On the sulphur abundance of blue compact dwarf galaxies

On the Sulphur Abundance of Blue Compact Dwarf Galaxies Carolina Kehrig

1,2

, José M. Vílchez

1,

Enrique

1 Pérez ,

2

Eduardo Telles & François Cuisinier

3

1 – Instituto de Astrofísica de Andalucía (Granada, Spain), 2 - Observatório Nacional (Rio de Janeiro, Brazil) 3 – Departamento de Astronomia, UFRJ (Rio de Janeiro, Brazil) Motivation The sulphur and oxygen abundances give us information about the evolutionary stage of the starburst region since their evolution tells us what mass range of stars dominates in the production of these two elements, and whether that mass range has changed with time. Therefore the analysis of the strong near–IR [SIII] and [SII] lines (??9069, 9532 Å and ??6717, 6731 Å) is fundamental if we want to study the sulphur in HII regions. However the number of HII galaxies for which measurements exist of these lines, together with optical emission lines, is still small. This is mainly because of the difficulty of observing [SIII] 9069,9532 due to atmospheric absorption in this range. This implies that published S++ abundances, the dominant sulphur ion, are mostly based on measures of the weak [SIII]6312. Given the large uncertainties associated with observing [SIII]6312 and the importance of S++ in determining the sulphur abundance, it is clear that more observations of the strong near–IR [SIII] lines are desirable. With this motivation, we have obtained CCD observations of [SIII] in a number of HII galaxies. Another important point is regarding to sulphur abundance parameter S23 (Vílchez & Esteban 1996) defined as: S23 = ( [SII]??6717,6731 + [SIII]??9069,9532 ) / Hß A calibration of S23 versus sulphur abundance should enable sulphur abundance determinations in HII regions where the weak temperature sensitive line [SIII]6312 is undetectable. In addition, the parameter S23 can also be used to derive oxygen abundance, and this calibration presents two important advantages against the R23 : it remains single-valued up to abundances close to solar and it is almost independent of the degree of ionization of the nebula.

SUMMARY We have analysed long-slit spectrophotometric observations of a sample of 35 HII galaxies in the red (6000 Å - 1µm), including the nebular [SIII] lines ??9069,9532. For 6 of the 35 objects observed it has been possible to measure the auroral [SIII] line at ?6312. All these galaxies were observed previously in the blue (Kehrig et al 2004) and it has therefore been possible to derive directly two line temperatures: t([OIII])obs from the [OIII]4363 line (Kehrig 2003) and t([SIII])obs from the [SIII]6312 line. In this work we present the physical conditions and chemical abundances to the 6 galaxies with the [SIII]6312 Å measured. Despite the observed dispersion in the temperatures the final result for the abundances is in accordance with the expected values for HII galaxies.

Pysical Conditions & Abundances Object

IIZW40

Tol0226-390

ne ([SII])

171±73

233± 116

= 278

=170

= 100*

276±86

te ([SIII])obs

1.11±0.12

1.85± 0.22

1.67±0.19

1.82±0.50

1.36±0.29

1.38±0.39

te ([OIII])expa 1.13±0.14

2.02± 0.26

1.81±0.22

1.99±0.60

1.44±0.35

1.46±0.47

te ([OIII])obs

1.31±0.07

1.16± 0.05

1.32±0.07

1.27±0.06

1.32±0.07

1.21±0.06

te ([SIII])expa 1.26±0.06

1.14± 0.04

1.27±0.06

1.22±0.05

1.27±0.06

1.17±0.05

12 +log(O+/H+) 7.12±0.09

7.82± 0.08

7.31±0.08

…

7.54±0.09

…

12 +log(S+/H+)

5.29±0.04

5.93± 0.03

5.56±0.03

5.79±0.04

5.72±0.03

5.70±0.03

12 +log(S++/H+) 6.48±0.09

5.75± 0.07

6.01±0.07

6.12±0.19

6.07±0.17

6.01±0.25

ICF(S+ + S++)b 1.50± 0.11

1.05± 0.04

1.28±0.08

1.18±0.05

1.10±0.05

1.36±0.07

12 +log(S/H)

6.69±0.09

6.17± 0.04

6.25±0.06

6.36±0.13

6.27±0.12

6.31±0.17

12 +log(N+/H+) 5.95±0.04

6.69± 0.03

6.07±0.04

6.26±0.04

6.18±0.04

6.30±0.04

12 +log(N+/O+) 10.84±0.10

10.87± 0.09

10.76±0.09

…

10.64±0.09

…

5.78±0.11

5.43± 0.08

5.26±0.08

5.67±0.22

5.60±0.19

5.65±0.30

-0.20±0.01

-0.03± 0.05

-0.23±0.01

-0.05±0.01

0.00±0.02

-0.08±0.09

12 +log(Ar++/H+) log(S23)

Tol1924-416

Tol0538-416

Cam0840+1201

CTS1008

* Tol0226-390 and Tol0538-416 present very high values for the t([SIII])obs; both galaxies present a poor measurement of the [SIII]6312 line . In any case, we have to take into account that it is difficult to apply homogenous HII region-model results to these galaxies. Further observations with higher spectral resolution and larger aperture telescopes are needed. * Despite the observed dispersion in the temperatures the final result for the abundances is in accordance with the expected values for HII galaxies.

