Molecular Line Observations in Two Dusty Star-Forming Galaxies at z = 6.9

Abstract: SPT0311-58 is the most massive infrared luminous system discovered so far during the Epoch of Reionization (EoR). In this paper, we present a detailed analysis of the molecular interstellar medium at z = 6.9, through high-resolution observations of the CO(6-5), CO(7-6), CO(10-9), CI, and p-H2O(211-202) lines and dust continuum emission with the Atacama Large Millimeter/submillimeter Array (ALMA). The system consists of a pair of intensely star-forming gravitationally lensed galaxies (labelled West and East). The intrinsic far-infrared luminosity is (16 $\pm$ 4)$\times\rm 10^{12} \ \rm L_{\odot}$ in West and (27 $\pm$ 4)$\times\rm 10^{11} \ \rm L_{\odot}$ in East. We model the dust, CO, and [CI] using non-local thermodynamic equilibrium radiative transfer models and estimate the intrinsic gas mass to be (5.4 $\pm$ 3.4)$\times\rm 10^{11} \ \rm M_{\odot}$ in West and (3.1 $\pm$ 2.7)$\times\rm 10^{10} \ \rm M_{\odot}$ in East. We find that the CO spectral line energy distribution in West and East are typical of high-redshift sub-millimeter galaxies (SMGs). The CO-to-H2 conversion factor ($\alpha_{CO}$) and the gas depletion time scales estimated from the model are consistent with the high-redshift SMGs in the literature within the uncertainties. We find no evidence of evolution of depletion time with redshift in SMGs at z > 3. This is the most detailed study of molecular gas content of a galaxy in the EoR to-date, with the most distant detection of H2O in a galaxy without any evidence for active galactic nuclei in the literature.

• The paper presents an in-depth molecular ISM analysis for two DSFGs at z=6.9, including the most distant p-H2O detection in a non-AGN galaxy.
• It employs high-resolution ALMA spectral line observations to derive intrinsic far-infrared luminosities and gas masses, providing key insights into early universe star formation.
• The study reveals that star formation efficiencies in these high-redshift galaxies are typical of starbursts, offering constraints for models of galaxy evolution during the Epoch of Reionization.

Overview of Molecular Line Observations in Dusty Star-Forming Galaxies at $z$ = 6.9

This paper presents an in-depth analysis of the molecular interstellar medium (ISM) within two dusty star-forming galaxies (DSFGs), identified as SPT0311-58 West and East, at a redshift of $z = 6.9$ using data from high-resolution observations from the Atacama Large Millimeter/submillimeter Array (ALMA). This study highlights the prominence of SPT0311-58 as one of the most massive infrared-luminous systems discovered during the Epoch of Reionization. The focus is on characterizing the dust and gas contents of these galaxies, using spectral line observations, particularly of carbon monoxide (CO) and ionized carbon ([C II]) lines, and inferring the intrinsic properties of their ISM.

Key Findings

• Luminosity and Gas Mass: The galaxies exhibit intrinsic far-infrared luminosities of (16 $\pm$ 4)$\times\rm 10^{12} \ \rm L_{\odot}$ in West and (27 $\pm$ 4)$\times\rm 10^{11} \ \rm L_{\odot}$ in East. The intrinsic gas masses are estimated to be (5.4 $\pm$ 3.4)$\times\rm 10^{11} \ \rm M_{\odot}$ and (3.1 $\pm$ 2.7)$\times\rm 10^{10} \ \rm M_{\odot}$, respectively.
• Spectral Line Analysis: CO spectral line energy distribution (SLED) derived for the West galaxy shows characteristics typical of high-redshift sub-millimeter galaxies (SMGs). A significant finding is the most distant detection of water (p-$\rm H_{2}$O) in a non-AGN galaxy, connecting water excitation to star formation at high redshifts.
• Star Formation and Depletion: The study finds no evidence for an evolution of gas depletion times with redshift for SMGs with $z > 3$. The depletion times derived for SPT0311-58 are typical of high-redshift starbursts, revealing insights into the star formation efficiencies during the early universe.

Implications and Future Directions

From a theoretical perspective, the study contributes to our understanding of the physical conditions in the ISM of early DSFGs. The findings on the molecular gas content and the CO SLED provide important constraints for models of galaxy formation and evolution during the Epoch of Reionization. Practically, the paper demonstrates the vital role of ALMA in probing the birthplaces of the earliest generations of stars and galaxies. The work suggests future studies could expand upon these findings by observing a larger sample of high-redshift galaxies to confirm trends in ISM properties and further refine estimates of molecular gas conversion factors with respect to environmental conditions at extreme redshifts.

The results from this study are pivotal as they place the observed galaxies within the broader context of cosmic history, allowing researchers to extrapolate conditions in the early universe. SPT0311-58 serves as a critical probe into the conditions present during the Epoch of Reionization, providing evidence of vigorous star formation and complex physical processes taking root during this formative period of cosmic history. The detailed analysis informs theoretical models and supports further high-resolution observations of young, massive galaxies in their formative stages.