Globular cluster numbers in dark matter haloes in a dual formation scenario: an empirical model within EMERGE

TL;DR

This paper develops an empirical model linking globular cluster numbers to dark matter halo masses, incorporating dual formation pathways, and explains observed correlations, scatter, and age distributions in galaxy systems.

Contribution

It introduces a dual formation scenario for GCs within an empirical model, accounting for younger populations and scatter related to smooth accretion.

Findings

Linear GC-halo mass relation is due to hierarchical merging.

Scatter correlates with smooth accretion, affecting GC counts.

GC age distribution and inversion are reproduced by the model.

Abstract

We present an empirical model for the number of globular clusters (GCs) in galaxies based on recent data showing a tight relationship between dark matter halo virial masses and GC numbers. While a simple base model forming GCs in low-mass haloes reproduces this relation, we show that a second formation pathway for GCs is needed to account for observed younger GC populations. We confirm previous works that reported the observed linear correlation as being a consequence of hierarchical merging and its insensitivity to the exact GC formation processes at higher virial masses, even for a dual formation scenario. We find that the scatter of the linear relation is strongly correlated with the relative amount of smooth accretion: the more dark matter is smoothly accreted, the fewer GCs a halo has compared to other haloes of the same mass. This scatter is smaller than that introduced by halo mass measurements, indicating that the number of GCs in a galaxy is a good tracer for its dark matter mass. Smooth accretion is also the reason for a lower average dark matter mass per GC in low-mass haloes. Finally, we successfully reproduce the observed general trend of GCs being old and the tendency of more massive haloes hosting older GC systems. Including the second GC formation mechanism through gas-rich mergers leads to a more realistic variety of GC age distributions and also introduces an age inversion in the halo virial mass range $\log M_\mathrm{vir}/\mathrm{M}_\odot = 11{-}13$.