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Register a new userGlobular Cluster Formation in Mergers
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Francois Schweizer
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Mergers of gas-rich galaxies lead to gravitationally driven increases in gas pressure that can trigger intense bursts of star and cluster formation. Although star formation itself is clustered, most newborn stellar aggregates are unbound associations and disperse. Gravitationally bound star clusters that survive for at least 10-20 internal crossing times (~20-40 Myr) are relatively rare and seem to contain <10% of all stars formed in the starbursts. The most massive young globular clusters formed in present-day mergers exceed omega Cen by an order of magnitude in mass, yet appear to have normal stellar initial mass functions. In the local universe, recent remnants of major gas-rich disk mergers appear as protoelliptical galaxies with subpopulations of typically 100-1000 young metal-rich globular clusters in their halos. The evidence is now strong that these second-generation globular clusters formed from giant molecular clouds in the merging disks, squeezed into collapse by large-scale shocks and high gas pressure rather than by high-velocity cloud-cloud collisions. Similarly, first- generation metal-poor globular clusters may have formed during cosmological reionization from low-metallicity giant molecular clouds squeezed by the universal reionization pressure.
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Here we test the idea that new globular clusters (GCs) are formed in the same gaseous (wet) mergers or interactions that give rise to the young stellar populations seen in the central regions of many early-type galaxies. We compare mean GC colors with the age of the central galaxy starburst. The red GC subpopulation reveals remarkably constant mean colors independent of galaxy age. A scenario in which the red GC subpopulation is a combination of old and new GCs (formed in the same event as the central galaxy starburst) can not be ruled out; although this would require an age-metallicity relation for the newly formed GCs that is steeper than the Galactic relation. However, the data are also well described by a scenario in which most red GCs are old, and few, if any, are formed in recent gaseous mergers. This is consistent with the old ages inferred from some spectroscopic studies of GCs in external systems. The event that induced the central galaxy starburst may have therefore involved insufficient gas mass for significant GC formation. We term such gas-poor events damp mergers.
Our numerical simulations first demonstrate that the pressure of ISM in a major merger becomes so high ($>$ $10^5$ $rm k_{rm B}$ K $rm cm^{-3}$) that GMCs in the merger can collapse to form globular clusters (GCs) within a few Myr. The star formation efficiency within a GMC in galaxy mergers can rise up from a few percent to $sim$ 80 percent, depending on the shapes and the temperature of the GMC. This implosive GC formation due to external high pressure of warm/hot ISM can be more efficient in the tidal tails or the central regions of mergers. The developed clusters have King-like profile with the effective radius of a few pc. The structural, kinematical, and chemical properties of these GC systems can depend on orbital and chemical properties of major mergers.
Primordial clouds are supposed to host the so-called population III stars. These stars are very massive and completely metal-free. The final stage of the life of population III stars with masses between 130 and 260 solar masses is a very energetic hypernova explosion. A hypernova drives a shock, behind which a spherically symmetric very dense supershell forms, which might become gravitationally unstable, fragment, and form stars. In this paper we study under what conditions can an expanding supershell become gravitationally unstable and how the feedback of these supershell stars (SSSs) affects its surroundings. We simulate, by means of a 1-D Eulerian hydrocode, the early evolution of the primordial cloud after the hypernova explosion, the formation of SSSs, and the following evolution, once the SSSs start to release energy and heavy elements into the interstellar medium. Our results indicate that a shell, enriched with nucleosynthetic products from SSSs, propagates inwards, towards the center of the primordial cloud. In a time span of a few Myr, this inward-propagating shell reaches a distance of only a few parsec away from the center of the primordial cloud. Its density is extremely high and its temperature very low, thus the conditions for a new episode of star formation are achieved. We study what fraction of these two distinct populations of stars can remain bound and survive until the present day. We study also under what conditions can this process repeat and form multiple stellar populations. We extensively discuss whether the proposed scenario can help to explain some open questions of the formation mechanism of globular clusters.
Globular clusters are compact, gravitationally bound systems of up to a million stars. The GCs in the Milky Way contain some of the oldest stars known, and provide important clues to the early formation and continuing evolution of our Galaxy. More generally, GCs are associated with galaxies of all types and masses, from low-mass dwarf galaxies to the most massive early-type galaxies which lie in the centres of massive galaxy clusters. GC systems show several properties which connect tightly with properties of their host galaxies. For example, the total mass of GCs in a system scales linearly with the dark matter halo mass of its host galaxy. Numerical simulations are at the point of being able to resolve globular cluster formation within a cosmological framework. Therefore, GCs link a range of scales, from the physics of star formation in turbulent gas clouds, to the large-scale properties of galaxies and their dark matter. In this Chapter we review some of the basic observational approaches for GC systems, some of their key observational properties, and describe how GCs provide important clues to the formation of their parent galaxies.
We propose and investigate a new formation mechanism for globular clusters in which they form within molecular clouds that are formed in the shocked regions created by galactic winds driven by successive supernova explosions shortly after the initial burst of massive star formation in the galactic centers. The globular clusters have a radial distribution that is more extended than that of the stars because the clusters form as pressure-confined condensations in a shell that is moving outward radially at high velocity. In addition the model is consistent with existing observations of other global properties of globular clusters, as far as comparisons can be made.
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