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Warm molecular gas in M51 : mapping the excitation temperature and mass of H_2 with the Spitzer infrared spectrograph

Brunner, Gregory and Sheth, Kartik and Armus, Lee and Wolfire, Mark and Vogel, Stuart and Schinnerer, Eva and Helou, George and Dufour, Reginald and Smith, John-David and Dale, Daniel A. (2008) Warm molecular gas in M51 : mapping the excitation temperature and mass of H_2 with the Spitzer infrared spectrograph. Astrophysical Journal, 675 (1). pp. 316-329. ISSN 0004-637X.

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We have mapped the warm molecular gas traced by the H_2 S(0)-H_2 S(5) pure rotational mid-infrared emission lines over a radial strip across the nucleus and disk of M51 (NGC 5194) using the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. The six H_2 lines have markedly different emission distributions. We obtained the H_2 temperature and surface density distributions by assuming a two-temperature model: a warm (T = 100–300 K) phase traced by the low J [S(0)-S(2)] lines and a hot phase (T = 400–1000 K) traced by the high J [S(2)-S(5)] lines. The lowest molecular gas temperatures are found within the spiral arms (T ~ 155 K), while the highest temperatures are found in the inter-arm regions (T > 700 K). The warm gas surface density reaches a maximum of 11 M⊙ pc^(−2) in the northwest spiral arm, whereas the hot gas surface density peaks at 0.24 M⊙ pc^(−2) at the nucleus. The spatial offset between the peaks in the different phases suggests that the warm phase is more efficiently heated by star formation activity and the hot phase is more efficiently heated by nuclear activity. The warm H_2 is found in the dust lanes of M51 and is generally spatially coincident with the cold molecular gas traced by CO emission, consistent with excitation of the warm phase in dense photodissociation regions. The hot H_2 is most prominent in the nuclear region. Here, the hot H_2 coincides with [O IV] (25.89 μm) and X-ray emission indicating that shocks and/or X-rays are responsible for exciting this phase.

Item Type:Article
Related URLs:
URLURL TypeDescription
Sheth, Kartik0000-0002-5496-4118
Armus, Lee0000-0003-3498-2973
Wolfire, Mark0000-0003-0030-9510
Schinnerer, Eva0000-0002-3933-7677
Helou, George0000-0003-3367-3415
Smith, John-David0000-0003-1545-5078
Dale, Daniel A.0000-0002-5782-9093
Additional Information:© 2008 The American Astronomical Society. Received 2007 July 13; accepted 2007 October 10. The author graciously acknowledges the Spitzer Science Center Visiting Graduate Student Fellowship program and committee for providing support for this research. The author would like to specifically acknowledge the program coordinators, Phil Appleton and Alberto Noriega-Crepso. The authors would also like to thank the anonymous referee who helped to clarify our results and improve our discussion. Partial support for the completion and preparation for publication of this study by the author was provided by AURA grant GO10822.1 to Rice University.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Association of Universities for Research in Astronomy (AURA)UNSPECIFIED
Subject Keywords:galaxies: individual (M51); galaxies: ISM; ISM: molecules
Issue or Number:1
Record Number:CaltechAUTHORS:BRUapj08
Persistent URL:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:13692
Deposited By: Tony Diaz
Deposited On:09 Jul 2009 18:05
Last Modified:24 Feb 2020 10:30

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