Globular cluster content and evolutionary history of NGC 147

¹ Special Astrophysical Observatory, Russian Academy of Sciences, N. Arkhyz, KChR 369167, Russia e-mail: sme@sao.ru
² Laboratoire d'Astrophysique de Toulouse-Tarbes, Université de Toulouse, CNRS, 14 avenue E. Belin, 31400 Toulouse, France

Received: 7 November 2008
Accepted: 28 January 2009

Abstract

Context. Globular clusters are representative of the oldest stellar populations. It is thus essential to have a complete census of these systems in dwarf galaxies, from which more massive galaxies are progressively formed in the hierarchical scenario.

Aims. We present the results of spectroscopic observations of eight globular cluster candidates in NGC 147, a satellite dwarf elliptical galaxy of M 31. Our goal is to make a complete inventory of the globular cluster system of this galaxy, determine the properties of their stellar populations, and compare these properties with those of systems of globular clusters in other dwarf galaxies.

Methods. The candidates were identified on Canada-France-Hawaii telescope photographic plates. Medium resolution spectra were obtained with the SCORPIO spectrograph at the prime focus of the 6 m telescope of the Russian Academy of Sciences. They were analyzed using predictions of stellar population synthesis models.

Results. We were able to confirm the nature of all eight candidates, three of which (GC5, GC7, and GC10) are indeed globular clusters, and to estimate evolutionary parameters for the two brightest ones and for Hodge II. The bright clusters GC5 and GC7 appear to have metallicities ([Z/H] ~ -1.5 -1.8) that are lower than the oldest stars in the galaxy. The fainter GC Hodge II has a metallicity [Z/H] = -1.1, similar to that of the oldest stars in the galaxy. The clusters GC5 and GC7 have low alpha-element abundance ratios. The mean age of the globular clusters in NGC 147 is 9 ± 1 Gyr. We also measured the radial velocities of Hodge II and IV, and derived a mass of NGC 147 in good agreement with the value from the literature. The frequency, S_n = 6.4, and mass fraction, of globular clusters in NGC 147 appear to be higher than those for NGC 185 and 205.

Conclusions. Our results indicate that the bright clusters GC5, GC7, and Hodge III formed in the main star-forming period ~8-10 Gyr ago, while the fainter clusters Hodge I and II formed together with the second generation of field stars.