5 Surface photometry

5.1 Procedure and results

Surface photometry of the observed galaxies was performed using the method described by González-Serrano & Pérez-Fournon ([1991]) and González-Serrano et al. ([1993]). The procedure consists of a Fourier analysis of the intensity variations along elliptical contours fitted to the galaxy intensity distribution. For a given semimajor axis the following ellipse parameters were obtained: centre coordinates, ellipticity, position angle of the major axis, mean intensity and the first four Fourier components. Of these, the third (cos$3\theta$) and fourth (cos$4\theta$) give information of non-elliptical isophotes and their distortions (see González-Serrano & Pérez-Fournon [1991] and references therein). After an ellipse was fitted, the semimajor axis was increased by a fixed factor, proceeding to the next ellipse. In this way radial profiles were obtained for all the ellipse parameters, including the surface brightness profile. In most cases the surface photometry analysis was started at the first pixel from the nucleus of the galaxy and finished when the ellipses reached the sky background level. Some galaxies were saturated in their nuclear parts and the analysis was started at larger radii from the centre.

Before the method was applied each galaxy image was cleaned from other objects, bad pixels and cosmic ray hits that could contaminate the intensity distribution of the galaxy. This was done automatically by building a mask image that indicates the pixels to be ignored in the ellipse fitting process. When the galaxy under study had a nearby companion the cleaning task was more complicated since the companion, if bright and closeby, contributed to the light distribution of the main galaxy over a large area. In these cases we have applied an iterative procedure in order to subtract the contaminating galaxy from the main image. This technique is explained in detail in González-Serrano & Pérez-Fournon ([1991]), where a discussion on the reliability and errors is also given. Typical errors of ellipse parameters are below $\sim 3^\circ$ for PA, below $\sim 0.02$ for ellipticity, and $\sim 0.5\%$ for the B₄ parameter. Deviations lower than these errors were found from the comparison of the PA and ellipticity profiles of B2 1217+29 (NGC 4278) and B2 1610+29 (NGC 6086) with those presented in Peletier et al. ([1990]).

The resulting surface brightness profiles (expressed in V magnitudes corrected for Galactic extinction) and isophote parameters' profiles, including ellipticity, position angle and B₄, are presented in Fig. 2 and listed in Table 4. The profiles in the inner part of the galaxies are not reliable, as they are affected by the seeing. The seeing radius is indicated in each profile by a small vertical hash. The procedure of quantitative surface photometry was applied to 59 of the galaxies in the sample, including 53 ellipticals, 4 spirals and 2 irregulars. The galaxies excluded were 4 ellipticals (B2 1141+37, B2 1339+26, B2 1502+26 and B2 1511+26), 2 spirals (B2 1106+26 and B2 2240+29), and 5 irregulars (B2 0207+38, B2 0209+37, B2 1303+31, B2 1318+34 and B2 2320+32). The four ellipticals excluded belong to multiple systems (three dumbells and a tight chain of four galaxies) in which the secondary galaxies seriously affected the light distribution of the main galaxy. The reason to exclude seven spirals/irregulars was that the presence of features like bars, spiral arms, dust fringes and/or double nuclei, produced a large deviation of the galaxies relative to an elliptical model.

Figure 2: Radial profiles for V-band surface brightness, ellipticity, position angle and B4 parameter for the observed B2 galaxies. The seeing radius is indicated in each profile by a small vertical hash

5.2 Surface brightness profiles of the elliptical galaxies

The surface brightness profile of 45 of the 53 ellipticals reasonably fit an r^1/4 law. For one of these galaxies, B2 0149+35, the flat surface brightness profile at the central part is due to the presence of nuclear dust. Three galaxies show large excesses relative to the r^1/4 law in the outer parts (B2 0120+33, B2 0326+39, and B2 1447+27). These excesses are at least of one magnitude at the faintest isophote. The remaining 8 ellipticals are described below. For five of them the surface brightness profile is largely affected by the seeing due to a combination of a small galaxy size and a poor seeing. These galaxies are B2 0843+31, B2 1204+34, B2 1358+30, B2 1455+28 and B2 1457+29, and their surface brightness profiles ($\mu_V$) appear severely curved on the $\mu_V-r^{1/4}$ plots (Fig. 2). We recall that the first of these sources is an uncertain identification. The three remaining ellipticals have bright nuclei which strongly affect the surface brightness profiles. These galaxies are B2 1101+38, B2 1652+39 and B2 1833+32, and their $\mu_V$profiles clearly deviate from a de Vaucouleurs law. For these threee galaxies there is evidence of nuclear activity in the optical range from the literature. B2 1101+38 and B2 1652+39 are catalogued Markarian galaxies (Mrk 421 and Mrk 501 respectively), and hence have nuclei with blue/UV excess. Both are classified as BL Lac in Hewitt & Burbidge ([1993], see references for the sources therein). B2 1833+32 is a broad-line radio galaxy (Corbett et al. [1997] and references therein).

For the 45 galaxies obeying the r^1/4 law the surface brightness profiles were fitted with this law, and the effective radius $r_{\rm e}$ and the surface brightness at that radius $\mu_{\rm e}$ were obtained from the fits. Table 5 lists the values of both parameters, along with the ellipticity and position angle at the effective radius, and the minimum and maximum radii used for the r^1/4 fits. Columns 4-7 list $\mu_{\rm e}$ in mag arcsec^-2, corrected for Galactic extinction, $r_{\rm e}$in arcsecs and their errors. Columns 10-12 list $\mu_{\rm e}$ in mag arcsec^-2, including the k-correction from Coleman et al. ([1980]) and the (1+z)^-4 correction, and $r_{\rm e}$ and its error in kpc.

