The M/L* ratio of young star clusters in galactic mergers

We point out a strong time evolution of the mass- to- light conversion factor eta commonly used to estimate masses of dense star clusters from observed cluster radii and stellar velocity dispersions. We use a gasdynamical model coupled with the Cambridge stellar evolution tracks to compute line- of- sight velocity dispersions and halflight radii weighted by the luminosity. Stars at birth are assumed to follow the Salpeter initial mass function in the range of 0.15 - 17 M,. We find that eta and hence the estimated cluster mass, increases by factors as large as 3 over timescales of 20 million years. Increasing the upper mass limit to leads to a sharp rise of similar 50 M, amplitude but in as little as 10 million years. Fitting truncated isothermal ( Michie- King) models to the projected light profile leads to overestimates of the concentration parameter c of deltac approximate to 0.3 compared to the same functional fit applied to the projected mass density.