Carbon star survey in the Local Group^*

VIII. Probing the stellar halo of NGC 147

¹ INAF, Osservatorio Astronomico di Roma, Viale del Parco Mellini 84, 00136 Roma, Italia
² Département de Physique, Université de Montréal, C.P.6128, Succursale Centre-Ville, Montréal, Québec, H3C 3J7, Canada e-mail: demers@astro.umontreal.ca

Corresponding author: P. Battinelli, battinel@oarhp1.rm.astro.it

Received: 27 June 2003
Accepted: 12 January 2004

Abstract

We have imaged a 42 field centered on the spheroidal galaxy NGC 147 to study its AGB population and determine the main structural properties of this M 31 companion. This field was observed through two broadband (R and I) and two narrowband (CN and TiO) filters, following the standard approach of our systematic survey of C stars in nearby galaxies. We identified 288 C stars in NGC 147. The average I-magnitude of such sample of C stars is with . If we adopt a distance modulus , derived from the luminosity of both the tip of the red giant branch (TRGB) and the horizontal branch (HB) by Han et al., we conclude that the average absolute I-magnitude of C stars is  = –4.39, thus slightly under luminous with respect to what was found for other galaxies in our survey. From the radial surface profile we derive a scale length of 4.1 ± and a tidal radius r_t = 33.9 ± , a value quite larger than previous estimates. The C star population shows a scale length remarkably similar, thus suggesting that intermediate-age stars in NGC 147 are well mixed with the older population. Finally, the overall  ratio is 0.24 ± 0.02 and its local values increase monotonically from the center up to 3.5 scale lengths (beyond which counts become too low to yield reliable  ratios). Such radial behaviour implies a corresponding decrease of about 0.4 dex in [Fe/H], confirming a well defined metallicity gradient in NGC 147.