Nuclear Star Clusters From Clustered Star Formation

Abstract

Photometrically distinct nuclear star clusters (NSCs) are common in late-type-disk and spheroidal galaxies. The formation of NSCs is inevitable in the context of normal star formation in which a majority of stars form in clusters. A young, mass-losing cluster embedded in an isolated star-forming galaxy remains gravitationally bound over a period determined by its initial mass and the galactic tidal field. The cluster migrates radially toward the center of the galaxy and becomes integrated in the NSC if it reaches the center. The rate at which the NSC grows by accreting young clusters can be estimated from empirical cluster formation rates and dissolution times. We model cluster migration and dissolution and find that the NSCs in late-type disks and in spheroidals could have assembled from migrating clusters. The resulting stellar nucleus contains a small fraction of the stellar mass of the galaxy; this fraction is sensitive to the high-mass truncation of the initial cluster mass function (ICMF). The resulting NSC masses are consistent with the observed values, but generically, the final NSCs are surrounded by a spatially more extended excess over the inward-extrapolated exponential (or Sérsic) law of the outer galaxy. We suggest that the excess can be related to the pseudobulge phenomenon in disks, though perhaps not all of the pseudobulge mass assembles this way. Comparison with observed NSC masses can be used to constrain the truncation mass scale of the ICMF and the fraction of clusters suffering prompt dissolution. We infer truncation mass scales of 10 6 M ( 10 5 M ) without (with 90%) prompt dissolution. Since the NSC assembly is collisionless and non-dissipative, no relation to the process responsible for central black hole assembly in more massive galaxies is expected.

The nuclear cluster of the Milky Way: our primary testbed for the interaction of a dense star cluster with a massive black hole

“…There are two prevailing formation scenarios for NSCs. They could have formed by infalling star clusters (e.g., Tremaine et al 1975, Agarwal & Milosavljević 2011, Antonini et al 2012, Gnedin et al 2014, or by the accretion of gas clouds that formed the stars in situ (e.g. Milosavljević 2004, Pflamm-Altenburg & Kroupa 2009.…”

The formation of stellar nuclear discs in bar-induced gas inflows

“…Mechanisms that have been proposed for driving gas to small radii in such systems include nested bars (Shlosman, Frank & Begelman 1989), the magnetorotational instability (Milosavljević 2004), and cloud-cloud mergers (Bekki 2007). Agarwal & Milosavljević (2011) proposed that NDs can form out of the debris of infalling star clusters and Portaluri et al (2013) showed that the available photometric and kinematic data are still consistent with this idea. However, detailed modelling (De Lorenzi et al 2013) and comparison to simulations (Hartmann et al 2011) of the kinematics of the nuclear star cluster in NGC 4244 (Seth et al 2008) reveal that gas dissipation had to have played a major role in the formation of its nuclear cluster, indicating that some gas must be able to reach the inner ∼ 10 parsecs.…”

The Imprint of Massive Black Hole Mergers on the Correlation Between Nuclear Star Clusters and Their Host Galaxies

“…Previous calculations assumed NSC formation to take place in isolated galaxies (e.g., Agarwal & Milosavljević 2011;Antonini 2013;Gnedin et al 2014;Arca-Sedda & Capuzzo-Dolcetta 2014;Aharon & Perets 2015), thus neglecting the possible role of galaxy mergers, as well as in-situ star formation processes. Also, these early idealized attempts did not explore the possible interplay between MBH and NSC evolution.…”