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

72 3 • IFS of a GEHR in NGC 6946
Interestingly, knots HK83-013 and HK83-014 have strong continuum contribution, but
very low Hα and Hβ emission, and their emission peaks have an offset as compared to their
respective continua. This effect may be due to the fact that the underlying population of
these knots is slightly older. Note that knot HK83-014 has a supernova associated.
As for the continuum emission near 8500 Å of Figure 3.16, the morphology is very similar
to that of the blue part of Figure 3.15. In fact, the continuum peak of the most important
knots in the red matches the ones from the blue. In some structures, the intensity of the red
continuum is more marked, as the case of the source between knots HK83-013 and HK83-016.
Nevertheless, the overall continuum morphology of the whole region matches the one of
the emission line maps, where the complex extends through the west region of the FOV,
while the surroundings of the east (south and north) region have very low surface brightness.
This is clear in Figure 3.3, where it is seen that the region is located at the end of the NE
arm of NGC 6946.
3.4.4 Reddening correction and c(Hβ) map
We have used the value of the Balmer decrement derived from Hα/Hβ to estimate the
reddening coefficient, c(Hβ) for each fiber spectrum. The Hi series are used to determine
the extinction, comparing the observed line ratios with the expected theoretical values. Case
B (optically thick in all the Lyman lines) is the best simple approximation to describe the
physical conditions in the ionization of the gas. This method takes advantage of the fact that
the ratio between the emissivities of two hydrogen recombination lines, which depends on
electron temperature and density, is almost constant. As an example, the ratio between the
emissivity of Hα and Hβ is 2.86 for the case B with n e = 100 cm −3 and T e = 10000 K, and
this value varies less than 10% in the range of interest of temperatures and densities for an Hii
region. Although the variation of these values is small, we have used an iterative method to
estimate them, taking as starting values those derived from the measured [Sii] λλ 6717,6731 Å
and [Oiii] λλ 4959,5007 Å. The theoretical values have been calculated based on the data by
Storey and Hummer (1995). We have used a mean value of n e = 10 2 cm −3 and T e = 8000 K
as characteristic values for the region to obtain the reddening map and the extinction law
given by Cardelli et al. (1989) with R V = 3.1.
Normally, all the available Balmer lines are used to estimate c(Hβ), an then a least square
fit of the measured ratios, F(λ)/F(Hβ), to the theoretical ones is performed. Nevertheless,
Hγ and Hδ lines have very low signal-to-noise in zones of low surface brightness, and are
very difficult to measure for automatic fitting routines, yielding high uncertainties in the
computed c(Hβ) value. Thus, we have used only the ratio Hα/Hβ to derive the extinction,
since these two lines are very easily detected and measured. See Appendix A for a detailed
explanation on the reddening correction and equations involved in the relation between c(Hβ)
and the visual extinction A V .