The Stellar Structures around Disk Galaxies

We present a brief summary of our current results on the stellar distribution and population gradients of the resolved stars in the surroundings of ~50 nearby disk galaxies, observed with space- (Hubble & Spitzer) and ground-based telescopes (Subaru, VLT, BTA, Palomar, CFHT & INT). We examine the radial (in-plane) and vertical (extraplanar) distributions of resolved stars as a function of stellar age and metallicity by tracking changes in the color-magnitude diagram of face-on and edge-on galaxies. Our data show, that the scale length and height of a stellar population increases with age, with the oldest detected stellar populations identified at a large galactocentric radius or extraplanar height, out to typically a few kpc. In the most massive of the studied galaxies there is evidence for a break in number density and color gradients of evolved stars, which plausibly correspond to the thick disk and halo components of the galaxies. The ratio of intermediate-age to old stars in the outermost fields correlate with the gas fraction, while relative sizes of the thick-to-thin disks anticorrelate with galactic circular velocity.Comment: To appear in the proceedings for the IAUS 241 'Stellar Populations as Building Blocks of Galaxies' held in La Palma, Spain, December 10-16 200

The Stellar Content of the Polar Rings in the Galaxies NGC 2685 and NGC 4650A

We present the results of stellar photometry of polar-ring galaxies NGC 2685 and NGC 4650A, using the archival data obtained with the Hubble Space Telescope's Wide Field Planetary Camera 2. Polar rings of these galaxies were resolved into ~800 and ~430 stellar objects in the B, V and Ic bands, considerable part of which are blue supergiants located in the young stellar complexes. The stellar features in the CM-diagrams are best represented by isochrones with metallicity Z = 0.008. The process of star formation in the polar rings of both galaxies was continuous and the age of the youngest detected stars is about 9 Myr for NGC 2685 and 6.5 Myr for NGC 4650A.Comment: 21 pages, 9 figures, AJ 2004 February, accepte

Stellar Subsystems of the Galaxy NGC 2366

Hubble Space Telescope archive data are used to perform photometry of stars in seven fields at the center and periphery of the galaxy NGC2366. The variation of the number density of stars of various ages with galactocentric radius and along the minor axis of the galaxy are determined. The boundaries of the thin and thick disks of the galaxy are found. The inferred sizes of the subsystems of NGC2366 ($Z_{thin} = 4$ kpc and $Z_{thick} = 8$ kpc for the thin and thick disks, respectively) are more typical for spiral galaxies. Evidence for a stellar halo is found at the periphery of NGC2366 beyond the thick disk of the galaxy.Comment: 16 pages, 4 figures, Astronomy Reports, 2008,v. 52, n.1, p. 1

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).Comment: 18 pages, 9 figures. accepted to Astrofizic