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The molecular gas in Luminous Infrared Galaxies: a new emergent picture

Published online by Cambridge University Press:  21 March 2013

Padelis P. Papadopoulos ,
Zhi-Yu Zhang ,
Axel Weiss ,
Paul van der Werf ,
Kate Isaak ,
Yu Gao ,
Manolis Xilouris  and
Thomas R. Greve
Padelis P. Papadopoulos
Affiliation:
Max Planck Institute for Radioastronomy, Auf dem Hügel 69, D-53121 Bonn, Germany, email: [email protected]
Zhi-Yu Zhang
Affiliation:
Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, Jiangsu 210008, China
Axel Weiss
Affiliation:
Max Planck Institute for Radioastronomy, Auf dem Hügel 69, D-53121 Bonn, Germany, email: [email protected]
Paul van der Werf
Affiliation:
Leiden Observatory, Leiden University, NL-2300 RA Leiden, The Netherlands
Kate Isaak
Affiliation:
Research & Scientific Support, European Space Agency, ESTEC, NL-2201, The Netherlands
Yu Gao
Affiliation:
Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, Jiangsu 210008, China
Manolis Xilouris
Affiliation:
Institute for Astronomy, Astrophysics, Space Applications & Remote Sensing, National Observatory of Athens, P. Penteli, 15236 Athens, Greece
Thomas R. Greve
Affiliation:
Department of Physics and Astronomy, University College London, London WC1E 6BT, UK
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Abstract

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Results from a large, multi-J CO, 13CO, and HCN line survey of Luminous Infrared Galaxies (LIRGs: LIR≥ 1010 L⊙) in the local Universe (z≤0.1), complemented by CO J=4–3 up to J=13–12 observations from the Herschel Space Observatory (HSO), paints a new picture for the average conditions of the molecular gas of the most luminous of these galaxies with turbulence and/or large cosmic ray (CR) energy densities UCR rather than far-UV/optical photons from star-forming sites as the dominant heating sources. Especially in ULIRGs (LIR>1012 L⊙) the Photon Dominated Regions (PDRs) can encompass at most a few % of their molecular gas mass while the large UCR∼ 103 UCR, Galaxy, and the strong turbulence in these merger/starbursts, can volumetrically heat much of their molecular gas to Tkin∼ (100-200) K, unhindered by the high dust extinctions. Moreover the strong supersonic turbulence in ULIRGs relocates much of their molecular gas at much higher average densities (≥104 cm−3) than in isolated spirals (∼ 102–103 cm−3). This renders low-J CO lines incapable of constraining the properties of the bulk of the molecular gas in ULIRGs, with substantial and systematic underestimates of its mass possible when only such lines are used. Finally a comparative study of multi-J HCN lines and CO SLEDs from J=1–0 up to J=13–12 of NGC 6240 and Arp 193 offers a clear example of two merger/starbursts whose similar low-J CO SLEDs, and LIR/LCO,1−0 and LHCN, 1−0/LCO,1-0 ratios (proxies of the so-called SF efficiency and dense gas mass fraction), yield no indications about their strongly diverging CO SLEDs beyond J=4–3, and ultimately the different physical conditions in their molecular ISM. The much larger sensitivity of ALMA and its excellent site in the Atacama desert now allows the observations necessary to assess the dominant energy sources of the molecular gas and its mass in LIRGs without depending on the low-J CO lines.

Keywords

techniques: spectroscopic — galaxies: ISM — galaxies: starburst — ISM: molecules — cosmic rays — ISM: radio lines
Type
Contributed Papers
Information
Proceedings of the International Astronomical Union , Volume 8 , Symposium S292: Molecular Gas, Dust, and Star Formation in Galaxies , August 2012 , pp. 209 - 214
Copyright
Copyright © International Astronomical Union 2013

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