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METAL ABUNDANCES of KISS GALAXIES. V. NEBULAR ABUNDANCES of FIFTEEN INTERMEDIATE LUMINOSITY STAR-FORMING GALAXIES Alec S

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METAL ABUNDANCES of KISS GALAXIES. V. NEBULAR ABUNDANCES of FIFTEEN INTERMEDIATE LUMINOSITY STAR-FORMING GALAXIES Alec S Draft version September 30, 2018 Preprint typeset using LATEX style emulateapj v. 5/2/11 METAL ABUNDANCES OF KISS GALAXIES. V. NEBULAR ABUNDANCES OF FIFTEEN INTERMEDIATE LUMINOSITY STAR-FORMING GALAXIES Alec S. Hirschauer1, John J. Salzer1, Fabio Bresolin2, Ivo Saviane3, Irina Yegorova3 1Astronomy Department, Indiana University, Bloomington, IN 47405 2Institute for Astronomy, University of Hawaii, Honolulu, HI 96822 and 3European Southern Observatory, Alonso de Cordova 3107, Santiago, Chile Draft version September 30, 2018 ABSTRACT We present high S/N spectroscopy of 15 emission-line galaxies (ELGs) cataloged in the KPNO International Spectroscopic Survey (KISS), selected for their possession of high equivalent width [O iii] lines. The primary goal of this study was to attempt to derive direct-method (Te) abundances for use in constraining the upper-metallicity branch of the R23 relation. The spectra cover the full optical region from [O ii]λλ3726,3729 to [S iii]λλ9069,9531 and include the measurement of [O iii]λ4363 in 13 objects. From these spectra, we determine abundance ratios of helium, nitrogen, oxygen, neon, sulfur, and argon. We find these galaxies to predominantly possess oxygen abundances in the range of 8.0 . 12+log(O/H) . 8.3. We present a comparison of direct-method abundances with empirical SEL techniques, revealing several discrepancies. We also present a comparison of direct-method oxygen abundance calculations using electron temperatures determined from emission lines of O++ and S++, finding a small systematic shift to lower Te (∼1184 K) and higher metallicity (∼0.14 dex) for sulfur- derived Te compared to oxygen-derived Te. Finally, we explore in some detail the different spectral activity types of targets in our sample, including regular star-forming galaxies, those with suspected AGN contamination, and a local pair of low-metallicity, high-luminosity compact objects. Subject headings: galaxies: abundances { galaxies: starburst { H ii regions 1. INTRODUCTION therefore a difficult observational task. Reliable observa- Analysis of emission-line spectra provides the principal tions of high quality are thus relatively rare in the liter- diagnostic tools for determining gas-phase physical char- ature. acteristics of star-forming galaxies. Heavy element abun- For the majority of cases in which direct metallic- dances are valuable in the analysis of star formation his- ity determinations are not possible, relations utiliz- tories (e.g., Kennicutt 1998; Contini et al. 2002), chem- ing consistently detectable strong lines have been de- ical evolution (e.g., Steidel et al. 1996; Kobulnicky & veloped. These are either empirical relations that Kewley 2004; Lamareille et al. 2006), and general galactic are tied to direct-method abundances, theoretical re- evolution trends such as the luminosity-metallicity rela- lations that are based on photoionization models, or tion (e.g., Rubin et al. 1984; Lee et al. 2004; van Zee et al. semi-empirical relations that are a mixture of the 2006), and the mass-metallicity relation (e.g., Lequeux two (e.g., Moustakas et al. 2010). The most com- et al. 1979; Tremonti et al. 2004). Determination of this mon such strong-emission-line (SEL) method is R23 = gas-phase property remains a difficult task for all but the ([O ii]λλ3726,3729+[O iii]λλ4959,5007)/Hβ, first intro- most nearby and luminous H ii regions and emission-line duced by Pagel et al. (1979). R23 provides an estimate galaxies (ELGs), and is intrinsically limited both by our of total cooling and thus represents a proxy for global ability to accurately measure intensity ratios of nebular metallicity (Kewley & Dopita 2002), but is famously emission lines and by our calibration of the conversion of double-valued at high- and low-abundance and includes observational data into abundance measurements. an ambiguous turnaround region. Calibration of this re- Metallicities determined using the temperature of the lation relies upon comparison to accurate measurements electron gas (the so-called direct- or T -method), con- of metallicity, derived via the direct-method or through arXiv:1507.06256v1 [astro-ph.GA] 22 Jul 2015 e sidered to be the most accurate and robust available, photoionization models (e.g., Kewley et al. 2001). Many are calculated using measurements of often exceedingly R23 calibrations of the latter sort are available in the weak temperature-sensitive auroral emission lines such literature (e.g., Edmunds & Pagel 1984; Skillman et al. as [O iii]λ4363 (Osterbrock & Ferland 2006). In com- 1989; Zaritsky et al. 1994). As Te-method abundances puting the chemical composition of photoionized nebulae are more readily determined for lower-abundance sys- via the direct-method, the limiting factor is typically the tems, the R23 relation is much better calibrated using measurement precision of the electron gas temperature. direct-method data below the turnaround region than