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2. Observations and photometry

Images of the six observed irregular dwarf galaxies are reproduced from the Digital POSS in Fig. 1 (click here). We performed observations of the galaxies in March 10, 1994 at the prime focus of the 6 meter telescope (scale 8.6 arcsec/mm). A CCD chip of tex2html_wrap_inline782 pixels was used with a readout noise of 12 e and a pixel size of tex2html_wrap_inline784 microns, which yielded a total field of tex2html_wrap_inline786 arcsec. For each of the galaxies we obtained B and V frames with a typical exposure tex2html_wrap_inline792 s under a seeing of FWHM = 1.3 - 1.5 arcsec. The frames were calibrated using the equatorial standard stars from Landolt (1992).

Figure 1: Reproductions from the Digital POSS-I of the six irregular galaxies. Each chart is tex2html_wrap_inline796 arcmin. North at top, East at left

Figure 2: The V frames of NGC 4163, NGC 4190, UGC 7298, UGC 7577, UGC 8308, and UGC 8320 after a subtraction of a smoothed frame. North and East are indicated by arrows. The brightest stars from Table 1 (click here) are marked. Tick marks are given for every 5 arcsec

After the initial processing of the frames we made aperture photometry of stars using the PC VISTA package (Treffers & Richmond 1990) and the UMAG package of Georgiev (1995), which allows us to reduce the inhomogeneity of the local background around the measured stars.

The V images of the observed galaxies are presented in Fig. 2 (click here). To show the structure of the central part as well as of the periphery of each galaxy, we subtracted a frame smoothed with a window of tex2html_wrap_inline802. Based on the results of our photometry the Colour-Magnitude diagrams were derived for the brightest stars of the galaxies. We reproduce them in Fig. 3 (click here), where star-like objects are indicated with filled squares and slightly diffuse ones with open squares. The brightest blue (B- V < 0.4) and red (B - V > 1.6) stars are marked in Fig. 2 (click here). Their apparent B magnitudes and B - V colours are presented in Table 1 (click here). The complete tables of the photometric data are accessible via FTP address:

Figure 3: Colour-Magnitude diagrams for the six dwarf galaxies. Dark squares are star-like objects, open squares are slightly diffuse ones, crosses are the blue stars which have been used for distance determination


star B B - V star B B - V
NGC 4163 UGC 7577
B0 18.76 0.05 B1 20.26 -0.17
B1 20.01 -0.10 B2 20.48 -0.19
B2 20.37 0.09 B3 20.56 0.12
B3 20.65 -0.21 (R1) 22.49 1.58
NGC 4190 UGC 8308
B1 19.84 0.13 B1 20.60 0.04
B2 20.02 -0.09 B2 21.07 0.08
B3 20.64 -0.01 B3 21.19 -0.13
R1 21.80 1.67 R1 22.16 1.87
UGC 7298 UGC 8320
B1 21.93 0.20 B1 19.37 0.17
B2 22.45 0.11 B2*) 20.21 (0.21)
B3 22.47 0.07 B3 20.76 0.08
R1 23.33 1.60 B4 21.10 0.00
B5 21.40 0.12
R1 22.97 2.30
*) The star No 44 from Hopp & Schulte-Ladbeck
is situated outside our CCD frame.
Table 1: B, V photometry of the brightest stars in six irregular galaxies


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