HCN luminosity is a tracer of dense molecular gas, n(H 2 ) k 3 ; 10 4 cm À3 , associated with star-forming giant molecular cloud (GMC) cores. We present the results and analysis of our survey of HCN emission from 65 infrared galaxies, including nine ultraluminous infrared galaxies (ULIGs, L IR k10 12 L ), 22 luminous infrared galaxies (LIGs, 10 11 L < L IR P10 12 L ), and 34 normal spiral galaxies with lower IR luminosity (most are large spiral galaxies). We have measured the global HCN line luminosity, and the observations are reported in Paper I. This paper analyzes the relationships between the total far-IR luminosity (a tracer of the star formation rate), the global HCN line luminosity (a measure of the total dense molecular gas content), and the CO luminosity (a measure of the total molecular content). We find a tight linear correlation between the IR and HCN luminosities L IR and L HCN (in the log-log plot) with a correlation coefficient R ¼ 0:94, and an almost constant average ratio L IR =L HCN ¼ 900 L (K km s À1 pc 2 ) À1 . The IR-HCN linear correlation is valid over 3 orders of magnitude including ULIGs, the most luminous objects in the local universe. The direct consequence of the linear IR-HCN correlation is that the star formation law in terms of dense molecular gas content has a power-law index of 1.0. The global star formation rate is linearly proportional to the mass of dense molecular gas in normal spiral galaxies, LIGs, and ULIGs. This is strong evidence in favor of star formation as the power source in ultraluminous galaxies since the star formation in these galaxies appears to be normal and expected given their high mass of dense star-forming molecular gas.The HCN-CO correlation is also much tighter than the IR-CO correlation. We suggest that the nonlinear correlation between L IR and L CO may be a consequence of the stronger and perhaps more physical correlations between L IR and L HCN and between L HCN and L CO . Thus, the star formation rate indicated by L IR depends on the amount of dense molecular gas traced by HCN emission, not the total molecular gas traced by CO emission. One of the main arguments in favor of an active galactic nucleus (AGN) as the power source in ULIGs is the anomalously high ratio L IR =L CO or L IR /M(H 2 ) or high star formation rate per M of gas, compared with that from normal spiral galaxies. This has been interpreted as indicating that a dust-enshrouded AGN is required to produce the very high luminosity. Viewed in terms of the dense gas mass the situation is completely different. The ratio L IR =L HCN or L IR =M dense , a measure of the star formation rate per solar mass of dense gas, is essentially the same in all galaxies including ULIGs. The ratio L IR =M dense is virtually independent of galaxy luminosity and on average L IR =M dense % 90L =M , about the same as in GMC cores but much higher than in GMCs. We find that ULIGs simply have a large quantity of dense molecular gas and thus produce a prodigious starburst that heats the dust, produces the I...

We investigate the CO excitation of normal (near-IR selected BzK) star-forming (SF) disk galaxies at z = 1.5 using IRAM Plateau de Bure observations of the CO[2-1], CO , and CO[5-4] transitions for four galaxies, including VLA observations of CO[1-0] for three of them, with the aim of constraining the average state of H 2 gas. By exploiting previous knowledge of the velocity range, spatial extent, and size of the CO emission, we measure reliable line fluxes with a signal-to-noise ratio >4-7 for individual transitions. While the average CO spectral line energy distribution (SLED) has a subthermal excitation similar to the Milky Way (MW) up to CO[3-2], we show that the average CO emission is four times stronger than assuming MW excitation. This demonstrates that there is an additional component of more excited, denser, and possibly warmer molecular gas. The ratio of CO[5-4] to lower-J CO emission is lower than in local (ultra-)luminous infrared galaxies (ULIRGs) and high-redshift starbursting submillimeter galaxies, however, and appears to be closely correlated with the average intensity of the radiation field U and with the star formation surface density, but not with the star formation efficiency. The luminosity of the CO transition is found to be linearly correlated with the bolometric infrared luminosity over four orders of magnitudes. For this transition, z = 1.5 BzK galaxies follow the same linear trend as local spirals and (U)LIRGs and high-redshift star-bursting submillimeter galaxies. The CO[5-4] luminosity is thus empirically related to the dense gas and might be a more convenient way to probe it than standard high-density tracers that are much fainter than CO. We see excitation variations among our sample galaxies that can be linked to their evolutionary state and clumpiness in optical rest-frame images. In one galaxy we see spatially resolved excitation variations, where the more highly excited part of the galaxy corresponds to the location of massive SF clumps. This provides support to models that suggest that giant clumps are the main source of the high-excitation CO emission in high-redshift disk-like galaxies.

