Molecular Gas in Extreme Star-Forming Environments: The Starbursts Arp 220 and NGC 6240 as Case Studies

Abstract

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.

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