We used the SPIRE/FTS instrument aboard the Herschel Space Observatory (HSO) to obtain the Spectral Line Energy Distributions (SLEDs) of CO from J=4-3 to J=13-12 of Arp 193 and NGC 6240, two classical merger/starbursts selected from our molecular lin
e survey of local Luminous Infrared Galaxies (LIRGs: L_{IR}>=10^{11} L_{sol}). The high-J CO SLEDs are then combined with ground-based low-J CO, {13}CO, HCN, HCO+, CS line data and used to probe the thermal and dynamical states of their large molecular gas reservoirs. We find the two CO SLEDs strongly diverging from J=4-3 onwards, with NGC6240 having a much higher CO line excitation than Arp193, despite their similar low-J CO SLEDs and L_{FIR}/L_{CO,1-0}, L_{HCN}/L_{CO} (J=1-0) ratios (proxies of star formation efficiency and dense gas mass fraction). In Arp193, one of the three most extreme starbursts in the local Universe, the molecular SLEDs indicate a small amount ~(5-15)% of dense gas (n>=10^{4}cm^{-3}) unlike NGC6240 where most of the molecular gas (~(60-70)%) is dense n~(10^4-10^5)cm^{-3}. Strong star-formation feedback can drive this disparity in their dense gas mass fractions, and also induce extreme thermal and dynamical states for the molecular gas.In NGC6240, and to a lesser degree in Arp193, we find large molecular gas masses whose thermal states cannot be maintained by FUV photons from Photon Dominated Regions (PDRs). We argue that this may happen often in metal-rich merger/starbursts, strongly altering the initial conditions of star formation. ALMA can now directly probe these conditions across cosmic epoch, and even probe their deeply dust-enshrouded outcome, the stellar IMF averaged over galactic evolution.
Results from a large, multi-J CO, {13}CO, and HCN line survey of Luminous Infrared Galaxies (L_{IR}>=10^{10} L_{odot}) in the local Universe (z<=0.1), complemented by CO J=4--3 up to J=13--12 observations from the Herschel Space Observatory (HSO), pa
ints a new picture for the average conditions of the molecular gas of the most luminous of these galaxies with turbulence and/or large cosmic ray (CR) energy densities U_{CR} rather than far-UV/optical photons from star-forming sites as the dominant heating sources. Especially in ULIRGs (L_{IR}>10^{12} L_{odot}) the Photon Dominated Regions (PDRs) can encompass at most sim few% of their molecular gas mass while the large U_{CR} and the strong turbulence in these merger/starbursts, can volumetrically heat much of their molecular gas to T_{kin}sim(100-200)K, unhindered by the high dust extinctions. Moreover the strong supersonic turbulence in ULIRGs relocates much of their molecular gas at much higher average densities than in isolated spirals. This renders low-J CO lines incapable of constraining the properties of the bulk of the molecular gas in ULIRGs, with substantial and systematic underestimates of its mass possible when only such lines are used. A comparative study of multi-J HCN lines and CO SLEDs from J=1--0 up to J=13--12 of NGC 6240 and Arp 193 offers a clear example of two merger/starbursts whose similar low-J CO SLEDs, and L_{IR}/L_{CO,1-0}, L_{HCN, 1-0}/L_{CO,1-0} ratios, yield no indications about their strongly diverging CO SLEDs beyond J=4--3, and ultimately the different physical conditions in their molecular ISM. The much larger sensitivity of ALMA and its excellent site in the Atacama desert now allows the observations necessary to ....
Cosmic rays (CRs) control the thermal, ionization and chemical state of the dense H_2 gas regions that otherwise remain shielded from far-UV and optical stellar radiation propagating through the dusty ISM of galaxies. It is in such CR-dominated regio
ns (CRDRs) rather than Photon-dominated regions (PDRs) of H_2 clouds where the star formation initial conditions are set, making CRs the ultimate star-formation feedback factor in galaxies, able to operate even in their most deeply dust-enshrouded environments. CR-controlled star formation initial conditions naturally set the stage for a near-invariant stellar Initial Mass Function (IMF) in galaxies as long as their average CR energy density U_{CR} permeating their molecular ISM remains within a factor of ~10 of its Galactic value. Nevertheless, in the extreme environments of the compact starbursts found in merging galaxies, where U_{CR}sim(few)x10^{3}U_{CR,Gal}, CRs dramatically alter the initial conditions of star formation. In the resulting extreme CRDRs H_2 cloud fragmentation will produce far fewer low mass (<8 M_{sol}) stars, yielding a top-heavy stellar IMF. This will be a generic feature of CR-controlled star-formation initial conditions, lending a physical base for a bimodal IMF during galaxy formation, with a top-heavy one for compact merger-induced starbursts, and an ordinary IMF preserved for star formation in isolated gas-rich disks. In this scheme the integrated galactic IMFs (IGIMF) are expected to be strong functions of the star formation history of galaxies.
In this work we conclude the analysis of our CO line survey of Luminous Infrared Galaxies (LIRGs: L_{IR}>=10^{11}L_{sol}) in the local Universe (Paper,I), by focusing on the influence of their average ISM properties on the total molecular gas mass es
timates via the so-called X_{co}=M(H_2)/L_{co,1-0} factor. One-phase radiative transfer models of the global CO Spectral Line Energy Distributions (SLEDs) yield an X_{co} distribution with: <X_{co}>sim(0.6+/-0.2) M_{sol}(K km s^{-1} pc^2)^{-1} over a significant range of average gas densities, temperatures and dynamical states. The latter emerges as the most important parameter in determining X_{co}, with unbound states yielding low values and self-gravitating states 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 (>=10^{4} cm^{-3}) rather than a simple one-phase analysis we find that {it near-Galactic X_{co} (3-6), M_sol,(K,km^{-1},pc^2)^{-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 X_{co} to dominate the average value for the entire galaxy. ......... ...this may have thus resulted to systematic underestimates of molecular gas mass in ULIRGs.
We report results from a large molecular line survey of Luminous Infrared Galaxies (L_{IR} >= 10^{11} L_sol) in the local Universe (z<=0.1), conducted during the last decade with the James Clerk Maxwell Telescope (JCMT) and the IRAM 30-m telescope. T
his work presents the CO and {13}CO line data for 36 galaxies, further augmented by multi-J total CO luminosities available for other IR-bright galaxies from the literature. This yields a sample of N=70 galaxies with the star-formation (SF) powered fraction of their IR luminosities spanning L_{IR} (10^{10}-2x10^{12}) L_sol 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 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)x10^9 M_sol) gas mass reservoirs under conditions that would otherwise amount only ~1% of mass per typical SF molecular cloud in the Galaxy. Some of the highest excitation CO SLEDs are found in the so-called Ultra Luminous Infrared Galaxies and seem irreducible to ensembles of ordinary SF-powered regions. Highly supersonic turbulence and high cosmic ray (CR) energy densities rather than far-UV/optical photons or SNR-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.....
