N-emitters as possible sign-posts of GC formation

Abstract

Based on the finding of unusual chemical abundance ratios of N-emitters, which resemble those of globular cluster (GC) stars, their compactness, high ISM densities and other properties, it has been suggested that N-emitters could indicate the formation sites of globulars. A recent statistical study of the N-emitter population has quantified the frequency $f_N$ of these rare objects and their redshift evolution (Morel et al. 2025). Using these results we here test if N-emitters trace the formation of GCs and use the observed cosmic star-formation rate density evolution to predict the cosmological evolution of the GC population with time, their age distribution, and the total present-day stellar mass density formed in globulars. The predicted age distribution of GCs strongly resembles the typical asymmetric observed distributions in the Galaxy and ellipiticals, with a peak at $\sim 11.5-12$ Gyr and a longer tail extending to younger ages. We derive a total stellar mass density formed in N-emitters down to redshift zero of $(2-7) \times 10^5$ M$_{\odot}$ Mpc$^{-3}$, which matches within a factor $\sim 2$ the observed fraction of stellar mass found in the GC population at $z=0$. These results provide additional indirect arguments supporting the hypothesis that N-emitters could represent sign-posts of a short phase of GC formation.