Stellar disks and halos of the edge-on spiral galaxies: NGC 891, NGC   4144 and NGC 4244
  The results of the stellar photometry of the images ACS/WFC and WFPC2 of the HST are used to study stellar population and spatial distribution of stars in three edge-on galaxies: NGC 891, NGC 4144 and NGC 4244. The measuring of the number density of the old stars revealed two stellar substructures in these galaxies: thick disk and halo. The borders of these substructures consisting mainly of red giants, are determined by the change of number density gradient of the old stars. The revealed halos have flattened shapes and extend up to 25 kpc from the galaxy planes. The obtained results of number density distributions of different type stars perpendicular to the galaxy planes allow us to verify our stellar model of spiral galaxies. Using the determination of the tip of red giant branch (TRGB method) we have derived the following distances: D = 9.82 Mpc (NGC 891), D = 7.24 Mpc (NGC 4144), D = 4.29 Mpc (NGC 4244). 
N. A. Tikhonov, O. A. Galazutdinova