Surprising increase of electron temperature in metal-rich star-forming region

Abstract

The electron temperature is a crucial parameter for the determination of the gas-phase metallicity of galaxies. Low electron temperature is expected for metal-rich galaxies, theoretically. We report the discovery that temperature, as measured through auroral-to-strong line ratios of O$^+$, trends in reverse directions at 12+log(O/H) $\geq$ 8.7. This trend remains consistent regardless of the emission line fitting method employed and is not attributable to contamination or dust attenuation correction. Notably, this phenomenon is not observed in other low-ionization ions, such as S$^+$ and N$^+$, which also probe electron temperature. The results are verified in two independent datasets. We analyze the potential cause for the high [OII] auroral-to-strong line ratios at high metallicities, finding that no specific reason could account for that. This finding challenges the fundamental principles of the direct $T_e$ method for metallicity measurement, warranting further investigation into its physical interpretation.

Figures (5)

• Figure 1: Left: Electron temperatures measured from [O ii] vs. those from [S ii] based on MaNGA () and Legacy () stacked spectra. Right: Electron temperatures measured from [O ii] vs. those from [N ii] based on MaNGA () and Legacy () stacked spectra. Each data point with error bars demonstrates a metallicity-ionization parameter bin and is color-coded by metallicity derived from strong line calibrations. The error bars correspond to the 1 $\sigma$ uncertainty of the temperature measurements. The black line shows the 1:1 line. The three dashed lines are the $T_e$ ([S ii]) versus $T_e$ ([O ii]) relation from mendez2023densityzurita2021barberg2020chaos, respectively.
• Figure 2: $T_e$ vs $T_e$ with the model considering the contributions of the recombination lines of both [O ii]$\lambda \lambda$ 7320,7330 and [N ii]$\lambda$ 5755 . The grey dots represent stacked observed data from MaNGA and Legacy. The colored lines are the temperatures derived from the model, color-coded by their gas-phase metallicities. The lines offset from the 1:1 line with the increasing of ionization parameters.
• Figure 3: Variations of strong-to-auroral line ratios with different electron densities, normalized by the corresponding ratios at $n_e=1 {\rm cm}^{-3}$, assuming the electron temperature is 10,000K.
• Figure 4: Part of the stacked spectra around [S ii]$\lambda \lambda$ 4069,4076 (top), [N ii]$\lambda$ 5755 (middle) and [O ii]$\lambda \lambda$ 7320,7330 (bottom) after continuum subtraction. Here, the binning is only by the strong-line metallicity. Each spectrum is normalized by its strong line flux of the corresponding ion: the top panels are normalized by [S ii]$\lambda \lambda$ 6716,6731 flux, the middle panels are normalized by [N ii]$\lambda$ 6584 flux, and the bottom panels are normalized by [O ii]$\lambda \lambda$ 3726,3729 flux. The shaded regions are the sidebands for fitting the auroral lines. All the spectra presented here are corrected for dust attenuation using the extinction curve from 2019ApJ...886..108F, assuming an intrinsic Balmer decrement H$\alpha$ / H$\beta$ = 2.86.
• Figure 5: Line ratio comparisons between data and models. In each panel, the blue grids represent the photoionization model with different 12+log(O/H) and log(U), the red and orange grids represent the shock model with different B/n and velocity. Dots represent stacked data, color-coded by their strong-line metallicity. [O ii]$_{a-t-s}$ is [O ii]$\lambda \lambda$ 7320,7330 /[O ii]$\lambda \lambda$ 3726,3729 , [S ii]$_{a-t-s}$ is [S ii]$\lambda \lambda$ 4069,4076 /[S ii]$\lambda \lambda$ 6716,6731 .