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We use the first systematic data sets of CO molecular line emission in z∼ 1–3 normal star-forming galaxies (SFGs) for a comparison of the dependence of galaxy-averaged star formation rates on molecular gas masses at low and high redshifts, and in different galactic environments. Although the current high- z samples are still small and biased towards the luminous and massive tail of the actively star-forming ‘main-sequence’, a fairly clear picture is emerging. Independent of whether galaxy-integrated quantities or surface densities are considered, low- and high- z SFG populations appear to follow similar molecular gas–star formation relations with slopes 1.1 to 1.2, over three orders of magnitude in gas mass or surface density. The gas-depletion time-scale in these SFGs grows from 0.5 Gyr at to 1.5 Gyr at . The average corresponds to a fairly low star formation efficiency of 2 per cent per dynamical time. Because star formation depletion times are significantly smaller than the Hubble time at all redshifts sampled, star formation rates and gas fractions are set by the balance between gas accretion from the halo and stellar feedback. In contrast, very luminous and ultraluminous, gas-rich major mergers at both low and high z produce on average four to 10 times more far-infrared luminosity per unit gas mass. We show that only some fraction of this difference can be explained by uncertainties in gas mass or luminosity estimators; much of it must be intrinsic. A possible explanation is a top-heavy stellar mass function in the merging systems but the most likely interpretation is that the star formation relation is driven by global dynamical effects. For a given mass, the more compact merger systems produce stars more rapidly because their gas clouds are more compressed with shorter dynamical times, so that they churn more quickly through the available gas reservoir than the typical normal disc galaxies. When the dependence on galactic dynamical time-scale is explicitly included, disc galaxies and mergers appear to follow similar gas-to-star formation relations. The mergers may be forming stars at slightly higher efficiencies than the discs. [ABSTRACT FROM AUTHOR]

ABSTRACT In order to understand which process (e.g. galactic winds, cold accretion) is responsible for the cool ( T ∼ 104 K) halo gas around galaxies, we embarked on a programme to study the star formation properties of galaxies selected by their Mg ii absorption signature in quasar spectra. Specifically, we searched for the Hα line emission from galaxies near very strong z ≃ 2 Mg ii absorbers (with rest-frame equivalent width Å) because these could be the signposts of outflows or inflows. Surprisingly, we detect Hα from only four hosts out of 20 sightlines (and two out of the 19 H i-selected sightlines), despite reaching a star formation rate (SFR) sensitivity limit of 2.9 M⊙ yr−1 (5σ) for a Chabrier initial mass function. This low success rate (4/20) is in contrast with our z ≃ 1 survey where we detected 66 per cent (14/21) of the Mg ii hosts (down to 0.6 M⊙ yr−1; 5σ). Taking into account the difference in sensitivity between the two surveys, we should have been able to detect ≥11.4 (≥7.6) of the 20 z ≃ 2 hosts - assuming that SFR evolves as ∝(1 + z)γ with γ= 2.5 (or γ= 0) respectively - whereas we found only four galaxies. Interestingly, all the z = 2 detected hosts have observed SFRs ≳ 9 M⊙ yr−1, well above our sensitivity limit, while at z = 1 they all have SFR < 9 M⊙ yr−1, an evolution that is in good agreement with the evolution of the SFR main sequence, i.e. with γ= 2.5. Moreover, we show that the z = 2 undetected hosts are not hidden under the quasar continuum after stacking our data. They also cannot be outside our surveyed area as this latter option runs against our sample selection criteria ( Å) and the known Wλ2796r-impact parameter relation for low-ionization ions. Hence, strong Mg ii absorbers could trace star-formation-driven winds in low-mass haloes ( Mh≤ 1010.6 M⊙), provided that the winds do not extend beyond 20 kpc in order not to violate the evolution of the absorber number density d N/d z (Mg ii). Alternatively, our results imply that z = 2 galaxies traced by strong Mg ii absorbers do not form stars at a rate expected (3-10 M⊙ yr−1) for their (halo or stellar) masses, supporting the existence of a transition in accretion efficiency at Mh≃ 1011 M⊙. This scenario can explain both the detections and the non-detections. [ABSTRACT FROM AUTHOR]

