The implications of dust for high-redshift protogalaxies and the formation of binary disks

TL;DR

This study uses high-resolution simulations to explore how dust influences the chemistry, dynamics, and observability of the first galaxies, especially regarding molecular formation, Lyman alpha emission, and dust continuum detection.

Contribution

It provides the first detailed simulation-based analysis of dust's role in early galaxy evolution, including molecular chemistry, galaxy morphology, and observational signatures.

Findings

Dust enhances molecular formation (H2 and HD) in early galaxies.

UV background flux affects molecular dissociation and galaxy evolution.

Dust masses of 10^8 M_sun are needed for ALMA detection at z~5.

Abstract

Numerical simulations suggest that the first galaxies are formed in protogalactic halos with virial temperatures >= 10^4 K. It is likely that such halos are polluted with trace amounts of metals produced by the first generation of stars. The presence of dust can significantly change the chemistry and dynamics of early galaxies. In this article, we aim to assess the role of dust on the thermal and dynamical evolution of the first galaxies in the presence of a background UV flux, and its implications for the observability of Lyman alpha emitters and sub-mm sources. We have performed high resolution cosmological simulations using the adaptive mesh refinement code FLASH to accomplish this goal. We have developed a chemical network appropriate for these conditions and coupled it with the FLASH code. The main ingredients of our chemical model include the formation of molecules, a multi-level treatment of atomic hydrogen, line trapping of Lyman alpha photons and, photoionization and photodissociation processes in a UV background. We found that the formation of molecules (H_{2} and HD) is significantly enhanced in the presence of dust grains. The presence of a background UV flux strongly influences the formation of molecules by photodissociating them. We explore the evolution after a major merger, leading to the formation of a binary disk. These disks have gas masses of ~10^{7} M_sun at a redshift of 5.4. Each disk lies in a separate subhalo as a result of the merger event. The disks are supported by turbulent pressure due to the highly supersonic turbulence present in the halo. The presence of dust does not significantly reduce the Lyman alpha emission. The emission of Lyman alpha is extended and originates from the envelope of the halo due to line trapping effects. We also find that dust masses of 10^8 M_sun are required to observe the dust continuum emission from z 5 galaxies with ALMA.