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3. Data reduction and photometric calibrations

tex2html_wrap_inline1694 and I images were taken at the Cassegrain focus of the 2.2 m MPI/ESO telescope with EFOSC2, CCD19 (tex2html_wrap_inline1698 with a scale of tex2html_wrap_inline1700) during 3 photometric nights in la Silla, Chile, in October 1994. Typical exposure times were 900 s in B, 300 s in V and 600 s in I.

Figure 1: Grey-scale V images of 9 Karachentsev's pairs of galaxies in our sample. North is to the top and East to the left. For the pair CPG548 the size of the image is 5.8tex2html_wrap_inline1710 5.8'. For the other 8 images the size is 2.9tex2html_wrap_inline1710 2.9'

Figure 2: Grey-scale V images of 5 Karachentsev's pairs of galaxies in our sample. North is to the top and, East to the left. For the pair CPG587 the size of the image is 5.8tex2html_wrap_inline1710 5.8'. For the other 4 images the size is 2.9tex2html_wrap_inline1710 2.9'

Observational conditions were photometric with an average seeing of 1.2''.

Initial reductions followed standard procedures, and were performed with IRAF. The bias level was subtracted from each image which were then divided by the averaged flat field exposure for the night. In Figs. 1 (click here) and 2 (click here) we show the grey-scale V images of the 14 pairs in our sample.

Standard stars from Landolt's list (1992), MarkA, T Phe and SAO 98 were observed for extinction and calibration purposes. To calibrate the images we used the following equations:





where tex2html_wrap_inline1732 and I are the standard magnitudes, b, v and i are the instrumental magnitudes, X is the airmass of the observation and the coefficients A, C, D, E and F are the transformation coefficients. B was obtained from the relation B = b + m0 - mxX, where m0 is m0 = -A1 - A3((b-v) - E1)/E3 and mx = (A2 - A3E2/E3). Similar relations were derived for V and I.

In Table 1 we present the values of the extinction and color coefficients, mx and m0, and the rms of the fit obtained for each filter. Mean extinction coefficients provided by ESO were used for comparison.

Two-dimensional surface photometry was accomplished using the last version of GALPHOT image analysis package developed and kindly provided by W. Freudling. This package runs in the IRAF environment and uses the ellipse-fitting routine available in the ISOPHOTE package of STSDAS following the method described by Jedrzejewski (1987). Prior to ellipse fitting, the sky background is removed and field stars and other features are automatically masked outside the outer isophotes of the galaxy.


Filter m0 mx rms
B -0.0027 0.2402 0.006
V -0.0026 0.1531 0.012
I -0.0027 0.0649 0.009
Table 1: Calibration coefficients

Stars overlapping galaxies were carefully removed by extracting elliptical models which were substituted using an interpolation across the region. For pairs with overlapping galaxies such as CPG018, CPG083, CPG091, CPG099, CPG547, CPG577, we used the following procedure: first we built a model of the pair component a, called model a using the ELLIPSE and BMODEL. Then we subtracted this model from the original image, creating an image 2. From image 2 we built a model for galaxy b, called model b. Model b was subtracted from the original image, creating image 3, which was then used to do photometry of galaxy a (see Fig. 3 (click here)a for an example of image 3). With the same procedure we have created images 4, 5 which was used to do photometry of galaxy b (see Fig. 3 (click here)b for an example of image 5). In order to test this method we have applied it to synthesized images. Good results were obtained when the overlapped area was less than 10% of the total area. The goal of this procedure was to recover the overlapping external regions of the galaxies. For the cases where there were remaining features in the center of the subtracted galaxies, we masked these features before doing photometry of the companions. Photometry of CPG018 and CPG083 should be used with caution since they present a common envelope.

Figure 3: a) Image of CPG547 after the model subtraction of galaxy b; b) Image of CPG547 after the model subtraction of galaxy a)

The radial surface luminosity profiles of the galaxies were fitted by an exponential law of the form

where tex2html_wrap_inline1788 is the surface luminosity density (in tex2html_wrap_inline1790 pc-2) corresponding to the surface brightness, tex2html_wrap_inline1794 (in mag arcsec-2), and r is the major axis radius. The fit parameters are then the central surface brightness, tex2html_wrap_inline1800, and the exponential scale length, tex2html_wrap_inline1802. The value of tex2html_wrap_inline1804 will depend on the morphological type of the galaxy, being 1 for spiral disks, and 1/4 for ellipticals or spiral bulges (de Vaucouleurs 1948). The following relations were used in order to fit the surface brightness versus radius and obtain the fit parameters:

where tex2html_wrap_inline1802 is the disk scale length and tex2html_wrap_inline1810 is the disk central surface brightness, tex2html_wrap_inline1812 and tex2html_wrap_inline1814 are the effective radius and the surface brightness at tex2html_wrap_inline1812 radius and surface brightness at tex2html_wrap_inline1812. We present the radial surface brightness distributions and the exponential fits in Figs. 4 (click here) and 5 (click here).

