On the link between nuclear star cluster and globular cluster system mass, nucleation fraction and environment

Abstract

We present a simple model for the host mass dependence of the galaxy nucleation fraction ( 𝑓 𝑛𝑢𝑐 ), the galaxy's nuclear star cluster (NSC) mass and the mass in its surviving globular clusters (𝑀 𝐺𝐶,𝑜𝑏𝑠 ). Considering the mass and orbital evolution of a GC in a galaxy potential, we define a critical mass limit (𝑀 𝐺𝐶,𝑙𝑖𝑚 ) above which a GC can simultaneously in-spiral to the galaxy centre due to dynamical friction and survive tidal dissolution, to build up the NSC. The analytic expression for this threshold mass allows us to model the nucleation fraction for populations of galaxies. We find that the slope and curvature of the initial galaxy size-mass relation is the most important factor (with the shape of the GC mass function a secondary effect) setting the fraction of galaxies that are nucleated at a given mass. The well defined skewnormal 𝑓 𝑛𝑢𝑐 −𝑀 𝑔𝑎𝑙 observations in galaxy cluster populations are naturally reproduced in these models, provided there is an inflection in the initial size-mass relation at 𝑀 𝑔𝑎𝑙 ∼ 10 9.5 M . Our analytic model also predicts limits to the 𝑀 𝑔𝑎𝑙 − 𝑀 𝐺𝐶,𝑡𝑜𝑡 and 𝑀 𝑔𝑎𝑙 − 𝑀 𝑁 𝑆𝐶 relations which bound the scatter of the observational data. Moreoever, we illustrate how these scaling relations and 𝑓 𝑛𝑢𝑐 vary if the star cluster formation efficiency, GC mass function, galaxy environment or galaxy size-mass relation are altered. Two key predictions of our model are: 1) galaxies with NSC masses greater than their GC system masses are more compact at fixed stellar mass, and 2) the fraction of nucleated galaxies at fixed galaxy mass is higher in denser environments. That a single model framework can reproduce both the NSC and GC scaling relations provides strong evidence that GC in-spiral is an important mechanism for NSC formation.

Disentangling the formation mechanisms of nuclear star clusters

“…Recently, Neumayer et al (2020) suggested a transition of the dominant NSC formation channel from GC accretion to in situ formation at galaxy masses of M gal ∼ 10 9 M based on the functional shape of the nucleation fraction of galaxies and stellar population properties of the NSCs of low-mass early-type galaxies (ETGs, Paudel et al 2011;Spengler et al 2017) that tend to be more metal-poor than the expected metallicity of the host as suggested by the mass-metallicity relation (Gallazzi et al 2005;Kirby et al 2013). Similar trends were found in dwarf galaxies, where both the metallicity of NSCs and hosts were measured (Leaman et al 2020;Fahrion et al 2020a;Johnston et al 2020).…”

“…In this paper, we have employed a semi-analytic model of NSC formation (Leaman & van de Ven 2021) to determine the dominant NSC-formation channel (either the in situ star formation or the accretion of GCs) in a sample of 119 nearby galaxies in the Local Volume, the Fornax and the Virgo clusters, with galaxy masses from 3 × 10 7 to 3 × 10 11 M , whose NSC and GC system masses have been previously determined. We quantified the fraction of the NSC mass assembled via in situ formation, f in,NSC , in each NSC and explored its dependence on various properties of the host galaxy and its GC system.…”

“…photometric centre (e.g. KKs58 and DDO216; Fahrion et al 2020a;Leaman et al 2020). Although there are various processes that can prohibit NSC formation or destroy NSCs, these are not included in our model and we refer readers to LvdV21 for a detailed discussion on how these relate to the nucleation fraction of galaxies.…”