Mass-to-Light Ratios and the Age of Galaxies

Abstract

We present a method for constraining the present age of spiral galaxies or equivalently the value of the Hubble constant H _o , for a given cosmological model. This is achieved by calculating a dynamical measure of the disc mass-to-light ratio, M _D /L _B , for a sample of spiral galaxies, (with accurate rotation curves and colours) and by comparing the resulting tight relation between M _disc /L _B and (B-V) ^T _o , with theoretical predictions of galaxy evolutionary models.

The observed mass-to-light ratios are proportional to H _o , but the model predictions are proportional to galaxy age, t _⋆ which is inversely proportional to H _o (for the Friedmann models). Consistency between theory and observations can only be produced, for a given value of Ω _o , within a narrow range of combinations of H _o and t _⋆.

For the case of Ω _o = 1, we find consistency for t_⋆ = 9 ± 1 Gyr and H _o = 75 ± 8kms ⁻¹ Mpc ⁻¹.