4 Surface photometry

A direct inspection of the broad band images and the contour maps shows that many of these galaxies exhibit morphological peculiarities. To conduct a spatially resolved study of the stellar populations and the distribution of dust, we constructed (B-V), (V-R) and (B-I) colour maps. The colour maps were constructed after matching the PSFs in the two bands to avoid artefacts. This was achieved by degrading the better of the PSFs in the two bands by using a Gaussian smoothing filter. The colour maps thus obtained were examined to identify features like star forming regions, dust lanes, etc. and study their photometric properties and locations with respect to the underlying galaxy. See Figs. 1a-j. Grey scales have been chosen to maximize the contrast over the range of colour indices.

The surface brightness distribution and the variation of the position angle and ellipticity of the isophotes of each galaxy were obtained using the ISOPHOTE package within STSDAS. Ellipses were fit to the galaxy images in all the four pass bands after masking out any foreground stars. The ellipse fitting was done using the algorithm proposed by Jedrejewski ([1987]). The deviation of an isophote from a perfect elliptical shape can be estimated by expanding the difference in intensity between the isophote and the corresponding fitted ellipse as a Fourier series in the eccentricity anomaly along the isophote. A negative coefficient B4 of the cos(4 $\phi$ ) term indicates boxiness while a positive coefficient indicates the presence of a disk. Though this technique was initially used in the analysis of elliptical galaxies, in recent years, it has been successfully used in identifying structures like bars, isophotal twists, dust lanes, etc. in disk galaxies also (Wozniak et al. [1995]; Wozniak & Pierce [1991]). Analysis of a sample of MBG starbursts has been carried out by Barth et al. ([1995]) using ellipse fitting techniques. Non-circular motions of the gas increase the rate of collisions, as well as the radial transport of gas to the galactic center and are seen as isophotal twists. Scoville & Hersh ([1979]) suggest that this mechanism could be responsible for enhancing the gas density in the nucleus and triggering star formation there. Generally, all the parameters of the ellipses, viz ellipticity, position angle and center, were allowed to vary. However, in Mrk 743, which shows a double nucleus, the center was kept fixed at a value derived from the outermost isophotes. Ellipses were fit right up to the central pixel in all cases except in Mrk 743 due to its double nucleus and in Mrk 363 which does not show a well defined nucleus in the B band. For these galaxies, the minimum radius of fitting was decided on the basis of the extent of the central structure. The results of the ellipse fitting viz. the radial variation of the surface brightness, the ellipticity (e) and the position angle (PA) are presented in Figs. 2a-i. The variation in colour is presented in Figs. 3a-j. From the ellipticity of the outermost fitted isophotes, we derive the angle of projection of the galaxy on the plane of the sky using the equation given by Tully ([1988]).

$\begin{figure} \includegraphics []{ds7815f1a.ps}\end{figure}$

Figure 1: a) Clockwise from top left, the B and I band isophotes, the residual image and the (B-I) image. The contours are plotted between 19-23 mag/ $^{\prime\prime}$ ² at intervals of 0.5 mag/ $^{\prime\prime}$ ² for the B band and between 17-22 mag/ $^{\prime\prime}$ ² with the same interval for the I band. The same order is followed in the figures a) to j). For (B-I) images, dark is blue and white is red. For Mrk 87, the B and I images are presented instead of the isophotes to depict the inner ring clearly. North is at the the top and east is to the left. Residual images in the R band are given only for those galaxies which show some fine structure

