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

142 4 • Long-slit spectrophotometry of multiple knots of Hii galaxies
The parameter O3N2, defined by Alloin et al. (1979) depends on strong emission lines of
[Oiii] and [Nii]. We used the calibration due to Pettini and Pagel (2004) and, as we can see
in Figure 4.17 it has a very similar behaviour to that of N2.
The combination of the S 23 and O 23 parameters gives S 23 /O 23 , defined by Díaz and Pérez-
Montero (2000), a parameter that increases monotonically with the oxygen abundance up to
oversolar regime and which is very useful to study variations over wide ranges of metallicity
(e.g. disks). We applied the calibration found in Pérez-Montero and Díaz (2005) and, in this
case, there is almost a perfect coincidence with the values found by the direct method in
knots B and C. Again, we find a similar value in knots D and A.
The Ar 3 O 3 parameter, defined and calibrated by Stasińska (2006) as the ratio of [Ariii]
7136 Å and [Oiii] 5007 Å emission lines, predicts identical values for the metallicity in knots
B and C, although noticeably higher than the directly derived ones. A rather high value is
found in knot A and no data on the [Ariii] line in knot D were available.
Finally, the N 2 O 2 parameter, defined by Pérez-Montero and Díaz (2005) as the ratio
between [Nii] and [Oii] emission lines, can be used to obtain the N/O ratio. Using this
parameter, very little difference is found in N/O among the four observed knots, which are
also consistent with the directly derived ratio.
For the sake of comparison, Figure 4.18 shows the corresponding total oxygen abundances
as derived from the strong-line methods used in the previous case and the oxygen abundances
calculated from the electron temperatures measured in the three knots of J1657. As it can
be seen, O 23 and N2 parameters clearly overestimate the oxygen abundance, particularly in
the case of knots B and C. The behaviour of the rest of the parameters is very similar as for
IIZw71, except for Ar 3 O 3 , which has a better estimation of the abundance for all the knots.
4.4.3 The stellar population
The study of the stellar content in the observed knots of J1657 and IIZw71 was carried
out using the STARLIGHT code, which calculates the combination of stellar libraries and the
extinction law that reproduces the spectral distribution of energy, to derive the properties
of the stellar population in each of the knots. The used Population Synthesis stellar libraries
and the method to fit the observed spectra of each of the knots is described in section 4.3.1.
In Figures 4.19 and 4.20 we show the age distributions of the mass fraction (upper) and
the visual light fraction (bottom) for the knots of IIZw71 and J1657, respectively. All of them
present a very young stellar population with ages below 10 Myr (IIZw71) or around this value
(J1657), responsible for the ionization of the surrounding gas, although most of the stellar
mass belongs to populations older than 100 Myr in almost all cases for IIZw71. Interestingly,
for J1657, practically all the mass of the knots comes from a very old population of 8.3 Gyr.
The estimated total stellar mass and the fraction by mass of the stellar population with an
age younger than 10 million years, responsible for the ionization of the gas, are listed in