4 Analysis

   
4.1 Surface brightness profiles

We have analyzed the surface brightness profile of Mrk 86 in different bands (BVRJHK; see Fig. 3). These surface brightness profiles have been obtained using the IRAF task ELLIPSE. Background galaxies and foreground stars were interactively masked. We obtained the surface brightness profiles over the original images, excepting in the outer regions of the R-band profile where a median filter of $5\times5$ pixels (0 $.\!\!^{\prime\prime}$333/pixel) was applied. Due to the contribution of about 60 high surface brightness regions, we only fitted the isophotes with major axis larger than 25 $^{\prime \prime }$. Using the mean of the isophotal centers measured between 80 and 120 $^{\prime \prime }$ in the R-band image we have estimated the galaxy coordinates given in Sect. 2 (see also Fig. 2).

  

Table 4: Photometric magnitudes, colors and H$\alpha$ fluxes

 

Table 4: continued

  

Table 5: Emission-line fluxes

   
4.2 Knot positions and sizes

In order to derive the positions and sizes of the regions in the neighborhood of Mrk 86 we have developed a program called COBRA (see Appendix). Briefly, this program selects the image section where the region of interest is placed. Then, the light profiles in both image axes are fitted using a straight line, in order to account for the underlying emission, and two Gaussians reproducing the knot emission profile. The center of the knot is estimated as the maximum of this latter component. The positions derived for all these regions are given in Table 3 (Cols. 3 and 4). They have been numbered in reverse declination order. Most of these regions were identified on the Johnson R-band image, the deepest of those shown in Fig. 1 (For R=25.6 the signal-to-noise ratio is 30). However, some regions showed intense H$\alpha$ emission, but practically no optical or near-infrared continuum emission (see Table 3). Since more than thirty new objects are identified in this R-band image relative to the Gunn-r image used in GZG, we have introduced a new notation for the knots. In this sense, the regions numbered as #9, #15, #16, #31 and #49 in GZG, now are, respectively, #18, #26, #27, #42 and #70 (see Fig. 2 and Table 3).

The determination of the boundaries of these knots is not an obvious task.

Some authors simply locate the position of the knots by using a weighted mean of the pixels surrounding a maximum of intensity and then they use large enough circular apertures to perform the photometry. This simple approach assume circular geometry for the knots, a prerequisite which is not always fulfilled. An autonomous feature, commonly an H II region, can be also defined as the region which is inside its outermost closed contour (Petrosian et al. 1997) or selecting all pixel that are connected above a given threshold using predetermined background and noise properties of the frame (Mazzarella et al. 1993). All these methods, rewieved in Fuentes-Masip (1997), are difficult to apply in overcrowded fields or when there is a very intense and variable background emission. We have applied a new method which, using interactively the COBRA code, is able to eliminate the emission of adyacent regions and, specially, the low-frequency contamination. This contamination is quite relevant in this object due to the contribution of the underlying stellar population emission to the surface brightness of the galaxy.

After the underlying emission was subtracted, we estimated the number of pixels above different thresholds (see Appendix for a complete description). From the relation between the number of pixels and the threshold used we determine the physical size of the region at the e-folding of the equivalent two-dimensional Gaussian. The contours derived in this way will be not circular (see Fig. 2). This method avoids sistematic effects introduced in the contours and sizes determination due to changes in the underlying emission from one region to another.

The equivalent radii derived are given in Table 3 (Col. 5). Finally, this procedure allows to take into account the effect of the Point Spread Function (PSF hereafter) over the sizes derived. Thus, applying $\sigma^{2}=\sigma_{\rm measured}^{2}-\sigma_{\rm seeing}^{2}$, we obtain the radii corrected from the PSF effect (see Col. 6). The e-folding radius of the PSF in the H$\alpha$ and R-band images was $\sigma_{\rm seeing}=0\hbox{$.\!\!^{\prime\prime}$ }72$. The radii at the e², e³-folding was, respectively, $\sqrt{2}$ and $\sqrt{3}$ times the e-folding radii derived.

   
4.3 Object classification

We have spectroscopic data for only 22 of the 85 regions detected in the neighborhood of Mrk 86 (see Table 5), 20 of them in the low resolution spectra and two, #18 and #40, in the high resolution ones. All these regions seem to belong to the galaxy, with heliocentric emission-lines velocities within the range 400-540kms^-1 (see GZG). However, the nature of the remaining 63 objects is much more uncertain.

Fortunately, there are many other criteria that provide important clues about the nature of these objects, 1) if the host galaxy and some of these objects show line-emission (e.g., H$\alpha$emission) within the wavelength range covered by a narrow-band filter ( $FWHM\sim50-100$ Å) they will probably have similar recession velocities within a range of $\Delta v\sim1000-2000$ kms^-1; 2) if one of these objects is placed in a high surface brightness region of the galaxy, and it has an extended morphology, it will probably belong to the host galaxy. On the other hand, regions that do not show line emission and 3) are placed in galaxy outer regions, or 4) show point-like morphology, should not be classified as belonging to the host galaxy.