Observations have revealed prodigious amounts of star formation in starburst galaxies as traced by dust and molecular emission, even at large redshifts. Recent work shows that for both nearby spiral galaxies and distant starbursts, the global star formation rate, as indicated by the infrared luminosity, has a tight and almost linear correlation with the amount of dense gas as traced by the luminosity of HCN. Our surveys of Galactic dense cores in HCN 1−0 emission show that this correlation continues to a much smaller scale, with nearly the same ratio of infrared luminosity to HCN luminosity found over 7-8 orders of magnitude in L IR , with a lower cutoff around 10 4.5 L ⊙ of infrared luminosity. The linear correlation suggests that we may understand distant star formation in terms of the known properties of local star-forming regions. Both the correlation and the luminosity cutoff can be explained if the basic unit of star formation in galaxies is a dense core, similar to those studied in our Galaxy.

We report systematic HCN J ¼ 1-0 (and CO) observations of a sample of 53 infrared (IR) and/or CO-bright and/or luminous galaxies, including seven ultraluminous infrared galaxies, nearly 20 luminous infrared galaxies, and more than a dozen of the nearest normal spiral galaxies. This is the largest and most sensitive HCN survey of galaxies to date. All galaxies observed so far follow the tight correlation between the IR luminosity L IR and the HCN luminosity L HCN initially proposed by Solomon, Downes, & Radford, which is detailed in a companion paper. We also address here the issue of HCN excitation. There is no particularly strong correlation between L HCN and the 12 m luminosity; in fact, of all the four IRAS bands, the 12 m luminosity has the weakest correlation with the HCN luminosity. There is also no evidence of stronger HCN emission or a higher ratio of HCN and CO luminosities L HCN =L CO for galaxies with excess 12 m emission. This result implies that mid-IR radiative pumping, or populating, of the J ¼ 1 level of HCN by a mid-IR vibrational transition is not important compared with the collisional excitation by dense molecular hydrogen. Furthermore, large velocity gradient calculations justify the use of HCN J ¼ 1-0 emission as a tracer of high-density molecular gas (k3 ; 10 4 = cm À3 ) and give an estimate of the mass of dense molecular gas from HCN observations. Therefore, L HCN may be used as a measure of the total mass of dense molecular gas, and the luminosity ratio L HCN =L CO may indicate the fraction of molecular gas that is dense.

We report single-dish multi-transition measurements of the 12 CO, HCN, and HCO + molecular line emission as well as HNC J=1-0 and HNCO in the two ultraluminous infra-red galaxies Arp 220 and NGC 6240. Using this new molecular line inventory, in conjunction with existing data in the literature, we compiled the most extensive molecular line data sets to date for such galaxies. The many rotational transitions, with their different excitation requirements, allow the study of the molecular gas over a wide range of different densities and temperatures with significant redundancy, and thus allow good constraints on the properties of the dense gas in these two systems. The mass (∼ (1 − 2) × 10 10 M ⊙ ) of dense

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.

