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 \pm 0.08 Mpc for M 81 , 2.12 \pm 0.10 Mpc for NGC 55 , and 2.00 \pm 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 .