N-emitters as possible sign-posts of GC formation

TL;DR

This study investigates whether N-emitters can serve as indicators of globular cluster formation by analyzing their properties, evolution, and relation to the overall GC population, supporting their potential as sign-posts of GC formation.

Contribution

The paper provides a theoretical framework linking N-emitters to GC formation, predicting their age distribution and stellar mass density, and compares these with observations to support their role as sign-posts.

Findings

Predicted GC age distribution peaks at 11.5-12 Gyr.

Total stellar mass density in N-emitters matches observed GC mass fraction within a factor of 2.

N-emitters likely indicate a short phase of globular cluster 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.