Pagel s Method to derive the O/H ratio in galaxies

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1 Pagel s Method to derive the O/H ratio in galaxies Manuel Peimbert Antonio Peimbert César Esteban Jorge García-Rojas Silvia Torres-Peimbert María Teresa Ruiz Leticia Carigi Tonantzintla, August 2005

2 Direct Abundance Determination Z=1/25 Z sun Problems: [OIII] 4363 weak, even in the best cases of local low-z gals; most star-forming gals studied at high redshift are not very metal-poor--no HOPE, given typical fluxes and S/N (10-20s Ha detections) (figure from van Zee 2000)

3 R23 Relations A variety of empirical calibrations of the R23 relation have been attempted over the last 25 years. At the low metallicity end, most of the data available for an empirical calibration of the R23 relation lie within a small range of ionization parameter. Thus, it is not possible to calibrate the R23 relation based on observations alone. Slide from Van Zee 2005.

4 z~3 LBG Metallicities: R 23 Measure [OIII] and Hb in the K-band with KeckII/NIRSPEC Measure [OII] in the H- band Relative flux-calibration is difficult, dust extinction Measurements for fewer than 10 gals at z~3 (Pettini et al. 2001; Kobulnicky & Koo 2000)

5 z~3 LBG Metallicities: R 23 LBG nebular O/H ~ Z sun, 2-4 mags brighter than local L-Z relation (Pettini et al. 2001)

6 R23 Relations A variety of empirical calibrations of the R23 relation have been attempted over the last 25 years. At the low metallicity end, most of the data available for an empirical calibration of the R23 relation lie within a small range of ionization parameter. Thus, it is not possible to calibrate the R23 relation based on observations alone. Slide from Van Zee 2005.

7 IZw 18

8 H II Regions in M101

9 M 101 NOAO

10 Pagel s R 23 Method DE Dopita & Evans, 1986 EP Edmunds & Pagel, 1984 MRS McCall et al., 1985 TPPF TPPF + t 2 TPPF Torres-Peimbert et al., 1989

11 HII regions with electron temperature abundances. Slide from: Mustakas 2005

12 Comparison with O/H from calibration of strong lines Abundances derived based on direct measurement of T are lower than those derived from a variety of popular calibrations of R23 = ([O II] + [O III])/Hβ Slide from Garnett 2005.

13 Abundance Determination Methods Any line ratio from two ions of different elements can be used to determine their abundance ratio Permitted Lines (Recombination Lines) [H, He, C, O] Forbidden Lines (Collisionally Excited Lines) [O, N, S, Ar, Cl, etc.]

14 N e and T e Line Intensity Dependence Temperature Dependence Density Dependence Recombination Lines T -α α -1 N e N i Visible and UV CELs T -1/2 exp(-ε/t) ε N e N i : N e < 10 5

15 t 2 T 0 = Determinations T e N e N i dv N e N i dv t 2 = (T e -T 0 ) 2 N e N i dv T 02 N e N i dv T e (4363/5007) = T 0 [1 + (91300/T 0-3) t 2 /2] T e (Bac/Hβ) = T 0 ( t 2 ) T e (He I lines) = T 0 (1 k t 2 ) k 1.8 T e (4649/5007) = f 1 (T 0, t 2 )

16 Abundance Determinations using CELs_V, and assuming t It is not easy to determine t 2. One of the methods used to determine t 2 is to find the value necessary to reconcile the measured RL intensities with the measured CEL intensities. Observations where t 2 can be measured using different methods [T(Bac), T(He), LRs/LCEs, etc.] usually find consistent values for all determinations.

17 O II Recombination Lines, Multiplet 1 Due to their faint intensity < I (Hβ) and to blends with other lines, it is usually not possible to measure all 8 lines of this multiplet. Peimbert, 2003, ApJ, 584, 735

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19 Giant extragalactic H II region, NGC 604 Esteban, Peimbert, Torres-Peimbert, Rodriguez 2002

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21 Intensity Dependence on Density Peimbert, Peimbert, & Ruiz, 2005, ApJ, in press.

