Age estimations of M 31 globular clusters from their spectral energy distributions

This paper presents accurate spectral energy distributions (SEDs) of 15 M 31 globular clusters (GCs) confirmed by spectroscopy and/or high spatial-resolution imaging, as well as 30 M 31 globular cluster candidates detected by Mochejska et al. (1998, AcA, 48, 455). Most of these candidates have $m_V > 18$, deeper than previous searches, and these candidates have not yet been confirmed to be globular clusters. The SEDs of these clusters and candidates are obtained as part of the BATC Multicolor Survey of the Sky, in which the spectrophotometrically-calibrated CCD images of M 31 in 13 intermediate-band filters from 4000 to 10 000 Å were observed. These filters are specifically designed to exclude most of the bright and variable night-sky emission lines including the OH forest. In comparison to the SEDs of true GCs, we find that some of the candidate objects are not GCs in M 31. SED fits show that theoretical simple stellar population (SSP) models can fit the true GCs very well. We estimate the ages of these GCs by comparing with SSP models. We find that, the M 31 clusters range in age from a few ten Myr to a few Gyr old, as well as old GCs, confirming the conclusion that has been found by Barmby et al. (2000, AJ, 119, 727), Williams & Hodge (2001, ApJ, 548, 190), Beasley et al. (2004, AJ, 128, 1623), Burstein et al. (2004, ApJ, 614, 158) and Puzia et al. (2005, A&A, 434, 909) in their investigations of the SEDs of M 31 globular clusters