Globular cluster formation with multiple stellar populations from hierarchical star cluster complexes

Abstract

Most old globular clusters (GCs) in the Galaxy are observed to have internal chemical abundance spreads in light elements. We discuss a new GC formation scenario based on hierarchical star formation within fractal molecular clouds. In the new scenario, a cluster of bound and unbound star clusters ('star cluster complex', SCC) that have a power-law cluster mass function with a slope (β) of 2 is first formed from a massive gas clump developed in a dwarf galaxy. Such cluster complexes and β = 2 are observed and expected from hierarchical star formation. The most massive star cluster ('main cluster'), which is the progenitor of a GC, can accrete gas ejected from asymptotic giant branch (AGB) stars initially in the cluster and other low-mass clusters before the clusters are tidally stripped or destroyed to become field stars in the dwarf. The SCC is initially embedded in a giant gas hole created by numerous supernovae of the SCC so that cold gas outside the hole can be accreted onto the main cluster later. New stars formed from the accreted gas have chemical abundances that are different from those of the original SCC. Using hydrodynamical simulations of GC formation based on this scenario, we show that the main cluster with the initial mass as large as [2 − 5] × 10 5 M ⊙ can accrete more than 10 5 M ⊙ gas from AGB stars of the SCC. We suggest that merging of hierarchical star cluster complexes can play key roles in stellar halo formation around GCs and self-enrichment processes in the early phase of GC formation.

The Recent Growth History of the Fornax Cluster Derived from Simultaneous Sloshing and Gas Stripping: Simulating the Infall of NGC 1404

“…Especially if the third infall scenario is the correct case, this gives two opportunities for NGC 1404 and NGC 1399 to interact and exchange globular clusters. It should be noted here that our simulations differ greatly from that in Bekki et al (2003) in terms of the galaxy orbit. In their simulation, the orbit radius does not extend past 60 kpc compared to reaching between 280-450 kpc in our simulations.…”

“…Another indication for an ongoing merger between NGC 1404 and Fornax is the globular cluster content of both systems. In particular, Forbes et al (1998) and Bekki et al (2003) both find that NGC 1399 has rich globular cluster content (high specific frequency) whilst NGC 1404 has poor globular cluster content (low specific frequency) compared to the average for elliptical galaxies, suggesting that NGC 1399 may have stripped NGC 1404 of some its globular clusters as it undergoes a merger. Under this argument it would imply that NGC 1404 has already fallen through the cluster once already.…”

“…An interesting point to note is that the second/third infall could offer an explanation to the globular cluster content debate of NGC 1404 and NGC 1399. It has been suggested through measurements of their specific frequency that NGC 1404 has lost some of its globular clusters to NGC 1399 (Forbes et al 1998, Bekki et al 2003 due to a possible interaction via tidal stripping. Our proposed scenario requires that NGC 1404 passed by NGC 1399 at least once, where the loss of globular clusters could have happened.…”

Gould Belt members in X-ray RAVE: cross-matching RAVE stars with 3XMM point sources

“…The GB consists of X-ray active young stars with distances up to 600 pc (Olano 1982;Fresneau et al 1996;Poppel 1997;Guillout et al 1998). The age interval varies greatly in the literature: most authors state that it is between 20-30 and 70-80 Myr (Ogorodnikov 1965;Olano 1982;Poppel 1997;Guillout et al 1998;Bekki 2009), however, Frogel & Stothers (1977)'s review suggests a wider interval: 30-220 Myr. Using Strömgren photometry Torra et al (2000) determined stars with ages ¡ 90 Myr belong in the GB.…”

“…This causes controversial opinions to form about its formation. Bobylev (2014) states that the GB is a separate structure from the Milky Way, whereas Bekki (2009) says that a dark matter clump collided with a gas cloud to trigger GB's formation. Comerón (2001) observed a GB-like structure in M83 which is approximately the same size with the GB, is separate from the spiral structure of the galaxy, lies at a similar distance from the galactic center and has the same order of age (log age/yr=7).…”