The observations of 18 dwarf irregular galaxies in Canes Venatici were carried out in 1995 February using a TEC 1024 1024 chip at the prime focus of the 2.56 m Nordic Optical Telescope (NOT) at La Palma. The CCD size was 24.58 24.58 mm, which yielded a total field of 3 3 arcmin. We observed the galaxies under good seeing conditions in Johnson B and V bands. The observational log is presented in Table 1.
The initial processing of the data was performed with IRAF (CCDRED package) using bias, dark and twilight flatfield frames obtained during the observing nights. Cosmic events filtering was done with CCDPHOT package.
The CCD frames of the 18 observed dwarf galaxies are presented either in B or in V band in Figs. 2-19. Most of the galaxies have been resolved into stars for the first time.
The photometry of stellar objects was carried out using MIDAS implementation of the point spread function (PSF) photometry programs DAOPHOT and ALLSTAR (Stetson 1987). The PSF was obtained for each frame from bright, well isolated stars, then PSF fitting of all stellar objects found in the frame was performed simultaneously by ALLSTAR. Object lists for each galaxy were combined according to positional identity (with a deviation of no more than 1.5 pixels) of the star images in different bands. The standard photometric error in and SHARP parameters of ALLSTAR were taken into account during the star selection process. Mostly objects with SHARP and a standard photometric error were kept.
Our ALLSTAR magnitudes are based on the first estimate of stellar magnitudes with small aperture (4-8 pixels) to exclude contamination by neighbouring stars. Therefore, an aperture correction was made. We obtained this correction using PSF stars by plotting growth curves for each image. All objects found, except PSF stars, were subtracted from the images before this process. The aperture correction was then calculated as the difference between the asymptotic magnitude and the magnitude within a small aperture from the first estimate in ALLSTAR. We estimate mean errors of this correction as: in B, in V and in I band.
For the calibration, the photometric standards from Landolt's
(1992) list were
observed during the run. An aperture photometry of standard
stars was done to obtain their magnitudes.
After correction for atmospheric extinction, the following
equations for transformation into Johnson-Cousins standard system were derived:
where the capital letters are Johnson-Cousins magnitudes and the small letters are instrumental ones. We estimate the following zero-point errors of this transformation: in V, in B-V and in V-I. Adding quadratically these errors to those of the aperture correction, we obtain the resulting zero-point errors: in V, in B-V and in V-I.
The results of the stellar photometry are presented in Tables 2-19 which can also be accessed in computer-readable form in the CDS. All the tables are organized as follow: (1) the ordinal number of the star; (2), (3) - coordinates of the star in pixels; (4) - the apparent magnitude in V band; (5) - the (B-V) colour index; (6), (7) - the formal DAOPHOT error in V and (B-V) respectively; (8) - the mean of B and V SHARP value.
To determine the total magnitude of a galaxy, concentric circular apertures were used. The total magnitude in each band was defined to be the magnitude at which the curve of growth flattened. Background stars and distant galaxies were clipped before. We estimate the resulting error as in B and in V.
Figure 2: The B frame of UGC 6782. As for all other images, North is top, and East is left. The upper right corner corresponds to of the frame coordinates given in the tables; X increases to East and Y to South
Figure 3: The V frame of UGC 7131
Figure 4: The V frame of UGC 7356
Figure 5: The B frame of UGC 7559
Figure 6: The B frame of UGC 7599
Figure 7: The V frame of UGC 7605
Figure 8: The V frame of UGC 7639
Figure 9: The B frame of UGC 7698
Figure 10: The V frame of UGCA 290
Figure 11: The V frame of UGCA 292
Figure 12: The B frame of UGC 7866
Figure 13: The B frame of UGC 7949
Figure 14: The B frame of UGC 8024
Figure 15: The V frame of K 215
Figure 16: The V frame of UGC 8638
Figure 17: The V frame of UGC 8651
Figure 18: The V frame of UGC 8760
Figure 19: The V frame of UGC 8833