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New and archival interferometric 12CO(1->0) datasets from six nearby galaxies are combined with H_2 2.122um and H-alpha maps to explore in detail the interstellar medium in different star-forming galaxies. We investigate the relation between warm (H_2 at T~2000 K) and cold (CO at T~50 K) molecular gas from 100 pc to 2 kpc scales. On these scales, the ratio of warm-to-cold molecular hydrogen correlates with the fnu(60um)/fnu(100um) ratio, a ratio that tracks the star formation activity level. This result also holds for the global properties of galaxies from a much larger sample drawn from the literature. The trend persists for over three orders of magnitude in the mass ratio, regardless of source nuclear activity.
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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 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.
We present ALMA Cycle-0 observations of the CO (6-5) line emission (rest-frame frequency = 691.473 GHz) and of the 435$mu m$ dust continuum emission in the nuclear region of NGC 34, a local luminous infrared galaxy (LIRG) at a distance of 84 Mpc (1 = 407 pc) which contains a Seyfert 2 active galactic nucleus (AGN) and a nuclear starburst. The CO emission is well resolved by the ALMA beam ($rm 0.26times 0.23$), with an integrated flux of $rm f_{CO~(6-5)} = 1004; (pm 151) ; Jy; km; s^{-1}$. Both the morphology and kinematics of the CO (6-5) emission are rather regular, consistent with a compact rotating disk with a size of 200 pc. A significant emission feature is detected on the red-shifted wing of the line profile at the frequency of the $rm H^{13}CN; (8-7)$ line, with an integrated flux of $rm 17.7 pm 2.1 (random) pm 2.7 (sysmatic); Jy;km; s^{-1}$. However, it cannot be ruled out that the feature is due to an outflow of warm dense gas with a mean velocity of $rm 400; km; s^{-1}$. The continuum is resolved into an elongated configuration, and the observed flux corresponds to a dust mass of $rm M_{dust} = 10^{6.97pm 0.13}; M_{sun}$. An unresolved central core ($rm radius simeq 50; pc$) contributes $28%$ of the continuum flux and $19%$ of the CO (6-5) flux, consistent with insignificant contributions of the AGN to both emissions. Both the CO (6-5) and continuum spatial distributions suggest a very high gas column density ($rm >= 10^4; M_{sun}; pc^{-2}$) in the nuclear region at $rm radius <= 100; pc$.
We investigate the relation between the detection of the $11.3,mu$m PAH feature in the nuclear ($sim 24-230,$pc) regions of 22 nearby Seyfert galaxies and the properties of the cold molecular gas. For the former we use ground-based (0.3-0.6 resolution) mid-infrared (mid-IR) spectroscopy. The cold molecular gas is traced by ALMA and NOEMA high (0.2-1.1) angular resolution observations of the CO(2-1) transition. Galaxies with a nuclear detection of the $11.3,mu$m PAH feature contain more cold molecular gas (median $1.6times 10^7,M_odot$) and have higher column densities ($N({rm H}_2) = 2 times 10^{23},{rm cm}^{-2}$) over the regions sampled by the mid-IR slits than those without a detection. This suggests that molecular gas plays a role in shielding the PAH molecules in the harsh environments of Seyfert nuclei. Choosing the PAH molecule naphthalene as an illustration, we compute its half-life in the nuclear regions of our sample when exposed to 2.5keV hard X-ray photons. We estimate shorter half-lives for naphthalene in nuclei without a $11.3,mu$m PAH detection than in those with a detection. The Spitzer/IRS PAH ratios on circumnuclear scales ($sim$ 4 $sim$ 0.25-1.3kpc) are in between model predictions for neutral and partly ionized PAHs. However, Seyfert galaxies in our sample with the highest nuclear H$_2$ column densities are not generally closer to the neutral PAH tracks. This is because in the majority of our sample galaxies, the CO(2-1) emission in the inner $sim$ 4 is not centrally peaked and in some galaxies traces circumnuclear sites of strong star formation activity. Spatially resolved observations with the MIRI medium-resolution spectrograph (MRS) on the James Webb Space Telescope will be able to distinguish the effects of an active galactic nucleus (AGN) and star formation on the PAH emission in nearby AGN.
We have conducted two-component, non-LTE modeling of the CO lines from J = 1-0 through J = 13-12 in 87 galaxies observed by the Herschel SPIRE Fourier Transform Spectrometer (FTS). We find the average pressure of the cold molecular gas, traced especially by CO J = 1-0, is $sim 10^{5.0 pm 0.5}$ K cm$^{-3}$. The mid- to high-J lines of CO trace higher-pressure gas at $10^{6.5 pm 0.6}$ K cm$^{-3}$; this pressure is slightly correlated with LFIR. Two components are often necessary to accurately fit the Spectral Line Energy Distributions (SLEDs); a one-component fit often underestimates the flux of CO J = 1-0 and the mass. If low-J lines are not included, mass is underestimated by an order of magnitude. Even when modeling the low-J lines alone or using an $alpha_{CO}$ conversion factor, the mass should be considered to be uncertain to a factor of at least 0.4 dex, and the vast majority of the CO luminosity will be missed (median, 65%). We find a very large spread in our derived values of $alpha_{CO}$, though they do not have a discernible trend with LFIR; the best fit is a constant 0.7 M$_{odot}$/ (K kms$^{-1}$ pc$^2$), with a standard deviation of 0.36 dex, and a range of 0.3-1.6 M$_{odot}$/ (K kms$^{-1}$ pc$^2$). We find average molecular gas depletion times ($tau_{dep}$) of $10^8$ yr that decrease with increasing SFR. Finally, we note that the J = 11-10/J = 1-0 line flux ratio is diagnostic of the warm component pressure, and discuss the implications of this comprehensive study of SPIRE FTS extragalactic spectra for future study post-Herschel.
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