ISM properties of a Massive Dusty Star-Forming Galaxy discovered at z ~ 7 (1705.07912v2)

Abstract: We report the discovery and constrain the physical conditions of the interstellar medium of the highest-redshift millimeter-selected dusty star-forming galaxy (DSFG) to date, SPT-S J031132-5823.4 (hereafter SPT0311-58), at $z=6.900 +/- 0.002$. SPT0311-58 was discovered via its 1.4mm thermal dust continuum emission in the South Pole Telescope (SPT)-SZ survey. The spectroscopic redshift was determined through an ALMA 3mm frequency scan that detected CO(6-5), CO(7-6) and CI, and subsequently confirmed by detections of CO(3-2) with ATCA and [CII] with APEX. We constrain the properties of the ISM in SPT0311-58 with a radiative transfer analysis of the dust continuum photometry and the CO and [CI] line emission. This allows us to determine the gas content without ad hoc assumptions about gas mass scaling factors. SPT0311-58 is extremely massive, with an intrinsic gas mass of $M_{\rm gas} = 3.3 \pm 1.9 \times10^{11}\,M_{\odot}$. Its large mass and intense star formation is very rare for a source well into the Epoch of Reionization.

ISM Properties of a Massive Dusty Star-Forming Galaxy Discovered at $z \sim 7$

In a paper, the interstellar medium (ISM) characteristics of a high-redshift, millimeter-selected dusty star-forming galaxy (DSFG) were explored. This galaxy, SPT-S J031132-5823.4, herein referred to as SPT0311-58, was detected at an extraordinary redshift of $z = 6.900 \pm 0.002$. The discovery was made through its millimeter thermal dust emission via the South Pole Telescope (SPT)-SZ survey, with redshift conformation provided by CO and CI line emissions detected using ALMA and ATCA.

Observational Insights

SPT0311-58 encompasses significant complexity in its ISM structure. This was characterized using a multi-faceted approach comprising radiative transfer modeling of dust continuum and line emissions. It revealed an enormous intrinsic gas mass of $$M_{\text{gas}} = 3.3 \pm 1.9 \times 10^{11}\, \text{M}_\odot$$. This substantial mass and the galaxy's vigorous star formation are exceptionally rare during the Epoch of Reionization (EoR).

Inferences from optical surveys and theoretical models have suggested that DSFGs are unlikely to have formed substantial dust masses this early in cosmic time, yet the discovery of galaxies like SPT0311-58 challenges these assumptions. Observations estimated an apparent FIR luminosity of $$L_{\text{FIR}}=4.1\pm0.7\times10^{13}\, \text{L}_\odot$$, reflecting intense star-forming activity.

ISM Modeling and Line Emissions

The radiative transfer analysis applied here integrates the characteristics of both the dust continuum and the molecular lines, eschewing reliance on gas mass scaling factors. This method discerned two main gas components: a widespread cold dust component and a concentrated warm dust component, each contributing to specific line and continuum properties.

Notably, the identification of CO transitions and carbon lines allowed for an estimation of gas density and temperature, providing insight into the ISM conditions within SPT0311-58. For the cold gas, the temperature was determined to be $T_{\text{dust}} = 36 \pm 7 \, \text{K}$, while the warm gas exhibited a higher temperature of approximately $115 \pm 54 \, \text{K}$.

Theoretical and Practical Implications

The discovery of SPT0311-58 offers substantial implications for our understanding of early universe DSFGs. The presence of this galaxy type at $z \sim 7$ suggests more rapid dust and metallicity evolution than previously anticipated. The analysis of its ISM conditions implies potential revisions in models of high-redshift galaxy formation and ISM enrichment processes.

The investigation further contributes to the understanding of the CO-to-H2 conversion factor ($\alpha_{\text{CO}}$) in environments with high star formation, offering a recalibration to $$\alpha_{\text{CO}} = 4.8 \pm 2.9 \, \text{M}_\odot(\text{K km s}^{-1}\text{pc}^2)^{-1}$$. This value significantly exceeds typical low-redshift starburst assumptions, reflecting the dominance of dense gas in this early universe context.

Conclusion and Future Prospects

The findings from SPT0311-58 provide pivotal insights into the ISM properties of DSFGs during the EoR. As high-redshift observational capabilities improve, further detailed studies of such galaxies will be crucial. These endeavors will enhance comprehension of galaxy formation and evolution during the universe's first billion years and may inform more accurate models of cosmic reionization processes.

Future explorations, particularly with high-sensitivity instruments like the next-generation Very Large Array and extended ALMA arrays, may offer deeper insights into the spatial and chemical complexities of such early-stage galaxies, further elucidating the nature and role of massive DSFGs in the nascent universe.