Variation in the stellar initial mass function from the chromospheric activity of M dwarfs in early-type galaxies

TL;DR

This study uses Lyα emission from M dwarfs to independently test the variation of the stellar initial mass function in early-type galaxies, finding evidence that the IMF varies with environment.

Contribution

It introduces a novel method using Lyα emission to probe the IMF, providing an independent check on previous optical absorption line studies.

Findings

Lyα flux ratio correlates with IMF variations in galaxies.

Detected Lyα emission originates from stars in early-type galaxies.

Results support non-universality of the IMF across different environments.

Abstract

Mass measurements and absorption line studies indicate that the stellar initial mass function (IMF) is bottom-heavy in the central regions of many early-type galaxies, with an excess of low mass stars compared to the IMF of the Milky Way. Here we test this hypothesis using a method that is independent of previous techniques. Low mass stars have strong chromospheric activity characterized by non-thermal emission at short wavelengths. Approximately half of the UV flux of M dwarfs is contained in the $\lambda{}1215.7$ Ly$\alpha$ line, and we show that the total Ly$\alpha$ emission of an early-type galaxy is a sensitive probe of the IMF with a factor of $\sim 2$ flux variation in response to plausible variations in the number of low mass stars. We use the Cosmic Origins Spectrograph on the Hubble Space Telescope to measure the Ly$\alpha$ line in the centers of the massive early-type galaxies NGC1407 and NGC2695. We detect Ly$\alpha$ emission in both galaxies and demonstrate that it originates in stars. We find that the Ly$\alpha$ to i-band flux ratio is a factor of 2.0$\pm$0.4 higher in NGC1407 than in NGC2695, in agreement with the difference in their IMFs as previously determined from gravity-sensitive optical absorption lines. Although a larger sample of galaxies is required for definitive answers, these initial results support the hypothesis that the IMF is not universal but varies with environment.