The Disk and Extraplanar Regions of NGC 55

The stellar content of the nearby SB(s)m galaxy NGC 55 is investigated using images covering the visible and near-infrared wavelength regions. A well-defined plume, which stellar evolution models suggest contains stars with masses near the RSG - AGB transition, is detected in CMDs of the disk, and it is concluded that star formation in the thin disk of NGC 55 has ocurred at a significant rate for at least the past 0.1 - 0.2 Gyr. The disk also contains a large population of old (log(t_yr) ~ 10) stars, and it is argued that a stable disk has been in place in NGC 55 for a significant fraction of the age of the Universe. At projected distances in excess of 2 kpc off of the disk plane the brightest AGB stars have ages 10(+3)(-2) Gyr. Thus, despite indications that dust and gas are present in the envelope surrounding the NGC 55 disk, the AGB content suggests that recently formed stars do not occur in large numbers in the extraplanar region. The mean metallicity of extraplanar RGB stars is in excellent agreement with that measured in the extraplanar HII regions EHR 1 and 2, suggesting that the age-metallicity relation in this part of NGC 55 has been flat for at least a few Gyr. Finally, the RGB-tip occurs near i' = 23.1 in the extraplanar region, and a distance modulus of 26.5 is computed from this feature