Figure 4: IUE spectrum in the range 1150-1975 Å obtained with the SWP camera in the low dispersion mode. The spectrum has been smoothed using a 2 pixels boxcar filter. The Si IV$\lambda$$\lambda$1394, 1403 Å and C IV$\lambda$1548 Å spectral lines are shown with a clear P Cygni profile in the case of the C IV$\lambda$1548 Å line

In Table 3 we mark those regions identified spectroscopically as belonging to Mrk 86 with an S letter in Col. 8. Those regions showing photometric H$\alpha$line-emission are marked with an E letter, and with an N those regions with extended morphology placed at short galactocentric distances. Extended objects placed at large galactocentric distances have been marked with a B (probably background galaxies). Finally, three point-like sources have been classified as F type (probably foreground stars; #4, #61 & #67).

It is worthwhile to check whether these point-like objects without emission lines belong to the galaxy. Their colors (see Sect. 6.2) indicate that objects #4, #61 and #67 are likely old type stars. Column 13 of Table 4 shows the R-band magnitudes of the regions after being subtracted from the galaxy background. Using a distance modulus of 29.2 we derive absolute magnitudes for these stars between 5 and 8 magnitudes too bright to belong to the galaxy. We conclude that objects #4, #61 and #67 are effectively foreground stars. In this work we will study only the S, E and N type objects.

Figure 5: Model predictions. The behavior of the evolutionary synthesis models is shown. We give the change in the B-V and V-J colors, H$\alpha$ luminosity and H$\alpha$ equivalent width for two different metallicities and burst strengths. Solid-lines represent models with two fifths solar metallicity, and dashed-lines represent solar metallicity models. Thick-lines are for b=0.01 burst strength models and thin-lines are for pure burst models. These models have been computed assuming a fraction of 15 per cent of escaping Lyman photons

   
4.4 Optical and nIR photometry

Using the contours obtained with COBRA, we have measured aperture BVRJHK magnitudes and B-V, V-R, R-J, J-H, J-Kcolors for all the regions in Table 4 (Cols. 2-7 for magnitudes, and 8-12 for colors). These apertures include both knot and underlying stellar population emission. Before measuring these colors we degraded the BVRHK images to the worst seeing J-band image ( $FWHM\sim1.8\hbox{$^{\prime\prime}$ }$). In addition, we measured integrated magnitudes in R-band for these regions, subtracted from the underlying emission as determined by COBRA (Table 4; Col. 13). Background subtracted H$\alpha$ fluxes were also measured and they are given in Table 4 (Col. 14). Due to the very bad seeing of the [O III]$\lambda$5007 Å image we do not include photometric [O III]$\lambda$5007 Å fluxes in our analysis.

The magnitudes and colors shown in Table 4 are measured quantities and they have not been corrected for extinction.

   
4.5 Optical spectroscopy

The relatively small slit width employed in these observations, prevents us from obtaining knot total emission line fluxes. We will derive the emission line ratios in order to characterize the ionized gas properties in the galaxy star-forming regions. In Table 5 we give the line intensities measured in a region of $4\hbox{$.\!\!^{\prime\prime}$ }30\times2\hbox{$.\!\!^{\prime\prime}$ }65$ centered in the maximum of the emission knot section covered by the slit. These line intensities have been corrected for internal extinction when gas color excesses, $E(B-V)_{\rm gas}$, were given. The $E(B-V)_{\rm gas}$values have been deduced from the H$\alpha$-H$\beta$ and ${\rm H}\beta-{\rm H}\gamma$ Balmer line ratios measured, and assuming the line ratios predicted for the recombination case-B by Osterbrock (1989). H$\beta$ line intensities have been measured deblending the absorption and emission components using the IRAF-STSDAS NGAUSSFIT task. The line intensities were measured separately in both b (blue) and r (red) spectra. When the line-ratios measured in both arms, typically [O III]$\lambda$5007/H$\beta$, matched for a given region, we merged both data sets.

4.6 Ultraviolet spectroscopy

The aperture used in the UV spectra taken from the IUE Final Archive ( $20\hbox{$^{\prime\prime}$ }\times10\hbox{$^{\prime\prime}$ }$) was centered in the brightest visual knot (#26) with position angle PA = $117\hbox{$^\circ$ }$. The UV spectra of this knot shows the strong absorption lines of Si IV $\lambda\lambda1394,1403$ Å and C IV $\lambda1548$ Å (see Fig. 4). Although this spectrum has been smoothed, the signal-to-noise ratio prevents us from carrying out a quantitative analysis. However, this spectrum contains valuable information, showing a clear P Cygni profile on the C IV $\lambda1548$ Å line, typical of the fast and dense winds of O-type supergiants.