GN-z11 in context: possible signatures of globular cluster precursors at redshift 10

Abstract

The first JWST spectroscopy of the luminous galaxy GN-z11 simultaneously both established its redshift at z = 10.6 and revealed a rest-ultraviolet spectrum dominated by signatures of highly-ionized nitrogen, which has so far defied clear interpretation. Here we present a reappraisal of this spectrum in the context of both detailed nebular modeling and nearby metal-poor reference galaxies. The N IV] emission enables the first nebular density measurement in a star-forming galaxy at z > 10, and reveals evidence for extremely high densities n e 10 5 cm −3 . We definitively establish with a suite of photoionization models that regardless of ionization mechanism and accounting for depletion and this density enhancement, an ISM substantially enriched in nitrogen ([N/O] = +0.52) is required to reproduce the observed lines. A search of local UV databases confirms that nearby metal-poor galaxies power N IV] emission, but that this emission is uniformly associated with lower densities than implied in GN-z11. We compare to a unique nearby galaxy, Mrk 996, where a high concentration of Wolf-Rayet stars and their CNO-processed wind ejecta produce a UV spectrum remarkably similar to that of both GN-z11 and the Lyc-leaking super star cluster in the Sunburst Arc. Collating this evidence in the context of Galactic stellar abundances, we suggest that the peculiar nitrogenic features prominent in GN-z11 may be a unique signature of intense and densely clustered star formation in the evolutionary chain of the present-day globular clusters, consistent with in-situ early enrichment with nuclear-processed stellar ejecta on a massive scale. Combined with insight from local galaxies, these and future JWST data open a powerful new window onto the physical conditions of star formation and chemical enrichment at the highest redshifts.Keywords: High-redshift galaxies (734) -Blue compact dwarf galaxies ( 165) -Galaxy abundances (574) -Globular star clusters (656) * Carnegie Fellow large populations of globular clusters in the present-day Universe, whose typical metallicities ([Fe/H] ∼ −2.5-−1; Harris 1996) and inferred ages (uncertain, but ranging from ∼ 10 to 13 Gyr; Vandenberg et al. 1996; Brodie & Strader 2006) establish many as potentially forming at z 5 (e.g. Renzini 2017).Detailed study of globulars has also provided a mysterious clue about the character of the star formation episodes that produced them. Essentially all globulars differ from a single simple stellar population: and in particular, a substantial fraction of their stars (their 'enriched' or 'second' population) contain variations in light and heavy element abundances consistent with the yields of high-temperature nuclear burning (specifically, the CNO process and proton-capture chains, which produce abundance N, Na, and Al) that sharply distinguish them from field star populations (e.g. Osborn 1971; Smith

N-enhancement in GN-z11: First evidence for supermassive stars nucleosynthesis in proto-globular clusters-like conditions at high redshift?

“…The same agreement holds for log(C/O), although we note that no high values are found in 47 Tuc. This comparison empirically supports the possibility that the bright N emission lines compared to faint O lines in GN-z11 may result from the contamination of the gas of a massive and compact -proto-GC-like -star cluster by CNO-processed material from the same polluters as those required to explain GC abundance patterns (see also Senchyna et al 2023). 2023) fiducial model and from their more conservative assumptions respectively (with upper boundaries which are not well defined).…”

“…The main unknown here is the n e , which cannot be determined using standard optical diagnostics (e.g. using [O ii] λ3726,3729 doublet) from the available JWST observations of GN-z11, but which can be estimated from density-sensitive UV lines (Senchyna et al 2023). The extreme compactness of GN-z11 clearly indicates higher-than-average densities.…”

“…To put our hypothesis in the GC observational context (see also Senchyna et al 2023), we chose four Galactic GCs that cover a large range of metallicity and for which C, N, and O abundance measurements are simultaneously available, and we show in Fig. 1 the values of log(N/O) and log(C/O) as a function of log(O/H) in their individual stars and in MW field halo stars.…”

“…2023) fiducial model and from their more conservative assumptions respectively (with upper boundaries which are not well defined). The black pentagon and asterisk indicate the location of Senchyna et al (2023) photoionization modeling results for GN-z11. Left: Abundance ratios in individual stars of 4 GCs (NGC 6397, Carretta et al 2005;NGC 2808, Carlos et al 2023NGC 6752, Carretta et al 2005, and47 Tuc, Mészáros et al 2020), and in MW field stars within the same metallicity range as the 4 GCs (Israelian et al 2004;Akerman et al 2004;Fabbian et al 2009); for the GCs we connect the points by line segments to guide the eye.…”

“…The detection of unusually bright NIII] and NIV] UV emission lines in the JWST/NIRSpec spectrum of GN-z11 (Bunker et al 2023) revealed exceptional nitrogen enrichment in this highz galaxy. The N/O abundance ratio in the interstellar medium (ISM) of GN-z11 derived by Cameron et al (2023, see also Senchyna et al 2023) is more than four times solar (log(N/O) > ∼ −0.25). This is more than one order of magnitude higher than what is usually found in low-redshift galaxies and Hii regions of similar low metallicity (12+log(O/H) < ∼ 8.0), and even slightly higher than in galaxies with super-solar metallicities (e.g.…”