L. S. Pilyugin

Oxygen abundance determination in H II regions: The strong line intensities-abundance calibration revisited

The problem of oxygen abundance determination in H II regions based on the strong oxygen line intensities-oxygen abundance empirical calibration is revisited. A compilation of spectroscopic data of H II regions in spiral and irregular galaxies with a measured [O III] λ4363 line intensity has been carried out, resulting in a sample containing more than 700 individual measurements. Methods are devised to select out only high-precision measurements from that original sample. Te-based oxygen abundances have been recomputed in the same way for all H II regions with high-precision measurements. That sample of Te abundances is used to recalibrate the empirical relations between the oxygen abundance and the strong oxygen line intensities for both high (the upper branch of the O/H-R23 diagram) and low (the lower branch) metallicities, within the framework of the P method, where P is the excitation parameter. Concerning high-metallicity H II regions, an alternative way for deriving oxygen abundances using only measurements of the strong nebular oxygen lines is proposed. The method is based on a tight correlation between the flux in the auroral [O III] λ4363 line and the fluxes in the nebular [O II] λλ3727, 3729 and [O III] λλ4959, 5007 lines, called the ff relation. This relation is also used to select out high-metallicity H II regions with high-precision measurements. In contrast to previous work, the new upper branch P calibration is based only on abundances. It is found that P abundances usually agree well with the ff abundances, although faint (log R23 -0.5) low-excitation (P 0.25) H II regions may show systematic differences that can be as large as ~0.1 dex. As for the newly derived low-metallicity P calibration, it is shown to be robust. The calibrations derived from the sample containing all abundance determinations and from that containing only recent (since 1995) measurements are found to be in very good agreement. For both low- and high-metallicity H II regions, the new calibration gives P abundances that agree with abundances to within 0.1 dex.

Oxygen and nitrogen abundances in nearby galaxies - Correlations between oxygen abundance and macroscopic properties

We performed a compilation of more than 1000 published spectra of HII regions in spiral galaxies. The oxygen and nitrogen abundances in each HII region were recomputed in a homogeneous way, using the P-method. The radial distributions of oxygen and nitrogen abundances were derived. The correlations between oxygen abundance and macroscopic properties are examined. There is a significant difference between the L-Z relationship obtained here and that based on the oxygen abundances determined through the R_23-calibrations. The oxygen abundance of NGC 5457 recently determined using direct measurements of Te (Kennicutt, Bresolin&Garnett 2003) agrees with the L-Z relationship derived here, but is in conflict with the L-Z relationship derived with the R_23-based oxygen abundances. The obtained L-Z relation for spirals is compared to that for irregulars. Our sample of galaxies shows evidence that the slope of the O/H-M_B relationship for spirals is slightly more shallow than that for irregulars. The effective oxygen yields were estimated for spiral and irregular galaxies. The effective oxygen yield increases with increasing luminosity from M_B=-11 to M_B=-18 (or with increasing rotation velocity from Vrot=10 km/s to Vrot=100 km/s) and then remains approximately constant. Irregular galaxies from our sample have effective oxygen yields lowered by a factor of 3 at maximum, i.e. irregular galaxies usually keep at least 1/3 of the oxygen they manufactured during their evolution.We performed a compilation of more than 1000 published spectra of H  regions in spiral galaxies. The oxygen and nitrogen abundances in each H  region were recomputed in a homogeneous way, using the P-method. The radial dis- tributions of oxygen and nitrogen abundances were derived. The correlations between oxygen abundance and macroscopic properties are examined. We found that the oxygen abundance in spiral galaxies correlates with its luminosity, rotation velocity, and morphological type: the correlation with the rotation velocity may be slightly tighter. There is a significant difference be- tween the luminosity-metallicity relationship obtained here and that based on the oxygen abundances determined through the R23-calibrations. The oxygen abundance of NGC 5457 recently determined using direct measurements of Te (Kennicutt et al. 2003) agrees with the luminosity-metallicity relationship derived in this paper, but is in conflict with the luminosity-metallicity relationship derived with the R23-based oxygen abundances. The obtained luminosity-metallicity relation for spiral galaxies is compared to that for irregular galaxies. Our sample of galaxies shows evidence that the slope of the O/H − MB relationship for spirals (−0.079 ± 0.018) is slightly more shallow than that for irregular galaxies (−0.139 ± 0.011). The effective oxygen yields were estimated for spiral and irregular galaxies. The effective oxygen yield increases with increasing luminosity from MB ∼− 11 to MB ∼− 18 (or with increasing rotation velocity from Vrot ∼ 10 km s −1 to Vrot ∼ 100 km s −1 ) and then remains approximately constant. Irregular galaxies from our sample have effective oxygen yields lowered by a factor of 3 at maximum, i.e. irregular galaxies usually keep at least 1/3 of the oxygen they manufactured during their evolution.

