The stellar IMF determined in early-type galaxies from a non-degenerate set of optical line indices

TL;DR

This study uses optical spectral features to measure the initial mass function in early-type galaxies, revealing a correlation between IMF slope and galaxy velocity dispersion, and introduces new IMF-sensitive indices to improve accuracy.

Contribution

The paper introduces new optical indices sensitive to the low-mass end of the IMF and demonstrates their effectiveness in constraining the IMF slope in elliptical galaxies.

Findings

IMF slope steepens with increasing velocity dispersion

New CaH1 index effectively isolates dwarf star contribution

Good agreement with previous IMF measurements and models

Abstract

We investigate the optical spectral region of spectra of 1000 stars searching for IMF-sensitive features to constrain the low-mass end of the initial mass function (IMF) slope in elliptical galaxies. We use the MILES stellar library in the wavelength range [3500-7500] {\AA} to select indices that are sensitive to cool dwarf stars and that only weakly depend on age and metallicity. In this wavelength range, the response of a change in the effective temperature of the cool red giant population is similar to the response of a change in the number of dwarf stars in the galaxy. We investigate the degeneracy between IMF variation and Teff,RGB and show that it is possible to break it with the new IMF indicators defined here. In particular, we define a CaH1 index around {\lambda}6380 {\AA} that arises purely from cool dwarfs, does not strongly depend on age and is anti-correlated with [a/Fe]. This index allows the determination of the the IMF slope when combined with TiO lines and age- and metallicity-dependent features. The use of several indicators is crucial to break degeneracies between IMF, age, abundance pattern and Teff of the RGB population. We measure line-index strengths of the optical IMF indicators in the SSP models and compare these with index strengths of the same features in stacked SSDSS early-type galaxy spectra with varying velocity dispersions. Using 10 different indicators, we find a clear trend of a steepening IMF with increasing velocity dispersion described by the equation x=(2.3+-0.1)*log{\sigma}200+(2.13+-0.15), where x is the IMF slope and {\sigma}200 is the central stellar velocity dispersion measured in units of 200km/s. We compute the 'IMF mismatch' parameter as the ratio of stellar mass-to-light ratio predicted from the x-{\sigma}200 relation to that inferred from SSP models assuming a Salpeter IMF and find good agreement with independent published results.