2006/10/31 by T. R. Greve, Panagiotis Papadopoulos, P. P. Papadopoulos +3 · 3 citations
Chemistry · Physics and Astronomy · #Astronomy #Astrophysics #Astrophysics and Star Formation Studies #Atomic physics #Excited state #Galaxies: Formation, Evolution, Phenomena #Galaxy #Line (geometry) #Molecular cloud #Physics #Spectroscopy and Laser Applications #Star formation #Stars #astro-ph
paper · pdf · doi:10.1088/0004-637x/692/2/1432
published as Astrophys.J.692:1432-1446,2009 · 38 pages (preprint ApJ style), 3 figures, accepted for ApJ
arxiv created 2008/10/13 · openalex publication_date 2009/02/20 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/06
We report single-dish multitransition measurements of the 12 CO, HCN, and HCO + molecular line emission as well as HNC J = 1–0 and HNCO in the two ultraluminous IR galaxies Arp 220 and NGC 6240. Using this new molecular line inventory, in conjunction with existing data in the literature, we compiled the most extensive molecular line data sets to date for such galaxies. The many rotational transitions, with their different excitation requirements, allow the study of the molecular gas over a wide range of different densities and temperatures with significant redundancy, and thus allow good constraints on the properties of the dense gas in these two systems. The mass (∼ (1–2) × 10 10 M ☉ ) of dense gas (≳10 5−6 cm −3 ) found accounts for the bulk of their molecular gas mass, and is consistent with most of their IR luminosities powered by intense starbursts while self-regulated by O, B star cluster radiative pressure onto the star-forming dense molecular gas. The highly excited HCN transitions trace a gas phase ∼ (10–100) × denser than that of the subthermally excited HCO + lines (for both galaxies). These two phases are consistent with an underlying density-size power law found for Galactic giant molecular clouds (but with a steeper exponent), with HCN lines tracing denser and more compact regions than HCO + . Whether this is true in IR-luminous, star-forming galaxies in general remains to be seen, and underlines the need for observations of molecular transitions with high critical densities for a sample of bright (U)LIRGs in the local universe—a task for which the HI-FI instrument on board Herschel is ideally suited to do.