Globular Clusters and Streaming Velocities: Testing the new formation channel in high-resolution cosmological simulations

Abstract

The formation of globular clusters and their relation to the distribution of dark matter have long puzzled astronomers. One of the most recently-proposed globular cluster formation channels ties ancient star clusters to the large-scale streaming velocity of baryons relative to dark matter in the early Universe. These streaming velocities affect the global infall of baryons into dark matter halos, the high-redshift halo mass function, and the earliest generations of stars. In some cases, streaming velocities may result in dense regions of dark-matter-free gas that becomes Jeans unstable, potentially leading to the formation of compact star clusters. We investigate this hypothesis using cosmological hydrodynamical simulations that include a full chemical network and the formation and destruction of H 2 , a process crucial for the formation of the first stars. We find that high-density gas in regions with significant streaming velocities -which constitute approximately 1% of the Universe -is indeed somewhat offset from the centers of dark matter halos, but this offset is typically significantly smaller than the virial radius. Gas outside of dark matter halos never reaches Jeans-unstable densities in our simulations. We postulate that low-level (Z ≈ 10 −3 Z ) metal enrichment by Population III supernovae may enable cooling in the extra-virial regions, allowing gas outside of dark matter halos to cool to the CMB temperature and become Jeans-unstable. Follow-up simulations that include both streaming velocities and metal enrichment by Population III supernovae are needed to understand if streaming velocities provide one path for the formation of globular clusters in the early Universe.

$\mathrm{H_2}$ cooling and gravitational collapse of supersonically induced gas objects

“…We find that the enclosed mass of S1 is less than Jeans mass at all radii at z = 25. This is consistent with the result of a recent study by Schauer et al (2021a), who also find similar dense gas clumps located outside of its closest DM halos. However, Schauer et al (2021a) argue that none of the gas clumps satisfies the Jeans instability condition for collapse because of low gas number density of typically ∼ 1 − 10 cm −3 .…”

“…This is consistent with the result of a recent study by Schauer et al (2021a), who also find similar dense gas clumps located outside of its closest DM halos. However, Schauer et al (2021a) argue that none of the gas clumps satisfies the Jeans instability condition for collapse because of low gas number density of typically ∼ 1 − 10 cm −3 . As we shall show in the next section, the particular gas cloud S1 in our simulation goes run-away collapse via Jeans instability.…”

“…The nature of SIGOs in cosmological simulations has been investigated in previous studies (e.g., Popa et al 2016;Chiou et al 2018Chiou et al , 2019Chiou et al , 2021Lake et al 2021). In a recent study by Schauer et al (2021a), the formation and chemo-thermal evolution of SIGOs are studied in detail. They conclude that H 2 cooling alone is not sufficient for the gas clumps to condense down to low enough temperatures for gravitational collapse.…”

LYRA II: Cosmological dwarf galaxy formation with inhomogeneous Population III enrichment

“…Additionally, Schauer et al (2020) show that including streaming velocities can alter the expected mass of the first star forming halos. Schauer et al (2021) proceed to postulate SF outside of the first halos with the aid of metals that escape the virial radius and provide additional cooling avenues in the intergalactic medium.…”