6 Results

   
6.1 Underlying stellar population

The surface brightness profiles given in Fig. 3 show a clear exponential dominance at regions outer than $\sim1.25$kpc. We have obtained the parameters of this exponential component on the deepest R-band profile yielding a scale, $\alpha$, of $0.88\pm0.02$kpc and an extrapolated central surface brightness, $\mu_{\mathrm{E,0}}$, of $21.50\pm0.06$magarcsec^-2. Papaderos et al. (1996a) obtained quite different values, $\alpha=0.74\,\pm\,0.06$ kpc and $\mu_{\mathrm{E,0}}=19.42\pm0.03$magarcsec^-2. This difference is not surprising considering that the R-band profile given Papaderos et al. (1996a) only reaches galactocentric distances of 2kpc.

The color profiles in the galaxy outer regions, where the underlying stellar population dominates the total light profile, are shown in Fig. 6. No significant color gradients are observed at distances larger than 1.2-1.3kpc, whereas at shorter distances a progressive and swallow blueing is derived. This blueing is probably related with an increment in the light contamination from high surface brightness star-forming regions associated to the plateau component. In Fig. 7 we show the H$\alpha$ profile compared with the surface brightness profile in the B-band of the plateau component (see Papaderos et al. 1996a). From this figure it is quite clear that the progresive blueing observed at distances shorter than 40 $^{\prime \prime }$ is due to currently star-forming regions located in the plateau component.

Figure 7: H$\alpha$ profile obtained using the isophotes measured in the R-band image ( solid-line). The surface brightness profile of the plateau component as parametrized by Papaderos et al. (1996a) in the B-band is also shown ( dot-dashed-line)

   
6.2 Star-forming regions

Figure 8: a) R-band luminosity distribution for the S, E and N-type regions. b) H$\alpha$ luminosity distribution for the galaxy emission-line regions ( S and E types). c) Radius distribution for the S, E and N-type regions. d) H$\alpha$ and continuum R-band luminosities comparison. Lines of constant EW(H$\alpha$) are drawn. e) R-band apparent magnitude vs. radius

In Sect. 4.3 we have classified the regions observed in the Mrk 86 neighborhood as S, E, N, F and B objects (S, spectroscopically confirmed; E, emission-line regions; N, extended and diffuse; F, foreground stars; B, background galaxies).

The optical-nIR colors measured for the F objects seem to indicate that the #4, #61 and #67 regions are, respectively, M0-M3, K2-K5 and G7-K0 spectral type foreground stars. Besides, between the B type objects (background galaxies), we find two very red objects, #2 and #3 regions, with V-K colors of about 5 and 4 ^m, respectively.

Hereafter, we present the results for the study of the S, E and N-type regions. First, we have analyzed their R-band luminosities in Fig. 8a. We observe that this distribution has a clear maximum at M_R=-9.5^m. This distribution can not be well fitted using any standard power-law luminosity function (see, e.g. Elson & Fall 1985, for the LMC star-clusters LF).

In Fig. 8b we show the H$\alpha$ luminosity distribution of the galaxy emission-line regions. The H$\alpha$luminosities used were corrected for internal extinction. In those cases were spectroscopic data were not available we used a mean color excess of $E(B-V)_{\rm gas}=0.34^{\rm m}$. The distribution obtained is nearly flat in the luminosity range 10^37.7-10^38.7ergs^-1, with most of the H II regions showing H$\alpha$ luminosities in the range 10³⁶-10³⁹ergs^-1. This luminosity range corresponds to star formation rates between $6\ 10^{-5}$ and 0.06 $M_{\hbox{$\odot$ }}$yr^-1 (Gil de Paz et al. 2000b). The spatial resolution of the H$\alpha$ image is about 40pc. Therefore, the scarcity of H II regions fainter than 10³⁶ergs^-1 can not be explained as an effect of the spatial resolution (see Kennicutt et al. 1989).

