Full spectral fitting of Milky Way and M 31 globular clusters: ages and metallicities ^⋆

¹ Núcleo de Astrofísica Teórica, Universidade Cruzeiro do Sul, R. Galvão Bueno 868, 01506-000 São Paulo, Brazil
e-mail: paula.coelho@cruzeirodosul.edu.br
² Research School of Astronomy and Astrophysics, The Australian National University, Cotter Road, Weston, ACT 2611, Australia
e-mail: abrito@mso.anu.edu.au
³ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, 3122 Victoria, Australia
e-mail: dforbes@swin.edu.au
⁴ UCO/Lick Observatory, University of California, Santa Cruz, CA 95064, USA
e-mail: brodie@ucolick.org

Received: 4 September 2012
Accepted: 2 November 2012

Abstract

Context. The formation and evolution of disk galaxies are long standing questions in astronomy. Understanding the properties of globular cluster systems can lead to important insights on the evolution of its host galaxy.

Aims. We aim to obtain the stellar population parameters – age and metallicity – of a sample of M 31 and Galactic globular clusters. Studying their globular cluster systems is an important step towards understanding their formation and evolution in a complete way.

Methods. Our analysis employs a modern pixel-to-pixel spectral fitting technique to fit observed integrated spectra to updated stellar population models. By comparing observations to models we obtain the ages and metallicities of their stellar populations. We apply this technique to a sample of 38 globular clusters in M 31 and to 41 Galactic globular clusters, used as a control sample.

Results. Our sample of M 31 globular clusters spans ages from 150 Myr to the age of the Universe. Metallicities [Fe/H] range from –2.2 dex to the solar value. The age-metallicity relation obtained can be described as having two components: an old population with a flat age-[Fe/H] relation, possibly associated with the halo and/or bulge, and a second one with a roughly linear relation between age and metallicity, higher metallicities corresponding to younger ages, possibly associated with the M 31 disk. While we recover the very well known Galactic GC metallicity bimodality, our own analysis of M 31’s metallicity distribution function (MDF) suggests that both GC systems cover basically the same [Fe/H] range yet M 31’s MDF is not clearly bimodal. These results suggest that both galaxies experienced different star formation and accretion histories.