A giant elliptical galaxy with a lightweight initial mass function

Abstract

We present new observations of the closest known strong-lensing galaxy, the σ ≈ 330 km s− 1 giant elliptical ESO325−G004, made with the ESO Very Large Telescope. The low redshift of the lens (zl = 0.035) results in arcs being formed at a small fraction of the effective radius (REin = 2.85 arcsec ≈ Reff/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. However, 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 have detected multiple spectral lines, from two bright parts of the arc system, and measured a source redshift of zs = 2.141. Combined with lens modelling constraints, this yields a total mass inside the Einstein radius of 1.50 ± 0.06 × 1011 M⊙. We estimate the range of possible contribution of dark matter to the lensing mass, using halo profile statistics from cosmological N-body simulations. Subtracting this component yields a stellar mass-to-light ratio for the lens of M*/LF814W =3.14+0.24−0.42(M/L)⊙,F814W. Using VIMOS, we have also obtained very high signal-to-noise spectroscopy for the lens galaxy. Fitting models to this spectrum confirms that ESO325−G004 has a very old stellar population. For a Milky-Way-like (Kroupa) initial mass function (IMF), the stellar population fit yields a predicted stellar mass-to-light ratio of ϒMW = 3.01 ± 0.25 (M/L)⊙, F814W. Hence, the mass attributable to stars with a Kroupa IMF is consistent with the lensing estimate. By contrast, a Salpeter (or heavier) IMF is disfavoured at the 99.8 per cent confidence level. A ‘heavyweight’ IMF, with a mass twice as large as the Kroupa case, is firmly excluded for this galaxy. Such an IMF has been proposed for more distant elliptical lenses, and also to explain strong dwarf-star-sensitive spectral features, in particular the Na I 8200 Å doublet. A FORS2 far-red spectrum shows that this feature is as strong in ESO325−G004 as it is in other high-σ ellipticals, suggesting tension between dwarf-star indicators and lensing-mass constraints for this galaxy.