We investigate in detail the white dwarf cooling sequence of the globular cluster Messier 4 .
In particular we study the influence of various systematic uncertainties , both observational and theoretical , on the determination of the cluster age from the white dwarf cooling sequence .
These include uncertainties in the distance to the cluster and the extinction along the line of sight , as well as the white dwarf mass , envelope and core compositions and the white dwarf–main sequence mass relation .
We find that fitting to the full two-dimensional colour-magnitude diagram offers a more robust method for age determination than the traditional method of fitting the one-dimensional white dwarf luminosity function .
After taking into account the various uncertainties , we find a best fit age of 12.1 Gyr , with a 95 % lower limit of 10.3 Gyr .
We also perform fits using two other sets of cooling models from the literature .
The models of Chabrier et al ( 2000 ) yield an encouragingly similar result , although the models of Salaris et al ( 2000 ) do not provide as good a fit .
Our results support our previous determination of a delay between the formation of the Galactic halo and the onset of star formation in the Galactic disk .