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Register a new userNew fully empirical calibrations of strong-line metallicity indicators in star forming galaxies
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We derive new empirical calibrations for strong-line diagnostics of gas phase metallicity in local star forming galaxies by uniformly applying the Te method over the full metallicity range probed by the Sloan Digital Sky Survey (SDSS). To measure electron temperatures at high metallicity, where the auroral lines needed are not detected in single galaxies, we stacked spectra of more than 110,000 galaxies from the SDSS in bins of log[O II]/H$beta$ and log[O III]/H$beta$. This stacking scheme does not assume any dependence of metallicity on mass or star formation rate, but only that galaxies with the same line ratios have the same oxygen abundance. We provide calibrations which span more than 1 dex in metallicity and are entirely defined on a consistent absolute Te metallicity scale for galaxies. We apply our calibrations to the SDSS sample and find that they provide consistent metallicity estimates to within 0.05 dex.
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We present a study of the consequences of an initial mass function that is stochastically sampled on the main emission lines used for gas-phase metallicity estimates in extra-galactic sources. We use the stochastic stellar population code SLUG and the photoionisation code Cloudy to show that the stochastic sampling of the massive end of the mass function can lead to clear variations in the relative production of energetic emission lines such as [OIII] relative to that of Balmer lines. We use this to study the impact on the Te, N2O2, R23 and O3N2 metallicity calibrators. We find that stochastic sampling of the IMF leads to a systematic over-estimate of O/H in galaxies with low star formation rates (< $10^{-3}$ M$_odot$/yr) when using the N2O2, R23 and O3N2 strong-line methods, and an under-estimate when using the Te method on galaxies of sub-solar metallicity. We point out that while the SFR(Ha)-to-SFR(UV) ratio can be used to identify systems where the initial mass function might be insufficiently sampled, it does not provide sufficient information to fully correct the metallicity calibrations at low star formation rates. Care must therefore be given in the choice of metallicity indicators in such systems, with the N2O2 indicator proving most robust of those tested by us, with a bias of 0.08 dex for models with SFR = $10^{-4}$ M$_odot$/yr and solar metallicity.
We determine the gas-phase oxygen abundance for a sample of 695 galaxies and H II regions with reliable detections of [O III]4363, using the temperature-sensitive Te method. Our aims are to estimate the validity of empirical methods such as R23, R23-P, log([N II]/Halpha) (N2), log[([O III]/Hbeta)/([N II]/Halpha)] (O3N2), and log([S II]/Halpha) (S2), and especially to re-derive (or add) the calibrations of R23, N2, O3N2 and S2 indices for oxygen abundances on the basis of this large sample of galaxies with Te-based abundances. We select 531 star-forming galaxies from the SDSS-DR4, and 164 galaxies and H II regions from literature for such study. Their (O/H) abundances obtained from Te are within 7.1<12+log(O/H)<8.5 mostly. For roughly half of the SDSS samples, the Bayesian abundances obtained by the MPA/JHU group are overestimated by ~0.34 dex compared with the Te-based (O/H) measurements, possibly due to the treatment of nitrogen enrichment in the models they used. R23 and R23-P methods systematically overestimate the O/H abundance by a factor of ~0.20 dex and ~0.06 dex, respectively. The N2 index, rather than the O3N2 index, provides relatively consistent O/H abundances with the Te-method, but with some scatter. The relations of N2, O3N2, S2 with log(O/H) are consistent with the photoionization model calculations of Kewley & Doptita (2002), but R23 does not match well. Then we derive analytical calibrations for O/H from R23, N2, O3N2 and S2 indices on the basis of this large sample of galaxies, especially including the excitation parameter P as an additional parameter in the N2 calibration. These can be used as calibration references in the future studies about metallicities of galaxies.
Star-forming galaxies display a close relation among stellar mass, metallicity and star-formation rate (or molecular-gas mass). This is known as the fundamental metallicity relation (FMR) (or molecular-gas FMR), and it has a profound implication on models of galaxy evolution. However, there still remains a significant residual scatter around the FMR. We show here that a fourth parameter, the surface density of stellar mass, reduces the dispersion around the molecular-gas FMR. In a principal component analysis of 29 physical parameters of 41,338 star-forming galaxies, the surface density of stellar mass is found to be the fourth most important parameter. The new four-dimensional (4D) fundamental relation forms a tighter hypersurface that reduces the metallicity dispersion to 50% of that of the molecular-gas FMR. We suggest that future analyses and models of galaxy evolution should consider the FMR in a 4D space that includes surface density. The dilution time scale of gas inflow and the star-formation efficiency could explain the observational dependence on surface density of stellar mass. AKARI is expected to play an important role in shedding light on the infrared properties of the new 4D FMR.
Star-forming galaxies display a close relation among stellar mass, metallicity and star-formation rate (or molecular-gas mass). This is known as the fundamental metallicity relation (FMR) (or molecular-gas FMR), and it has a profound implication on models of galaxy evolution. However, there still remains a significant residual scatter around the FMR. We show here that a fourth parameter, the surface density of stellar mass, reduces the dispersion around the molecular-gas FMR. In a principal component analysis of 29 physical parameters of 41,338 star-forming galaxies, the surface density of stellar mass is found to be the fourth most important parameter. The new four-dimensional fundamental relation forms a tighter hypersurface that reduces the metallicity dispersion to 50% of that of the molecular-gas FMR. We suggest that future analyses and models of galaxy evolution should consider the FMR in a four-dimensional space that includes surface density. The dilution time scale of gas inflow and the star-formation efficiency could explain the observational dependence on surface density of stellar mass.
We present updated metallicity relations for the spectral database of star-forming galaxies (SFGs) found in the KPNO International Spectroscopic Survey (KISS). New spectral observations of emission-line galaxies (ELGs) obtained from a variety of telescope facilities provide oxygen abundance information. A nearly four-fold increase in the number of KISS objects with robust metallicities relative to our previous analysis provides for an empirical abundance calibration to compute self-consistent metallicity estimates for all SFGs in the sample with adequate spectral data. In addition, a sophisticated spectral energy distribution (SED) fitting routine has provided robust calculations of stellar mass. With these new and/or improved galaxy characteristics, we have developed luminosity-metallicity ($L$-$Z$) relations, mass-metallicity ($M_{*}$-$Z$) relations, and the so-called Fundamental Metallicity Relation (FMR) for over 1,450 galaxies from the KISS sample. This KISS $M_{*}$-$Z$ relation is presented for the first time and demonstrates markedly lower scatter than the KISS $L$-$Z$ relation. We find that our relations agree reasonably well with previous publications, modulo modest offsets due to differences in the SEL metallicity calibrations used. We illustrate an important bias present in previous $L$-$Z$ and $M_{*}$-$Z$ studies involving direct-method ($T_{e}$) abundances that may result in systematically lower slopes in these relations. Our KISS FMR shows consistency with those found in the literature, albeit with a larger scatter. This is likely a consequence of the KISS sample being biased toward galaxies with high levels of activity.
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