EVIDENCE FOR CO SHOCK EXCITATION IN NGC 6240 FROM HERSCHEL SPIRE SPECTROSCOPY
- Kapteyn Astronomical Institute, P.O. Box 800, NL-9700 AV Groningen (Netherlands)
- Leiden Observatory, Leiden University, P.O. Box 9513, NL-2300 RA Leiden (Netherlands)
- Max-Planck-Institut fuer Radioastronomie, Auf dem Huegel 16, Bonn, D-53121 (Germany)
- Max-Planck-Institut fuer Astronomie, Koenigstuhl 17, Heidelberg, D-69117 (Germany)
- Naval Research Laboratory, Remote Sensing Division, Washington, DC 20375 (United States)
- ESA Astrophysics Missions Division, ESTEC, P.O. Box 299, NL-2200 AG Noordwijk (Netherlands)
- Department of Radio and Space Science, Onsala Observatory, Chalmers University of Technology, SE-43992 Onsala (Sweden)
- Spitzer Science Center, California Institute of Technology, MS 220-6, Pasadena, CA 91125 (United States)
- University of Crete, Department of Physics, GR-71003 Heraklion (Greece)
- Observatoire de Paris, LERMA (CNRS:UMR8112), 61 Av. de l'Observatoire, F-75014, Paris (France)
- Department of Astronomy, University of Virginia, 530 McCormick Road, Charlottesville, VA 22904 (United States)
- Purple Mountain Observatory, Chinese Academy of Sciences, 2 West Beijing Road, Nanjing 210008 (China)
- Universidad de Alcala Henares, Departamente de Fisica, Campus Universitario, E-28871 Alcala de Henares, Madrid (Spain)
We present Herschel SPIRE FTS spectroscopy of the nearby luminous infrared galaxy NGC 6240. In total 20 lines are detected, including CO J = 4 - 3 through J = 13 - 12, 6 H{sub 2}O rotational lines, and [C I] and [N II] fine-structure lines. The CO to continuum luminosity ratio is 10 times higher in NGC 6240 than Mrk 231. Although the CO ladders of NGC 6240 and Mrk 231 are very similar, UV and/or X-ray irradiation are unlikely to be responsible for the excitation of the gas in NGC 6240. We applied both C and J shock models to the H{sub 2} v = 1-0 S(1) and v = 2-1 S(1) lines and the CO rotational ladder. The CO ladder is best reproduced by a model with shock velocity v{sub s} = 10 km s{sup -1} and a pre-shock density n{sub H} = 5 Multiplication-Sign 10{sup 4} cm{sup -3}. We find that the solution best fitting the H{sub 2} lines is degenerate. The shock velocities and number densities range between v{sub s} = 17-47 km s{sup -1} and n{sub H} = 10{sup 7}-5 Multiplication-Sign 10{sup 4} cm{sup -3}, respectively. The H{sub 2} lines thus need a much more powerful shock than the CO lines. We deduce that most of the gas is currently moderately stirred up by slow (10 km s{sup -1}) shocks while only a small fraction ({approx}< 1%) of the interstellar medium is exposed to the high-velocity shocks. This implies that the gas is rapidly losing its highly turbulent motions. We argue that a high CO line-to-continuum ratio is a key diagnostic for the presence of shocks.
- OSTI ID:
- 22078349
- Journal Information:
- Astrophysical Journal Letters, Vol. 762, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 2041-8205
- Country of Publication:
- United States
- Language:
- English
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