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N-emitters as possible sign-posts of GC formation

D. Schaerer, R. Marques-Chaves, H. Atek, N. Prantzos, C. Charbonnel, M. Talia, I. Morel, M. Dessauges-Zavadsky

TL;DR

The paper tests whether N-emitters are sign-posts of globular-cluster (GC) formation by combining the measured redshift evolution of the N-emitter fraction $f_N(z)$ with the cosmic star-formation rate density $ρ_{ m SFR}(z)$ to predict GC formation rate density $ρ_{ m GC}$, GC age distribution, and the present-day GC mass density $ρ_⋆({\rm GC},z)$. It adopts $ρ_{ m GC} = f_N ε_{ m GC} ρ_{ m SFR}$ with $ε_{ m GC} ≈ 1$, using $f_N(z)$ from Morel2025 and $ρ_{ m SFR}(z)$ from Shuntov2025 (with GLIMPSE high-z variants). The predicted GC formation peaks at $z \approx 3{-}4$, corresponding to ages $\approx 11.5{-}12$ Gyr, and yields an asymmetric age distribution with a tail to younger ages; the predicted present-day GC mass density is $(2{-}7)\times10^5$ M$_\odot$ Mpc$^{-3}$, in agreement with the observed GC mass fraction within a factor of ~2 when GC mass loss is considered. Overall, the results support N-emitters as sign-posts of a short GC-formation phase and provide a simple, empirically grounded link between high-redshift N-emitters and the GC population, while acknowledging uncertainties (AGN contamination, $ε_{ m GC}$, and $f_N$ completeness).

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.

N-emitters as possible sign-posts of GC formation

TL;DR

The paper tests whether N-emitters are sign-posts of globular-cluster (GC) formation by combining the measured redshift evolution of the N-emitter fraction with the cosmic star-formation rate density to predict GC formation rate density , GC age distribution, and the present-day GC mass density . It adopts with , using from Morel2025 and from Shuntov2025 (with GLIMPSE high-z variants). The predicted GC formation peaks at , corresponding to ages Gyr, and yields an asymmetric age distribution with a tail to younger ages; the predicted present-day GC mass density is M Mpc, in agreement with the observed GC mass fraction within a factor of ~2 when GC mass loss is considered. Overall, the results support N-emitters as sign-posts of a short GC-formation phase and provide a simple, empirically grounded link between high-redshift N-emitters and the GC population, while acknowledging uncertainties (AGN contamination, , and completeness).

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 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 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 M Mpc, which matches within a factor the observed fraction of stellar mass found in the GC population at . These results provide additional indirect arguments supporting the hypothesis that N-emitters could represent sign-posts of a short phase of GC formation.

Paper Structure

This paper contains 10 sections, 2 equations, 4 figures.

Table of Contents

  1. Introduction
  2. From N-emitters to GC populations
  3. N-emitter statistics to predict the average cosmological GC population
  4. Predicted age distribution of GCs
  5. Predicted cosmic GC mass density
  6. Discussion
  7. Overall picture linking N-emitters and GCs
  8. Caveats and open questions
  9. Conclusions
  10. New constraints on the N-emitter fraction at $z \sim 2-5$ from VANDELS

Figures (4)

  • Figure 1: Observed cosmic star-formation rate density evolution from Shuntov2025COSMOS-Web:-Ste and from the GLIMPSE survey at $z>9$Chemerynska2025The-first-GLIMP, and predicted GC formation rate, $\rho_{\rm GC}$, as a function redshift (dash-dotted and green dotted lines). The dashed line shows the observed increase of the N-emitter fraction, $f_N$, with redshift from Morel2025Discovery-of-ne, extrapolated down to $z=2$ and scaled by $1/10$ for convenience.
  • Figure 2: Predicted and observed age distribution of GCs. Observational data for GCs from the Milky Way from Kruijssen2019The-formation-a and for the massive elliptical galaxy NGC 1407, from Valenzuela2024Galaxy-archaeol and Usher2019The-SLUGGS-surv, are shown by solid lines. Dash-dotted lines show the predicted age distributions from our simple model, adopting the cosmic SFR density history from Shuntov2025COSMOS-Web:-Ste for different values of $z{\rm min}$z_ min$=1.5$, 2, and 3. The dotted line shows the same $z{\rm min}$z_ min$=2$ and the higher $\rho{\rm SFR}$ρ_ SFR$(z)$ values inferred from the GLIMPSE survey.
  • Figure 3: Predicted cumulative stellar mass density evolution (SMD) in stars, _⋆ $\rho_\star$, and globular clusters, _⋆ $\rho_\star$(GC), as a function of redshift. The SMDs are derived from the star-formation and globular cluster formation rate densities, _ SFR $\rho_{\rm SFR}$ and _ GC $\rho_{\rm GC}$, respectively, shown in Fig. \ref{['fig_sfrd']}. The shaded area indicates the range between $(1-4)$ times the observed cosmic stellar mass density in GCs derived at $z \sim 0$ by Harris2013A-Catalog-of-Gl, to account for the higher initial masses of GCs.
  • Figure 4: Redshift evolution of the fraction of N-emitters (circles) as a function of redshift, determined by Morel2025Discovery-of-ne from the JWST NIRSpec PRISM spectra (black circles and limits) and from the VANDELS survey (green circles and limits). Uncertainties show the 68% confidence range of the fractions, and the bin width along the x-axis. Blue lines show the fraction of rest-UV line emitters $4 \sigma$ (solid) or $3 \sigma$ detection (dotted) thresholds, respectively. The fraction of objects showing rest-optical emission lines is shown by the red line, and all emission lines galaxies (UV and/or optical) in magenta. The black dash-dotted line shows the fit to the $z>3$ data, Eq. \ref{['eq_fn']}, obtained by Morel2025Discovery-of-ne.