Emission Line Ratios for the Circumgalactic Medium and the "bimodal" Nature of Galaxies

Huanian Zhang, Dennis Zaritsky, Jessica Werk, Peter Behroozi

Research output: Contribution to journalArticlepeer-review

9 Scopus citations


We find significantly different diagnostic emission line ratios for the circumgalactic gas associated with galaxies of stellar masses above and below 1010.4 M o using Sloan Digital Sky Survey spectroscopy. Specifically, in a sample of 17,393 galaxies, intersected by 18,535 lines of sight at projected radii between 10 and 50 kpc, we stack measured fluxes for nebular strong emission lines, [O iii] λ5007, Hα, and [N ii] λ6583, and find that the gas surrounding the lower-mass galaxies exhibits similar line ratios to those of gas ionized by star formation and that surrounding the higher-mass galaxies similar to those of gas ionized by active galactic nucleus or shocks. This finding highlights yet another characteristic of galaxies that is distinctly different above and below this stellar mass threshold, but one that is more closely connected to the gas accretion behavior hypothesized to be responsible for this dichotomy.

Original languageEnglish (US)
Article numberL4
JournalAstrophysical Journal Letters
Issue number1
StatePublished - Oct 10 2018


  • galaxies: halos
  • galaxies: kinematics and dynamics
  • galaxies: structure
  • intergalactic medium

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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