We report the results of a pilot study with the Expanded Very Large Array (EVLA) of 12 CO J = 1-0 emission from four submillimetre-selected galaxies at z = 2.2-2.5, each with an existing detection of 12 CO J = 3-2, one of which comprises two distinct spatial components. Using the EVLA's most compact configuration, we detect strong, broad [medians: 990 km s −1 full width at zero intensity; 540 km s −1 full width at half-maximum (FWHM)] J = 1-0 line emission from all of our targets -coincident in position and velocity with their J = 3-2 emission. The median linewidth ratio, σ 1-0 /σ 3-2 = 1.15 ± 0.06, suggests that the J = 1-0 is more spatially extended than the J = 3-2 emission, a situation confirmed by our maps which reveal velocity structure in several cases and typical sizes of ∼16 kpc FWHM. The median brightness temperature (T b ) ratio is r 3−2/1−0 = 0.55 ± 0.05, consistent with local galaxies with L IR > 10 11 L , noting that our value may be biased high because of the J = 3-2 based sample selection. Naively, this suggests gas masses roughly two times higher than estimates made using higher J transitions of CO, with the discrepancy due entirely to the difference in assumed T b ratio. We also estimate molecular gas masses using the 12 CO J = 1-0 line and the observed global T b ratios, assuming standard underlying T b ratios for the non-star-forming and star-forming gas phases as well as a limiting star formation efficiency for the latter in all systems, i.e. without calling upon X CO (≡ α). Using this new method, we find a median molecular gas mass of (2.5 ± 0.8) × 10 10 M , with a plausible range stretching up to three times higher. Even larger masses cannot be ruled out, but are not favoured by dynamical constraints: the median dynamical mass within R ∼ 7 kpc for our sample is (2.3 ± 1.4) × 10 11 M or ∼6 times more massive than UV-selected galaxies at this epoch. We examine the Schmidt-Kennicutt (S-K) relation for all the distant galaxy populations for which CO J = 1-0 or J = 2-1 data are available, finding small systematic differences between galaxy populations. These have previously been interpreted as evidence for different modes of star formation, but we argue that these differences are to be expected, given the still considerable uncertainties, certainly when considering the probable excitation biases due to the molecular lines used, and the possibility of sustained S-K offsets during the evolution of individual gas-rich systems. Finally, we discuss the morass of degeneracies surrounding molecular gas E-mail: rji@roe.ac.uk C 2011 The Authors Monthly Notices of the Royal Astronomical Society C 2011 RAS R. J. Ivison et al. mass estimates, the possibilities for breaking them, and the future prospects for imaging and studying cold, quiescent molecular gas at high redshifts.

Detailed studies of the spectral energy distribution (SED) of normal galaxies have increasingly been used to understand the physical mechanism dominating their integrated emission, mainly owing to the availability of high quality multi-wavelength data from the UV to the far-infrared (FIR). However, systems hosting dust-enshrouded nuclear starbursts and/or an accreting supermassive black hole (an active galactic nucleus or AGN) are especially challenging to study. This is due to the complex interplay between the heating by massive stars and the AGN, the absorption and emission of radiation from dust, as well as the presence of the underlying old stellar population. We used the latest release of CIGALE, a fast state-of-the-art galaxy SED-fitting model relying on energy balance, to study the influence of an AGN in a self consistent manner in estimating both the star formation rate (SFR) and stellar mass in galaxies, as well as to calculate the contribution of the AGN to the power output of the host. Using the semi-analytical galaxy formation model , we created a suite of mock galaxy SEDs using realistic star formation histories (SFH). We also added an AGN of Type-1, Type-2, or intermediate-type whose contribution to the bolometric luminosity can be variable. We performed an SED-fitting of these catalogues with CIGALE, assuming three different SFHs: a single-exponentially-decreasing (1τ-dec), a double-exponentiallydecreasing (2τ-dec), and a delayed SFH. Constraining the overall contribution of an AGN to the total infrared luminosity (frac AGN ) is very challenging for frac AGN < 20%, with uncertainties of ∼5-30% for higher fractions depending on the AGN type, while FIR and sub-mm are essential. The AGN power has an impact on the estimation of M * in Type-1 and intermediate-type AGNs but has no effect on galaxies hosting Type-2 AGNs. We find that in the absence of AGN emission, the best estimates of M * are obtained using the 2τ-dec model but at the expense of realistic ages of the stellar population. The delayed SFH model provides good estimates of M * and SFR, with a maximum offset of 10% as well as better estimates of the age. Our analysis shows that the under-estimation of the SFR increases with frac AGN for Type-1 systems, as well as for low contributions of an intermediate AGN type, but it is quite insensitive to the emission of Type-2 AGNs up to frac AGN ∼ 45%. A lack of sampling the FIR, or sub-mm domain systematically over-estimates the SFR (<20%), independent of the contribution of the AGN. Similarly, the UV emission is critical in accurately retrieving both the M * for Type-1 and intermediate-type AGN and the SFR of all three AGN types. We show that the presence of AGN emission introduces a scatter to the SFR-M * main sequence relation derived from SED-fitting, which is driven by the uncertainties on M * . Finally, we used our mock catalogues to test the popular IR SED-fitting code DIR and show that frac AGN is under-estimated but that the SFR is recovered well for Type-1 and intermedia...

