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3. Results

3.1. The virtual aperture photometry and the second order photometric calibration

The present data were compared with aperture photometry available in the literature by integrating the counts in concentric circular rings around the galaxy centres to provide curves of growth up to the diameter of the reference photometry. This operation was made without subtracting stars from the frames since, unless otherwise specified, the aperture photometry is generally not corrected for stellar contamination. Galaxies observed at the 2.2-m telescope in non photometric conditions and with no aperture photometry available in the literature were re-observed at the 3.5 m telescope. The growth curve was used to derive the photometric zero point for galaxies observed in non photometric periods (as indicated in Table 5), and provided a general check of the intrinsic photometric accuracy of the current work. The virtual photometry measurements obtained in this work are consistent in the average with the aperture photometry available in the literature (88 measurements): tex2html_wrap_inline2212 and tex2html_wrap_inline2214. The most discrepant measurements are those at small apertures (tex2html_wrap_inline2216) due to a combination of seeing effects and galaxy centering, which might be quite inaccurate in the objects with no bright nuclei such as dwarf irregular galaxies.

We estimate that the overall photometric accuracy of our data, including systematic errors on the zero point determination is tex2html_wrap_inline2218 0.1 mag.

To facilitate comparison with optical data available in the literature, integrated magnitudes were determined in circular apertures from the star-subtracted frames to the diameter of the 25.5 mag arcsectex2html_wrap_inline2220 blue isophote as described and tabulated in Table 3.

3.2. Photometry along elliptical annuli

Using the star-subtracted frames, the surface brightness profiles were re-determined by averaging the brightness distribution in concentric elliptical annuli of fixed centre, position angle and ellipticity. The ellipses were fitted by eye to the K' band 21 mag arcsectex2html_wrap_inline2224  isophotes of the galaxy under study (for some low-surface brightness galaxies, a weaker isophote had to be used: the values are given in parentheses in Table 5; see Sect. 4). This simple technique was preferred to more sophisticated procedures to be as consistent as possible with the method we used in optical CCD studies (see Gavazzi et al. 1995a and references therein). Given the irregular shape of Im and BCD galaxies, elliptical profiles give just a crude representation of their real profiles (see Sect. 4). Starting from an inner ellipse of size comparable with the seeing disk, a set of annuli, increasing in major axis by fixed amounts was drawn. In each annulus the total number of counts, pixels and associated statistical uncertainties were computed following Gavazzi et al. (1994).
The isophotal major radii (tex2html_wrap_inline2226) in the H and K' bands were determined from the azimuthally integrated profiles as the radii at which the surface brightness reaches 21.5 mag arcsectex2html_wrap_inline2232. The values of tex2html_wrap_inline2234 given in this work are not corrected for galaxy inclination.
Magnitudes were derived by integrating the elliptical light profiles up to the radius corresponding to the 21.5 mag arcsectex2html_wrap_inline2236  isophote (tex2html_wrap_inline2238 and tex2html_wrap_inline2240). Since in some dwarf galaxies 21.5 mag arcsectex2html_wrap_inline2242  is too close to the peak brightness to measure a meaningful diameter, we also give tex2html_wrap_inline2244 and tex2html_wrap_inline2246. These entries, which are generally less accurate than tex2html_wrap_inline2248 and tex2html_wrap_inline2250 because of the higher noise in the data at the 22.0 mag arcsectex2html_wrap_inline2252  isophote, can nevertheless be fairly well determined in the low surface brightness galaxies where longer integration times were used. These magnitudes are observed quantities and are not corrected for extinction. The photometric parameters determined along elliptical rings are summarized in Table 5 (only available in electronic form), as follows:
Column 1: VCC name.
Column 2: adopted filter.
Column 3: total integration time per position (in sec); this corresponds to the product of the exposure time of the elementary integration (generally tex2html_wrap_inline2254) tex2html_wrap_inline2256 the number of (added) elementary integrations.
Column 4: number of frames per galaxy (combined with a median filter).
Column 5: adopted mosaic, as defined in Sect. 2.2. Some galaxies (marked tex2html_wrap_inline2258) were serendipitously observed in the sky frames of other targets. S indicates large galaxies where an ad hoc designed mosaic was used.
Column 6: pixel size, in arcseconds: these are 0.64 and 1.61 arcsec/pixel for the 2.2-m telescope and 0.81 arcsec/pixel for the 3.5-m telescope.
Column 7: photometric quality: galaxies marked ``tex2html_wrap_inline2260'' were observed in non-photometric periods; diameters and magnitudes are not determined; for galaxies marked ``r'' the zero-point was determined using reference aperture photometry.
Column 8: position angle of the galaxy major axis (measured counterclockwise from N).
Column 9: ellipticity (1-b/a).
Column 10: observed major (tex2html_wrap_inline2264) radius (in arcsec) determined at the tex2html_wrap_inline2266  magnitude isophote.
Column 11: observed magnitude (tex2html_wrap_inline2268) integrated within the 21.5 mag arcsectex2html_wrap_inline2270 elliptical isophote.
Column 12: concentration index tex2html_wrap_inline2272, defined as the ratio between the radii that contain 75% and 25% of tex2html_wrap_inline2274.
Column 13: observed major (tex2html_wrap_inline2276) radius (in arcsec) determined at the tex2html_wrap_inline2278  magnitude isophote.
Column 14: observed magnitude (tex2html_wrap_inline2280) integrated within the 22.0 mag arcsectex2html_wrap_inline2282 elliptical isophote.
Column 15: concentration index tex2html_wrap_inline2284, defined as the ratio between the radii that contain 75% and 25% of tex2html_wrap_inline2286.

For galaxies observed at both the 2.2-m and the 3.5-m telescope, Table 5 reports 2 lines. The 3.5-m telescope data refer in fact to the combined images.

Grey-level and contour representations of the K' brightness distribution of the galaxies studied are given in Fig. 3 (click here) alongside surface brightness and (where possible) colour profiles. Table 6 gives the parameters of the differential H and K' band radial surface brightness profiles obtained by azimuthal integration along elliptical annuli. Table 6 (only available in electronic form) is arranged as follow:
Column 1: VCC name.
Column 2: adopted filter.
Column 3: radius (on the major axis) of the elliptical annulus, in arcseconds.
Column 4: surface brightness at the given radius.

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