Thick disks and halos of spiral galaxies M 81, NGC 55 and NGC 300

By using images from the HST/WFPC2/ACS archive, we have analyzed the spatial distribution of the AGB and RGB stars along the galactocentric radius of nearby spiral galaxies M 81, NGC 300 and NGC 55. Examining color-magnitude diagrams and stellar luminosity functions, we gauge the stellar contents of the surroundings of the three galaxies. The red giant population (RGB) identified at large galactocentric radii yields a distance of 3.85 +/- 0.08 Mpc for M 81, 2.12 +/- 0.10 Mpc for NGC 55, and 2.00 +/- 0.13 Mpc for NGC 300, and a mean stellar metallicity of -0.65, -1.25, and -0.87 respectively. We find that there are two number density gradients of RGB stars along the radius, which correspond to the thick disk and halo components of the galaxies. We confirm the presence of a metallicity gradient of evolved stars in these galaxies, based on the systematic changes of the color distribution of red giant stars. These results imply that the thick disk might be a general feature of spiral galaxies, and endorse a further investigation of the outer stellar edges of nearby spirals, which is critical in constraining the origin and evolution of galaxies.