Carbon star survey in the Local Group

Abstract

We have imaged a 42$' \times 28'$ 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 $\langle I \rangle = 20.31$ with $\sigma = 0.40$. If we adopt a distance modulus $\mu_0 = 24.39$, derived from the luminosity of both the tip of the 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 $\langle M_I\rangle$ = –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 0.4 dex in [Fe/H], confirming a well defined metallicity gradient in NGC 147.