The O3N2 and N2 abundance indicators revisited: improved calibrations based on CALIFA andT_e-based literature data

Abstract

The use of integral field spectroscopy is since recently allowing to measure the emission line fluxes of an increasingly large number of star-forming galaxies, both locally and at high redshift. Many studies have used these fluxes to derive the gas-phase metallicity of the galaxies by applying the so-called strong-line methods. However, the metallicity indicators that these datasets use were empirically calibrated using few direct abundance data points (T e -based measurements). Furthermore, a precise determination of the prediction intervals of these indicators is commonly lacking in these calibrations. Such limitations might lead to systematic errors in determining the gas-phase metallicity, especially at high redshift, which might have a strong impact on our understanding of the chemical evolution of the Universe. The main goal of this study is to review the most widely used empirical oxygen calibrations, O3N2 and N2, by using new direct abundance measurements. We pay special attention to (1) the expected uncertainty of these calibrations as a function of the index value or abundance derived and (2) the presence of possible systematic offsets. This is possible thanks to the analysis of the most ambitious compilation of T e -based H ii regions to date. This new dataset compiles the T e -based abundances of 603 H ii regions extracted from the literature but also includes new measurements from the CALIFA survey. Besides providing new and improved empirical calibrations for the gas abundance, we also present a comparison between our revisited calibrations with a total of 3423 additional CALIFA H ii complexes with abundances derived using the ONS calibration from the literature. The combined analysis of T e -based and ONS abundances allows us to derive their most accurate calibration to date for both the O3N2 and N2 singleratio indicators, in terms of all statistical significance, quality, and coverage of the parameters space. In particular, we infer that these indicators show shallower abundance dependencies and statistically significant offsets compared to others'. The O3N2 and N2 indicators can be empirically applied to derive oxygen abundances calibrations from either direct abundance determinations with random errors of 0.18 and 0.16, respectively, or from indirect ones (but based on a large amount of data), reaching an average precision of 0.08 and 0.09 dex (random) and 0.02 and 0.08 dex (systematic; compared to the direct estimations), respectively.

NGC 6845: metallicity gradients and star formation in a complex compact group

“…These authors found a dispersion of 0.16 and 0.18 dex in their calibration for the N2 and O3N2, respectively (see equations 2 and 4 from Marino et al 2013).…”

“…Radial velocity distribution along both tidal tails of NGC 6845A. Note that regions #1 and #11 have velocities similar to that of NGC 6845B and are unlikely to be part of NGC 6845A tail between NGC 6845A and NGC 6845C has a gradient of α (A−C) =-0.006±0.001 dex kpc −1 both measured using the N2 method proposed by Marino et al (2013). The value for the slope in the fits between NGC 6845A and NGC 6845B is similar to that found for NGC 92 (α=-0.0017±0.0021, see Torres-Flores et al 2014).…”

“…Following the N2 index proposed by Marino et al (2013) Chien et al (2007) for the northern tidal tail of the merging galaxy pair NGC 4676. These authors found values for the oxygen metallicity in the range 12+log(O/H)= 8.86-8.97, but they used the R23 method.…”

“…Because a couple of regions do not display the Hβ emission line (sources #8 and #31), the metallicity gradient and a Oxygen abundances estimated by using the N2 calibrator proposed by Marino et al (2013) b Oxygen abundances estimated by using the O3N2 calibrator proposed by Marino et al (2013) conclusions obtained in this paper were made using the values derived through the N2 index proposed by Marino et al (2013). The N2 and O3N2 calibrators provided a similar trend in the oxygen abundance distribution for the system NGC 6845A (in the sense of the slope of the fit).…”

“…11 shows the abundance gradient of the northeast side of NGC 6845A covering 66 kpc from the nucleus of NGC 6845A. The solid black line represents the fit made for the oxygen abundance following the N2 index calibrated by Marino et al (2013). We have made a fit for the inner (R < R25) and outer regions (R > R25) separately.…”