Furthermore, the emission strength of the temperature- above (Pilyugin 2000; Pilyugin & Thuan 2005). The primary goal of this study is to attempt to derive sensitive auroral lines scales with increasing Te, and are thus preferentially detected in the hotter nebulae of low- direct-method (Te) abundances of star-forming galaxies abundance H ii regions and ELGs. Determination of for use in constraining the upper-metallicity branch of the electron temperature for high-metallicity systems is the R23 relation. We utilize the superior light-gathering capability of the Keck 10-m telescope to observe a spe- [email protected] cially selected set of strong-lined ELGs from the KISS 2 Hirschauer et al. sample (Salzer et al. 2000, 2001). These observations ionized oxygen (O++) and sulfur (S++) to test the ef- were planned with the hope of detecting and measur- fect of sampling different zones of ionization within the ing the temperature-sensitive [O iii]λ4363 emission line nebula. Finally, we scrutinize our sample to better un- in higher-metallicity star-forming systems situated above derstand the physical limitations inherent to studies of the R23 relation turnaround region. With a robust cali- this nature, compare direct-method abundances calcu- bration of R23, it becomes possible for the full range of lated using electron temperatures characteristic of differ- ELG metallicities to be reliably studied, rather than be- ent ionization zones, and explore in more detail the three ing limited only to the brightest and most metal-poor ob- categories of objects uncovered in our observations. Our jects. Accurate abundances for high-metallicity targets results are summarized in x6. allows for the comprehensive understanding of a more representative sample of star-forming galaxies in the lo- 2. OBSERVATIONS AND DATA REDUCTION cal Universe. Furthermore, the refinement of reliable em- 2.1. Sample Selection pirical metallicity estimation methods from our observa- tions of the total integrated light of the ELGs is particu- Our spectroscopic targets for this project were poten- larly useful in the study of higher-redshift galaxies, where tially metal-rich galaxies identified by the KPNO In- spectral observations are global by necessity and the de- ternational Spectroscopic Survey (KISS; Salzer et al. tection of auroral lines becomes observationally difficult. 2000). KISS used low-dispersion objective-prism spectra Calibration of the R23 method using global spectra of to identify ELG candidates via detection of line emis- local star-forming galaxies will provide for a more con- sion in galaxies with redshifts of less than 0.095. The sistent approach to metallicity relations and chemical en- \red" survey (Salzer et al. 2001; Gronwall et al. 2004b; richment studies across cosmic time. Jangren et al. 2005) cataloged objects by means of the There are often offsets in metallicity estimates when Hα line, while the \blue" survey (Salzer et al. 2002) dis- employing direct- and SEL-methods, where in general Te- tinguished objects through strong [O iii]λ5007 line emis- method abundances are low as compared to techniques sion. The KISS sample is comprised of a wide variety of utilizing ratios of strong lines (e.g., Kennicutt et al. 2003; ELGs, including starburst nucleus galaxies, H ii galax- Bresolin et al. 2005; Zurita & Bresolin 2012). Biases in ies, irregular galaxies with significant star formation, and the sample selection of observational targets used to cal- blue compact dwarfs (BCDs). In addition, the survey de- ibrate some empirical relations can be a major contribu- tected low-ionization nuclear emission-line regions (LIN- tor to such abundance discrepancies. The magnitude of ERs) and Seyfert galaxies as well as quasars whose red- these offsets is typically of order a few tenths of a dex. A shifts place their emission lines into the objective-prism more comprehensively populated R23 relation, including spectral bandpass. Te-method abundances of the upper-metallicity branch, The galaxies discussed in this paper were chosen from will help to reduce such discrepancies. An alternative the KISS Hα-selected catalog with selection criteria approach to resolving this disparity involves a reexam- based on a combination of [O iii] line strength and coarse ination of the Te-method. In computing direct-method abundance estimates (Melbourne & Salzer 2002; Salzer metallicities, the electron temperature can be estimated et al. 2005b). We were specifically interested in objects from ions of similar electronic structure to O++, such as for which [O iii]λ4363 might be detectable in order to ob- doubly-ionized sulfur (e.g., Garnett 1989; Bresolin et al. tain the nebular electron temperature Te, which is mea- 2005; P´erez-Montero et al. 2006). Models show that the sured using the line ratio [O iii]λ4363/[O iii]λλ4959,5007 S++ zone straddles the zones of both O+ and O++ (Gar- (e.g., Izotov et al. 1994). Additionally, we are interested nett 1992). The electron temperature derived from [S iii] in galaxies with high enough oxygen abundance to be emission lines may therefore be a better single represen- located above the turnaround region of the R23 relation tation of the temperature of the nebula as a whole.
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