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

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.