Shocks or Photoionization: Direct Temperature Measurements of the Low-Ionization Gas in Quiescent Galaxies

The ionization mechanism of the low-ionization gas in quiescent red sequence galaxies has been a long-standing puzzle. Direct temperature measurements would put strong constraints on this issue. We carefully selected a sample of quiescent red sequence galaxies from SDSS. We bin them into three bins with different [N II]/H{\alpha} and [N II]/[O II] ratios, and we measure the temperature-sensitive [O III] {\lambda}4363, [N II] {\lambda}5755, [S II] {\lambda}{\lambda}4068,4076, and [O II] {\lambda}{\lambda}7320,7330 lines in the stacked spectra. The [S II] doublet ratios indicate the line-emitting gas is in the low density regime (~10-100 cm$^{-3}$).We found the temperatures in the S+ zones to be around 8000K, the temperatures in the O+ zones to be around $1.1-1.5\times10^4$K, and the temperatures in the N+ zones to be around $1-1.4\times10^4$K. The [O III] {\lambda}4363 line is not robustly detected. We found that the extinction corrections derived from Balmer decrements would yield unphysical relationships between the temperatures of the S+ zones and O+ zones, indicating that the extinction is significantly overestimated by the measured Balmer decrements. We compared these line ratios with model predictions for three ionization mechanisms: photoionization by hot evolved stars, shocks, and turbulent mixing layers. For both the photoionization and shock models, the hot temperatures inferred from [S II] and [N II] coronal-to-strong line ratios require metallicities to be significantly subsolar. However, the [N II]/[O II] line ratios require them to be supersolar. None of the models could simultaneously explain all of the observed line ratios, neither could their combinations do.