  

Table 5: Photometric and structural parameters of the r^1/4 galaxies

Figure 3: $\delta /r$ deviations along semimajor axis of the galaxies exhibiting significant off-centering

5.3 Isophotal properties of the elliptical galaxies

Five galaxies classified as ellipticals in Table 3 were noted to show significant distortions relative to an elliptical model from the contour maps in Fig. 1. These galaxies are B2 0116+31, B2 0120+33, B2 0648+27, B2 1037+37 and B2 1833+32. The distortion of B2 1037+30 is due to a prominence of bright surface brightness towards the NE. B2 1833+32 shows also a tail towards the East at a high surface brightness level. In addition two galaxies show extensions of low surface brightness, in the form of a tail (B2 0222+36) and a shell (B2 0258+35). B2 0149+35 is the only elliptical to clearly feature nuclear dust. The dust is revealed on the contour map in Fig. 1 and it is responsible for the flatness of the inner part of the surface brightness profile. B2 0924+30 may present also nuclear dust although this needs confirmation.

  

Table 6: Isophotal peculiarities

  

Table 7: Emission line galaxies

The galaxies B2 0116+31, B2 0120+33, B2 0648+27, and B2 1358+30 show isophote twisting, which we define by the presence of changes in the PA of the semimajor axis larger than 20 degrees per semidecade in radius. These changes were measured at distances larger than 3 arcsec to avoid the nuclear parts. B2 0222+36 and B2 1037+30 also show twists, but they could be related to the presence of the tails. The isophote twisting measured in B2 0149+35 is probably due to the presence of a nuclear dust lane. The galaxy B2 1626+39 shows also PA twist, but the shape of the inner isophotes could be affected by the presence of multiple nuclei in the galaxy. B2 0326+39, B2 1113+24, B2 1217+29, B2 1257+28 and B2 1317+33 also show strong changes in PA, but for these cases the twists are measured over isophotes with low eccentricity, where the errors in position angle are large, and thus the twisting may not be real. Furthermore the PA profile for B2 0326+39 and B2 1317+33 could be affected by nearby galaxies. Therefore only B2 116+31, B2 0120+33, B2 0648+27, and B2 1358+30 do unambiguously reveal isophote twisting.

Another interesting parameter derived from the isophote analysis is the B₄ term of the Fourier series. This parameter should be zero for a perfect ellipse and indicates "boxy isophotes'' if B₄ is negative and "disky isophotes'' if B₄ is positive. We have considered as boxy (disky) those galaxies having a B₄ parameter lower (higher) than $\sim -0.02$ (+0.02) in more than 5 adjacent isophotes. Galaxies with noisy profiles and/or |B₄| values in the range 0.01 - 0.02 were considered as marginal cases of boxy/disky isophotes. The B₄ profiles are shown in Fig. 2. Seven galaxies of the sample appear to have boxy isophotes. The evidence for boxiness is clear for B2 0116+31, B2 1204+24, B2 1621+38, B2 1626+39, B2 1726+31, and B2 1833+32. B2 0149+35 shows a negative B₄ parameter but this is most probably due to the dust; in fact the galaxy has also a high B₃ term, which is a typical diagnostic of the presence of dust. Five other galaxies show indications of boxiness although at a marginal level.

The opposite behaviour, i.e. disky isophotes, is found for 12 galaxies which are B2 0116+31, B2 0222+36, B2 0331+39, B2 0838+32, B2 0843+31, B2 1037+30, B2 1316 + 29, B2 1346+26, B2 1358+30, B2 1527+30, B2 1652+29 and B2 1833+32. Six additional galaxies are marginally disky. We note that for the galaxy B2 0838+32 several nearby nuclei/companions were masked out and this may affect the B₄ profile. For B2 1037+30 the isophote analysis was obtained over a rather small area of the galaxy, due to the masking of a large prominence of difusse emission and this could also affect the B₄ profile.

The separation $\delta$ between the isophote centre at semimajor axis rand the centre of the inner isophote is a standard parametrization of non-concentric isophotes or off-centering, and it is an indication of the effects of tidal interactions (Paper II and references therein). Profiles of $\delta /r$ are obtained from the surface analysis method and measurement errors are around 1-5% depending on the galaxy brightness. In our sample, nine galaxies show off-centering at levels of $\sim 7-20$%. Two of them (B2 0838+32 and B2 1626+39) have multiple nuclei and B2 0149+35 has a dust lane that can be the responsible of this off-centering. Another galaxy, B2 0222+36, may have off-centering, but this needs confirmation. Profiles of the $\delta /r$ deviations for the significant cases are shown in Fig. 3.

A summary of the morphological and isophotal peculiarities of the galaxies derived from the surface photometry is given in Table 6. Cases for which evidence for a particular property is marginal are indicated by "yes?'' in the corresponding column and need confirmation through deep or high-resolution imaging.

5.4 Surface photometry of the spiral/irregular galaxies

We have obtained the surface photometry of six galaxies classified as spirals or irregulars from the contour plots, and we present their surface brightness profiles as well as the profiles for ellipticity, position angle and B₄ in Fig. 2. Three of the spiral galaxies (B2 0722+30, B2 0944+39 and B2 1039+27) have B₄ profiles clearly showing disky isophotes, with a high amplitude, related to the presence of disks, arms or bars. These features, responsible for the positive B₄ terms, are revealed as humps on the surface brightness profiles.