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Astron. Astrophys. 333, 803-808 (1998)

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3. Analysis technique

The isophotal shape analysis was carried out using the `ellipse' task within "STSDAS" 2, which is based on the method described in detail by Jedrzejewski (1987). The dust lane crosses the galaxy close to the center and hence determination of its center accurately is difficult. The I band image is least prone to the dust extinction, so we have determined the center of the median filtered I band image and used the same center for isophotal shape analysis of all the well aligned frames. While fitting ellipses to the isophotes, the center thus determined was kept fixed. We have derived the surface brightness and shape parameters of this galaxy by masking out the regions occupied by the dust lane and foreground stars. Previous work on this galaxy include a comparison of optical image with the radio map to look for the orientation of radio source with respect to the prominent dust lane on the galaxy (Mollenhoff et al. 1992), and its aperture photometry in optical broad band filters by Boisson et al. (1994). Detailed surface photometry of this galaxy has not been reported in the literature. We have used the aperture photometry of Boisson et al. (1994) to calibrate our frames. Simulated aperture photometry was performed using circular apertures to determine zero point constants for calibration. The surface brightness profile and shape parameter profiles are shown in Fig.1. The parameter b4 characterizes the deviation of isophote from the pure ellipse and it appears as the co-efficient of the fourth order cosine term of the Fourier expansion of the ellipse fitting procedure.

[FIGURE] Fig. 1. The R band profiles of surface brightness, ellipticity, position-angle, amplitude of residual cos4 [FORMULA] co-efficient of the isophotal deviation from the best fitting ellipse ([FORMULA]), and color-index profiles.

3.1. Generation of color-index and extinction maps

Color-index map: To calculate the mass of the dust residing in the galaxy we have generated the color-index maps (B-V, V-R, B-I, V-I) using the broad band images. These maps were used to determine the dust distribution and color-excess in the dusty region. The B-I color map is shown in Fig. 2.

[FIGURE] Fig. 2. B-I color index image of the galaxy NGC 2076; darker shades represent redder regions. East is up and north is to the right.

Extinction maps: The amount of extinction is determined by comparing the actual light distribution collected from the galaxy with that expected in the absence of dust lane. The extinction map was generated by dividing the original galaxy frame by a smooth model of it. The smooth model was obtained using the isophotal parameters generated by the ellipse fitting process. While fitting ellipses the regions occupied by dust and foreground stars were masked and ignored. The fits were carried out with all the fitting parameters allowed to vary, except the center. By applying polynomial fit to the fitted data a smooth model of the galaxy was obtained. The model images were used to get the extinction map at each wavelength in the following manner


where, I stands for the ADU counts. This gives the desired extinction map in magnitude scale.
Extinction curve: The extinction maps in different filters were used to determine the extinction due to the dust lane. We chose a rectangular box of size three times the seeing disk, and slided it over the whole region covered by dust to determine the numerical values of local extinction [FORMULA] = B, V, R, I). The local color-excess or selective extinction were also calculated using the relation


For determining the ratio of total to selective extinction we have used the method discussed by Brosch et al. (1990) and Goudfrooij et al. (1994b). Linear regression between the different extinction values were calculated and the best fitting slopes were assigned to be the average of the slope of [FORMULA] Vs. [FORMULA] and the reciprocal slope of [FORMULA] and [FORMULA]. The slope of regression represents the relation between extinction at the two spectral bands. These values are used to derive the average extinction curves for the areas occupied by dust.

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© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998