SPIDER VIII - Constraints on the Stellar Initial Mass Function of Early-type Galaxies from a Variety of Spectral Features

TL;DR

This study uses spectral features from a large galaxy sample to investigate how the stellar initial mass function varies with galaxy velocity dispersion, revealing a trend towards more low-mass stars in more massive galaxies.

Contribution

It provides new empirical constraints on the IMF variation with galaxy properties, especially velocity dispersion, and compares unimodal and bimodal IMF models using spectral and dynamical data.

Findings

IMF becomes more bottom-heavy with increasing sigma0

Fraction of low-mass stars increases from 20% to 80% as sigma0 rises

Bimodal IMF models are consistent with stellar mass-to-light ratios

Abstract

We perform a spectroscopic study to constrain the stellar Initial Mass Function (IMF) by using a large sample of 24,781 early-type galaxies from the SDSS-based SPIDER survey. Clear evidence is found of a trend between IMF and central velocity dispersion, sigma0, evolving from a standard Kroupa/Chabrier IMF at 100km/s towards a more bottom-heavy IMF with increasing sigma0, becoming steeper than the Salpeter function at sigma0>220km/s. We analyze a variety of spectral indices, corrected to solar scale by means of semi-empirical correlations, and fitted simultaneously with extended MILES (MIUSCAT) stellar population models. Our analysis suggests that sigma0, rather than [alpha/Fe], drives the IMF variation. Although our analysis cannot discriminate between a single power-law (unimodal) and a low-mass (<0.5MSun) tapered (bimodal) IMF, we can robustly constrain the fraction in low-mass stars at birth, that is found to increase from 20% at sigma0~100km/s, up to 80% at sigma0~300km/s. Additional constraints can be provided with stellar mass-to-light (M/L) ratios: unimodal models predict M/L significantly larger than dynamical M/L, across the whole sigma0 range, whereas a bimodal IMF is compatible. Our results are robust against individual abundance variations. No significant variation is found in Na and Ca in addition to the expected change from the correlation between [alpha/Fe] and sigma0. [Abridged]