2016/05/10 by Rodrigo Herrera-Camus, R. Herrera-Camus, Alberto D. Bolatto +34 · 5 citations
Physics and Astronomy · #Astronomy #Astrophysics #Astrophysics and Star Formation Studies #Electron density #Galaxies: Formation, Evolution, Phenomena #Galaxy #Infrared #Interstellar medium #Ion #Ionization #Line (geometry) #Physics #Plasma #Star formation #Stellar, planetary, and galactic studies #Surface brightness #astro-ph.GA
paper · pdf · doi:10.3847/0004-637x/826/2/175
18 pages, 9 figures, accepted for publication in The Astrophysical Journal
arxiv created 2016/05/10 · openalex created_date 2016/06/24 · openalex publication_date 2016/07/29 · arxiv updated 2017/03/08 · openalex updated_date 2026/08/05
ABSTRACT The [N ii ] 122 and 205 μ m transitions are powerful tracers of the ionized gas in the interstellar medium. By combining data from 21 galaxies selected from the Herschel KINGFISH and Beyond the Peak surveys, we have compiled 141 spatially resolved regions with a typical size of ∼1 kpc, with observations of both [N ii ] far-infrared lines. We measure [N ii ] 122/205 line ratios in the ∼0.6–6 range, which corresponds to electron gas densities of n e ∼ 1–300 cm −3 , with a median value of n e = 30 cm −3 . Variations in the electron density within individual galaxies can be as high as a factor of ∼50, frequently with strong radial gradients. We find that n e increases as a function of infrared color, dust-weighted mean starlight intensity, and star-formation rate (SFR) surface density (Σ SFR ). As the intensity of the [N ii ] transitions is related to the ionizing photon flux, we investigate their reliability as tracers of the SFR. We derive relations between the [N ii ] emission and SFR in the low-density limit and in the case of a log-normal distribution of densities. The scatter in the correlation between [N ii ] surface brightness and Σ SFR can be understood as a property of the n e distribution. For regions with n e close to or higher than the [N ii ] line critical densities, the low-density limit [N ii ]-based SFR calibration systematically underestimates the SFR because the [N ii ] emission is collisionally quenched. Finally, we investigate the relation between [N ii ] emission, SFR, and n e by comparing our observations to predictions from the MAPPINGS-III code.