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.

Aims. Within the framework of the DustPedia project we investigate the properties of cosmic dust and its interaction with the stellar radiation (originating from different stellar populations) for 814 galaxies in the nearby Universe, all observed by the Herschel Space Observatory. Methods. We take advantage of the widely used galaxy SED fitting code CIGALE, properly adapted to include the state-of-the-art dust model THEMIS. For comparison purposes an estimation of the dust properties is provided by approximating the emission at far-infrared and sub-millimeter wavelengths with a modified blackbody. Using the DustPedia photometry we determine the physical properties of the galaxies, such as, the dust and stellar mass, the star-formation rate, the bolometric luminosity as well as the unattenuated and the absorbed by dust stellar light, for both the old (> 200 Myr) and young (≤ 200 Myr) stellar populations. Results. We show how the mass of stars, dust, and atomic gas, as well as the star-formation rate and the dust temperature vary between galaxies of different morphologies and provide recipes to estimate these parameters given their Hubble stage (T ). We find a mild correlation between the mass fraction of the small a-C(:H) grains with the specific star-formation rate. On average, young stars are very efficient in heating the dust, with absorption fractions reaching as high as ∼ 77% of the total, unattenuated luminosity of this population. On the other hand, the maximum absorption fraction of old stars is ∼ 24%. Dust heating in early-type galaxies is mainly due to old stars, up to a level of ∼ 90%. Young stars progressively contribute more for 'typical' spiral galaxies and they become the dominant source of dust heating for Sm type and irregular galaxies, donating up to ∼ 60% of their luminosity to this purpose. Finally, we find a strong correlation of the dust heating fraction by young stars with morphology and the specific star-formation rate.

We present FIR[50 − 300 µm]−CO luminosity relations (i.e., log L FIR = α log L CO + β) for the full CO rotational ladder from J = 1 − 0 up to J = 13 − 12 for a sample of 62 local (z ≤ 0.1) (Ultra) Luminous Infrared Galaxies (LIRGs; L IR[8−1000 µm] > 10 11 L ) using data from Herschel SPIRE-FTS and ground-based telescopes. We extend our sample to high redshifts (z > 1) by including 35 (sub)millimeter selected dusty star forming galaxies from the literature with robust CO observations, and sufficiently well-sampled FIR/sub-millimeter spectral energy distributions (SEDs) so that accurate FIR luminosities can be deduced. The addition of luminous starbursts at high redshifts enlarge the range of the FIR−CO luminosity relations towards the high-IR-luminosity end while also significantly increasing the small amount of mid-J/high-J CO line data (J = 5 − 4 and higher) that was available prior to Herschel. This new data-set (both in terms of IR luminosity and J-ladder) reveals linear FIR−CO luminosity relations (i.e., α 1) for J = 1 − 0 up to J = 5 − 4, with a nearly constant normalization (β ∼ 2). In the simplest physical scenario this is expected from the (also) linear FIR−(molecular line) relations recently found for the dense gas tracer lines (HCN and CS), as long as the dense gas mass fraction does not vary strongly within our (merger/starburst)-dominated sample. However from J = 6 − 5 and up to the J = 13 − 12 transition we find an increasingly sub-linear slope and higher normalization constant with increasing J. We argue that these are caused by a warm (∼ 100 K) and dense (> 10 4 cm −3 ) gas component whose thermal state is unlikely to be maintained by star formation powered far-UV radiation fields (and thus is no longer directly tied to the star formation rate). We suggest that mechanical heating (e.g., supernova driven turbulence and shocks), and not cosmic rays, is the more likely source of energy for this component. The global CO spectral line energy distributions (SLEDs), which remain highly excited from J = 6 − 5 up to J = 13 − 12, are found to be a generic feature of the (U)LIRGs in our sample, and further support the presence of this gas component.