The white dwarf cooling sequence of the globular cluster Messier 4

We present the white dwarf sequence of the globular cluster M4, based on a 123 orbit Hubble Space Telescope exposure, with a limiting magnitude of Vsimilar to30 and Isimilar to28. The white dwarf luminosity function rises sharply for I>25.5, consistent with the behavior expected for a burst population. The white dwarfs of M4 extend to approximately 2.5 mag fainter than the peak of the local Galactic disk white dwarf luminosity function. This demonstrates a clear and significant age difference between the Galactic disk and the halo globular cluster M4. Using the same standard white dwarf models to fit each luminosity function yields ages 7.3+/-1.5 of Gyr for the disk and 12.7+/-0.7 Gyr for M4 (2 sigma statistical errors).