Single-colour diagnostics of the mass-to-light ratio - I. Predictions from galaxy formation models

Accurate galaxy stellar masses are crucial to better understand the physical mechanisms driving the galaxy formation process. We use synthetic star formation and metal enrichment histories predicted by the GALFORM galaxy formation model to investigate the precision with which various colours (m(a) - m(b)) can alone be used as diagnostics of the stellar mass-to-light ratio. As an example, we find that, at z = 0, the intrinsic (B-f435w -V-f606w) colour can be used to determine the intrinsic rest-frame V-band stellar mass-to-light ratio (log(10) Gamma(V) = log(10)[(M/M-circle dot)/(L-V/L-V circle dot)]) with a precision of sigma(lg Gamma) similar or equal to 0.06 when the initial mass function and redshift are known beforehand. While the presence of dust, assuming a universal attenuation curve, can have a systematic effect on the inferred mass-to-light ratio using a single-colour relation, this is typically small as it is often possible to choose a colour for which the dust reddening vector is approximately aligned with the (m(a) - m(b))- log(10) Gamma(V) relation. The precision with which the stellar mass-to-light ratio can be recovered using a single-colour diagnostic rivals implementations of spectral energy distribution (SED) fitting using more information but in which simple parametrizations of the star formation and metal enrichment histories are assumed. To facilitate the wide use of these relations, we provide the optimal observer frame colour to estimate the stellar mass-to-light ratio, along with the associated parameters, as a function of redshift (0 < z < 1.5) for two sets of commonly used filters.