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Published October 20, 2014 | Published
Journal Article Open

Star Formation Relations and CO Spectral Line Energy Distributions across the J-ladder and Redshift

Abstract

We present FIR [50-300 μm]–CO luminosity relations (i.e., log L_FIR = α log L'_co + β) for the full CO rotational ladder from J = 1-0 up to J = 13-12 for a sample of 62 local (z ≤ 0.1) (Ultra) Luminous Infrared Galaxies (LIRGs; L_(IR[8-1000 μm]) > 10^(11) L ☉) using data from Herschel SPIRE-FTS and ground-based telescopes. We extend our sample to high redshifts (z > 1) by including 35 submillimeter selected dusty star forming galaxies from the literature with robust CO observations, and sufficiently well-sampled FIR/submillimeter spectral energy distributions (SEDs), so that accurate FIR luminosities can be determined. The addition of luminous starbursts at high redshifts enlarge the range of the FIR–CO luminosity relations toward the high-IR-luminosity end, while also significantly increasing the small amount of mid-J/high-J CO line data (J = 5-4 and higher) that was available prior to Herschel. This new data set (both in terms of IR luminosity and J-ladder) reveals linear FIR–CO luminosity relations (i.e., α 1) for J = 1-0 up to J = 5-4, with a nearly constant normalization (β ~ 2). In the simplest physical scenario, this is expected from the (also) linear FIR–(molecular line) relations recently found for the dense gas tracer lines (HCN and CS), as long as the dense gas mass fraction does not vary strongly within our (merger/starburst)-dominated sample. However, from J = 6-5 and up to the J = 13-12 transition, we find an increasingly sublinear slope and higher normalization constant with increasing J. We argue that these are caused by a warm (~100 K) and dense (>10^4 cm^(–3)) gas component whose thermal state is unlikely to be maintained by star-formation-powered far-UV radiation fields (and thus is no longer directly tied to the star formation rate). We suggest that mechanical heating (e.g., supernova-driven turbulence and shocks), and not cosmic rays, is the more likely source of energy for this component. The global CO spectral line energy distributions, which remain highly excited from J = 6-5 up to J = 13-12, are found to be a generic feature of the (U)LIRGs in our sample, and further support the presence of this gas component.

Additional Information

© 2014 The American Astronomical Society. Received 2014 May 8; Accepted 2014 July 15; Published 2014 October 2. The authors gratefully acknowledge financial support under the "DeMoGas" project. The project DeMoGas is implemented under the "ARISTEIA" Action of the "Operational Programme Education and Lifelong Learning." The project is co-funded by the European Social Fund (ESF) and National Resources. T.R.G. acknowledges support from an STFC Advanced Fellowship. T.R.G. was also supported by Chinese Academy of Sciences Fellowship for Young International Scientists (grant no. 2012y1ja0006). Z.Y.Z. acknowledges support from the European Research Council (ERC) in the form of Advanced Grant,cosmicism. We are indebted to P. P. Papadopoulos for extensive discussions and comments on the paper (He ho'okele wa'a no ka la 'ino). Basic research in infrared astronomy at the Naval Research Laboratory is funded by the Office of Naval Research. J.F. also acknowledges support from the NHSC/JPL. The research presented here has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Finally, we would like to thank the anonymous referee for a useful and constructive referee report that helped improve the paper.

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