Crash Course II: Overview of Methods for Measuring Chemical Abundances and Modeling Ionized Gas Emission. Liese van Zee Indiana University

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1 Crash Course II: Overview of Methods for Measuring Chemical Abundances and Modeling Ionized Gas Emission Liese van Zee Indiana University

2 Overview: Elemental abundances from intensity- weighted emission- line measurements Direct abundance measurements Strong- line abundance calibraeons Conclusions/QuesEons

3 OpEcal Spectroscopy

4 ObservaEonal Concerns What part of the galaxy are you observing? Central regions can be contaminated by AGN Reddening correceons cannot account fully for dust- enshrouded HII regions Poor spaeal resolueon (large apertures) will mix stellar populaeons (age and metallicity) Poor spaeal resolueon (large apertures) will mix HII regions and diffuse ionized gas Are your spectra scienefically valid? DifferenEal refraceon Slit/aperture placement

5 Direct Abundance DeterminaEons Calculate reddening and underlying stellar absorpeon from Balmer line raeos (temperature dependent) Calculate electron temperature from [O III] raeos Calculate electron density from [S II] raeos Use FIVEL or other program to calculate emissivity coefficients. Derive ionic abundances Derive atomic abundances

6 Reddening and Balmer AbsorpEon

7 Reddening and Balmer AbsorpEon

8 Five Level Atom: Electron Temperature [O III] λ S 0 [N II] λ2321 λ5755 λ D λ5007 λ4959 } 3 P λ6583 λ Energy level diagrams for the 2p 2 ground configuraeon of [O III] and [N II].

9 Electron Temperature Measure raeo of line intensiees of transieons from very different energy levels e.g., [O III] 4363 Å, 4959 Å, and 5007 Å; [N II] 5755 Å, 6548 Å, and 6584 Å For [O III], every excitaeon to 1 S is followed by emission at 4363 Å or 2321 Å. Similarly, every emission at 4363 Å is followed by emission at 4959 Å or 5007 Å. Can calculate emissivity coefficients, which have a very strong temperature dependence: j(4959) + j(5007) j(4363) = 7.73 exp( / T ) (N e / T 1/2 )

10 Electron Temperature

11 [S III] lines yield similar results as [O III] [S III] lines at 9531, 9069, 6312 Å can be used instead of [O III]. However, night sky lines are a significant problem at the red- end of the opecal spectrum. Hirschauer et al. 2015

12 Electron Temperature, ConEnued Cau/ons: (1) 1- zone model is usually not appropriate. Use nebular models to approximate: T e (O + ) = 2 (T e (O ++ ) ) - 1 where T e = electron temperature in units of 10 4 K (Stasińska 1990) (2) High density gas can show relaevely strong 4363 Å because other lines are collisionally de- excited (3) Temperature fluctuaeons can bias results

13 CriEcal Density Collisional de- excitaeon happens as open as radiaeve de- excitaeon In other words, the mean Eme between collisions is comparable to or less than the radiaeve lifeeme. Density measurements may be made by using lines from the same ion with different criecal densiees. e.g., [O II] 3726 Å and 3729 Å; [S II] 6716 Å and 6731 Å

14 Five Level Atom: Density CalculaEons 1/2 3/2 3/2 5/2 λ3729 [O II] λ P 2 D [S II] λ6731 λ6716 3/2 1/2 5/2 3/2 3/2 4 S 3/2 Energy level diagrams for the 2p 3 ground configuraeon of [O II] and 3p 3 ground configuraeon of [S II]

15 Electron Density low density limit

16 Direct Abundance DeterminaEons Calculate reddening and underlying stellar absorpeon from Balmer line raeos (temperature dependent). Calculate electron temperature from [O III] raeos. Calculate electron density from [S II] raeos. Use FIVEL or other program to calculate emissivity coefficients (which depend on temperature and density). Derive ionic abundances: N(ion) N(H ) = j Hβ j ion I(ion) I(H β ) Derive atomic abundances: N(atom) = Σ N(ion) Concerns about ionization states that are not observed: ionization correction factors (ICFs)

