1998/07/30 by Jing Wang, Timothy M. Heckman, Matthew D. Lehnert · 26 citations
Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysics and Star Formation Studies #Brightness #Brightness temperature #Galaxies: Formation, Evolution, Phenomena #Galaxy #Halo #Line (geometry) #Milky Way #Scaling #Spectral line #Unified Model #astro-ph
paper · pdf · doi:10.1086/306489
published in The Astrophysical Journal 509(1), 93-102 (IOP Publishing) · AASTeX style latex file, two figures
arxiv created 1998/07/30 · openalex publication_date 1998/12/10 · openalex created_date 2016/06/24 · arxiv updated 2016/08/30 · openalex updated_date 2026/08/06
The "diffuse ionized medium" (DIM) makes up a significant fraction of the mass and ionization requirements of the interstellar medium of the Milky Way and is now known to be an energetically significant component in most normal star-forming galaxies. Observations of the ionized gas in starburst galaxies have revealed the presence of gas with striking similarities to the DIM in normal galaxies: relatively low surface brightness and strong emission from low-ionization forbidden lines like [S II] λλ6716, 6731. In this paper we analyze Hα images and long-slit spectra of samples of normal and starburst galaxies to better understand the nature of this diffuse, low surface brightness gas. We find that in both samples there is a strong inverse correlation between the Hα surface brightness (Σ Hα ) and the [S II]/Hα line ratio at a given location in the galaxy. However, the correlation for the starbursts is offset brightward by an order of magnitude in Hα surface brightness at a given line ratio. In contrast, we find that all the galaxies (starburst and normal alike) define a universal relation between line ratio and the relative Hα surface brightness (Σ Hα /Σ e , where Σ e is the mean Hα surface brightness within the galaxy half-light radius). We show that such a universal correlation is a natural outcome of a model in which the DIM is photoionized gas that has a characteristic thermal pressure ( P ) that is proportional to the mean rate of star formation per unit area in the galaxy (Σ SFR ). Good quantitative agreement with the data follows if we require the constant of proportionality to be consistent with the values of P and Σ SFR in the local disk of the Milky Way. Such a scaling between P and Σ SFR may arise either because feedback from massive stars heats the ISM or because Σ SFR is determined (or limited) by the mean gas pressure.