THE IONIZED GAS IN NEARBY GALAXIES AS TRACED BY THE [NII] 122 AND 205 μm TRANSITIONS
- Department of Astronomy, University of Maryland, College Park, MD 20742 (United States)
- Department of Physics and Astronomy, University of Toledo, 2801 West Bancroft Street, Toledo, OH 43606 (United States)
- Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544 (United States)
- Zentrum für Astronomie der Universität Heidelberg, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, D-69120 Heidelberg (Germany)
- Department of Astronomy, The Ohio State University, 4051 McPherson Laboratory, 140 West 18th Avenue, Columbus, OH 43210 (United States)
- Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA (United Kingdom)
- Department of Astronomy, University of Massachusetts, Amherst, MA 01003 (United States)
- Department of Physics, Reed College, Portland, OR 97202 (United States)
- Spitzer Science Center, California Institute of Technology, MC 314-6, Pasadena, CA 91125 (United States)
- Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden (Netherlands)
- Center for Astrophysics and Space Sciences, Department of Physics, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093 (United States)
- European Southern Observatory, Karl Schwarzschild Strasse 2, D-85748 Garching (Germany)
- Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611 (Australia)
- Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH (United Kingdom)
- Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg (Germany)
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 {sub e} ∼ 1–300 cm{sup −3}, with a median value of n {sub e} = 30 cm{sup −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 {sub e} increases as a function of infrared color, dust-weighted mean starlight intensity, and star-formation rate (SFR) surface density (Σ{sub 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{sub SFR} can be understood as a property of the n {sub e} distribution. For regions with n {sub 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 {sub e} by comparing our observations to predictions from the MAPPINGS-III code.
- OSTI ID:
- 22679574
- Journal Information:
- Astrophysical Journal, Vol. 826, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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