17 IonizaEon FracEons Levesque et al. 2010, AJ, 139, 713

18 IonizaEon CorrecEon Factors Nitrogen: N + similar to O +, so use N(O + )/N(O) to esemate ICF for N(N + ): N/O = N + /O + Neon: Ne ++ similar to O ++, so use N(O ++ )/N(O) to esemate ICF for N(Ne ++ ): Ne/O = Ne ++ /O ++ Sulfur: S +++ and S ++ are present in the O ++ zone, so the ICF is more complicated. Thuan et al. (1995) recommend the following ICF: x = O + /O Argon: Ar ++ spans both the O + and O ++ zone, so again the ICF is more complicated. Thuan et al. (1995) recommend the following ICF: x = O + /O (if no [Ar IV] emission line)

19 Results: Abundance Gradients in Normal Spiral Galaxies NGC 0628 Berg et al. 2015, ApJ, 806, 16

20 Results: Elemental Abundance RaEos Alpha elements (Ne, S, Ar) behave as expected Berg et al. 2015, ApJ, 806, 16

21 Results: Nitrogen Abundances N/O deviates from secondary nitrogen produceon at low metalliciees. Significant scaver. Similar N/O raeos are seen in outer disks of spiral galaxies. * Not all of the data shown here are from direct abundance calculaeons.

22 HII Regions in Normal Dwarfs Easy Impossible? Difficult

23 OpEcal Spectroscopy

24 DiagnosEc Diagrams HII Regions AGN The BPT diagram (Baldwin, Phillips, and Terlevich 1981, PASP, 93, 5). Most HII regions follow a common locus, since the balance of photo- ionizaeon and cooling leads to a limited range of physical condieons. Shim & Ranga- Ram 2013, ApJ, 765, 26

25 Abundance trends in BPT Diagram As illustrated by these MAPPINGS- IV models, the shape of the curve is a combinaeon of hardness of the radiaeon field, the ionizaeon parameter, and the metallicity (which effects the cooling efficiency of the HII region) Dopita et al. 2013, ApJS, 208, 10

26 Strong Line Abundances: R23 RelaEons A variety of empirical calibraeons of the R23 relaeon have been avempted over the last 30 years. At the low metallicity end, most of the data available for an empirical calibraeon of the R23 relaeon lie within a small range of ionizaeon parameter. Thus, it is not possible to calibrate the R23 relaeon based on observaeons alone.

27 Which branch is it? Using [N II]/[O II] to Decide [N II]/[O II] can be used to break the high- low- abundance degeneracy of R23 calibraeons, or as a separate abundance indicator on its own (e.g., Dopita et al. 2000). However, nitrogen is a primary element at low metallicity, so the [N II]/[O II] raeo is no longer an accurate abundance esemator for low- Z HII regions.

28 HII Regions in NGC logO/H = logO/H = logO/H = Note change in [O II] and [O III] line strengths with corresponding decrease in [N II] and [S II] lines.

29 [N II]- Log(O/H) DiagnosEc Low metallicity HII regions have a large scaver in this plot, in part due to the wide range of observed ionizaeon parameters. Like [N II]/[O II], [N II]/Hα is best used as a diagnosec for R23 calibraeons at low- Z, not as an abundance indicator on its own.

