The molecular gas in luminous infrared galaxies - I. CO lines, extreme physical conditions and their drivers

Abstract

We report results from a large molecular line survey of luminous infrared galaxies (LIRGs; L IR 10 11 L ) in the local Universe (z ≤ 0.1), conducted during the last decade with the James Clerk Maxwell Telescope and the IRAM 30-m telescope. This work presents the CO and 13 CO line data for 36 galaxies, further augmented by multi-J total CO line luminosities available for other infrared (IR) bright galaxies from the literature. This yields a combined sample of N = 70 galaxies with the star formation (SF) powered fraction of their IR luminosities spanning L ( * ) IR ∼ (10 10 -2 × 10 12 ) L and a wide range of morphologies. Simple comparisons of their available CO spectral line energy distributions (SLEDs) with local ones, as well as radiative transfer models, discern a surprisingly wide range of average interstellar medium (ISM) conditions, with most of the surprises found in the high-excitation regime. These take the form of global CO SLEDs dominated by a very warm (T kin 100 K) and dense (n ≥ 10 4 cm −3 ) gas phase, involving galaxy-sized (∼(few) × 10 9 M ) gas mass reservoirs under conditions that are typically found only for ∼(1-3) per cent of mass per typical SF molecular cloud in the Galaxy. Furthermore, some of the highest excitation CO SLEDs are found in ultraluminous infrared galaxies (ULIRGs; L IR ≥ 10 12 L ) and surpass even those found solely in compact SF-powered hot spots in Galactic molecular clouds. Strong supersonic turbulence and high cosmic ray energy densities rather than far-ultraviolet/optical photons or supernova remnant induced shocks from individual SF sites can globally warm the large amounts of dense gas found in these merger-driven starbursts and easily power their extraordinary CO line excitation. This exciting possibility can now be systematically investigated with Herschel and the Atacama Large Milimeter Array (ALMA). As expected for an IR-selected (and thus SF rate selected) galaxy sample, only few 'cold' CO SLEDs are found, and for fewer still a cold low/moderate-density and gravitationally bound state (i.e. Galactic type) emerges as the most likely one. The rest remain compatible with a warm and gravitationally unbound low-density phase often found in ULIRGs. Such degeneracies, prominent when only the low-J SLED segment (J = 1-0, 2-1 and 3-2) is available, advise against using its CO line ratios and the so-called X co = M(H 2 )/L co (1-0) factor as SF mode indicators, a practice that may have led to the misclassification of the ISM environments of IR-selected gas-rich discs in the distant Universe. Finally, we expect that the wide range of ISM conditions found among LIRGs will strongly impact the X co factor, an issue we examine in detail in Paper II.

A Total Molecular Gas Mass Census in Z ∼ 2–3 Star-Forming Galaxies: Low-J Co Excitation Probes of Galaxies’ Evolutionary States

“…We evaluate the integrated Schmidt-Kennicutt relation (the correlation betweenL FIR and ¢ L CO ) for our sample and find no significant difference between the slope and offset of the relation when using the different CO lines, in line with some previous results from aggregated samples thatdid not have both lines for every object (e.g., Greve et al 2014), although the results in the literature are mixed (e.g., Yao et al 2003;Bayet et al 2009;Kamenetzky et al 2015). We find that the index of the integrated Schmidt-Kennicutt relation is consistent withÑ 1, even when we include low-z U/LIRGs from Papadopoulos et al (2012) andGreve et al (2014 in our analysis. However, when we include the more normal low-z IRbright sample from Yao et al (2003) in our analysis (which also has both CO(1-0) and CO(3-2) detections), the index increases toÑ 1.2; this index is similar to the low-z objects on their own, indicating that they dominate the fit due to the large scatter in high-z measurements.…”

“…In Table 4 we list both the offset and the index to our fits for AGNs and SMGs, analyzed both separately and together, and in combination with local U/LIRGs from Papadopoulos et al (2012) and Greve et al (2014) and the low-z IR-bright galaxies from Yao et al (2003), which generally reach lower luminosities. 17 All fits are from an ordinary least-squares bisector linear regression.…”

Neutral carbon and CO in 76 (U)LIRGs and starburst galaxy centers

“…As already noted, high 12 CO/ 13 CO ratios have been found before in starburst galaxies (Martín et al 2010;Henkel et al 2014), but their values are not nearly as high as the ones we find here. Even worse, such very high [ 13 CO]/[ 12 CO] abundances are actually irreconcilable with observed J = 2−1 isotopic flux ratios, and the model also predicts J = (1−0) isotopic flux ratios to be higher than those in the J = 2−1 transition by factors of two to three times, which is contradicted by observation (see, e.g., Papadopoulos et al 2012;and Davis 2014). Thus, our assumption that the [C I] and 12 CO line emission is characterized by a single temperature and a single density breaks down in galaxies with decreasing [C I] (1−0)/ 13 CO (2−1) and [C I] (2−1)/(1−0) flux ratios.…”

“…We find mean values X CO = (0.21 ± 0.10) × 10 20 cm −2 / K kms −1 , and X C = (1.0 ± 0.6) × 10 20 cm −2 / K kms −1 . The mean CO-to-H 2 conversion factor derived from this sample is very close to the mean values found in the two-phase analysis by K14 and suggested earlier by Papadopoulos et al (2012) and Yao et al (2003). This mean value of X CO is a full order of magnitude lower than the value usually assumed for galaxy disks (the "standard" conversion factor), and a factor of two or more below the value often applied instead to active galaxy centers.…”

“…For the distant high-luminosity sources, most of these derive from JCMT or IRAM 30 m observations. In general, these objects have (CO) sizes that are smaller than the observing beam (see Papadopoulos et al 2012). For the nearer and more extended galaxies, we used JCMT or SEST observations with a beam size of 22".…”