paper

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

arXiv:1808.02045 · doi:10.1093/mnras/sty2143

Abstract

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]/Hα and [N II]/[O II] ratios, and we measure the temperature-sensitive [O III] λ4363, [N II] λ5755, [S II] λλ4068,4076, and [O II] λλ7320,7330 lines in the stacked spectra. The [S II] doublet ratios indicate the line-emitting gas is in the low density regime (~10-100 cm).We found the temperatures in the S+ zones to be around 8000K, the temperatures in the O+ zones to be around K, and the temperatures in the N+ zones to be around K. The [O III] λ4363 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.

20 pages, 17 figures. Accepted by MNRAS

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