Constraining the low-mass end of the Initial Mass Function with Gravitational Lensing

TL;DR

This study uses gravitational lensing of the Einstein Cross to constrain the low-mass end of the stellar Initial Mass Function, finding that a Salpeter IMF overestimates stellar mass and slopes with Gamma>0 are unlikely.

Contribution

It introduces a non-parametric lensing methodology to constrain the IMF's low-mass slope, avoiding assumptions about galaxy dynamics or population decomposition.

Findings

Salpeter IMF overpredicts stellar mass in the lens.

Slopes with Gamma>0 are ruled out at 90% confidence.

Method is robust against dust, population models, and galaxy structure uncertainties.

Abstract

The low-mass end of the stellar Initial Mass Function (IMF) is constrained by focusing on the baryon-dominated central regions of strong lensing galaxies. We study in this letter the Einstein Cross (Q2237+0305), a z=0.04 barred galaxy whose bulge acts as lens on a background quasar. The positions of the four quasar images constrain the surface mass density on the lens plane, whereas the surface brightness (H-band NICMOS/HST imaging) along with deep spectroscopy of the lens (VLT/FORS1) allow us to constrain the stellar mass content, for a range of IMFs. We find that a classical single power law (Salpeter IMF) predicts more stellar mass than the observed lensing estimates. This result is confirmed at the 99% confidence level, and is robust to systematic effects due to the choice of population synthesis models, the presence of dust, or the complex disk/bulge population mix. Our non-parametric methodology is more robust than kinematic estimates, as we do not need to make any assumptions about the dynamical state of the galaxy or its decomposition into bulge and disk. Over a range of low-mass power law slopes (with Salpeter being Gamma=+1.35) we find that at a 90% confidence level, slopes with Gamma>0 are ruled out.