It should be noticed that some of these H$\alpha$ emitting regions showing very faint or null continuum emission (#7, #9, #12, #50), seem to be pure gas regions photoionized by distant stellar clusters. These regions are in many cases associated with the rim of expanding bubbles (see Martin 1998, GZG).

   

Table 6: Data for the Mrk 86 starburst component

	Coordinates
RA(J2000)	08$^{\rm h}$13$^{\rm m}$14.69$^{\rm s}$
DEC(J2000)	+45$^\circ$59$^\prime$21.9 $^{\prime \prime }$
	Photometry
B-V	$0.39\pm0.06$
V-R	$0.54\pm0.25$
V-J	$1.59\pm0.02$
J-H	$0.77\pm0.08$
J-K'	$0.85\pm0.03$
MR	$-14.0\pm0.2~~~$
Radius	4.3 $^{\prime \prime }$
	Spectroscopic indexes
Slit	#4b	#1b
D4000	1.38	1.38
Mg2	0.06	0.04
H$\delta$ $^{\dagger}$	6.0 Å	6.2 Å
Fe5270	1.20 Å	-
Fe5406	0.74 Å	-
Region	$21\hbox{$.\!\!^{\prime\prime}$ }45\times2\hbox{$.\!\!^{\prime\prime}$ }65$	$14\hbox{$.\!\!^{\prime\prime}$ }3\times2\hbox{$.\!\!^{\prime\prime}$ }65$

$^{\dagger}$ Equivalent width in absorption.

Now, in Fig. 8c we show the radius distribution measured in the R-band image (given by the e-folding radius; see Appendix). These radii have been corrected for the PSF contribution (see Sect. 4.2). This distribution shows a maximum at about 1 arcsec, that corresponds to a FWHM of about 55pc.

In Fig. 8d, the R-band and H$\alpha$ luminosities are compared. The lines drawn suggest that these regions have equivalent widths of H$\alpha$ ranging between 100-500 Å. Since both, H$\alpha$ and R-band fluxes have been measured subtracted from the contribution of the underlying population, these EW(H$\alpha$) are pure H II region equivalent widths. In this figure the symbol size is proportional to the extinction corrected B-V color, using larger symbols for bluer regions. Finally, we have compared (see Fig. 8e) the knot apparent magnitude and the physical radius, both measured on the R-band image using the COBRA program. If all these star-forming regions were optically thin at these wavelengths and they had similar star densities we would expect the flux to be proportional to the cube of the radius. In Fig. 8e the best fit to the function $F_R\propto R^{3}$ is also shown.

   
6.3 Central starburst

Finally, we have studied the region classified by Papaderos et al. (1996a) as the starburst component (see Fig. 2). This component appears as a bright, extended and not very well defined region in the broad-band BVRJHK images (see Fig. 1).

The integrated blue color measured by Papaderos et al. (1996a) in the galaxy central parts, $B-R\simeq0.9$ ( $0.93\pm0.26$ in our work), similar to that observed in Scd and Im galaxy types (Fukugita et al. 1995), indicates the existence of a young stellar population superimposed on the evolved underlying component. However, the absence of significant gas emission (see the H$\alpha$ image in Fig. 1) suggests an intermediate aged dominating population. In Table 6 we give the colors and spectroscopic indexes measured for this region (see Trager et al. 1998) at two different slit positions. The integrated optical-near-infrared colors shown in Table 6 have been measured on the R-band image using an aperture with radius 4.3 $^{\prime \prime }$ (1 e-folding), as given by the COBRA program. This aperture is marked in Fig. 2 with a thick-lined contour. These colors have not been corrected for extinction. The M_R absolute magnitude has been measured subtracted from underlying emission. The spectroscopic indexes have been measured in the integrated spectra of the starburst section covered by the slits #4b and #1b. (see Table 2; see also Fig. 1 of GZG).

In Paper II we will derive the physical properties of this region by comparing the optical-nIR colors obtained with the predictions of our evolutionary synthesis models. In addition, the spectroscopic indexes measured will be compared with those prediced by the Bruzual & Charlot (priv. comm.) evolutionary synthesis models.