A giant elliptical galaxy with a lightweight initial mass function
  We present new VLT observations of the closest-known strong-lensing galaxy,
the sigma=330 km/s giant elliptical ESO325-G004. The low redshift of the lens
(z_lens=0.035) results in arcs being formed at a small fraction of the
effective radius, (R_Ein ~ R_Eff/4). At such small radii, stars dominate the
lensing mass, so that lensing provides a direct probe of the stellar
mass-to-light ratio, with only small corrections needed for dark matter. The
redshift of the galaxy lensed by ESO325-G004 was unknown until now, so the
lensing mass was not securely determined. Using X-SHOOTER, we measure a secure
source redshift of z_src=2.141. Combined with the lensing configuration, this
yields a total mass inside the Einstein radius of (1.50+/-0.06) 10^11 M_sun. We
estimate the range of possible contribution of dark matter to the lensing mass,
using statistics from cosmological N-body simulations. Subtracting this
component yields a stellar M/L ratio for the lens of 3.14^+0.24_-0.42 (M/L)_sun
(in F814W band). Using VIMOS, we have also obtained very high-S/N spectroscopy
for the lens galaxy. The VIMOS data confirm that ESO325-G004 has a very old
stellar population. For a Kroupa IMF, the stellar population fit yields a
predicted stellar M/L ratio of 3.01+/-0.25 (M/L)_sun. Hence the mass
attributable to stars with this IMF is consistent with the lensing estimate. By
contrast, a Salpeter or heavier IMF is disfavoured at the 99.8% confidence
level. A "heavyweight" IMF, with a mass twice as large as the Kroupa case, is
firmly excluded. Such an IMF has been proposed for more distant elliptical
lenses, and also to explain strong dwarf-star spectral features, in particular
the NaI 8200-Ang doublet. A FORS2 far-red spectrum shows that this feature is
as strong in ESO325-G004 as it is in other high-sigma ellipticals, suggesting
tension between dwarf-star indicators and lensing-mass constraints for this
galaxy.