* - low density limit ; a – assuming that t(S2+)=0.83t(O2+)+0.17(Garnett 1992); b- Barker(1980) and French(1981)

Line Intensities Line

i(?)a

IIZW40

[OI]6300 … 5.8±0.7 [SIII]6312 … 3.6±0.8 [OI]6364 … … [NII]6548 … … Ha 103 103±16.0 [NII]6584 … 26.0±4.0 HeI 6678 … 11.1±3.0 [SII]6717 … 25.1±0.3 [SII]6731 … 20.0±1.0 HeI 7065 … 16.1±0.4 [ArIII]7136 … 28.7±0.6 [OII]7320 … 3.8±0.2 [OII]7330 … 3.4±0.2 Pa14 2.3 2.3±0.2 Pa13 2.9 2.8±0.1 Pa12 3.9 4.8±0.9 Pa11 4.8 5.3±0.6 Pa10 6.4 6.8±0.6 [SIII]9069 … 50.8±0.3 Pa9 8.8 9.4±1.6 [SIII]9532 … 74.3±1.5* Pa8 12.8 13.8±3.0c

Tol0226-390 Tol1924-416 Tol0538-416 Cam0840+1201 CTS1008 25.8±0.7 7.7±0.4 12.0±1.3 50.0±10.0 103 ±30.0 116.0±8.0 15.0±3.0 88.0±4.0 73.0±3.0 16.0±0.1 33.0±2.0 10.0±0.4 11.5±0.9 … … … … … 43.6±0.9* 7.7±0.8 117.9±11.4 13:b

18.3±0.5 4.7±0.7 3.4±0.5 14.0±7.0 103 ±20.0 35.0±3.0 14.0±2.0 51.0±3.0 39.0±3.0 11.0±0.6 18.7±0.2 7.7±0.1 6.0±0.3 1.4±0.2 3.0±0.9 … 5.6±2.0 6.1±0.2 33.5±0.3 … 72.3±0.9* 16:b

14.2±1.4 7.7±2.6 4.9±2.4 17.5±2.5 103±14.5 50.1±3.5 12.9±1.3 82.5±2.7 60.6±4.4 10.8±1.5 55.3±1.7 … … … … … … … 48.9±0.8* … 73.4±2.5* 13:b

19.9±2.0 6.4±2.1 … 20.0±3.0 103±50.0 45.0±2.0 … 78.0±1.0 54.0±0.7 11.4±0.4 29.0±1.0 10.0±0.5 8.9±0.6 … … … … … 34.1±0.9* … 154.4±3.8* 21:b

13.7±1.5 5.8±1.9 … 22.0±6.0 103 ±190.0 50.0±10.0 13.0±1.0 54.0±2.0 46.0±1.0 13.0±1.0 33.0±1.0 … … … … … … … 36.0±1.0* 14.0±3.0 137.1±17.8* 13:b

A comparison between the measured line temperatures of [OIII] and [SIII]. The dark dotdashed line is the model-deduced relation for the same ion-weighted temperatures from Garnett (1992).

.

a) i(?)=jP(?)/jHa adopting case B recombination ratios (Osterbrock 1989) b) severe blending with [SIII]9532; c) blended with [SIII]9532 *) measured [SIII] fluxes more affected by atmospheric absorption Note – Intensities relative to Ha = 1000.0; : indicates uncertain value

The parameter S23 plotted against the oxygen and sulphur abundances of the galaxies.

Our measurements agree with the observed global relation in Pérez-Montero & Díaz (2003). But there are non-negligible deviations between the fit and the observed points which might affect the calculation of sulphur abundances in the regime of high excitation. The validity of this relation should be further explored, including third parameter dependence.

HII Galaxies Red Spectra

References Bresolin, F. , Garnett, D. R. & Kennicutt R.C. , astro-ph/0407065

Acknowledgements Conselho Nacional de Desenvolvimento Científico e Tecnológico in Brazil (CNPq) Instituto de Astrofísica de Andalucía Royal Astronomical Society

We have added our observed HII galaxies data points to the empirical calibration of the metallicity parameter, S23 = ([SII]+[SIII])/Hß (Pérez-Montero & Díaz 2003). This calibration presents two important advantages over the commonly used one based on the optical oxygen lines: it remains single-valued up to abundances close to solar and is almost independent of the ionization parameter of the nebula. The sulphur abdundance is also included (with a 1dex bias.)

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