New Improved Calibration Relations for the Determination of Electron Temperatures and Oxygen and Nitrogen Abundances in H II Regions

New improved empirical calibrations for the determination of electron temperatures and oxygen and nitrogen abundances in H II regions from the strong emission lines of oxygen, nitrogen, and sulfur are given. They are derived using spectra of H II regions with measured electron temperatures as calibrating data. Calibration relations are given separately for three classes of H II regions: cool, warm, and hot ones. Criteria for assigning a H II region to one of these classes are suggested. We find that classification ambiguities arise only in the case of hot H II regions with enhanced nitrogen abundances. The derived calibrations provide reliable abundances for H II regions: the mean difference between oxygen abundances determined from the calibrations and Te -based oxygen abundances is ~0.075 dex, while it is ~0.05 dex for nitrogen abundances.

Oxygen abundances in dwarf irregular galaxies and the metallicity-luminosity relationship

The low-luminosity dwarf irregular galaxies are considered. The oxygen abundances in HII regions of dwarf irregular galaxies were recalculated from published spectra through the recently suggested P - method. It has been found that the metallicity of low-luminosity dwarf irregular galaxies, with a few exceptions, correlates well with galaxy luminosity. The dispersion of oxygen abundances around the metallicity - luminosity relationship increases with decreasing of galaxy luminosity, as was found by Richer and McCall (1995). No relationship between the oxygen abundance and the absolute magnitude in the blue band for irregular galaxies obtained by Hidalgo-Gamez and Olofsson (1998) can be explained by the large uncertainties in the oxygen abundances derived through the Te - method, that in turn can be explained by the large uncertainties in the measurements of the strengths of the weak oxygen line [OIII]4363 used in the Te - method.

The metallicity extremes of the Sagittarius dSph: SALT spectroscopy of PNe

In this work we present the first spectroscopic results obtained with the Southern African Large Telescope (SALT) during its performance-verification phase. We find that the Sagittarius dwarf spheroidal galaxy (Sgr) contains a youngest stellar population with [O/H] ≈ -0.2 and age t > 1 Gyr, and an oldest population with [O/H] = -2.0. The values are based on spectra of two planetary nebulae (PNe), using empirical abundance determinations. We calculated abundances for O, N, Ne, Ar, S, Cl, Fe, C and He. We confirm the high abundances of PN StWr2-21 with 12 + log(O/H) = 8.57 ± 0.02 dex. The other PN studied, BoBn 1, is an extraordinary object in that the neon abundance exceeds that of oxygen. The abundances of S, Ar and Cl in BoBn 1 yield the original stellar metallicity, corresponding to 12 + log(O/H) = 6.72 ± 0.16 dex which is 1/110 of the solar value. The actual [O/H] is much higher: third dredge-up enriched the material by a factor of ∼12 in oxygen, ∼240 in nitrogen and ∼70 in neon. Neon as well as nitrogen and oxygen content may have been produced in the intershell of low-mass asymptotic giant branch (AGB) stars. Well defined broad WR lines are present in the spectrum of StWr2-21 and absent in the spectrum of BoBn 1. This puts the fraction of [WR]-type central PNe stars to 67 per cent for dSph galaxies.