22 Density Dependence Equations Based on H II Regions I ( ) I (sum) = N e /2800 I ( ) I (sum) = N e /2800 I ( ) I (sum) = N e /2800 I (4649) I (sum) = N e /2800

23 OII Recombination Lines, Multiplet 1 O ++ abundance depends on the total sum of the intensities from all 8 lines of the multiplet. The relative intensities of the lines within the multiplet are not constant. The relative intensities of the lines, depend on the populations of the 4 fine structure levels from the upper level of the transition. [ 2s 2 2p 2 ( 3 P)3s 4 D 0 3/2 ] When the electronic density is high, there is efficient collisional redistribution among the levels. When the electronic density is low, there is not.

24 NGC 6822 V Observations In order to determine the O/H ratio we have used these equations to correct the total intensity for the unobserved lines. After correcting for these lines, we measure O/H = 8.42 dex; this value is approximately 2 times higher than O/H = 8.16 dex: the value obtained using CELs, assuming t 2 = This value is consistent with the O/H value measured from A supergiants found in NGC Peimbert, Peimbert & Ruiz, 2005, ApJ, in press

25 H II Region and Stellar Oxygen Abundances a H II Regions H II Regions Stars Stars Object t 2 = t 2 > A Supergiants Sun+GCE NGC 6822 Solar vicinity SMC WLM 8.16_ _ _ _ _ _ _ Peimbert, Peimbert & Ruiz, 2005, ApJ, in press.

26 NGC 2666 NGC 2363

27 Calibration of Pagel s Method log O/H Recombination Lines Collisionally Excited Lines log R 23 Peimbert & Peimbert, RMAA, in press 2005

28 HII regions with electron temperature abundances. Slide from: Mustakas 2005

29 Temperatures and abundances from visual forbidden lines are potentially biased in metal-rich H II regions Stasinska 1978: H II regions with high O/H have temperature gradients because of strong cooling in interior by fine-structure lines Optical forbidden line emissivity weighted toward high-t regions How bad could it be? Slide from Garnett O/H = 2X solar O/H = solar

30 Stasinska 2005 A&A: Explored effects of cooling on measured T, abundances --> T[O III] derived from emission lines can be higher than average T(O ++ ) --> derived abundances lower than true for O/H > 8.6 However, we don t measure T[O III] in these objects O/H derived from T[O III] Slide from Garnett 2005.

31 We actually measure T[N II] T[N II] can be a good proxy for the actual T(N + ) under a variety of conditions Still a potential for biased abundances, but not as strong a case - depends on physical properties of real H II regions Direct comparison of IR emission lines with visible counterparts should help resolve this problem Slide from Garnett O/H based on T[N II]

32 Summary I Well observed Recombination Lines are better than Collisionally Excited Lines, to determine chemical abundances. The best studied Recombination Lines, that are suited for heavy element abundance determinations, are probably those of the multiplet 1 of O II and 4267 of C II. It is necessary to know the intensity of all the lines in the multiplet in order to accurately determine the O ++ abundance. The intensity ratios of lines within this multiplet do not coincide with LTE computations for densities N e < For these densities it is better to assume the ratios given by our equations. For H II regions the abundances determined from multiplet 1 of O II are approximately a factor of 2 higher than those determined using the [OIII]4363/5007 traditional method.

33 Summary II Recombination Line abundance determinations are consistent with solar abundance determinations, and with determinations made with NGC 6822 supergiants. If possible it is better to calibrate Pagel s method with O recombination lines. With present 6 to 10 meter telescopes it is posssible to calibrate the method in the 8.0 < 12 + log O/H < 9.0 range based on O recombination lines. If Pagel s method is calibrated with models it is better to adjust 3727 and 5007 to the models than 4363/5007. The calibration of Pagel s method with O recombination lines indicates that the amount of heavy elements in the universe is two times higher than that derived from 4363/5007 determinations.

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