Star Formation Relations and CO-Spectral Line Energy Distributions Across the J-Ladder and Redshift
Journal article, 2014
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 <= 0.1) (Ultra) Luminous InfraredGalaxies (LIRGs; LIR[8-1000 mu m] > 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.
ISM: molecules
galaxies: formation
galaxies: evolution
galaxies: starburst
galaxies: ISM
Author
T. R. Greve
University College London (UCL)
I. Leonidaki
National Observatory of Athens
E. M. Xilouris
National Observatory of Athens
A. Weiss
Max Planck Society
Z. Y. Zhang
European Southern Observatory (ESO)
Royal Observatory
P. van der Werf
Leiden University
Susanne Aalto
Chalmers, Earth and Space Sciences, Radio Astronomy and Astrophysics
L. Armus
Spitzer Science Center
T. Diaz-Santos
Spitzer Science Center
A. S. Evans
University of Virginia
National Radio Astronomy Observatory
J. Fischer
Naval Research Laboratory
Y. Gao
Chinese Academy of Sciences
E. Gonzalez-Alfonso
University of Alcalá
A. I. Harris
University of Maryland
C. Henkel
King Abdulaziz University
Max Planck Society
R. Meijerink
Leiden University
D. A. Naylor
University of Lethbridge
H. Smith
Harvard-Smithsonian Center for Astrophysics
M. Spaans
University of Groningen
G. J. Stacey
Cornell University
S. Veilleux
University of Maryland
F. Walter
Max Planck Society
Astrophysical Journal
0004-637X (ISSN) 1538-4357 (eISSN)
Vol. 794 2 Art. no. 142- 142Subject Categories
Astronomy, Astrophysics and Cosmology
Roots
Basic sciences
DOI
10.1088/0004-637x/794/2/142