STAR FORMATION RELATIONS AND CO SPECTRAL LINE ENERGY DISTRIBUTIONS ACROSS THE J-LADDER AND REDSHIFT

T. R. Greve, I. Leonidaki, E. M. Xilouris, A. Weiss, Z. -Y. Zhang, P. van der Werf, S. Aalto, L. Armus, T. Diaz-Santos, A. S. Evans, J. Fischer, Y. Gao, E. Gonzalez-Alfonso, A. Harris, C. Henkel, R. Meijerink, D. A. Naylor, H. A. Smith, M. Spaans, G. J. StaceyS. Veilleux, F. Walter

Research output: Contribution to journalArticlepeer-review

Abstract

We present FIR [50-300 mu m]-CO luminosity relations (i.e., log L-FIR = alpha log L'(CO) + beta) for the full CO rotational ladder from J = 1-0 up to J = 13-12 for a sample of 62 local (z 10(11) L-circle dot) 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., a similar or equal to 1) for J = 1-0 up to J = 5-4, with a nearly constant normalization (beta similar to 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 (similar to 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.

Original languageEnglish
Article number142
Number of pages16
JournalAstrophysical Journal
Volume794
Issue number2
DOIs
Publication statusPublished - 20 Oct 2014

Keywords

  • galaxies: evolution
  • galaxies: formation
  • galaxies: ISM
  • galaxies: starburst
  • ISM: molecules
  • ULTRALUMINOUS INFRARED GALAXIES
  • DENSE MOLECULAR GAS
  • LENSED SUBMILLIMETER GALAXIES
  • HERSCHEL-SPIRE SPECTROSCOPY
  • FREE-FREE EMISSION
  • FORMING GALAXIES
  • INTERSTELLAR-MEDIUM
  • NEARBY GALAXIES
  • STARBURST GALAXY
  • FORMATION LAW

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