$\begin{figure} \includegraphics []{ds7815f1b.ps} \end{figure}$

Figure 1: b) continued

$\begin{figure} \includegraphics []{7815f1c.eps} \end{figure}$

Figure 1: c) continued

$\begin{figure} \includegraphics []{ds7815f1d.ps} \end{figure}$

Figure 1: d) continued

$\begin{figure} \includegraphics []{ds7815f1e.ps} \end{figure}$

Figure 1: e) continued

$\begin{figure} \includegraphics []{ds7815f1f.ps} \end{figure}$

Figure 1: f) continued

$\begin{figure} \includegraphics []{ds7815f1g.ps} \end{figure}$

Figure 1: g) continued

$\begin{figure} \includegraphics []{7815f1h.eps} \end{figure}$

Figure 1: h) continued

$\begin{figure} \includegraphics []{ds7815f1i.ps} \end{figure}$

Figure 1: i) continued

$\begin{figure} \includegraphics []{ds7815f1j.ps} \end{figure}$

Figure 1: j) continued

$\begin{figure} \includegraphics [height=11cm]{ds7815f2a.ps} \includegraphics [height=11cm]{ds7815f2b.ps}\end{figure}$

Figure 2: a,b) Radial variation of surface brightness, ellipticity (e) and position angle (PA) in B, V, R, I from left to right

$\begin{figure} \includegraphics [height=11cm]{ds7815f2c.ps} \includegraphics [height=11cm]{ds7815f2d.ps} \end{figure}$

Figure 2: c,d) continued

$\begin{figure} \includegraphics [height=11cm]{ds7815f2e.ps} \includegraphics [height=11cm]{ds7815f2f.ps} \end{figure}$

Figure 2: e,f) continued

$\begin{figure} \includegraphics [height=11cm]{ds7815f2g.ps} \includegraphics [height=11cm]{ds7815f2h.ps} \end{figure}$

Figure 2: g,h) continued

$\begin{figure} \includegraphics [height=11cm]{ds7815f2i.ps} \end{figure}$

Figure 2: i) continued

$\begin{eqnarray} i &=& \cos^{-1} { \left\{ {{(b/a)^2 - 0.2^2}\over{1-0.2^2}} \r... ...+ 3\hbox{$^\circ$}\; \nonumber\\ e &=& 1-{b\over a}\cdot \nonumber\end{eqnarray}$

The results are tabulated in Table 1. The total light enclosed within the isophote at 24, 23.5, 23.5 and 22.5 mag/ $^{\prime\prime}$ ² in the B, V, R and I images respectively, was used to determine m_B, m_V, m_R and m_I by summing over the light in the fitted elliptical annuli. The magnitudes derived in this manner and colour indices for each galaxy are presented in Table 3. along with the spatial distribution of the starburst.

In order to identify non axisymmetric structure and other small scale features, unsharp masked images were constructed for each galaxy. For this, we produced models for the smooth light distribution of the galaxies in each band pass using the parameters derived from the isophotal analysis. This smoothed image was subtracted from the original image, to produce a residual image with enhanced features not apparent in the direct images. Such fine structure provides a wealth of information about the processes at play in the galaxy. We detect complex fine structure most prominently in the galaxies Mrk 1002, Mrk 363 and Mrk 213. For a detailed discussion refer to Sect. 4. Schweizer & Seitzer ([1988], [1992]) quantify such fine structure in E and S0 galaxies that are believed to be induced by mergers. Simulations by Hernquist ([1992], [1993]), also predict features like boxy isophotes and X-shaped fine structure for merger products.

$\begin{figure} \includegraphics []{ds7815f3a-d.eps}\end{figure}$

Figure 3: Radial distribution in the colour indices for the galaxies

$\begin{figure} \includegraphics []{ds7815f3e-h.eps} \end{figure}$

Figure 3: continued

$\begin{figure} \includegraphics []{ds7815f3ij.eps} \end{figure}$

Figure 3: continued

**Table 3:** Total magnitudes and colour indices uncorrected for galactic absorption and inclination effects. The last column describes the locations of the starburst, where nuc: nuclear, comp: companion galaxy
$\begin{tabular} {llllllllllll}\hline Mrk &\multicolumn{2}{c}{$B_{\rm T}$}&\multi... ...12.04&11.31& 13.43 & 0.63 & 0.58 & 0.77&nuc+bar end+comp \\ \hline\end{tabular}$

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