Oxygen abundances in the most oxygen-rich spiral galaxies

Oxygen abundances in the spiral galaxies expected to be richest in oxygen are estimated. The new abundance determinations are based on the recently discovered ff-relation between auroral and nebular oxygen line fluxes in HII regions. We find that the maximum gas-phase oxygen abundance in the central regions of spiral galaxies is 12+log(O/H)~8.75. This value is significantly lower than the previously accepted value. The central oxygen abundance in the Milky Way is similar to that in other large spirals.

The oxygen abundance distribution in M 101

The well-observed spiral galaxy M 101 was considered. The radial distributions of oxygen abundances determined in three dierent ways (with the classic Te{method, with the R23{method, and with the P {method) were compared. It was found that the parameters (the central oxygen abundance and the gradient) of the radial (O/H)P abundance distribution are close to those of the (O/H)Te abundance distribution. The parameters of the (O/H)R23 abundance distribution dier signicantly from those of the (O/H)Te abundance distribution: the central (O/H)R23 oxygen abundance is higher by around 0.4 dex and the gradient is steeper by a factor of around 1.5 as compared to those values in the (O/H)Te abundance distribution. The dispersion in (O/H)P abundance at xed radius is rather small,0.08 dex, and is equal to that in (O/H)Te abundance. The dispersion in (O/H)R23 abundance at xed radius is appreciably larger,0.16 dex, compared to that in (O/H)Te abundance. It has been shown that the extra dispersion in (O/H)R23 abundances is an artifact and reflects scatter in excitation parameter P at xed radius.

THE ABUNDANCE PROPERTIES OF NEARBY LATE-TYPE GALAXIES. I. THE DATA

We investigate the oxygen and nitrogen abundance distributions across the optical disks of 130 nearby late-type galaxies using around 3740 published spectra of H II regions. We use these data in order to provide homogeneous abundance determinations for all objects in the sample, including H II regions in which not all of the usual diagnostic lines were measured. Examining the relation between N and O abundances in these galaxies we find that the abundances in their centers and at their isophotal R {sub 25} disk radii follow the same relation. The variation in N/H at a given O/H is around 0.3 dex. We suggest that the observed spread in N/H may be partly caused by the time delay between N and O enrichment and the different star formation histories in galaxies of different morphological types and dimensions. We study the correlations between the abundance properties (central O and N abundances, radial O and N gradients) of a galaxy and its morphological type and dimension.

On the maximum value of the cosmic abundance of oxygen and the oxygen yield

We search for the maximum oxygen abundance in spiral galaxies. Because this maximum value is expected to occur in the centers of the most luminous galaxies, we have constructed the luminosity - central metallicity diagram for spiral galaxies, based on a large compilation of existing data on oxygen abundances of HII regions in spiral galaxies. We found that this diagram shows a plateau at high luminosities (-22.3<M_B<-20.3), with a constant maximum value of the gas-phase oxygen abundance 12+log(O/H) ~ 8.87. This provides strong evidence that the oxygen abundance in the centers of the most luminous metal-rich galaxies reaches the maximum attainable value of oxygen abundance. Since some fraction of the oxygen (about 0.08 dex) is expected to be locked into dust grains, the maximum value of the true gas+dust oxygen abundance in spiral galaxies is 12+log(O/H) ~ 8.95. This value is a factor of ~ 2 higher than the recently estimated solar value. Based on the derived maximum oxygen abundance in galaxies, we found the oxygen yield to be about 0.0035, depending on the fraction of oxygen incorporated into dust grains.

Abundance determination in H ii regions from spectra without the [O ii]λ3727+λ3729 line

We suggest an empirical calibration for determination of ox ygen and nitrogen abundances and electron temperature in H ii regions where the [Oii]λ3727+λ3729 line (R2) is not available. The calibration is based on the strong emission lines of O , N, and S (NS calibration) and derived using the spectra of H ii regions with measured electron temperatures as calibration datapoints. The NS calibration makes it possible to der ive abundances for Hii regions in nearby galaxies from the SDSS spectra where R2 line is out of the measured wavelength range, but can also be used for the oxygen and nitrogen abunda nces determinations in any H ii region independently whether the nebular oxygen line [O ii]λ3727+λ3729 is available or not. The NS calibration provides reliable oxygen and nitrogen ab undances for Hii regions over the whole range of metallicities.