A Method for Measuring Variations in the Stellar Initial Mass Function
  We present a method for investigating variations in the upper end of the
stellar Initial Mass Function (IMF) by probing the production rate of ionizing
photons in unresolved, compact star clusters with ages <~10 Myr and with
different masses. We test this method by performing a pilot study on the young
cluster population in the nearby galaxy NGC5194 (M51a), for which
multi-wavelength observations from the Hubble Space Telescope are available.
Our results indicate that the proposed method can probe the upper end of the
IMF in galaxies located out to at least ~10 Mpc, i.e., a factor ~200 further
away than possible by counting individual stars in young compact clusters. Our
results for NGC5194 show no obvious dependence of the upper mass end of the IMF
on the mass of the star cluster down to ~1000 M_sun, although more extensive
analyses involving lower mass clusters and other galaxies are needed to confirm
this conclusion.
