On the age and mass function of the globular cluster M 4: A different interpretation of recent deep HST observations^*

¹ European Space Agency, Space Telescope Operations Division, 3700 San Martin Drive, Baltimore MD 21218, USA
² European Southern Observatory, Karl–Schwarzschild-Str. 2, 85748 Garching, Germany e-mail: fparesce@eso.org
³ Osservatorio Astronomico di Teramo, Via Maggini, 64100 Teramo, Italy e-mail: straniero@te.astro.it
⁴ Dipartimento di Fisica, Università di Pisa, Via Buonarroti 2, 56127 Pisa, Italy e-mail: prada@df.unipi.it
⁵ Istituto Nazionale di Fisica Nucleare, Sezione di Pisa, 56010 Pisa, Italy
⁶ Osservatorio Astronomico di Teramo, Via Maggini, 64100 Teramo, Italy

Corresponding author: G. De Marchi, gdemarchi@rssd.esa.int

Received: 2 June 2003
Accepted: 30 September 2003

Abstract

Very deep images of the Galactic globular cluster M 4 (NGC 6121) through the F606W and F814W filters were taken in 2001 with the WFPC2 on board the HST. A first published analysis of this data set (Richer et al. [CITE]) produced the result that the age of M 4 is  Gyr (Hansen et al. [CITE]), thus setting a robust lower limit to the age of the universe. In view of the great astronomical importance of getting this number right, we have subjected the same data set to the simplest possible photometric analysis that completely avoids uncertain assumptions about the origin of the detected sources. This analysis clearly reveals both a thin main sequence, from which can be deduced the deepest statistically complete mass function yet determined for a globular cluster, and a white dwarf (WD) sequence extending all the way down to the  detection limit at . The WD sequence is abruptly terminated at exactly this limit as expected by detection statistics. Using our most recent theoretical WD models (Prada Moroni & Straniero [CITE]) to obtain the expected WD sequence for different ages in the observed bandpasses, we find that the data so far obtained do not reach the peak of the WD luminosity function, thus only allowing one to set a lower limit to the age of M 4 of ~9 Gyr. Thus, the problem of determining the absolute age of a globular cluster and, therefore, the onset of GC formation with cosmologically significant accuracy remains completely open. Only observations several magnitudes deeper than the limit obtained so far would allow one to approach this objective.