[BD2004c] 164 , the SIMBAD biblio

We have imaged a 42'x28' 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 <I≥20.31 with σ=0.40. If we adopt a distance modulus µ₀=24.39, 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 <M_I≥-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±0.1' and a tidal radius r_t=33.9±2.4', 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 C/M 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 C/M ratios). Such radial behaviour implies a corresponding decrease of about 0.4dex in [Fe/H], confirming a well defined metallicity gradient in NGC 147.