A Little Red Dot emerging from a nitrogen-rich UV-luminous galaxy with Very Massive Stars

Abstract

JWST is revealing abundant populations of high-$z$ galaxies with unforeseen properties: UV-luminous galaxies with high nitrogen enrichment, populations of very massive stars (VMSs; $>100\ M_\odot$), and early forms of massive black hole growth such as the Little Red Dots (LRDs). Here we present all these phenomena appearing simultaneously in COLA1, a luminous ($M_{\rm UV}=-21.3$) galaxy at $z=6.6$. New NIRSpec medium-resolution data reveal a blue UV continuum ($β_{\rm UV}=-2.5$) with strong emission lines including NIV] $\lambda1486$, stellar P Cygni profiles, and broad HeII $\lambda1640$. We fit the UV with stellar population models, and find that the models including VMSs (limited to $300\ M_\odot$) are statistically preferred. The best-fit stellar population has $M_*\approx10^8\ M_\odot$, and a young age of $2.4\pm0.8$ Myr. The rest-optical shows a redder continuum ($β_{\rm opt}=-1.1$) and broad Balmer lines with Balmer absorption akin to LRD spectra. By evolving the best-fit stellar population we find that the UV could dim by 1-2 mag in $\sim5$ Myr, and hence this system would be observed as a typical V-shaped LRD. We measure a supersolar $\rm log_{10}(N/O)=-0.07\pm0.05$ and subsolar $\rm log_{10}(C/O)=-0.64\pm0.11$; therefore $\rm log_{10}(N/C)=+0.58\pm0.12$. The independent density-sensitive NIV] $λ\lambda1483,1486$ and CIII] $λ\lambda1906/1909$ ratios require two zones with $n_e\gtrsim10^{6.5}$ cm$^{-3}$ and $n_e\lesssim10^{3.5}$ cm$^{-3}$, respectively. We estimate $\rm log_{10}(N/C)>1.3$ (2$σ$) for the dense zone, requiring extreme N/C enrichment mechanisms such as highly rotating $>100\ M_\odot$ VMSs, tidal disruption events, or supermassive stars. We speculate that runaway collisions of massive stars in a dense stellar cluster may have lead to the seeding of an intermediate-mass black hole resulting in the formation of an LRD.