Evidence for CO Shock Excitation in NGC 6240 from Herschel SPIRE Spectroscopy
- Creators
- Meijerink, R.
- Armus, L.
- Díaz-Santos, T.
- Lord, S.
Abstract
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 – 12,6H_2O 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_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_s = 10 km s^(–1) and a pre-shock density n_H = 5 × 10^4 cm^(–3). We find that the solution best fitting the H_2 lines is degenerate. The shock velocities and number densities range between v_s = 17-47 km s^(–1) and n H = 10^7-5 × 10^4 cm^(–3), respectively. The H_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^(–1)) shocks while only a small fraction (≾ 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.
Additional Information
© 2013 American Astronomical Society. Received 2012 July 27; accepted 2012 November 28; published 2012 December 13. We acknowledge the constructive comments by the referee Christine Wilson. The following institutes have provided hardware and software elements to the SPIRE project: University of Lethbridge, Canada; NAOC, Beijing, China; CEA Saclay, CEA Grenoble and LAM in France; IFSI, Rome, and University of Padua, Italy; IAC, Tenerife, Spain; Stockholm Observatory, Sweden; Cardiff University, Imperial College London, UCL-MSSL, STFCRAL, UK ATC Edinburgh, and the University of Sussex in the UK. Funding for SPIRE has been provided by the national agencies of the participating countries and by internal institute funding: CSA in Canada; NAOC in China; CNES, CNRS, and CEA in France; ASI in Italy; MEC in Spain; Stockholm Observatory in Sweden; STFC in the UK; andNASA in the USA. Additional funding support for some instrument activities has been provided by ESA. US authors acknowledge support from the NHSC. Basic research in IR astronomy at NRL is funded by the US ONR.Attached Files
Published - 2041-8205_762_2_L16.pdf
Submitted - 1211.6659v1.pdf
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Additional details
- Eprint ID
- 42473
- Resolver ID
- CaltechAUTHORS:20131114-151105271
- Canadian Space Agency (CSA)
- National Astronomical Observatories, Chinese Academy of Sciences (NAOC)
- Centre National d'Études Spatiales (CNES)
- Centre National de la Recherche Scientifique (CNRS)
- Commissariat à l'Energie Atomique (CEA)
- Agenzia Spaziale Italiana (ASI)
- Ministerio de Educación y Ciencia (MEC)
- Stockholm Observatory
- Science and Technology Facilities Council (STFC)
- NASA
- European Space Agency (ESA)
- NHSC
- Office of Naval Research (ONR)
- Created
-
2013-11-15Created from EPrint's datestamp field
- Updated
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2021-11-10Created from EPrint's last_modified field
- Caltech groups
- Infrared Processing and Analysis Center (IPAC)