  The centers of our galaxy and the nearby Messier 87 are known to contain
supermassive black holes, which support accretion flows that radiate across the
electromagnetic spectrum. Although the composition of the accreting gas is
unknown, it is likely a mix of ionized hydrogen and helium. We use a simple
analytic model and a suite of numerical general relativistic
magnetohydrodynamic accretion simulations to study how polarimetric images and
spectral energy distributions of the source are influenced by the
hydrogen/helium content of the accreting matter. We aim to identify general
trends rather than make quantitatively precise predictions, since it is not
possible to fully explore the parameter space of accretion models. If the
ion-to-electron temperature ratio is fixed, then increasing the helium fraction
increases the gas temperature; to match the observational flux density
constraints, the number density of electrons and magnetic field strengths must
therefore decrease. In our numerical simulations, emission shifts from regions
of low to high plasma beta -- both altering the morphology of the image and
decreasing the variability of the light curve -- especially in strongly
magnetized models with emission close to the midplane. In polarized images, we
find that the model gas composition influences the degree to which linear
polarization is (de)scrambled and therefore affects estimates for the resolved
linear polarization fraction. We also find that the spectra of
helium-composition flows peak at higher frequencies and exhibit higher
luminosities. We conclude that gas composition may play an important role in
predictive models for black hole accretion.
