Carbon star survey in the Local Group - VIII. Probing the stellar halo of NGC 147

We have imaged a 42' x 28' field centered on the spheroidal galaxy NGC 147 to Study its AGB Population and determine the main structural properties of this M 3 1 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 <1> = 20.31 with sigma = 0.40. If we adopt a distance modulus mu(0) = 24.39, derived from the luminosity of both the tip of the Led giant branch (TRGB) and the horizontal branch (HB) by Hart et al.. we conclude that the average absolute I-magnitude of C stars is <M-1> = -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(1) = 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 LIP 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 0.4 dcx in [Fe/H], confirming a well defined metallicity gradient in NGC 147.