The analysis of nebular spectra constitutes the best, and in some cases the
only one, method for the determination of chemical abundances in spiral and
irregular galaxies, as well as in sites of recent star formation. The
abundances of several elements like He, O, N and S can in principle be
determined since strong emission lines of these elements, some of them
in their dominant
ionization states, are present in the optical region of the spectrum. This
requires knowledge of the electron temperature which can be obtained by
measuring appropriate line ratios like [OIII]
[(4363) /
(
4959 +
5007)], [NII]
[(
5755) /
(
6548 +
6584)], [OII]
[(
7327) /
(
3727 +
3729)], or [SIII]
[(
6312) /
(
9069 +
9532)].
Unfortunately, these line ratios usually involve one intrisically weak line which can be detected and measured with confidence only for the brighter and hotter objects and in many cases - distant galaxies, low surface brightness objects, relatively low excitation regions - they become too faint to be observed.
In these cases, an empirical method based on the intensities of the easily
observable optical lines is widely used. The method, originally
proposed by
Pagel et al. (1979) and
Alloin et al. (1979),
relies on the variation of these lines with oxygen abundance.
Pagel et al. (1979)
defined an abundance parameter R23 = [([OII]
3727 + [OIII]
4959 +
5007) /
(H
)] which
increases with increasing abundance for abundances
lower than about 20% solar, and then reverses its behavior, decreasing with
increasing abundance, since above this value a higher oxygen abundance
leads to a more effective cooling, the electron temperature gets lower and
the optical emission lines get weaker.
In principle,
the calibration of the R23 versus oxygen
abundance relation can be
done empirically in the low metallicity regime where electron temperatures can
be derived directly, but requires the use of theoretical models for the so
called high abundance branch. Several different calibrations have been made
(Edmunds & Pagel 1984;
McCall et al. 1985;
Evans & Dopita 1986;
Skillman 1989;
McGaugh 1991)
as more observational data and more improved models have become
available. However, two problems that are difficult to solve still
remain. The first
one is related to the two-valued nature of the calibration, which can lead to
important errors in the derived abundances. The second one concerns the
dependence of R23 on the degree of ionization of the nebula
(see
Skillman 1989).
R23 also depends on density, but this
can be considered as a second order effect for low density regions
(nH 100
cm-3) which
constitute the majority of the extragalactic population. These two
facts, taken together, produce a large dispersion of the data
for values of logR23
0.8 and 12 + log(O/H)
8.0, with objects
with the same value of log R23 having actual
abundances which differ by
almost an order of magnitude. Unfortunately, a significant number of objects
(about 40% of the observed HII galaxies;
Díaz 1999) have
logR23
0.8 for
which the calibration is most uncertain, and this percentage is even
higher for HII regions in normal spiral galaxies.
Here we present an alternative abundance calibration based on the intensities
of the sulphur lines: [SII]
6716,
6731 and [SIII]
9069,
9532, through the use of the
sulphur abundance parameter S23
(Vílchez & Esteban
1996).
Spectroscopically, these lines are analogous to the optical oxygen lines defining R23 but, due to their longer wavelengths, their contribution to the cooling of the nebula should become important at a somewhat lower temperature. Yet, the lower abundance of sulphur makes these lines less significant than the [OIII] lines as a contributor to cooling. On the other hand, the sulphur lines are less sensitive to temperature, therefore the reversal in the relation between their intensities and the average nebular abundance is expected to occur at a higher metallicity and the relation should remain single valued longer.
From the observational point of view they present two important
advantages: first, the lines are easily detected both in high and low
excitation ionized regions
(Díaz et al. 1990)
and second, they are less
affected by reddening; moreover, they can be measured relative to nearby
hydrogen recombination lines,
H in the case of the [SII]
lines and Paschen lines
in the case of the [SIII] lines, which minimizes also any flux calibration
uncertainties. These lines are accessible spectroscopically with CCD
detectors up to a redshift of about 0.1.