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Resolving the Far-IR Line Deficit: Photoelectric Heating and Far-IR Line Cooling in NGC 1097 and NGC 4559
Astrophysical Journal (2012)
  • Kevin V. Croxall
  • J. D. Smith
  • M. G. Wolfire
  • H. Roussel
  • K. M. Sandstrom
  • B. T. Draine
  • G. Aniano
  • D. A. Dale
  • L. Armus
  • P. Beirão
  • G. Helou
  • A. D. Bolatto
  • P. N. Appleton
  • B. R. Brandl
  • Daniela Calzetti, University of Massachusetts - Amherst
  • A. F. Crocker
  • M. Galametz
  • B. A. Groves
  • C.-N. Hao
  • L. K. Hunt
  • B. D. Johnson
  • R, C, Kennicutt
  • J. Koda
  • O. Krause
  • Y. Li
  • S. E. Meidt
  • E. J. Murphy
  • N. Rahman
  • H.-W. Rix
  • M. Sauvage
  • E. Schinnerer
  • F. Walter
  • C. D. Wilson
The physical state of interstellar gas and dust is dependent on the processes which heat and cool this medium. To probe heating and cooling of the ISM over a large range of infrared surface brightness, on sub-kiloparsec scales, we employ line maps of [C \ii] 158 $\mu$m, [O \one] 63 $\mu$m, and [N \ii] 122 $\mu$m in NGC 1097 and NGC 4559, obtained with the PACS spectrometer onboard {\it Herschel}. We matched new observations to existing Spitzer-IRS data that trace the total emission of polycyclic aromatic hydrocarbons (PAHs). We confirm at small scales in these galaxies that the canonical measure of photoelectric heating efficiency, ([C \ii] + [O \one])/TIR, decreases as the far-infrared color, $\nu f_\nu$(70 $\mu$m)/$\nu f_\nu$(100 $\mu$m), increases. In contrast, the ratio of far-infrared (far-IR) cooling to total PAH emission, ([C \ii] + [O \one])/PAH, is a near constant $\sim$6% over a wide range of far-infrared color, 0.5 \textless\ $\nu f_\nu$(70 $\mu$m)/$\nu f_\nu$(100 $\mu$m) $\lesssim$ 0.95. In the warmest regions, where $\nu f_\nu$(70 $\mu$m)/$\nu f_\nu$(100 $\mu$m) $\gtrsim$ 0.95, the ratio ([C \ii] + [O \one])/PAH drops rapidly to 4%. We derived representative values of the local UV radiation density, $G_0$, and the gas density, $n_H$, by comparing our observations to models of photodissociation regions. The ratio $G_0/n_H$, derived from fine-structure lines, is found to correlate with the mean dust-weighted starlight intensity, $$ derived from models of the IR SED. Emission from regions that exhibit a line deficit is characterized by an intense radiation field, indicating that small grains are susceptible to ionization effects. We note that there is a shift in the 7.7 / 11.3 $\mu$m PAH ratio in regions that exhibit a deficit in ([C \ii] + [O \one])/PAH, suggesting that small grains are ionized in these environments.
  • galaxies: individual (NGC 1097,
  • NGC 4559),
  • galaxies: ISM,
  • ISM: lines and bands
Publication Date
Publisher Statement
This paper was harvested from and ArXiv identifier is arXiv:1201.1016
Citation Information
Kevin V. Croxall, J. D. Smith, M. G. Wolfire, H. Roussel, et al.. "Resolving the Far-IR Line Deficit: Photoelectric Heating and Far-IR Line Cooling in NGC 1097 and NGC 4559" Astrophysical Journal (2012)
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