Spectral Energy Distributions and Age Estimates of 172 Globular Clusters in M31

Abstract

In this paper, we present CCD multicolor photometry for 172 globular clusters (GCs), taken from the Bologna catalog (Battistini et al., published in 1987), in the nearby spiral galaxy M31. The observations were carried out by using the National Astronomical Observatories' 60/90 cm Schmidt telescope in 13 intermediate-band filters, which covered a range of wavelengths from 3800 to 10,000 Å. This provides a multicolor map of M31 in pixels of 17 × 17. By aperture photometry, we obtain the spectral energy distributions for these GCs. Using the relationship between the Beijing-Arizona-Taiwan-Connecticut intermediate-band system used for the observations and the UBVRI broadband system, the magnitudes in the B and V bands are derived. The computed V and B-V are in agreement with the values given by Battistini et al. and Barmby et al. (published in 2000). Finally, by comparing the photometry of each GC with theoretical stellar population synthesis models, we estimate ages of the sample GCs for different metallicities. The results show that nearly all our sample GCs have ages of more than 109 yr, and most of them are around 1010 yr old. In addition, we find that GCs fitted by the metal-poor model are generally older than ones fitted by the metal-rich model.