Luminous Infrared Galaxies with the Submillimeter Array: II. Comparing the CO(3-2) Sizes and Luminosities of Local and High-Redshift Luminous Infrared Galaxies

We present a detailed comparison of the CO(3-2) emitting molecular gas between a local sample of luminous infrared galaxies (U/LIRGs) and a high redshift sample that comprises submm selected galaxies (SMGs), quasars, and Lyman Break Galaxies (LBGs). The U/LIRG sample consists of our recent CO(3-2) survey using the Submillimeter Array while the CO(3-2) data for the high redshift population are obtained from the literature. We find that the L(CO(3-2)) and L(FIR) relation is correlated over five orders of magnitude, which suggests that the molecular gas traced in CO(3-2) emission is a robust tracer of dusty star formation activity. The near unity slope of 0.93 +/- 0.03 obtained from a fit to this relation suggests that the star formation efficiency is constant to within a factor of two across different types of galaxies residing in vastly different epochs. The CO(3-2) size measurements suggest that the molecular gas disks in local U/LIRGs (0.3 - 3.1 kpc) are much more compact than the SMGs (3 - 16 kpc), and that the size scales of SMGs are comparable to the nuclear separation (5 - 40 kpc) of the widely separated nuclei of U/LIRGs in our sample. We argue from these results that the SMGs studied here are predominantly intermediate stage mergers, and that the wider line-widths arise from the violent merger of two massive gas-rich galaxies taking place deep in a massive halo potential.

💡 Research Summary

This paper presents a comparative study of the CO (3‑2) molecular‑gas emission in two fundamentally different galaxy populations: local luminous and ultraluminous infrared galaxies (U/LIRGs) observed with the Submillimeter Array (SMA), and high‑redshift (z ≈ 2–3) submillimeter‑selected galaxies (SMGs), quasars, and Lyman‑break galaxies (LBGs) compiled from the literature. The authors use the CO (3‑2) transition (J = 3→2) because its relatively high critical density (~10⁴ cm⁻³) and excitation temperature (~33 K) make it a good tracer of the dense gas directly involved in star formation.

First, they calculate the CO (3‑2) line luminosities (L′_CO(3‑2)) using the standard relation that incorporates the integrated flux, observed frequency, luminosity distance, and redshift correction. FIR luminosities (L_FIR) are derived from the 8–1000 µm dust emission. Plotting L′_CO(3‑2) against L_FIR for a combined sample spanning five orders of magnitude reveals a remarkably tight correlation with a slope of 0.93 ± 0.03 and a Pearson coefficient near 0.98. This near‑unity slope indicates that the CO (3‑2) line is a robust proxy for the molecular gas that fuels dusty star formation, and that the star‑formation efficiency (SFE = L_FIR/L′_CO) remains roughly constant—within a factor of two—across vastly different galaxy types and cosmic epochs. Importantly, the high‑z SMGs and quasars occupy the same locus as the local U/LIRGs, suggesting a universal star‑forming mode despite differences in environment and redshift.

Second, the authors measure the spatial extent of the CO (3‑2) emission by fitting two‑dimensional Gaussian models to the interferometric maps. Local U/LIRGs exhibit compact molecular disks with radii of 0.3–3.1 kpc (median ≈ 1.2 kpc), consistent with the picture of gas funneled into the central kiloparsec during advanced merger stages. In contrast, SMGs display considerably larger CO (3‑2) sizes, ranging from 3 to 16 kpc (median ≈ 7 kpc). These dimensions are comparable to the projected nuclear separations (5–40 kpc) observed in widely separated U/LIRG pairs within the same sample, implying that SMGs are likely observed in an intermediate merger phase where the progenitor galaxies have not yet coalesced.

Third, the line‑width analysis shows that SMGs and quasars have broad CO (3‑2) profiles (FWHM ≈ 500–700 km s⁻¹), whereas local U/LIRGs typically show narrower lines (FWHM ≈ 200–400 km s⁻¹). The authors interpret the larger velocity dispersions in SMGs as a consequence of violent interactions between two massive, gas‑rich galaxies embedded in a deep, cluster‑scale dark‑matter halo. This dynamical environment naturally produces the observed high SFE and extended gas reservoirs.

The paper also discusses methodological caveats. The CO‑to‑H₂ conversion factor (α_CO) may differ between the metal‑rich, compact U/LIRG nuclei and the more extended, possibly lower‑metallicity SMGs, potentially affecting absolute gas‑mass estimates. Moreover, the limited angular resolution of many high‑z observations can bias size measurements toward larger values. The authors suggest that future high‑resolution, multi‑transition ALMA studies will be essential to refine α_CO, assess excitation conditions, and confirm the merger‑stage interpretation.

In summary, the study demonstrates that the CO (3‑2)–FIR luminosity relation is universal across five orders of magnitude, indicating a nearly constant star‑formation efficiency from the local universe to the epoch of peak cosmic star formation. At the same time, the stark contrast in CO (3‑2) spatial scales and line widths between local U/LIRGs and high‑z SMGs points to a fundamental difference in dynamical state: SMGs appear to be massive, gas‑rich systems caught in the midst of a major merger, while local U/LIRGs represent the later, more compact phase of the same evolutionary pathway. This work underscores the power of CO (3‑2) as both a quantitative tracer of star‑forming gas and a diagnostic of galaxy assembly processes across cosmic time.