A spatially resolved study of ionized regions in galaxies at different ...

66 3 • IFS of a GEHR in NGC 6946
the case of IFUs. These kind of studies only consider the results derived from the strongest
emission lines which have small proportional errors. However, we must be careful when the
aim of the work is the detailed and precise study since the errors introduced by the fitting
are higher for the weakest lines, including the auroral ones.
We have produced good continuum fits for each of the 993 spectra in the blue mosaic
wavelength range. This can be checked by creating maps (see section 3.4.2) of several regions
(free of emission lines) of the continuum after model subtraction. In other words, this kind
of maps are plots of the fit residuals, since the subtracted spectrum should be around zero
in the continuum, except for the emission lines. The fit residuals can be spatially checked
creating a map with the values obtained by adding the contribution of a small interval of the
continuum near an emission line for each spectrum (spaxel) in the RSS file. Two vertical lines
in the upper right panel of Figure 3.13 show the interval used for the maps of the residuals
plotted in the lower panels. The lower left panel represents the residual map in the spectral
range 6450-6500 Å (near the emission lines of Hii and [Nii] λλ 6548,6584 Å) for a fit to the
continuum using a template with only solar metallicity and a grid of 12 ages, as explained
before. The lower right panel shows the residual map for a template with metallicities 0.004,
0.008, and 0.02 in the same spectral range and same grid of ages. Two wavelength ranges of
the fit used to create the last map can be seen in the upper panels. Whenever the fit is good,
the sum over the small wavelength interval should be around zero. As it can be seen, the
map for a template with a single (solar) metallicity (lower left panel) gives higher residuals
than using the three-metallicities template (lower right panel). In the last one, the colormap
shows a very uniform distribution with almost all the values contained between -0.1 and 0.1,
and only a few regions with larger deviations. On the contrary, the former map has several
regions with strong differences between the continuum and the fit, reaching values as high
as 0.9 (notice that top values in the other map are below 0.3). It must be noted that the
deviations are related with spatial regions with strong emission, usually correlated with the
intense knots. The shape of the continuum near these zones cannot be properly matched
with one metallicity template alone, so a combination of them are needed. The best fit using
the lowest number of metallicities was using a combination of 0.004, 0.008 and 0.02.
3.4.2 Line intensities
As mentioned in last subsection, FIT3D is a package to fit and deblend emission lines.
Using this software, we fitted line profiles on each of the 993 blue spectra and 331 red spectra
in order to derive the integrated flux of each emission line. The program fits the data with a
given model computing a minimization of the reduced χ 2 and using a modified Levenberg-
Marquardt algorithm. The data is provided through a configuration file where the model is
described. The functions available to build the model are a one dimensional gaussian, a N-
order polynomial function and a spectrum background. A single Gaussian was fitted to each