confidence.

We analyse newly obtained Hubble Space Telescope (HST) imaging for two nearby strong lensing elliptical galaxies, SNL-1 (z = 0.03) and SNL-2 (z = 0.05), in order to improve the lensing mass constraints. The imaging reveals previously unseen structure in both the lens galaxies and lensed images. For SNL-1 which has a well resolved source, we break the mass-vs-shear degeneracy using the relative magnification information, and measure a lensing mass of 9.49 ± 0.15 × 10 10 M , a 7 per cent increase on the previous estimate. For SNL-2 the imaging reveals a bright unresolved component to the source and this presents additional complexity due to possible AGN microlensing or variability. We tentatively use the relative magnification information to constrain the contribution from SNL-2's nearby companion galaxy, measuring a lensing mass of 12.59 ± 0.30 × 10 10 M , a 9 per cent increase in mass. Our improved lens modelling reduces the mass uncertainty from 5 and 10 per cent to 2 and 3 per cent respectively. Our results support the conclusions of the previous analysis, with newly measured mass excess parameters of 1.17 ± 0.09 and 0.96 ± 0.10 for SNL-1 and SNL-2, relative to a Milky-Way like (Kroupa) initial mass function.

Strong gravitational lensing can provide accurate measurements of the stellar mass-to-light ratio Υ in low-redshift (z < ∼ 0.05) early-type galaxies, and hence probe for possible variations in the stellar initial mass function (IMF). However, true multiple imaging lens systems are rare, hindering the construction of large nearby lens samples. Here, we present a method to derive upper limits on Υ in galaxies with single close-projected background sources, where no counter-image is detected, down to some relative flux limit. We present a proof-of-principle application to three galaxies with integral field observations from different instruments. In our first case study, only a weak constraint on Υ is obtained. In the second, the absence of a detectable counterimage excludes stellar masses higher than expected for a Salpeter IMF. In the third system, the current observations do not yield a useful limit, but our analysis indicates that deeper observations should reveal a counter-image if the stellar mass is any larger than expected for a Milky Way IMF. We discuss how our method can help enlarge the current samples of low-z galaxies with lensing constraints, both by adding upper limits on Υ and by guiding follow-up of promising single-image systems in search of fainter counter-images.

We present new observations of two z = 0.12 strong-lensing elliptical galaxies, originally discovered from the SDSS-IV MaNGA survey, using the new FOCAS IFU spectrograph on the Subaru Telescope. For J1436+4943, our observations confirm the identification of this system as a multiple-image lens, in a cusp configuration, with Einstein radius θ Ein = 2.0 arcsec. For J1701+3722, the improved data confirm earlier hints of a complex source plane, with different configurations evident in different emission lines. The new observations reveal a previously unseen inner counter-image to the [O iii] arc found from MaNGA, leading to a smaller revised Einstein radius of θ Ein = 1.6 arcsec. The inferred projected masses within the Einstein apertures (3.7-4.7 kpc) are consistent with being dominated by stars with an initial mass function (IMF) similar to that of the Milky Way, and a dark matter contribution of ∼35 per cent as supported from cosmological simulations. These results are consistent with 'pure lensing' analyses of lower-redshift lenses, but contrast with claims for heavier IMFs from combined lensingand-dynamical studies of more distant early-type galaxies.

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, α. Combining the new lens and new upper limits, with the previously-discovered systems, we infer an average α = 1.06 ± 0.08 (marginalised 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 twenty-five observations is expected to yield a new strong-lens system. Our observational results are consistent with this expected yield.