ABSTRACT We present interferometric CO observations of 12 z∼ 2 submillimetre-faint, star-forming radio galaxies (SFRGs) which are thought to be ultraluminous infrared galaxies (ULIRGs) possibly dominated by warmer dust ( Tdust ≳ 40 K) than submillimetre galaxies (SMGs) of similar luminosities. Four other CO-observed SFRGs are included from the literature, and all the observations are taken at the Plateau de Bure Interferometer (PdBI) in the compact configuration. 10 of the 16 SFRGs observed in CO (63 per cent) are detected at >4σ with a mean inferred molecular gas mass of ∼2 × 1010 M⊙. SFRGs trend slightly above the local ULIRG LFIR- L′CO relation. Since SFRGs are about two times fainter in radio luminosity but exhibit similar CO luminosities to SMGs, this suggests SFRGs are slightly more efficient star formers than SMGs at the same redshifts. SFRGs also have a narrow mean CO linewidth, 320 ± 80 km s−1. Many SMGs have similarly narrow CO linewidths, but very broad features (∼900 km s−1) are present in a few SMGs and are absent from SFRGs. SFRGs bridge the gap between properties of very luminous >5 × 1012 L⊙ SMGs and those of local ULIRGs, and are consistent with intermediate-stage major mergers. We suspect that more moderate luminosity SMGs, not yet surveyed in CO, would show similar molecular gas properties to SFRGs. The active galactic nucleus (AGN) fraction of SFRGs is consistent with SMGs and is estimated to be 0.3 ± 0.1, suggesting that SFRGs are observed near the peak phase of star formation activity and not in a later, post-SMG enhanced AGN phase. The excitation analysis of one SFRG is consistent with the CO excitation observed in SMGs [turning over beyond 12CO( J= 4→3)]. This CO survey of SFRGs serves as a pilot project for the much more extensive survey of Herschel and SCUBA-2 selected sources which only partially overlap with SMGs. Better constraints on the CO properties of a diverse high- z ULIRG population are needed from ALMA to determine the evolutionary origin of extreme starbursts, and what role ULIRGs serve in catalyzing the formation of massive stellar systems in the early Universe. [ABSTRACT FROM AUTHOR]

Using the IRAM Plateau de Bure Interferometer (PdBI) we have searched for the upper fine structure line of neutral carbon [ ] and towards the submillimetre galaxies (SMGs) GN 20 (SMM J123711.9+622212, ) and GN 20.2 (SMM J123708.8+622202, ). The far-infrared continuum is detected at 8σ significance in GN 20, with a flux density of , while no continuum is detected in GN 20.2. Both sources are statistically undetected in both and lines; we derive line luminosity limits for both C i and CO of . Assuming carbon excitation temperatures of (the galaxies' measured dust temperatures), we infer C i mass limits of (GN 20) and . The derived C i abundance limits are for GN 20 and for GN 20.2, implying that the systems have Milky Way level carbon enrichment ( X[C i]/ X[H2]) or lower, similar to high-redshift carbon-detected systems (at ) but about 50 times less than the carbon enrichment of local starburst galaxies. Observations of GN 20 and GN 20.2 in high-resolution MERLIN+VLA (Multi-Element Radio Linked Interferometer Network+Very Large Array) radio maps of GOODS-N (Great Observatories Origin Deep Survey-North) are used to further constrain the sizes and locations of active regions. We conclude that the physical gas properties of young rapidly evolving systems like GN 20 and GN 20.2 are likely significantly different than starburst/ULIRG (ultraluminous infrared galaxy) environments in the local Universe yet similar to SMGs. Unless gravitationally amplified examples can be found, observations of galaxies like GN 20 will require the order of magnitude increase in sensitivity of the Atacama Large Millimetre Array (ALMA) to constrain their C i and high- J CO content, despite the fact that they are the brightest systems at . [ABSTRACT FROM AUTHOR]

In this paper, we present results from an Institut de Radio Astronomie Millimétrique (IRAM) Plateau de Bure millimetre-wave Interferometer (PdBI) survey for carbon monoxide (CO) emission towards radio-detected submillimetre galaxies (SMGs) with known optical and near-infrared spectroscopic redshifts. Five sources in the redshift rangewere detected, nearly doubling the number of SMGs detected in CO. We summarize the properties of all 12 CO-detected SMGs, as well as six sources not detected in CO by our survey, and use this sample to explore the bulk physical properties of the submillimetre galaxy (SMG) population as a whole. The median CO line luminosity of the SMGs is. Using a CO-to-H2 conversion factor appropriate for starburst galaxies, this corresponds to a molecular gas masswithin an∼ 2 kpc radius, approximately 4 times greater than the most luminous local ultraluminous infrared galaxies (ULIRGs) but comparable to that of the most extreme high-redshift radio galaxies (HzRGs) and quasi-sellar objects (QSOs). The median CO FWHM linewidth is broad,, and the SMGs often have double-peaked line profiles, indicative of either a merger or a disc. From their median gas reservoirsand star formation rates, we estimate a lower limit on the typical gas-depletion time-scale of≳ 40 Myr in SMGs. This is marginally below the typical age expected for the starbursts in SMGs and suggests that negative feedback processes may play an important role in prolonging the gas consumption time-scale. We find a statistically significant correlation between the far-infrared and CO luminosities of the SMGs, which extends the observed correlation for local ULIRGs to higher luminosities and higher redshifts. The non-linear nature of the correlation implies that SMGs have higher far-infrared to CO luminosity ratios and possibly higher star formation efficiencies (SFEs), than local ULIRGs. Assuming a typical CO source diameter of, we estimate a median dynamical mass offor the SMG sample. Both the total gas and stellar masses imply that SMGs are very massive systems, dominated by baryons in their central regions. The baryonic and dynamical properties of these systems mirror those of local giant ellipticals and are consistent with numerical simulations of the formation of the most massive galaxies. We have been able to impose a lower limit ofto the comoving number density of massive galaxies in the redshift range, which is in agreement with results from recent spectroscopic surveys and the most recent model predictions. [ABSTRACT FROM AUTHOR]