Ultra) luminous infrared galaxies ((U)LIRGs) are objects characterized by their extreme infrared (8-1000 μm) luminosities (L LIRG > 10 11 L and L ULIRG > 10 12 L ). The Herschel Comprehensive ULIRG Emission Survey (PI: van der Werf) presents a representative flux-limited sample of 29 (U)LIRGs that spans the full luminosity range of these objects (10 11 L L IR 10 13 L ). With the Herschel Space Observatory, we observe [C ii] 157 μm, [O i] 63 μm, and [O i] 145 μm line emission with Photodetector Array Camera and Spectrometer, CO J = 4-3 through J = 13-12, [C i] 370 μm, and [C i] 609 μm with SPIRE, and low-J CO transitions with ground-based telescopes.The CO ladders of the sample are separated into three classes based on their excitation level. In 13 of the galaxies, the [O i] 63 μm emission line is self absorbed. Comparing the CO excitation to the InfraRed Astronomical Satellite 60/100 μm ratio and to far infrared luminosity, we find that the CO excitation is more correlated to the far infrared colors. We present cooling budgets for the galaxies and find fine-structure line flux deficits in theand [C i] lines in the objects with the highest far IR fluxes, but do not observe this for CO 4 J upp 13. In order to study the heating of the molecular gas, we present a combination of three diagnostic quantities to help determine the dominant heating source. Using the CO excitation, the CO J = 1-0 linewidth, and the active galactic nucleus (AGN) contribution, we conclude that galaxies with large CO linewidths always have high-excitation CO ladders, and often low AGN contributions, suggesting that mechanical heating is important.

In this paper we present fluxes in the [C I] lines of neutral carbon at the centers of some 76 galaxies with far-infrared luminosities ranging from 10 9 to 10 12 L , as obtained with the Herschel Space Observatory and ground-based facilities, along with the line fluxes of the J = 7−6, J = 4−3, J = 2−1 12 CO, and J = 2−1 13 CO transitions. With this dataset, we determine the behavior of the observed lines with respect to each other and then investigate whether they can be used to characterize the molecular interstellar medium (ISM) of the parent galaxies in simple ways and how the molecular gas properties define the model results. In most starburst galaxies, the [C I] to 13 CO line flux ratio is much higher than in Galactic star-forming regions, and it is correlated to the total far-infrared luminosity. The [C I] (1−0)/ 12 CO (4−3), the [C I] (2−1)/ 12 CO (7−6), and the [C I] (2−1)/(1−0) flux ratios are correlated, and they trace the excitation of the molecular gas. In the most luminous infrared galaxies (LIRGs), the ISM is fully dominated by dense (n( H 2 ) = 10 4 −10 5 cm −3 ) and moderately warm (T kin ≈ 30 K) gas clouds that appear to have low [C • ]/[CO] and [ 13 CO]/[ 12 CO] abundances. In less luminous galaxies, emission from gas clouds at lower densities becomes progressively more important, and a multiple-phase analysis is required to determine consistent physical characteristics. Neither the 12 CO nor the [C I] velocity-integrated line fluxes are good predictors of molecular hydrogen column densities in individual galaxies. In particular, so-called X([C I]) conversion factors are not superior to X( 12 CO) factors. The methods and diagnostic diagrams outlined in this paper also provide a new and relatively straightforward means of deriving the physical characteristics of molecular gas in high-redshift galaxies up to z = 5, which are otherwise hard to determine.

Starburst galaxies are galaxies or regions of galaxies undergoing intense periods of star formation. Understanding the heating and cooling mechanisms in these galaxies can give us insight to the driving mechanisms that fuel the starburst. Molecular emission lines play a crucial role in the cooling of the excited gas. With Herschel Spectral and Photometric Imaging Receiver we have been able to observe the rich molecular spectrum towards the central region of NGC 253. Carbon monoxide (CO, J = 4−3 to 13−12) is the brightest molecule in the Herschel wavelength range and together with ground-based low-J observations, the line fluxes trace the excitation of CO. By studying the CO excitation ladder and comparing the intensities to models, we investigate whether the gas is excited by UV radiation, X-rays, cosmic rays, or turbulent heating. Comparing the 12 CO and 13 CO observations to large velocity gradient models and photon-dominated region (PDR) models we find three main interstellar medium (ISM) phases. We estimate the density, temperature, and masses of these ISM phases. By adding 13 CO, HCN, and HNC line intensities, we are able to constrain these degeneracies and determine the heating sources. The first ISM phase responsible for the low-J CO lines is excited by PDRs, but the second and third phases, responsible for the mid to high-J CO transitions, require an additional heating source. We find three possible combinations of models that can reproduce our observed molecular emission. Although we cannot determine which of these is preferable, we can conclude that mechanical heating is necessary to reproduce the observed molecular emission and cosmic ray heating is a negligible heating source. We then estimate the mass of each ISM phase; 6 × 10 7 M for phase 1 (low-J CO lines), 3 × 10 7 M for phase 2 (mid-J CO lines), and 9 × 10 6 M for phase 3 (high-J CO lines) for a total system mass of 1 × 10 8 M .

Context. The aromatic infrared bands (AIBs) observed in the mid infrared spectrum of galactic and extragalactic sources are attributed to polycyclic aromatic hydrocarbons (PAHs). Recently, two new approaches have been developed to analyze the variations of AIBs in terms of chemical evolution of PAH species: blind signal separation (BSS) and the NASA Ames PAH IR Spectroscopic Database fitting tool. Aims. We aim to study AIBs in a photo-dissociation region (PDR) since in these regions, as the radiation environment changes, the evolution of AIBs are observed. Methods. We observe the NGC 7023-north west (NW) PDR in the mid-infrared (10-19.5 μm) using the InfraRed Spectrometer (IRS), on board Spitzer, in the high-resolution, short wavelength mode. Clear variations are observed in the spectra, most notably the ratio of the 11.0 to 11.2 μm bands, the peak position of the 11.2 and 12.0 μm bands, and the degree of asymmetry of the 11.2 μm band. The observed variations appear to change as a function of position within the PDR. We aim to explain these variations by a change in the abundances of the emitting components of the PDR. A blind signal separation (BSS) method, i.e. a Non-Negative Matrix Factorization algorithm is applied to separate the observed spectrum into components. Using the NASA Ames PAH IR Spectroscopic Database, these extracted signals are fit. The observed signals alone were also fit using the database and these components are compared to the BSS components. Results. Three component signals were extracted from the observation using BSS. We attribute the three signals to ionized PAHs, neutral PAHs, and very small grains (VSGs). The fit of the BSS extracted spectra with the PAH database further confirms the attribution to PAH + and PAH 0 and provides confidence in both methods for producing reliable results. Conclusions. The 11.0 μm feature is attributed to PAH + while the 11.2 μm band is attributed to PAH 0 . The VSG signal shows a characteristically asymmetric broad feature at 11.3 μm with an extended red wing. By combining the NASA Ames PAH IR Spectroscopic Database fit with the BSS method, the independent results of each method can be confirmed and some limitations of each method are overcome.