30 HII regions with 12+log(O/H)=8.0 Note change in [O II] and [O III] line strengths with corresponding increase in [N II] and [S II] lines. Thus, these lines can vary significantly, even in similar metallicity HII regions. van Zee & Haynes 2006

31 Semi- empirical Abundances Abundance diagnostics: (1) Calculate both R23 and O3O2. (2) Locate HII region on McGaugh (1991) diagram. (3) Determine T[O III] necessary to obtain oxygen abundance (4) Calculate relative atomic abundances

32 Comparison of Direct McGaugh 1991 van Zee & Haynes 2006

33 Comparison of Direct Pilyugin 2000 van Zee & Haynes 2006

34 Pilyugin & Thuan 2005 UGCA log(O/H) = 8.0 PT05 empirical calibraeon is skewed by the available input data.

35 Aging HII Regions & IonizaEon Parameter As an HII region ages, its locaeon in the abundance diagnosec diagram ships. The expected emission line strengths for HII regions with 12+log(O/H) = 7.33 are shown (adapted from Stasińska & Leitherer 1996). The theoreecal locus of possible emission line raeos for HII regions with 12+log(O/H) = 7.33 covers a large fraceon of this diagnosec diagram. van Zee, Skillman, & Haynes 2006

36 Temperature- Abundance relaeons Fortunately, the expected temperature range for a given abundance is relaevely small. Real HII regions are unlikely to sample the full temperature- ionizaeon parameter locus shown in the previous figure. (see also Stasińska & Izotov 2003; Dopita et al. 2013) Observable extragalacec HII regions are likely to be even more confined within parameter space.

37 In other words: there are some limits to what is acceptable!

38 Concerns for Semi- Empirical Abundances CalibraEon of photo- ionizaeon models Aging HII regions; sparsely populated IMFs Plus all the issues associated with direct abundance measures.

39 Oxygen abundances from non- oxygen lines N2O2: log ([N II]/[O II]) ok at high abundances; suscepeble to assumed excenceon law and variaeons in N/O. Kewley & Dopita 2002 O3N2: log ([O III]/Hβ / [N II]/Hα) ok at high abundances, assumes normal ionizaeon parameter Pe{ni & Pagel 2004; Marino et al Ho et al. 2015, MNRAS, 448, 2030 Strong emission lines (SEL): combinaeons of muleple lines and photo- ionizaeon models pyqz: Dopita et al IZI: Blanc et al. 2015

40 Comparison of various line- raeo combinaeons and sensievity to ionizaeon parameter R23 N2O2 N2 O3N2 O3O2 R3 N2S2 S2 MAPPINGS IV photo- ionizaeon models Blanc et al. 2015, ApJ, 798, 99

41 SEL Methods Require Good Input Data! Based on the previous plots, [N II]/[O II] is very tempeng as a metallicity indicator, but it breaks down at the low metallicity end. Any SEL method may be similarly biased, as it is only as good as the input data.

42 Despite these Issues: Results! NGC4210 UGC Metallicity gradients (and ionizaeon parameter gradients) for two galaxies in the CALIFA Survey. Ho et al. 2015, MNRAS, 448, 2030

43 Effect of Different R23 CalibraEons on the Mass- Metallicity RelaEon Kewley & Ellison 2008

44 Relevance to High- redship? Almost all of the complicaeng details of Milky Way HII regions are lost when we consider observaeons of nearby galaxies Almost all of the complicaeng details of individual HII region complexes are lost when we consider observaeons of high- z sources However, our interpretaeon of observed line- strengths is based on our understanding of typical HII regions and photo- ionizaeon models Intensity- weighted line raeos will favor: For [O III]: lower metallicity (higher temperatures) and hard radiaeon fields, resuleng in an average oxygen abundance that may be lower than the true global average. For [N II]: higher metallicity regions, resuleng in an average metallicity that may be higher than the true global average.

45 Effect of SpaEal ResoluEon on [N II]/Hα metallicity gradient Yuan et al. 2013, ApJ, 767, 106

46 Conclusions Semi- empirical abundance calibraeons trace appropriate shape in metallicity- ionizaeon parameter space. Small range of physical parameters in most observed HII regions allow such calibraeons to work CauEon required, however, for analysis of low metallicity HII regions, parecularly for low ionizaeon parameter HII regions Open Issues SEll need work on strong- line abundance calibraeons ObservaEons of many more HII regions with [O III] 4363 Å, including low ionizaeon HII regions, to provide calibraeon checks. ObservaEons of other temperature sensieve lines.