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


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 L16

Subject Categories

Astronomy, Astrophysics and Cosmology


Basic sciences



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