Warm Molecular Gas in Luminous Infrared Galaxies

We present our initial results on the CO rotational spectral line energy distribution (SLED) of the J to J−1 transitions from J = 4 up to 13 from Herschel SPIRE spectroscopic observations of 65 luminous infrared galaxies (LIRGs) in the Great Observatories All-Sky LIRG Survey (GOALS). The observed SLEDs change on average from one peaking at J ≤ 4 to a broad distribution peaking around J ∼ 6−7 as the IRAS 60-to-100μm color, C(60/100), increases. However, the ratios of a CO line luminosity to the total infrared luminosity, LIR, show the smallest variation for J around 6 or 7. This suggests that, for most LIRGs, ongoing star formation (SF) is also responsible for a warm gas component that emits CO lines primarily in the mid-J regime (5 . J . 10). As a result, the logarithmic ratios of the CO line luminosity summed over CO (5−4), (6−5), (7−6), (8−7) and (10−9) transitions to LIR, logRmidCO, remain largely independent of C(60/100), and show a mean value of −4.13 (≡ logR midCO) and a sample standard deviation of only 0.10 for the SF-dominated galaxies. Including additional galaxies from the literature, we show, albeit with small number of cases, the possibility that galaxies, which bear powerful interstellar shocks unrelated to the current SF, and galaxies, in which an energetic active galactic nucleus contributes significantly to the bolometric luminosity, have their RmidCO higher and lower than R midCO, respectively. Subject headings: galaxies: active — galaxies: ISM — galaxies: star formation — infrared: galaxies — ISM: molecules — submillimeter: galaxies 1 Based on Herschel observations. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. 2 Infrared Processing and Analysis Center, California Institute of Technology, MS 100-22, Pasadena, CA 91125, USA; [email protected] 3 Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, China 4 Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, Nanjing 210008, China 5 Spitzer Science Center, California Institute of Technology, MS 220-6, Pasadena, CA 91125, USA 6 ESA Astrophysics Missions Division, ESTEC, PO Box 299, 2200 AG Noordwijk, The Netherlands 7 Gemini Observatory, 670 N. Aohoku Place, Hilo, HI 96720, USA 8 Astronomy Department, California Institute of Technology, Pasadena, CA 91125, USA 9 Department of Physics, University of Crete, GR-71003 Heraklion, Greece 10 IAASARS, National Observatory of Athens, GR-15236, Penteli, Greece 11 Chercheur Associé, Observatoire de Paris, F-75014, Paris, France 12 Department of Astronomy, University of Virginia, 530 McCormick Road, Charlottesville, VA 22904, USA 13 National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA 14 National Optical Astronomy Observatory, Tucson, AZ 85719, USA 15 ICREA and Institut de Ciències del Cosmos (ICC), Universitat de Barcelona (IEEC-UB), Mart́ı i Franquès 1, 08028, Barcelona, Spain 16 Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UK 17 University of Hawaii, Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822, USA 18 Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands