Bibcode
Herrera-Camus, R.; Bolatto, A.; Smith, J. D.; Draine, B.; Pellegrini, E.; Wolfire, M.; Croxall, K.; de Looze, I.; Calzetti, D.; Kennicutt, R.; Crocker, A.; Armus, L.; van der Werf, P.; Sandstrom, K.; Galametz, M.; Brandl, B.; Groves, B.; Rigopoulou, D.; Walter, F.; Leroy, A.; Boquien, M.; Tabatabaei, F. S.; Beirao, P.
Bibliographical reference
The Astrophysical Journal, Volume 826, Issue 2, article id. 175, 17 pp. (2016).
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8
2016
Journal
Citations
62
Refereed citations
57
Description
The [N ii] 122 and 205 μm 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
(ΣSFR). 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 ΣSFR 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.