The C star outer disk population of M 31 seen with the SLOAN filters

¹ Département de Physique, Université de Montréal, CP 6128, Succursale Centre-Ville, Montréal, Québec H3C 3J7, Canada e-mail: demers@astro.umontreal.ca
² INAF - Osservatorio Astronomico di Roma, Viale del Parco Mellini 84, 00136 Roma, Italia e-mail: battinel@oarhp1.rm.astro.it

Received: 14 December 2004
Accepted: 17 February 2005

Abstract

We employ the CFHT Megacam camera to survey ~one deg² of the southern outer disc of M 31, a region which includes the area where Battinelli et al. (2003, AJ, 125, 1298) have identified nearly one thousand C stars. In the outer M 31 region not previously surveyed, we identify 361 new C star candidates, having similar photometric properties to the known ones, and confirm the slight decrease in the luminosity of C stars with galactocentric distances. We show that the Sloan g', r', i' filters are a viable approach, comparable to (CN – TiO), to identify C stars. We find that the () colours of cool C stars can be so red that prohibitively long g' exposures are needed to acquire faint extragalactic C stars. This makes the Sloan filters a less promising approach to extend a C star survey to several Mpc. Our uniform large field survey detects the edge of M 31 disk at ~35 kpc. The intermediate-age population, represented by C stars, extends further to ~40 kpc.