## 2. Data-models comparison method## 2.1. Determination of the underlying population propertiesThe optical-near-infrared colors measured for the underlying stellar component (see Sects. 5 and 6.1 in Paper I) have been compared with those predicted by the Bruzual & Charlot (priv. comm.) evolutionary synthesis models. We have obtained the best-fitting model for each point in the color profiles (see Fig. 6 of Paper I) using a maximum likelihood estimator. This maximum likelihood estimator is defined as, where (with We have studied different star formation histories for the formation of this component. In particular, we have considered instantaneous and 1, 3 and 7 Gyr duration bursts and continuous star formation models. This comparison has been restricted to models with metallicity lower than the solar value. We are confident with this assumption since the gas metallicities derived in Sect. 3.4 for the galaxy star-forming regions are lower than one tenth solar. ## 2.2. Determination of the star-forming regions propertiesA more elaborated comparison method has been used in the case of the galaxy star-forming regions. This comparison method is fully described in Gil de Paz et al. (2000b). Briefly, it combines Monte Carlo simulations and a maximum likelihood estimator with Cluster and Principal Component Analysis. The maximum likelihood estimator employed is very similar to that described in Sect. 2.1, but replacing the and colors by the and colors. In addition, since these regions have intense H emission, we have included a new term, defined as +2.5log(). This term is equivalent to the H equivalent width (EW hereafter) term, 2.5log EW(H), used in Gil de Paz et al. (2000b). The magnitudes are those measured within the apertures given in Paper I. In order to properly derive this new term, we have computed the
fraction of H flux, i.e. the fraction
of Lyman photons, due to the stellar continuum measured within the
apertures. Two different approaches can be followed. First, we could
measure the H fluxes using these
apertures. However, since the H
emission is usually more extended than the continuum emisson, this
procedure would sistematically underestimate the
H flux (see, e.g. #8, #13, #18, #50,
#70 and #80 regions). Therefore, we have used an alternative method.
We measured the total H using the
COBRA program (see Paper I). Then, we assumed that
the fraction of photons emitted within the apertures relative to the
total emission is equivalent for the Lyman and Then, multiplying these flux ratios by the total H fluxes given in Table 4 of Paper I, we derive the H luminosities due to the continuum emission measured within the apertures. The H fluxes were corrected for
extinction using the color excesses
provided by the
H-H,
H-H
Balmer decrements. In addition, the broad-band magnitudes and colors
were corrected for extinction assuming that the extinction affecting
the stellar continuum and the gas extinction are related Thus, each star-forming region has a point associated in the
, ,
, ,
,
+2.5log()
six-dimensional space. However, the corresponding uncertainties
transform these points into probability distributions. Using a Monte
Carlo method with 10 Finally, we studied the clustering pattern present in these
distributions using a hierarchical clustering method (see Murtagh
& Heck 1987). This method allows to isolate different solutions in
the ( For the central starburst component we used a similar procedure.
However, since no H emission was
detected for this component, the
+2.5log()
term was not included in the maximum likelihood estimator. In
addition, we introduced the continuum color excess
as a free parameter. Color excesses
in the range 0.0- were studied, where
is the Galactic color excess. For
each of the 10 © European Southern Observatory (ESO) 2000 Online publication: October 2, 2000 |