Evidence for CO Shock Excitation in NGC 6240 from Herschel SPIRE Spectroscopy
Journal article, 2013
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 H2O 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 COladders
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 H2 v = 1–0 S(1) and v = 2–1 S(1)
lines and the CO rotational ladder. The CO ladder is best reproduced by amodel with shock velocity vs = 10 km s−1
and a pre-shock density nH = 5 × 104 cm−3. We find that the solution best fitting the H2 lines is degenerate. The shock velocities and number densities range between vs = 17–47 km s−1 and nH = 107–5×104 cm−3, respectively.
The H2 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.
galaxies: individual (NGC 6240)
galaxies: active
galaxies: starburst
infrared: galaxies
galaxies: nuclei
Author
R. Meijerink
University of Groningen
Leiden University
L. Kristensen
Leiden University
A. Weiss
Max Planck Society
P. van der Werf
Leiden University
F. Walter
Max Planck Society
M. Spaans
University of Groningen
A.F. Loenen
Leiden University
J. Fischer
Naval Research Laboratory
F. P. Israel
Leiden University
K. Isaak
European Space Research and Technology Centre (ESA ESTEC)
P. P. Papadopoulos
Max Planck Society
Susanne Aalto
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
L. Armus
Spitzer Science Center
V. Charmandaris
University of Crete
K.M. Dasyra
LERMA - Laboratoire d'Etudes du Rayonnement et de la Matiere en Astrophysique et Atmospheres
T. Diaz-Santos
Spitzer Science Center
A. Evans
National Radio Astronomy Observatory
University of Virginia
Yu Gao
Chinese Academy of Sciences
E. Gonzalez-Alfonso
University of Alcalá
R. Güsten
Max Planck Society
C. Henkel
King Abdulaziz University
Max Planck Society
C. Kramer
Institut de Radioastronomie Millimétrique (IRAM)
S. Lord
California Institute of Technology (Caltech)
J. Martin-Pintado
CSIC - Instituto de Estructura de la Materia (IEM)
D. A. Naylor
University of Lethbridge
D. B. Sanders
University of Hawaii
H. Smith
Harvard-Smithsonian Center for Astrophysics
L. Spinoglio
Istituto nazionale di astrofisica (INAF)
G. Stacey
Cornell University
S. Veilleux
University of Maryland
M.C. Wiedner
Centre national de la recherche scientifique (CNRS)
Astrophysical Journal Letters
2041-8205 (ISSN) 2041-8213 (eISSN)
Vol. 762 2 L16-L20 L16Subject Categories
Astronomy, Astrophysics and Cosmology
Roots
Basic sciences
DOI
10.1088/2041-8205/762/2/L16