THE MOLECULAR GAS IN LUMINOUS INFRARED GALAXIES. II. EXTREME PHYSICAL CONDITIONS AND THEIR EFFECTS ON THE Xco FACTOR

By P. Papadopoulos, P. P. van der Werf, E. Xilouris, K. Isaak, Yu Gao. 2012

134 citations recorded by the source corpus.

In this work, we conclude the analysis of our CO line survey of luminous infrared galaxies (LIRGs: LIR ≳ 1011 L☉) in the local universe (Paper I) by focusing on the influence of their average interstellar medium (ISM) properties on the total molecular gas mass estimates via the so-called Xco = M(H2)/Lco, 1–0 factor. One-phase radiative transfer models of the global CO spectral line energy distributions (SLEDs) yield an Xco distribution with 〈Xco〉 ∼ (0.6 ± 0.2) M☉ (K km s−1 pc2)−1 over a significant range of average gas densities, temperatures, and dynamic states. The latter emerges as the most important parameter in determining Xco, with unbound states yielding low values and self-gravitating states yielding the highest ones. Nevertheless, in many (U)LIRGs where available higher-J CO lines (J = 3–2, 4–3, and/or J = 6–5) or HCN line data from the literature allow a separate assessment of the gas mass at high densities (⩾104 cm−3) rather than a simple one-phase analysis, we find that near-Galactic Xco ∼ (3–6) M☉ (K km s−1 pc2)−1 values become possible. We further show that in the highly turbulent molecular gas in ULIRGs, a high-density component will be common and can be massive enough for its high Xco to dominate the average value for the entire galaxy. Using solely low-J CO lines to constrain Xco in such environments (as has been the practice up until now) may have thus resulted in systematic underestimates of molecular gas mass in ULIRGs, as such lines are dominated by a warm, diffuse, and unbound gas phase with low Xco but very little mass. Only well-sampled high-J CO SLEDs (J = 3–2 and higher) and/or multi-J observations of heavy rotor molecules (e.g., HCN) can circumvent such a bias, and the latter type of observations may have actually provided early evidence of it in local ULIRGs. The only way that the global Xco of such systems could be significantly lower than Galactic is if the average dynamic state of the dense gas is strongly gravitationally unbound. This is an unlikely possibility that must nevertheless be examined, with lines of rare isotopologues of high gas density tracers (e.g., H13CN, high-J 13CO lines) being very valuable in yielding (along with the lines of the main isotopes) such constraints. For less IR-luminous, disk-dominated systems, we find that the galaxy-averaged Xco deduced by one-phase models of global SLEDs can also underestimate the total molecular gas mass when much of it lies in an star-formation-quiescent phase extending beyond a central star-forming region. This is because such a phase (and its large Xco) remains inconspicuous in global CO SLEDs. Finally, detailed studies of a subsample of galaxies find ULIRGs with large amounts (∼109 M☉) of very warm (⩾100 K) and dense gas (≳105 cm−3), which could represent a serious challenge to photon-dominated regions as the main energy portals in the molecular ISM of such systems.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/f92062ed1351f4c47fdb0c6e70304022c039149a

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The CO-to-H2 Conversion Factor (2013, background): “At this stage, the modeling of optically thin isotopologues (e.g., Papadopoulos et al. 2012) may offer the best opportunity for direct XCO measurements at high redshift.”
Dusty Star Forming Galaxies at High Redshift (2014, background): “The large range in possible conversion factor in highz starbursts is similar to recent constraints from local ULIRGs, which suggest that local starbursts aswell can have bothMilky Way-like conversion factors, as well as much lower values (Papadopoulos et al., 2012).”
Cool Gas in High-Redshift Galaxies (2013, background): “…of local ISM conditions, including pressure, gas dynamics, and metallicity, and remains an active area of research for nearby galaxies (Ostriker & Shetty 2011; Narayanan et al. 2012; Narayanan & Hopkin 2012; Papadopoulos et al. 2012, Leroy et al. 2013, Schruba et al. 2012, Sandstrom et al. 2013).”
THE IMPACT OF INTERACTIONS, BARS, BULGES, AND ACTIVE GALACTIC NUCLEI ON STAR FORMATION EFFICIENCY IN LOCAL MASSIVE GALAXIES (2012, background): “…al. (2011) also previously reported significantly different star formation efficiencies in different regions of the interacting galaxy Arp 158, and Papadopoulos et al. (2012) argue in general that accurate modeling of the interstellar medium (ISM) of luminous infrared galaxies (LIRGs) requires…”
THE EVOLVING INTERSTELLAR MEDIUM OF STAR-FORMING GALAXIES SINCE z = 2 AS PROBED BY THEIR INFRARED SPECTRAL ENERGY DISTRIBUTIONS (2012, background): “…(starbursts) having, a value of αCO that is a factor of ∼ 6 smaller than that for local spiral galaxies12 (e.g., Downes & Solomon 1998), and seems to be true at high 12 We note that Papadopoulos et al. (2012) presented evidence that higher αCO values are possible in local ULIRGs redshift too.”
THE CO-TO-H2 CONVERSION FACTOR AND DUST-TO-GAS RATIO ON KILOPARSEC SCALES IN NEARBY GALAXIES (2012, background): “This explanation has been suggested by Downes et al. (1993) and Downes & Solomon (1998) to explain the properties of molecular gas in the nuclear disks of ultra-luminous infrared galaxies (ULIRGs) (see also recent work by Papadopoulos et al. 2012).”
Molecular line emission in NGC 1068 imaged with ALMA: I. An AGN-driven outflow in the dense molecular gas (2014, background): “Lower XCO factors have also been commonly assumed for ULIRGs; however, it is still debated whether this is a common feature of extreme starburst systems (e.g., see discussion in Papadopoulos et al. 2012).”
The molecular gas in Luminous Infrared Galaxies I: CO lines, extreme physical conditions, and their drivers (2011, background): “Finally weexpect that the wide range of ISM conditions found among LIRGs will strongly impact theXco factor, an issue we examine in detail in paper II (Papadopoulos et al. 2012).”
REGULARITY UNDERLYING COMPLEXITY: A REDSHIFT-INDEPENDENT DESCRIPTION OF THE CONTINUOUS VARIATION OF GALAXY-SCALE MOLECULAR GAS PROPERTIES IN THE MASS-STAR FORMATION RATE PLANE (2013, unknown): “As discussed in Section 5.3.1, the average offset from the integrated S-K law does not change for this sample when we use the αCO values given by Papadopoulos et al. (2012). tion of strong SBs.”
Massive molecular outflows and evidence for AGN feedback from CO observations (2013, background): “We measure a total integrated line flux of S CO,TOT = (10.80 ± 0.70) Jy km s−1 within ±2000 km s−1 from the systemic velocity, which is consistent with previous single-dish and interferometric observations (Evans et al. 2002; Solomon et al. 1997; Papadopoulos et al. 2012b).”