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We present a CO(3−2) study of four systems composed of six (ultra) luminous infrared galaxies (U/LIRGs), located at 0.28 <z < 0.44, that straddle the transition region between regular star-forming galaxies and starbursts. These galaxies benefit from previous multiwavelength analysis allowing in depth exploration of an understudied population of U/LIRGs at a time when the universe is experiencing a rapid decline in star formation rate density. We detect CO(3−2) emission in four targets and these galaxies fall between the loci of regular star-forming galaxies and starbursts on the Kennicutt–Schmidtt relation. Compared to low luminosity LIRGs and high luminosity ULIRGs at similar redshifts, we find they all have similar molecular gas budgets with the difference in their star formation rates (SFR) driven by the star formation efficiency (SFE). This suggests that at these redshifts large molecular gas reservoirs must coincide with an increased SFE to transition a galaxy into the starburst regime. We studied the structure and kinematics and found our four detections are either interacting or have disturbed morphology which may be driving the SFE. One of the CO(3−2) non-detections has a strong continuum detection, and has been previously observed in H α, suggesting an unusual interstellar medium for a ULIRG. We conclude that our sample of transitioning U/LIRGs fill the gap between regular star-forming galaxies and starbursts, suggest a continuous change in SFE between these two populations and the increased SFE may be driven by morphology and differing stages of interaction.

 

We investigate the fine-structure [Cii] line at 158μm as a molecular gas tracer by analyzing the relationship between molecular gas mass (M_mol) and [Cii] line luminosity (L_[CII]) in 11,125z≃ 6 star-forming, main-sequence galaxies from thesimbasimulations, with line emission modeled by the Simulator of Galaxy Millimeter/Submillimeter Emission. Though most (∼50%–100%) of the gas mass in our simulations is ionized, the bulk (>50%) of the [Cii] emission comes from the molecular phase. We find a sublinear (slope 0.78 ± 0.01)$\log L_{[\mathrm{C}\mathrm{II}]}–\log M_{\mathrm{mol}}$relation, in contrast with the linear relation derived from observational samples of more massive, metal-rich galaxies atz≲ 6. We derive a median [Cii]-to-M_molconversion factor ofα_[CII]≃ 18M_⊙/L_⊙. This is lower than the average value of ≃30M_⊙/L_⊙derived from observations, which we attribute to lower gas-phase metallicities in our simulations. Thus, a lower, luminosity-dependent conversion factor must be applied when inferring molecular gas masses from [Cii] observations of low-mass galaxies. For our simulations, [Cii] is a better tracer of the molecular gas than COJ= 1–0, especially at the lowest metallicities, where much of the gas isCO-dark. We find thatL_[CII]is more tightly correlated withM_molthan with star formation rate (SFR), and both the$\log L_{[\mathrm{C}\mathrm{II}]}–\log M_{\mathrm{mol}}$and$\log L_{[\mathrm{C}\mathrm{II}]}–\log \mathrm{SFR}$relations arise from the Kennicutt–Schmidt relation. Our findings suggest thatL_[CII]is a promising tracer of the molecular gas at the earliest cosmic epochs.

 

We studied the molecular gas properties of AzTEC/C159, a star-forming disk galaxy at $z=4.567$. We secured $^{12}$CO molecular line detections for the $J=2\to1$ and $J=5\to4$ transitions using the Karl G. Jansky VLA and the NOEMA interferometer. The broad (FWHM$\sim750\,{\rm km\,s}^{-1}$) and tentative double-peaked profiles of both $^{12}$CO lines are consistent with an extended molecular gas reservoir, which is distributed in a rotating disk as previously revealed from [CII] 158$\mu$m line observations. Based on the $^{12}$CO(2$\to$1) emission line we derived $L'_{\rm{CO}}=(3.4\pm0.6)\times10^{10}{\rm \,K\,km\,s}^{-1}{\rm \,pc}^{2}$, that yields a molecular gas mass of $M_{\rm H_2 }(\alpha_{\rm CO}/4.3)=(1.5\pm0.3)\times 10^{11}{\rm M}_\odot$ and unveils a gas-rich system with $\mu_{\rm gas}(\alpha_{\rm CO}/4.3)\equiv M_{\rm H_2}/M_\star=3.3\pm0.7$. The extreme star formation efficiency (SFE) of AzTEC/C159, parametrized by the ratio $L_{\rm{IR}}/L'_{\rm{CO}}=(216\pm80)\, {\rm L}_{\odot}{\rm \,(K\,km\,s}^{-1}{\rm \,pc}^{2})^{-1}$, is comparable to merger-driven starbursts such as local ultra-luminous infrared galaxies (ULIRGs) and SMGs. Likewise, the $^{12}$CO(5$\to$4)/CO(2$\to$1) line brightness temperature ratio of $r_{52}= 0.55\pm 0.15$ is consistent with high excitation conditions, similar to that observed in SMGs. We constrained the value for the $L'_{\text{CO}}-{\rm H}_2$ mass conversion factor in AzTEC/C159, i.e. $\alpha_{\text{CO}}=3.9^{+2.7}_{-1.3}{\rm \,M}_{\odot}{\rm \,K}^{-1}{\rm \,km}^{-1}{\rm \,s\,pc}^{-2}$, that is consistent with a self-gravitating molecular gas distribution as observed in local star-forming disk galaxies. Cold gas streams from cosmological filaments might be fueling a gravitationally unstable gas-rich disk in AzTEC/C159, which breaks into giant clumps forming stars as efficiently as in merger-driven systems and generate high gas excitation.