The Disk and Extraplanar Environment of NGC 247

Abstract

The stellar content of the spiral galaxy NGC 247 is investigated using deep visible and near-infrared images. The main-sequence turnoff (MSTO) in the inner 12 kpc of the disk corresponds to an age of ~6 Myr. A mean star formation rate (SFR) of 0.1 M☉ yr-1 during the past 16 Myr is computed from star counts. The color of the red supergiant plume does not change with radius, suggesting that the mean metallicity of young stars does not vary by more than ~0.1 dex. The number of bright main-sequence stars per local stellar mass density climbs toward larger radii out to a distance of 12 kpc; the scale lengths that characterize the radial distributions of young and old stars in the disk thus differ. The density of bright main-sequence stars with respect to projected H I mass gradually drops with increasing radius. The population of very young stars disappears in the outer disk; the MSTO at galactocentric radii between 12 and 15 kpc corresponds to ~16 Myr, while between 15 and 18 kpc the age is ≥40 Myr. Red giant branch (RGB) stars are resolved at a projected minor-axis galactocentric distance of ~12 kpc. There is a broad spread in metallicity among the RGB stars, with a mean [M/H] ~ -1.2. The RGB tip occurs at i' = 24.5 ± 0.1, indicating that the distance modulus is 27.9 ± 0.1. Luminous AGB stars with an age ~3 Gyr are also seen in this field.