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.
