Hα and free-free emission from the warm ionized medium

Ruobing Dong, B. T. Draine

Research output: Contribution to journalArticlepeer-review

21 Scopus citations


Recent observations have found the ratio of Hα to free-free radio continuum to be surprisingly high in the diffuse ionized interstellar medium (the so-called warm ionized medium), corresponding to an electron temperature of only ∼3000 K. Such low temperatures were unexpected in gas that was presumed to be photoionized. We consider a three-component model for the observed diffuse emission, consisting of a mix of (1) photoionized gas, (2) gas that is recombining and cooling, and (3) cool Hi gas. This model can successfully reproduce the observed intensities of free-free continuum, Hα, and collisionally excited lines such as [N ii]6583. To reproduce the low observed value of free-free to Hα, the polycyclic aromatic hydrocarbon abundance in the photoionized regions must be lowered by a factor ∼3, and ∼20% of the diffuse Hα must be reflected from dust grains, as suggested byWood &Reynolds.

Original languageEnglish (US)
JournalAstrophysical Journal
Issue number1
StatePublished - Jan 20 2011

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Atomic processes
  • ISM: general
  • ISM: lines and bands
  • ISM: structure


Dive into the research topics of 'Hα and free-free emission from the warm ionized medium'. Together they form a unique fingerprint.

Cite this