A new strong-lensing galaxy at z= 0.066: another elliptical galaxy with a lightweight IMF

Abstract

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 $\sigma$ = 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 previously-studied low-redshift lenses of similar $\sigma$, the derived mean mass excess factor (relative to a Kroupa IMF) is $\langle\alpha\rangle$ = 1.09$\pm$0.08. With all four systems, the intrinsic scatter in $\alpha$ for massive elliptical galaxies can be limited to $<0.32$, at 90 per cent confidence.