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Recent JWST observations of the z=10.6 galaxy GN-z11 have revealed a veryhigh gas-phase nitrogen abundance (higher than four times the solar value), avery small half-light radius(~ 60 pc), and a large stellar mass (M_s ~ 10^9M_sun) for its size. We consider that this object is a forming galactic nucleusor ultra-compact dwarf galaxy rather than a proto globular cluster, and therebyinvestigate the chemical abundance pattern using one-zone chemical evolutionmodels.The principal results of the models are as follows. The observed log(N/O) > -0.24, log (C/O)>-0.78, and 12+log (O/H) ~ 7.8 can be self-consistentlyreproduced by the models both with very short star formation timescales (< 10^7yr) and with top-heavy stellar initial mass functions (IMFs). The adoptedassumption of no chemical enrichment by massive (m>25 M_sun) core collapsesupernovae (CCSNe) is also important for the reproduction of high gas-phase log(N/O), because such CCSNe can decrease high log (N/O) of gas polluted by OB andWolf-Rayet stars. GN-z11 can have a significant fraction (>0.5) ofnitrogen-rich ([N/Fe]>0.5) stars, which implies a possible link betweennitrogen-rich stellar populations of the inner Galaxy and giant ellipticalgalaxies and high-z objects with high gas-phase log (N/O) like GN-z11.

A model for GN-z11: top-heavy stellar initial mass functions in forming galactic nuclei and ultra-compact dwarfs