I present a simple phenomenological model for the observed linear scaling of the stellar mass in old globular clusters ( GCs ) with z = 0 halo mass in which the stellar mass in GCs scales linearly with progenitor halo mass at z = 6 above a minimum halo mass for GC formation .
This model reproduces the observed M _ { GCs } - M _ { halo } relation at z = 0 and results in a prediction for the minimum halo mass at z = 6 required for hosting one GC : M _ { min } ( z = 6 ) = 1.07 \times 10 ^ { 9 } M _ { \odot } .
Translated to z = 0 , the mean threshold mass is M _ { halo } ( z = 0 ) \approx 2 \times 10 ^ { 10 } M _ { \odot } .
I explore the observability of GCs in the reionization era and their contribution to cosmic reionization , both of which depend sensitively on the ( unknown ) ratio of GC birth mass to present-day stellar mass , \xi .
Based on current detections of z \gtrsim 6 objects with M _ { 1500 } < -17 , values of \xi > 10 are strongly disfavored ; this , in turn , has potentially important implications for GC formation scenarios .
Even for low values of \xi , some observed high- z galaxies may actually be GCs , complicating estimates of reionization-era galaxy ultraviolet luminosity functions and constraints on dark matter models .
GCs are likely important reionization sources if 5 \lesssim \xi \lesssim 10 .
I also explore predictions for the fraction of accreted versus in situ GCs in the local Universe and for descendants of systems at the halo mass threshold of GC formation ( dwarf galaxies ) .
An appealing feature of the model presented here is the ability to make predictions for GC properties based solely on dark matter halo merger trees .