Shocks or photoionization: Direct temperature measurements of the low-ionization gas in quiescent galaxies

Renbin Yan

Research output: Contribution to journalArticlepeer-review

15 Scopus citations


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 Sloan Digital Sky Survey. We bin them into three bins with different [NII]/H α and [NII]/[O II] ratios, and we measure the temperature-sensitive [O III] λ4363, [N II] λ5755, [S II] λλ4068, 4076, and [OII] λλ7320, 7330 lines in the stacked spectra. The [SII] doublet ratios indicate the line-emitting gas is in the low-density regime (~10-200 cm-3). We found the temperatures in the S+ zones to be around 8000 K, the temperatures in the O+ zone to be around 1.1- 1.5 × 104 K, and the temperatures in the N+ zones to be around 1-1.4× 104 K. The [OIII] 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 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 [NII]/[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.

Original languageEnglish
Pages (from-to)476-493
Number of pages18
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
StatePublished - Nov 21 2018

Bibliographical note

Publisher Copyright:
© 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society.


  • (galaxies:) quasars: emission lines
  • CD
  • Galaxies: ISM
  • Galaxies: abundances
  • Galaxies: elliptical and lenticular

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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