O3N2

O3N2 is a strong-line diagnostic used in extragalactic astronomy to estimate the gas-phase oxygen abundance, or metallicity, in H II regions and star-forming galaxies. It combines the flux ratios of four emission lines that are close in wavelength: [O III] 5007, Hβ, [N II] 6584, and Hα. The index is defined as O3N2 = log10( ([O III] 5007 / Hβ) / ([N II] 6584 / Hα) ). Because the lines lie near each other in the spectrum, the measure is relatively insensitive to reddening and flux calibration.

The O3N2 index is calibrated to metallicity using empirical relationships derived from H II regions with direct electron temperature (Te) measurements. The original calibration by Pettini and Pagel (2004) gives 12 + log(O/H) ≈ 8.73 − 0.32 × O3N2 for O3N2 values typically in the range −1 to about 1.9. Subsequent work, such as Marino et al. (2013), provides updated coefficients and alternative calibrations. The exact conversion can vary with the chosen calibration, and uncertainties are typically on the order of 0.1–0.2 dex.

Applicability and limitations: O3N2 is most reliable at intermediate to low metallicities, where [O III] 5007 remains detectable. At high metallicities, [O III] weakens and the diagnostic loses dynamic range. The index can be influenced by the ionization parameter and the nitrogen-to-oxygen abundance (N/O) ratio, which can bias metallicity if not accounted for. While convenient for large surveys because it relies on strong lines, O3N2 provides an indirect metallicity estimate and is subject to systematic differences between calibrations.

See also: N2 index, R23, Te method.