Stacey, G. J. and Charmandaris, V. and Boulanger, F. and Wu, Yanling and Combes, F. and Higdon, S. J. U. and Smith, J. D. T. and Nikola, T. (2010) The Energetics of Molecular Gas in NGC 891 from H_2 and Far-infrared Spectroscopy. Astrophysical Journal, 721 (1). pp. 59-73. ISSN 0004-637X http://resolver.caltech.edu/CaltechAUTHORS:20101029-095715119
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We have studied the molecular hydrogen energetics of the edge-on spiral galaxy NGC 891, using a 34 position map in the lowest three pure rotational H_2 lines observed with the Spitzer Infrared Spectrograph. The S(0), S(1), and S(2) lines are bright with an extinction-corrected total luminosity of ~2.8 × 10^7 L_☉, or 0.09% of the total-infrared luminosity of NGC 891. The H_2 line ratios are nearly constant along the plane of the galaxy—we do not observe the previously reported strong drop-off in the S(1)/S(0) line intensity ratio in the outer regions of the galaxy, so we find no evidence for the very massive cold CO-free molecular clouds invoked to explain the past observations. The H_2 level excitation temperatures increase monotonically indicating that there is more than one component to the emitting gas. More than 99% of the mass is in the lowest excitation (T_(ex) ~ 125 K) "warm" component. In the inner galaxy, the warm H_2 emitting gas is ~16% of the CO(1-0)-traced cool molecular gas, while in the outer regions the fraction is twice as high. This large mass of warm gas is heated by a combination of the far-UV photons from stars in photodissociation regions (PDRs) and the dissipation of turbulent kinetic energy. Including the observed far-infrared [O I] and [C II] fine-structure line emission and far-infrared continuum emission in a self-consistent manner to constrain the PDR models, we find essentially all of the S(0) and most (70%) of the S(1) line arise from low excitation PDRs, while most (80%) of the S(2) and the remainder of the S(1) line emission arise from low-velocity microturbulent dissipation.
|Additional Information:||© 2010 American Astronomical Society. Received 2010 February 4; accepted 2010 July 12; published 2010 August 26. We thank an anonymous referee for many insightful comments that led to a much improved version of this manuscript. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under NASA contract 1407. Support for this work was provided by NASA through Contract Numbers 1257184 and 1311091 issued by JPL/Caltech. V.C. acknowledges partial support from the EU ToK grant 39965 and FP7-REGPOT 206469.|
|Subject Keywords:||dust, extinction; galaxies: individual (NGC 891); infrared: galaxies; infrared: ISM; ISM: molecules|
|Classification Code:||PACS: 98.52.Nr; 98.62.Qz; 95.85.Hp; 98.58.Bz; 98.58.Db|
|Official Citation:||G. J. Stacey et al 2010 ApJ 721 59 doi: 10.1088/0004-637X/721/1/59|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Jason Perez|
|Deposited On:||01 Nov 2010 18:05|
|Last Modified:||26 Dec 2012 12:34|
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