The globular cluster system mass–halo mass relation in the E-MOSAICS simulations

Abstract

Linking globular clusters (GCs) to the assembly of their host galaxies is an overarching goal in GC studies. The inference of tight scaling relations between GC system properties and the mass of both the stellar and dark halo components of their host galaxies are indicative of an intimate physical connection, yet have also raised fundamental questions about how and when GCs form. Specifically, the inferred correlation between the mass of a GC system (MGC) and the dark matter halo mass (Mhalo) of a galaxy has been posited as a consequence of a causal relation between the formation of dark matter mini-haloes and GC formation during the early epochs of galaxy assembly. We present the first results from a new simulation of a cosmological volume (L = 34.4 cMpc on a side) from the E-MOSAICS suite, which includes treatments of the formation and evolution of GCs within the framework of a detailed galaxy formation model. The simulated MGC-Mhalo relation is linear for halo masses >5 × 1011 M⊙, and is driven by the hierarchical assembly of galaxies. Below this halo mass, the simulated relation features a downturn, which we show is consistent with observations, and is driven by the underlying stellar mass-halo mass relation of galaxies. Our fiducial model reproduces the observed MGC-M⋆ relation across the full mass range, which we argue is more physically relevant than the MGC-Mhalo relation. We also explore the physical processes driving the observed constant value of ${M_{\rm GC}}/ {M_{\rm halo}}\sim 5\times 10^{-5}$ and find that it is the result of a combination of cluster formation physics and cluster disruption.

Kraken reveals itself – the merger history of the Milky Way reconstructed with the E-MOSAICS simulations

“…Secondly, we find some evidence that the specific frequency T N changes with redshift, consistent with models suggesting that GC formation is fundamentally a baryonic process that results in relations with host galaxy properties that evolve in the context of hierarchical galaxy formation and evolution (e.g. Kruijssen 2015;Choksi et al 2018;Pfeffer et al 2018;El-Badry et al 2019;Kruijssen et al 2019a;Bastian et al 2020).…”

“…However, the specific frequencies of the Kraken and Gaia-Enceladus exceed those of galaxies of a similar mass at z = 0, suggesting that the scaling relations between the number of GCs and the host galaxy mass evolve with redshift (e.g. Kruijssen 2015;Choksi & Gnedin 2019;Bastian et al 2020). We propose that the excess can be explained by the continued stellar mass growth of galaxies after z acc .…”

“…If the number of GCs per unit halo mass is higher at high redshift (as suggested by, e.g. Kruijssen 2015;Choksi & Gnedin 2019;El-Badry et al 2019;Bastian et al 2020), this assumption overestimates the galaxy mass. Likewise, the inclusion of ambiguous GCs as members also maximizes the galaxy mass.…”

“…Because these relations are close to linear, they are often expressed in terms of number ratios, such as the specific frequency normalized by stellar mass (T N ≡ N GC /M ) or by halo mass (η N ≡ N GC /M h ), or in terms of the ratio between the total GC system mass and the halo virial mass (η M ≡ M GCS /M h ). It is an important question whether these relations are fundamental and were imprinted at the time of GC formation (Spitler & Forbes 2009;Boylan-Kolchin 2017;Harris et al 2017;Burkert & Forbes 2020) or if they result from a combination of ongoing baryonic processes and further linearization by hierarchical galaxy assembly (Kruijssen 2015;Choksi et al 2018;van Dokkum et al 2018;El-Badry et al 2019;Bastian et al 2020). The key underlying question is how these metrics evolve with redshift.…”

What to expect when using globular clusters as tracers of the total mass distribution in Milky Way-mass galaxies

“…The simulations have also shown their ability to use GCs to predict properties of dwarf galaxies that now cause stellar streams (Hughes et al 2019) and have been shown to reproduce the GC colour-magnitude relation (Usher et al 2018). Another observable reproduced by the E-MOSAICS simulations is the relation between the number of GCs and the host galaxy mass (Kruijssen et al 2019a;Bastian et al 2020). In addition, the simulations have made predictions for future observables regarding the formation environments and initial demographics of GCs, to be tested with JWST and the 30-m class telescopes (Reina-Campos et al 2019;Pfeffer et al 2019a;Keller et al 2020).…”