3. Discussion

Following the usual approach developed by Sandage & Tammann (1974) and de Vaucouleurs (1978), we determined the galaxy distance moduli from the mean apparent magnitude of the three brightest blue stars, <B(3B)>, from the relation:

where is the integral magnitude of the parent galaxy, is the galactic extinction, and the numerical coefficients were calibrated (Karachentsev & Tikhonov 1994) from galaxies whose distances had been measured from cepheids. In the cases when we found red supergiant candidates, additional distance estimates were derived from a similar relation:

 

Parameter N 4163 N 4190 U 7298 U 7577 U 8308 U 8320

a₂₅ 1.7 1.6 1.2 4.2 1.1 3.7

13.93c 13.52c 15.83c 12.87 15.53c 12.73

0.45c 0.44c 0.40c 0.45c 0.48c 0.31c

0.0 0.0 0.04 0.0 0.0 0.0

V₀ 165 234 253 240 244 273

<B(3B)> 20.34 20.17 22.28 20.43 20.95 20.11

<B-V>₃ -0.07 0.01 0.13 -0.08 0.00 0.15

27.75 27.70 29.67 28.42 27.85 28.01

- 27.79 29.32 (28.71) 27.77 28.46

D 3.55 3.47 8.59 4.84 3.72 4.01

 

Some basic parameters of the galaxies are presented in Table 2 (click here). Its lines contain:

(1) - the standard angular diameter of the galaxy in arcmin from the Catalogue PGC-ROM (Paturel et al. 1992);

(2) - the integrated magnitude of the galaxy either from PGC or from our photometry (marked by "c'');

(3) - the integrated colour index of the galaxy within its standard diameter according our measurements;

(4) - the value of the galactic extinction from the Catalogue PGC-ROM;

(5) - the radial velocity (in km/s) corrected for motion with respect to the Local group centroid (Karachentsev & Makarov 1996);

(6,7) - the mean magnitude and mean colour of the three brightest blue stars;

(8,9) - the distance modulus of the galaxy via blue stars and red one;

(10) - the galaxy distance (in Mpc), corresponding to the blue modulus. Below we note some properties of the galaxies observed.

NGC 4163 = UGC 7199. This blue compact galaxy has a quite regular shape on its faint isophotes. The integrated colour index increases smoothly from 0.41 in the center up to 0.53 within the largest visible radii, which may indicate the presence of two stellar populations in the galaxy. The brightest blue object (=B0) has a star-like profile. However, we excluded it as a probable multiple system when the distance was derived.

NGC 4190 = UGC 7232 = VV 104. The galaxy seems to contain two overlapping regions of star formation. For this reason Vorontsov-Velyaminov (1959) included this peculiar object into the Atlas of interacting galaxies. The brightest blue stars are concentrated in the SW part of the galaxy. The distance estimate via the brightest red star (R1) agrees well with that from the blue ones.

UGC 7298. This small blue galaxy of a low surface brightness is distinctly resolved into stars, despite the fact that all of them are fainter than V = 21.5 mag. The galaxy distance, which is determined both via three blue stars and via the brightest red one, exceeds distances of both previous galaxies by more than two times. The distance estimate, 8.6 Mpc, is unusual for a galaxy with a radial velocity of only +253 km/s.

UGC 7577 = DDO 125. This and the next two galaxies are in the list of dwarf irregular galaxies of van den Bergh (1966). In spite of the large dimension of the galaxy ( arcmin), almost all its brightest stars are concentrated within our CCD frame. From the three brightest blue stars the galaxy distance modulus is 28.42 mag. The brightest red star (R1) yields 28.71 mag, which is in agreement with the blue modulus. However, its colour index, 1.58, does not formally satisfy the condition B-V > 1.6 adopted above.

UGC 8308 = DDO 167. The galaxy has an asymmetric shape and a low surface brightness. Its integral colour index increases smoothly from 0.29 in the central region up to 0.48 within the faintest isophotes. The distance estimates from the blue stars and the red one agree well with each other. But we have to note a large scatter in the apparent magnitudes of red stars, which makes the red modulus rather unreliable.

UGC 8320 = DDO 168. From the six considered galaxies this is the only one that have already been resolved into stars. According to Bresolin et al. (1993) its distance is 3.3 Mpc. Photometry of 86 brightest stars in B, R bands throughout the galaxy body was carried out by Hopp & Schulte-Ladbeck (1995). For the common stars in the B band we find good agreement between our and their photometry. Because of its large dimension,  arcmin, a large fraction of the galaxy stars fall outside our CCD frame. Therefore, estimating its distance via blue stars, we took the star No. 44 from Hopp & Schulte-Ladbeck's list as the second brightest blue star, B2. The derived distance value, 4.0 Mpc, exceeds slightly the estimate of Bresolin et al. (1993).