The Little Engines That Could? Globular clusters contribute significantly to reionization-era star formation

Metal-poor globular clusters (GCs) are both numerous and ancient, which indicates that they may be important contributors to ionizing radiation in the reionization era. Starting from the observed number density and stellar mass function of old GCs at |z| = 0, I compute the contribution of GCs to ultraviolet luminosity functions (UVLFs) in the high-redshift Universe (10 ≳ |z| ≳ 4). Even under absolutely minimal assumptions – no disruption of GCs and no reduction in GC stellar mass from early times to the present – GC star formation contributes non-negligibly to the UVLF at luminosities that are accessible to the Hubble Space Telescope (HST, M1500 ≈ −17). If the stellar masses of GCs were significantly higher in the past, as is predicted by most models explaining GC chemical anomalies, then GCs dominate the UV emission from many galaxies in existing deep-field observations. On the other hand, it is difficult to reconcile observed UVLFs with models requiring stellar masses at birth that exceed present-day stellar masses by more than a factor of 5. The James Webb Space Telescope will be able to directly detect individual GCs at |z| ∼ 6 in essentially all bright galaxies, and many galaxies below the knee of the UVLF, for most of the scenarios considered here. The properties of a subset of high-redshift sources with −19 ≲ M1500 ≲ −14 in HST lensing fields indicate that they may actually be GCs in formation.