Origins of scaling relations of globular cluster systems

Abstract

Globular cluster (GC) systems demonstrate tight scaling relations with the properties of their host galaxies. In previous work, we developed an analytic model for GC formation in a cosmological context and showed that it matches nearly all of the observed scaling relations across 4 orders of magnitude in host galaxy mass. Motivated by the success of this model, we investigate in detail the physical origins and evolution of these scaling relations. The ratio of the combined mass in GCs M GC to the host dark matter halo mass M h is nearly constant at all redshifts, but its normalization evolves by a factor of ∼10 from birth to z = 0. The relation is steeper than linear at halo masses M h 10 11.5 M , primarily due to non-linearity in the stellar mass-halo mass relation. The near constancy of the ratio M GC /M h , combined with the shape of the stellar mass-halo mass relation, sets the characteristic U−shape of the GC specific frequency as a function of host galaxy mass. The contribution of accreted satellite galaxies to the buildup of GC systems is a strong function of the host galaxy mass, ranging from ≈0% at M h ≈ 10 11 M to 80% at M h ≈ 10 15 M . The metal-poor clusters are significantly more likely to form ex-situ relative to the metal-rich clusters, but a substantial fraction of metal-poor clusters still form in-situ in lower mass galaxies. Similarly, the fraction of red clusters increases from ≈ 10% at M h = 10 11 M to ≈ 60% at M h ≈ 10 13 M , and flattens at higher M h . Clusters formation occurs essentially continuously at high redshift, while at low redshift galactic mergers become increasingly important for cluster formation.

The Fornax 3D project: Non-linear colour–metallicity relation of globular clusters

“…Harris et al (2017) used the same metallicities, but combined the griz photometry of Sinnott et al (2010) with U BVRI photometry available from Peng et al (2004) to derive a very similar quadratic relation using (g − I) colours. They also give conversion to (g − z) colours (see also Choksi & Gnedin 2019). Their CZR is offset to our red GCs and shallower at blue colours.…”

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

“…The subsequent merging of galaxies (along with their GC populations) would then act to fully linearize the relation. A related interpretation has recently been put forward by El-Badry et al (2019) and Choksi & Gnedin (2019). These authors suggest that the hierarchical growth of galaxies naturally leads to such a tight correlation, independent of the adopted cluster formation model, except at the low halo mass end, which may retain some memory of GC formation.…”

Discovery of new globular clusters in the Sagittarius dwarf galaxy

“…Since our near-IR surveys do not cover the southern extension of the Sgr galaxy, we predict that several more GCs have yet to be found, estimating a total population of at least 30 GCs in this galaxy. The predictions of recent theoretical studies of the GC populations in dwarf galaxies (e.g., Kruijssen et al 2019Kruijssen et al , 2020El Badry et al 2019;Choksi & Gnedin 2019;Hughes et al 2019) may be compared with the GCs of Sgr once the metallicities, luminosities, and ages of the newly discovered GCs are properly measured (Garro et al, in prep.). Table A.1 (except for Minni 327 which is outside the field of view).…”

“…Additional motivation for this work comes from recent theoretical studies of GC populations in galaxies (Kruijssen et al 2019(Kruijssen et al , 2020El Badry et al 2019;Choksi & Gnedin 2019;Hughes et al 2019) and from the comparison of GC systems in galaxies with different masses. For example, Kruijssen et al (2020) assigned 8 ± 3 massive GCs (with M > 10 5 M ) and a total halo mass M T = 10 10.94 ± 0.10 M to the Sgr dwarf galaxy.…”