Concerning the White Dwarf Cooling Age of M4: A Reply to De Marchi et al. on “A Different Interpretation of Recent Deep HST Observations”

We respond to the recent paper by De Marchi et al. on the white dwarf cooling age of M4. The authors question the data analysis and interpretation that led to our conclusions in Hansen et al. in 2002. In their paper, De Marchi et al. are unable to obtain photometry as deep as ours from the same data set and therefore assert that only a lower limit to the white dwarf cooling age for this cluster of ∼9 Gyr can be obtained. In this brief contribution we show that shortcomings in the data analysis and reduction techniques of De Marchi et al. are responsible for their inability to reach the photometry limits that our study reports. In a forthcoming paper in which the complete techniques for age determination with white dwarfs are laid out, we demonstrate that their method of fitting the luminosity function yields a spuriously low white dwarf cooling age.