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

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; $>100M_{\odot }$), 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 ($M_{\mathrm{U}\mathrm{V}}=-21.3$) galaxy at $z=6.6$. New NIRSpec medium-resolution data reveal a blue UV continuum (${\beta }_{\mathrm{U}\mathrm{V}}=-2.5$) with strong emission lines including NIV] $\lambda 1486$, stellar P Cygni profiles, and broad HeII $\lambda 1640$. We fit the UV with stellar population models, and find that the models including VMSs (limited to $300M_{\odot }$) are statistically preferred. The best-fit stellar population has $M_{\ast }\approx {10}^8{M}_{\odot }$, and a young age of $2.4\pm 0.8$ Myr. The rest-optical shows a redder continuum (${\beta }_{\mathrm{o}\mathrm{p}\mathrm{t}}=-1.1$) 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 $\sim 5$ Myr, and hence this system would be observed as a typical V-shaped LRD. We measure a supersolar $\mathrm{l}\mathrm{o}{\mathrm{g}}_{10}(\mathrm{N}\mathrm{/}\mathrm{O})=-0.07\pm 0.05$ and subsolar $\mathrm{l}\mathrm{o}{\mathrm{g}}_{10}(\mathrm{C}\mathrm{/}\mathrm{O})=-0.64\pm 0.11$; therefore $\mathrm{l}\mathrm{o}{\mathrm{g}}_{10}(\mathrm{N}\mathrm{/}\mathrm{C})=+0.58\pm 0.12$. The independent density-sensitive NIV] $\lambda \lambda 1483,1486$ and CIII] $\lambda \lambda 1906\mathrm{/}1909$ ratios require two zones with $n_e\gtrsim {10}^{6.5}$ cm${}^{-3}$ and $n_e\lesssim {10}^{3.5}$ cm${}^{-3}$, respectively. We estimate $\mathrm{l}\mathrm{o}{\mathrm{g}}_{10}(\mathrm{N}\mathrm{/}\mathrm{C})>1.3$ (2$\sigma$) for the dense zone, requiring extreme N/C enrichment mechanisms such as highly rotating $>100M_{\odot }$ 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.