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Register a new userWarm Molecular Gas in Luminous Infrared Galaxies
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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 $J le 4$ to a broad distribution peaking around $J sim,$6$-$7 as the IRAS 60-to-100 um color, $C(60/100)$, increases. However, the ratios of a CO line luminosity to the total infrared luminosity, $L_{rm IR}$, 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 ($5 lesssim J lesssim 10$). 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 $L_{rm IR}$, $log R_{rm midCO}$, remain largely independent of $C(60/100)$, and show a mean value of $-4.13$ ($equiv log R^{rm SF}_{rm midCO}$) 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 $R_{rm midCO}$ higher and lower than $R^{rm SF}_{rm midCO}$, respectively.
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Mid-infrared molecular hydrogen (H$_2$) emission is a powerful cooling agent in galaxy mergers and in radio galaxies; it is a potential key tracer of gas evolution and energy dissipation associated with mergers, star formation, and accretion onto supermassive black holes. We detect mid-IR H$_2$ line emission in at least one rotational transition in 91% of the 214 Luminous Infrared Galaxies (LIRGs) observed with Spitzer as part of the Great Observatories All-sky LIRG Survey (GOALS). We use H$_2$ excitation diagrams to estimate the range of masses and temperatures of warm molecular gas in these galaxies. We find that LIRGs in which the IR emission originates mostly from the Active Galactic Nuclei (AGN) have about 100K higher H$_2$ mass-averaged excitation temperatures than LIRGs in which the IR emission originates mostly from star formation. Between 10 and 15% of LIRGs have H$_2$ emission lines that are sufficiently broad to be resolved or partially resolved by the high resolution modules of Spitzers Infrared Spectrograph (IRS). Those sources tend to be mergers and contain AGN. This suggests that a significant fraction of the H$_2$ line emission is powered by AGN activity through X-rays, cosmic rays, and turbulence. We find a statistically significant correlation between the kinetic energy in the H$_2$ gas and the H$_2$ to IR luminosity ratio. The sources with the largest warm gas kinetic energies are mergers. We speculate that mergers increase the production of bulk in-flows leading to observable broad H$_2$ profiles and possibly denser environments.
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), paints 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 ....
We present an initial result from the 12CO (J=1-0) survey of 79 galaxies in 62 local luminous and ultra-luminous infrared galaxy (LIRG and ULIRG) systems obtained using the 45 m telescope at the Nobeyama Radio Observatory. This is the systematic 12CO (J=1-0) survey of the Great Observatories All-sky LIRGs Survey (GOALS) sample. The molecular gas mass of the sample ranges 2.2 x 10^8 - 7.0 x 10^9 Msun within the central several kiloparsecs subtending 15 beam. A method to estimate a size of a CO gas distribution is introduced, which is combined with the total CO flux in the literature. The method is applied to a part of our sample and we find that the median CO radius is 1-4 kpc. From the early stage to the late stage of mergers, we find that the CO size decreases while the median value of the molecular gas mass in the central several kpc region is constant. Our results statistically support a scenario where molecular gas inflows towards the central region from the outer disk, to replenish gas consumed by starburst, and that such a process is common in merging LIRGs.
We performed 12CO(1-0), 13CO(1-0), and HCN(1-0) single-dish observations (beam size ~14-18) toward nearby starburst and non-starburst galaxies using the Nobeyama 45 m telescope. The 13CO(1-0) and HCN(1-0) emissions were detected from all the seven starburst galaxies, with the intensities of both lines being similar (i.e., the ratios are around unity). On the other hand, for case of the non-starburst galaxies, the 13CO(1-0) emission was detected from all three galaxies, while the HCN(1-0) emission was weakly or not detected in past observations. This result indicates that the HCN/13CO intensity ratios are significantly larger (~1.15+-0.32) in the starburst galaxy samples than the non-starburst galaxy samples (<0.31+-0.14). The large-velocity-gradient model suggests that the molecular gas in the starburst galaxies have warmer and denser conditions than that in the non-starburst galaxies, and the photon-dominated-region model suggests that the denser molecular gas is irradiated by stronger interstellar radiation field in the starburst galaxies than that in the non-starburst galaxies. In addition, HCN/13CO in our sample galaxies exhibit strong correlations with the IRAS 25 micron flux ratios. It is a well established fact that there exists a strong correlation between dense molecular gas and star formation activities, but our results suggest that molecular gas temperature is also an important parameter.
We have observed three luminous infrared galaxy systems (LIRGS) which are pairs of interacting galaxies, with the Galaxy H$alpha$ Fabry-Perot system (GH$alpha$FaS) mounted on the 4.2m William Herschel Telescope at the Roque de los Muchachos Observatory, and combined the observations with the Atacama Large Millimeter Array (ALMA) observations of these systems in CO emission to compare the physical properties of the star formation regions and the molecular gas clouds, and specifically the internal kinematics of the star forming regions. We identified 88 star forming regions in the H$alpha$ emission data-cubes, and 27 molecular cloud complexes in the CO emission data-cubes. The surface densities of the star formation rate and the molecular gas are significantly higher in these systems than in non-interacting galaxies and the Galaxy, and are closer to the surface densities of the star formation rate and the molecular gas of extreme star forming galaxies at higher redshifts. The large values of the velocity dispersion also show the enhanced gas surface density. The HII regions are situated on the ${rm{SFR}}-sigma_v$ envelope, and so are also in virial equilibrium. Since the virial parameter decreases with the surface densities of both the star formation rate and the molecular gas, we claim that the clouds presented here are gravitationally dominated rather than being in equilibrium with the external pressure.
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