The formation mechanism of globular clusters (GCs) has long been debated by astronomers. It was recently proposed that supersonically induced gas objects (SIGOs)–which formed in the early Universe due to the supersonic relative motion of baryons and dark matter at recombination–could be the progenitors of early GCs. In order to become GCs, SIGOs must form stars relatively efficiently despite forming outside of dark matter halos. We investigate the potential for star formation in SIGOs using cosmological hydrodynamic simulations, including the aforementioned relative motions of baryons and dark matter, molecular hydrogen cooling in primordial gas clouds, and explicit star formation. We find that SIGOs do form stars and that the nascent star clusters formed through this process are accreted by dark matter halos on short timescales (∼a few hundred megayears). Thus, SIGOs may be found as intact substructures within these halos, analogous to many present-day GCs. From this result, we conclude that SIGOs are capable of forming star clusters with similar properties to globular clusters in the early Universe, and we discuss their detectability by upcoming JWST surveys.
Supersonically induced gas objects (SIGOs) are a class of early universe objects that have gained attention as a potential formation route for globular clusters. SIGOs have recently begun to be studied in the context of molecular hydrogen cooling, which is key to characterizing their structure and evolution. Studying the population-level properties of SIGOs with molecular cooling is important for understanding their potential for collapse and star formation, and for addressing whether SIGOs can survive to the present epoch. Here, we investigate the evolution of SIGOs before they form stars, using a combination of numerical and analytical analysis. We study timescales important to the evolution of SIGOs at a population level in the presence of molecular cooling. Revising the previous formulation for the critical density of collapse for SIGOs allows us to show that their prolateness tends to act as an inhibiting factor to collapse. We find that simulated SIGOs are limited by artificial two-body relaxation effects that tend to disperse them. We expect that SIGOs in nature will be longer lived compared to our simulations. Further, the fall-back timescale on which SIGOs fall into nearby dark matter halos, potentially producing a globular-cluster-like system, is frequently longer than their cooling timescale and the collapse timescale on which they shrink through gravity. Therefore, some SIGOs have time to cool and collapse outside of halos despite initially failing to exceed the critical density. From this analysis we conclude that SIGOs should form stars outside of halos in nonnegligible stream velocity patches in the universe.
more » « less- Award ID(s):
- 2007355
- NSF-PAR ID:
- 10395044
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 943
- Issue:
- 2
- ISSN:
- 0004-637X
- Format(s):
- Medium: X Size: Article No. 132
- Size(s):
- ["Article No. 132"]
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract -
more » « less
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 H2, a process crucial for the formation of the first stars. We find that high-density gas in regions with significant streaming velocities 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 ≈ 10−3Z ⊙) 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 cosmic microwave background 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. -
more » « less
Abstract Fuzzy dark matter (FDM) is a proposed modification for the standard cold dark matter (CDM) model motivated by small-scale discrepancies in low-mass galaxies. Composed of ultralight (mass ∼ 1022eV) axions with kiloparsec-scale de Broglie wavelengths, this is one of a class of candidates that predicts that the first collapsed objects form in relatively massive dark matter halos. This implies that the formation history of the first stars and galaxies would be very different, potentially placing strong constraints on such models. Here we numerically simulate the formation of the first stars in an FDM cosmology, following the collapse in a representative volume all the way down to primordial protostar formation including a primordial nonequilibrium chemical network and cooling for the first time. We find two novel results: first, the large-scale collapse results in a very thin and flat gas “pancake”; second, despite the very different cosmology, this pancake fragments until it forms protostellar objects indistinguishable from those in CDM. Combined, these results indicate that the first generation of stars in this model are also likely to be massive and, because of the sheet morphology, do not self-regulate, resulting in a massive Population III starburst. We estimate the total number of first stars forming in this extended structure to be 104over 20 Myr using a simple model to account for the ionizing feedback from the stars, and should be observable with the James Webb Space Telescope. These predictions provide a potential smoking gun signature of FDM and similar dark matter candidates.
-
more » « less
ABSTRACT Molecular hydrogen allows cooling in primordial gas, facilitating its collapse into Population III stars within primordial halos. Lyman–Werner (LW) radiation from these stars can escape the halo and delay further star formation by destroying H2 in other halos. As cosmological simulations show that increasing the background LW field strength increases the average halo mass required for star formation, we perform follow-up simulations of selected halos to investigate the knock-on effects this has on the Population III IMF. We follow 5 halos for each of the J21 = 0, 0.01, and 0.1 LW field strengths, resolving the pre-stellar core density of 10−6 g cm−3 (1018 cm−3) before inserting sink particles and following the fragmentation behaviour for hundreds of years further. We find that the mass accreted onto sinks by the end of the simulations is proportional to the mass within the ∼10−2 pc molecular core, which is not correlated to the initial mass of the halo. As such, the IMFs for masses above the brown dwarf limit show little dependence on the LW strength, although they do show variance in the number of low-mass clumps formed. As the range of background LW field strengths tested here covers the most likely values from literature, we conclude that the IMF for so-called Pop III.2 stars is not significantly different from the initial population of Pop III.1 stars. The primordial IMF therefore likely remains unchanged until the formation of the next generation of Population II stars.
-
null (Ed.)Our understanding of the formation and evolution of binary black holes (BBHs) is significantly impacted by the recent discoveries made by the LIGO/Virgo collaboration. Of utmost importance is the detection of the most massive BBH system, GW190521. Here we investigate what it takes for field massive stellar binaries to account for the formation of such massive BBHs. Whether the high mass end of the BH mass function is populated by remnants of massive stars that either formed at extremely low metallicities and avoid the pair-instability mass gap or increase their birth mass beyond the pair-instability mass gap through the accretion of gas from the surrounding medium. We show that assuming that massive stars at very low metallicities can form massive BHs by avoiding pair-instability supernova, coupled with a correspondingly high formation efficiency for BBHs, can explain the observed BH mass function. To this end, one requires a relation between the initial and final mass of the progenitor stars at low metallicities that is shallower than what is expected from wind mass loss alone. On the other hand, assuming pair-instability operates at all metallicities, one can account for the observed BH mass function if at least about 10% of the BHs born at very low metallicities double their mass before they merge because of accretion of ambient gas. Such BBHs will have to spend about a Gyr within a parsec length-scale of their parent atomic cooling halos or a shorter timescale if they reside in the inner sub-parsecs of their host dark matter halos. Future stellar evolution calculations of massive stars at very low metallicity and hydrodynamical simulations of gas accretion onto BBHs born in atomic cooling halos can shed light on this debate.more » « less
