The cold circumgalactic environment of MAMMOTH-I: dynamically cold gas in the core of an Enormous Ly-alpha Nebula (1911.05053v1)

Abstract: The MAMMOTH-I Nebula at redshift 2.3 is one of the largest known Ly-alpha nebulae in the Universe, spanning ~440 kpc. Enormous Ly-alpha nebulae like MAMMOTH-I typically trace the densest and most active regions of galaxy formation. Using sensitive low-surface-brightness observations of CO(1-0) with the Very Large Array, we trace the cold molecular gas in the inner 150 kpc of the MAMMOTH-I Nebula. CO is found in four regions that are associated with either galaxies or groups of galaxies that lie inside the nebula. In three of the regions, the CO stretches up to ~30 kpc into the circum-galactic medium (CGM). In the centermost region, the CO has a very low velocity dispersion (FWHM${\rm CO}$ ~ 85 km/s), indicating that this gas is dynamically cold. This dynamically cold gas coincides with diffuse restframe optical light in the CGM around a central group of galaxies, as discovered with the Hubble Space Telescope. We argue that this likely represents cooling of settled and enriched gas in the center of MAMMOTH-I. This implies that the dynamically cold gas in the CGM, rather than the obscured AGN, marks the core of the potential well of this Ly-alpha nebula. In total, the CO in the MAMMOTH-I Nebula traces a molecular gas mass of M${\rm H2}$ ~ 1.4 ($\alpha_{\rm CO}$/3.6) $\times$ 10$^{11}$ M$_{\odot}$, with roughly 50% of the CO(1-0) emission found in the CGM. Our results add to the increasing evidence that extended reservoirs of molecular gas exist in the CGM of massive high-z galaxies and proto-clusters.

• The paper identifies dynamically cold CO(1-0) emissions extending up to 30 kpc into the CGM of the MAMMOTH-I Nebula.
• The study estimates a molecular gas mass of ~1.4 × 10^11 M⊙, emphasizing the significant role of cold gas in massive galaxy formation.
• The findings challenge traditional models by revealing a low-velocity dispersion core, suggesting efficient cooling and settling in high-redshift environments.

Analyzing the Cold Circumgalactic Environment of MAMMOTH-I

Introduction

The paper focuses on the MAMMOTH-I Nebula, which is one of the most extensive known Ly$\alpha$ nebulae, located at a redshift of 2.3. Using the Very Large Array (VLA), the authors investigate the cold molecular gas within this nebula, which spans approximately 440 kpc. These nebulae are generally associated with the densest areas of galaxy formation, thus offering insights into early galaxy evolution and the circumgalactic medium (CGM) dynamics.

Observational Insights and Results

This paper's primary observational method involves the use of low-surface-brightness CO(1-0) line observations, capable of detecting cold molecular gas. The authors identify four distinct regions within MAMMOTH-I, characterized by either individual galaxies or groups of galaxies. Three of these regions show CO emissions reaching approximately 30 kpc into the CGM, while the central region exhibits extremely low velocity dispersion, indicating the dynamically cold nature of this gas.

1. Cold Molecular Gas Distribution: Roughly half of the CO(1-0) emission within MAMMOTH-I is found in the CGM. This gas is spatially and kinematically complex, with a notable concentration showing a low dispersion focus near the central galaxies.
2. Mass Estimation: The paper estimates the total mass of molecular gas traced by CO in the MAMMOTH-I Nebula to be approximately 1.4 $\times$ 10$^{11}$ M$_{\odot}$, a significant fraction of which is associated with the CGM component.
3. Complex Kinematics: In the central region of the nebula, the kinematic properties suggest a different physical state compared to the typical rotational velocities observed in high-redshift galaxies. The relatively stable kinematics underscore the gas's dynamically cold properties and suggest potential cooling and settling processes within the nebular core.

Theoretical Implications

The findings support an evolving understanding of high-redshift nebulae's CGM. The presence of dynamically cold gas that is enriched suggests complex interchange processes within these galaxies' halos:

• Gas Cooling Mechanisms: Dynamically cold molecular gas in the core implies efficient cooling, different from the hotter, more turbulent environments seen at larger scales. This scenario challenges existing models of circumgalactic dynamics and suggests that the cold molecular core could play a crucial role in galaxy evolution and star formation within massive proto-clusters.
• Feedback Effects and Metal Enrichment: The correlation between cold molecular gas and metal enrichment indicates feedback from active processes such as outflows from a central-supermassive black hole or starburst-driven winds. These processes might enrich the gas and facilitate its cooling, contributing to the nebula's mass build-up.

Future Research Directions

The nature of MAMMOTH-I invites further exploration into:

• Gas Dynamics and Evolution: Continued observations with instruments like the Atacama Large Millimeter/submillimeter Array (ALMA) can explore higher-$J$ CO transitions to better understand the excitation conditions and further elucidate the physical state of cold gas in these environments.
• Comparison with Other High-Redshift Structures: Studying other similar giant Ly$\alpha$ nebulae could provide comparative insights into the evolution of such systems and the role of circumgalactic and intergalactic media.
• Simulations and Modeling: Further computational modeling of cold gas dynamics in massive protogalactic nuclei and their interactions with enriched CGM could advance the theoretical framework and explain observed phenomena such as the dynamically cold core found in MAMMOTH-I.

Conclusion

This investigation into the MAMMOTH-I Nebula's cold circumgalactic environment contributes vital insights into the kinematics and thermodynamics of gas in early massive galaxies. The evidence for extended, cold molecular gas and its implications for galaxy evolution within dense proto-cluster environments are critical for advancing our understanding of high-redshift galaxy formation.