We present the IRAM-30 m observations of multiple-J CO (J up mostly from 3 up to 8) and [C I]( 3 P 2 → 3 P 1 ) ([C I](2-1) hereafter) line emission in a sample of redshift ∼ 2-4 submillimeter galaxies (SMGs). These SMGs are selected among the brightest-lensed galaxies discovered in the Herschel-Astrophysical Terahertz Large Area Survey (H-ATLAS). Forty-seven CO lines and 7 [C I](2-1) lines have been detected in 15 lensed SMGs. A non-negligible effect of differential lensing is found for the CO emission lines, which could have caused significant underestimations of the linewidths, and hence of the dynamical masses. The CO spectral line energy distributions (SLEDs), peaking around J up ∼ 5-7, are found to be similar to those of the local starburst-dominated ultra-luminous infrared galaxies and of the previously studied SMGs. After correcting for lensing amplification, we derived the global properties of the bulk of molecular gas in the SMGs using non-LTE radiative transfer modelling, such as the molecular gas density n H 2 ∼ 10 2.5 -10 4.1 cm −3 and the kinetic temperature T k ∼ 20-750 K. The gas thermal pressure P th ranging from ∼ 10 5 K cm −3 to 10 6 K cm −3 is found to be correlated with star formation efficiency. Further decomposing the CO SLEDs into two excitation components, we find a low-excitation component with n H 2 ∼ 10 2.8 -10 4.6 cm −3 and T k ∼ 20-30 K, which is less correlated with star formation, and a high-excitation one (n H 2 ∼ 10 2.7 -10 4.2 cm −3 , T k ∼ 60-400 K) which is tightly related to the on-going star-forming activity. Additionally, tight linear correlations between the far-infrared and CO line luminosities have been confirmed for the J up ≥ 5 CO lines of these SMGs, implying that these CO lines are good tracers of star formation. The [C I](2-1) lines follow the tight linear correlation between the luminosities of the [C I](2-1) and the CO(1-0) line found in local starbursts, indicating that [C I] lines could serve as good total molecular gas mass tracers for high-redshift SMGs as well. The total mass of the molecular gas reservoir, (1-30) × 10 10 M , derived based on the CO(3-2) fluxes and α CO(1-0) = 0.8M ( K km s −1 pc 2 ) −1 , suggests a typical molecular gas depletion time t dep ∼ 20-100 Myr and a gas to dust mass ratio δ GDR ∼ 30-100 with ∼ 20%-60% uncertainty for the SMGs. The ratio between CO line luminosity and the dust mass L CO /M dust appears to be slowly increasing with redshift for high-redshift SMGs, which need to be further confirmed by a more complete SMG sample at various redshifts. Finally, through comparing the linewidth of CO and H 2 O lines, we find that they agree well in almost all our SMGs, confirming that the emitting regions of the CO and H 2 O lines are co-spatially located.

We present a "super-deblended" far-infrared to (sub)millimeter photometric catalog in the Cosmic Evolution Survey (COSMOS), prepared with the method recently developed by Liu et al. 2018, with key adaptations. We obtain point spread function (PSF) fitting photometry at fixed prior positions including 88,008 galaxies detected in either VLA 1.4 GHz, 3 GHz and/or MIPS 24 µm images. By adding a specifically carved massselected sample (with an evolving stellar mass limit), a highly complete prior sample of 194,428 galaxies is achieved for deblending FIR/(sub)mm images. We performed "active" removal of non relevant priors at FIR/(sub)mm bands using spectral energy distribution (SED) fitting and redshift information. In order to cope with the shallower COSMOS data we subtract from the maps the flux of faint non-fitted priors and explicitly account for the uncertainty of this step. The resulting photometry (including data from Spitzer, Herschel, SCUBA2, AzTEC, MAMBO and NSF's Karl G. Jansky Very Large Array at 3 GHz and 1.4 GHz) displays well behaved quasi-Gaussian uncertainties, calibrated from Monte Carlo simulations and tailored to observables (crowding, residual maps). Comparison to ALMA photometry for hundreds of sources provide a remarkable validation of the technique. We detect 11,220 galaxies over the 100-1200 µm range, extending to z phot ∼ 7. We conservatively selected a sample of 85 z > 4 high redshift candidates, significantly detected in the FIR/(sub)mm, often with secure radio and/or Spitzer/IRAC counterparts. This provides a chance to investigate the first generation of vigorous starburst galaxies (SFRs∼ 1000M yr −1 ). The photometric and value added catalogs are publicly released.