Figure 4: Brightness profiles (tex2html_wrap_inline1820, tex2html_wrap_inline1822 and tex2html_wrap_inline1824) and exponential fits (solid lines), in units mag/tex2html_wrap_inline1826, as a function of the semi-major axis (a or a1/4) in units of arcsec, for both galaxies of 9 pairs of our sample in B, V, I filters. a) and b) on the top indicate the members of the pairs

Figure 5: The same as in Fig. 4 for five other pairs

The total magnitudes were calculated by extrapolating the luminosity profiles to infinity. For several galaxies, bulge and disk components were deconvolved from the surface brightness profile following a standard interactive procedure (Boroson 1981; Schombert & Bothun 1987; Marquez & Moles 1996). First, we have fitted the outer regions of the surface brightness profile using an exponential law. We have subtracted the fitted disk profile from the total profile obtaining the bulge profile. Then, the bulge profile was fitted using an r1/4 law. Finally, the fitted profiles (disk and bulge) were summed and the total fitted profile was compared with the total observed profile. After some iteractions the method converges and we obtain the lowest tex2html_wrap_inline1840 fit. The total magnitudes correspond to the lowest tex2html_wrap_inline1840. The major uncertainty on the total (asymptotic) magnitude values depends on the error on the surface brightness (tex2html_wrap_inline1794) calculated at the outermost point reached by the profile determination (due to the uncertainty on the sky level) and on the error due to the extrapolation procedure. Taking into account all contributions, the total uncertainties are 0.18, 0.13 and 0.12 for B, V and I, respectively. The total magnitudes B, V and I, were corrected for inclination following Burstein (1979); Binggeli (1980) and Franx et al. (1991) for the ellipticals. For the spirals we have assumed round disks. We have also corrected for galactic extinction using Burstein & Heiles (1984) sample. For the galaxies that are not in their sample we have followed the method in RC2 (de Vaucouleurs et al. 1976).

Figure 6: Comparison between our values of B magnitudes before corrections (galactic extinction and inclination corrections) and mB from RC3. Filled circles correspond to six galaxies that we had in common with RC3 and with magnitude differences tex2html_wrap_inline1862. The others symbols correspond to the pair CPG587 (triangles), the galaxy CPG575a (square), and the galaxy CPG580a (star) whose differences related to RC3 are discussed in the text. The error bars are those given in the RC3

In order to check the quality of our photometry we have compared our magnitudes with mB in the RC3 (de Vaucouleurs et al. 1991) and with other papers in the literature, specially with Marquez & Moles (1994 and 1996, MM hereafter) values since our magnitude determination followed similar procedure. In Fig. 6 (click here) we plotted the values of our B magnitudes before corrections (galactic extinction correction and inclination correction) and mB from RC3. CPG575b was already found as discrepant by MM and our result is closer to MM value than to RC3 (RC3 mB= 15.08, MM mB= 14.67, our value
mB = 14.14). CPG580a is discrepant in relation to RC3 by 0.86 magnitudes, but that could be due to presence of a warped disk which makes the photometric decomposition not straightforward. The other two discrepant galaxies are CPG587a and b. The distortion on the disks could also be responsible for this large difference in magnitudes (0.83 for CPG587a and 2.00 for CPG587b), however there is no additional magnitude determination in the literature for comparison. The other six galaxies that we have in common with RC3 have differences in magnitude tex2html_wrap_inline1862. CPG575a, for instance, is only 0.01 fainter in our determination than in RC3 and agrees with MM within 0.11 magnitudes, when no corrections are applied.

A few galaxies in our sample were studied by other authors. Reshetnikov et al. (1993) has done photometry of CPG099, CPG587 and CPG575 however, we can not compare directly the total magnitude values because they have used another photometric system (the R Cousins system). Rampazzo et al. (1995) studied CPG548 but they did not provide total magnitude values, only surface brightness profiles. They have remarked that the photometric profile of CPG548b (NGC 6964) follows an r1/4 law, indicating that this object is a bona fide elliptical which is in agreement with our results for this galaxy. Heraudeau & Simien (1996) have done the photometry of CPG548a galaxy extrapolating the brightness distribution by an exponential function. They obtained total magnitude values equal to 12.06 and 11.24 for V and I bands, respectively. Our total magnitude values for this object are different from these values maybe due to the differences in the decomposition method and the extrapolation procedure.

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