New insights on the formation of nuclear star clusters

Abstract

Nuclear Clusters (NCs) are common stellar systems in the centres of galaxies. Yet, the physical mechanisms involved in their formation are still debated. Using a parsecresolution hydrodynamical simulation of a dwarf galaxy, we propose an updated formation scenario for NCs. In this "wet migration scenario", a massive star cluster forms in the gas-rich disc, keeping a gas reservoir, and growing further while it migrates to the centre via a combination of interactions with other substructures and dynamical friction. A wet merger with another dense cluster and its own gas reservoir can occur, although this is not a pre-requisite for the actual formation of the NC. The merging process does significantly alter the properties of the NC (mass, morphology, star formation history), also quenching the on-going local star formation activity, thus leading to interesting observational diagnostics for the physical origin of NCs. A population of lower mass clusters co-exist during the simulation, but these are either destroyed via tidal forces, or have high angular momentum preventing them to interact with the NC and contribute to its growth. The proposed updated scenario emphasises the role of gas reservoirs associated with the densest star clusters formed in a gas-rich low-mass galaxy.

Discovery of Rr Lyrae Stars in the Nuclear Bulge of the Milky Way

“…The evidence supports the scenario where the nuclear stellar bulge was originally made out of a few globular clusters that merged through dynamical friction (Capuzzo-Dolcetta 1993; Guillard et al 2016), and as such it could well be the most massive and oldest surviving star cluster of our Galaxy. …”

“…There we can in principle resolve individual stars, probing a wide range of stellar ages and metallicities. There are two main scenarios proposed for the formation of the nuclear bulge of the Milky Way, and of all galactic nuclei in general: merging of globular clusters (Tremaine et al 1975;Capuzzo-Dolcetta 1993;Gnedin et al 2014;Guillard et al 2016), and fast gas accretion and star formation onto the central region (Milosavljević 2004;Schinnerer et al 2008). …”

DDO 216-A1: A Central Globular Cluster in a Low-luminosity Transition-type Galaxy∗

“…The general feature of the analysis, that in many cases clusters will be disrupted, but that the most massive will sometimes survive to be observed near the center of the host galaxy, is consistent with advanced numerical simulations of star formation in dwarfs (C. Christensen 2017, private communication). Guillard et al (2016) performed hydrodynamical simulations of a larger dwarf ( * M =10 9.5   M ) and observed exactly this behavior, producing a 10 8   M nuclear star cluster as the result of inspiral, gas accretion, and merging of an initially 10 4   M protocluster formed in the outskirts of the dwarf. Their simulated cluster arrives in the central part of the dwarf after ≈1 Gyr and is quenched by a final merger with another large cluster.…”

Asymmetric spatial distribution of subsolar metallicity stars in the Milky Way nuclear star cluster

“…While there is some variation of the resulting metallicity for the six fields, we find no trend to lower metallicities in Fields 4-6 compared to Fields 1-2, which suggests that we do not introduce a bias to the fraction of sub-solar metallicity stars when we perform out data selection. Guillard et al 2016), and thus also a broad metallicity distribution. The spatial anisotropy of sub-solar metallicity stars may indicate that some of them were brought to the Galactic centre from star cluster infall events.…”