3 Surface photometry and profile extraction

The main results of the present study are the surface brightness (SB) profiles as well as a set of isophotal and integral parameters, derived from these profiles. The SB distribution was analyzed by two methods. First, we calculated the equivalent light profile by means of PIPS, by slicing the smoothed galaxy image at predefined intensity levels and counting the pixel intensities in areas between successive isophotes. The differential counts reduced to unit area give the azimuthally averaged intensity profile of the galaxy as a function of the equivalent radius, as defined in the RC2 (de Vaucouleurs et al. [1976]). This procedure permits us to process the SB profile in faint outskirts of galaxies up to about 27.5 B mag/ $\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak\hfil \penalty50\h... ...\rlap{$\sqcap$ }$\sqcup$ } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi''$ on our snap-shot (60 s) data frames. A set of isophotal, effective and asymptotic photometric parameters were determined on the basis of the equivalent light profiles and of the light growth curves.

The second approach for the surface photometry should recover the shape and orientation of isophotes. For this purpose we used the ellipse fitting algorithm of Bender & Möllenhoff ([1987]) as available in the SURFPHOT package, running within MIDAS. The resulting fit was used to construct a smooth galaxy model. The subtraction of the elliptical model from the observed image allows us to detect underlying structures like spiral arms, dust, H II regions, if any.

Figure 1: Light distribution in four isolated dwarf ELGs. Surface gbrightness (SB), (B-R) colour index, minor-to-major axis ratio (b/a) and major axisposition angle (PA, 0 on positive x-axis increasing to positive y-axis) is shown versus the equivalent radius. Circles indicate data from B passband, crosses - from R passband. Profile types: HS 1240+3721 shows a pure exponential disk (prof: d); HS 1236+3937 shows multiple components - nuclear cusp + exponential disk (prof: nd); HS 1349+3942 shows almost everywhere concave curvature, i.e. is nonexponential (prof: nonexp); HS 1325+3255 has almost stellar light profile (prof: st) - a scaled star light profile is shown by dots, for comparison

As a result of the ellipse fitting we obtained a set of radial profiles: surface brightness (SB), minor-to-major axis ratio (b/a), position angle (PA) in each particular passband. The colour index profile was calculated by means of combining the B and Rsurface brightness profiles, which were obtained with one particular set of fitting ellipses applied both to the B and R frames. Four examples of the resulting profile-sets are shown in Fig. 1. They are selected in order to demonstrate the different profile types (see Sect. 5).

The observed photometric parameters, as measured from radial profiles, are given in Tables 4, 5. Table 4 summarizes the data of 125 galaxies, observed in B band, and contains also the B-R colour data of 79 galaxies.

The data are arranged in Table 4 as follows:

Column 1: The galaxy name according to Popescu et al. ([1996,1998]). Isolated galaxies are designated with an asterisk.

Column 2: Effective equivalent radius in arcseconds.

Columns 3, 4: Equivalent radii in arcseconds of the 25 and 26.5 B mag/ $\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak\hfil \penalty50\h... ...\rlap{$\sqcap$ }$\sqcup$ } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi''$ isophotes, respectively.

Column 5: Observed central surface brightness in B.

Column 6: Effective surface brightness in B, measured at $r_{\rm eff}$.

Column 7: Total B-magnitude, within the 25 B mag/ $\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak\hfil \penalty50\h... ...\rlap{$\sqcap$ }$\sqcup$ } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi''$ isophote.

Columns 8, 9: Asymptotic B- and R-magnitudes, respectively.

Columns 10, 11: B-R colour indices, measured at $r_{\rm eff}$ ( $CI_{\rm eff}$) and at the 25 B mag/ $\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak\hfil \penalty50\h... ...\rlap{$\sqcap$ }$\sqcup$ } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi''$ isophote (CI₂₅), respectively.

Column 11a: Colour gradient, when observed in colour index profiles, is indicated as follows: + marks positive colour gradient, i.e. the galaxy is getting redder with increasing radius; - marks the opposite case.

Columns 12, 13: Galaxy mean minor-to-major axis ratio (b/a), and position angle (PA), measured counterclockwise from North of the fitted ellipses, determined as the average between the 23 and 25 B mag/ $\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak\hfil \penalty50\h... ...\rlap{$\sqcap$ }$\sqcup$ } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi''$ isophotes.

Column 14: Light concentration index $c_{\rm in} = L(0.3r_{25})/L(r_{25})$ introduced by Doi et al. ([1993]) as the ratio of the integral light contained within the isophotes with equivalent radii of 0.3r₂₅ and r₂₅, respectively.

Column 15: Light concentration index $c_{31} = r(3/4L_{\rm T})/r(1/4L_{\rm T})$, defined by de Vaucouleurs ([1977]) as the ratio of equivalent radii which contain a fraction 3/4 and a fraction 1/4 of total light, respectively.

Table 5 adds the photometric data of 16 galaxies which were observed only in R band. The isophotal radii in Cols. 3, 4 refer to the 24 R mag/ $\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak\hfil \penalty50\h... ...\rlap{$\sqcap$ }$\sqcup$ } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi''$ and 25.5 R mag/ $\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak\hfil \penalty50\h... ...\rlap{$\sqcap$ }$\sqcup$ } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi''$ isophotes, respectively. Column 7 yields integral magnitude within 24 R mag/ $\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak\hfil \penalty50\h... ...\rlap{$\sqcap$ }$\sqcup$ } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi''$ isophote. Geometrical parameters (b/a) and (PA) are averaged between the 22 and 24 R mag/ $\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak\hfil \penalty50\h... ...\rlap{$\sqcap$ }$\sqcup$ } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi''$ isophotes. The concentration index in Col. 11 $c_{\rm in} = L(0.3r_{24})/L(r_{24})$refers to the 24 R mag/ $\ifmmode\hbox{\rlap{$\sqcap$ }$\sqcup$ }\else{\unskip\nobreak\hfil \penalty50\h... ...\rlap{$\sqcap$ }$\sqcup$ } \parfillskip=0pt\finalhyphendemerits=0\endgraf}\fi''$ isophote. Other parameters are as in Table 4. Uncertain data are marked with a colon. Typical measurement errors are indicated in the last row of Table 4. These errors are medians of the residuals of all multiple observations (in the B band) of individual ELGs.

The absolute magnitudes and linear sizes of the ELGs are listed in the left part of the Tables 6 and 7 (the content of the latter is explained in Sect. 5).

In addition to the filtering of the observed frames with an adaptive smoothing filter, we performed the adaptive Laplacian filtering, too. The Laplacian filter computes the second derivative (i.e. curvature) of the SB distribution. The Laplacian filtered images do not retain any photometric information; however they are proper to disentangle the inner morphology as multiple nuclei, H II regions, spiral arms, bridges, if any. Those (faint) structures are usually hidden by large luminosity gradients of the central regions of galaxies. The Laplacian filtered images were used for the morphological classification of the ELGs (Sect. 5).