2025A&A...702A.174M

We present JWST NIRSpec integral field spectroscopy observations of the z = 5.89 quasar NDWFS J1425+3254 from 0.6-5.3 μm, covering the rest-frame ultraviolet and optical at a spectral resolution of R ∼ 100. The quasar has a black hole mass of $ M_\mathrm{{BH}}=(1.4^{+3.1}_{-1.0})\times10^9 $M_⊙ and an Eddington ratio of $ L_\mathrm{{Bol}}/L_\mathrm{{Edd}}=0.3^{+0.6}_{-0.2} $, as implied from the broad Balmer Hα and Hβ lines. The quasar host has significant ongoing obscured star formation, as well as a quasar-driven outflow with velocity $ 6050^{+460}_{-630} $ km s^–1 and ionised outflow rate of $ 1650^{+130}_{-1230} $M_⊙ yr^–1. This is possibly one of the most extreme outflows in the early Universe. The data also reveal that two companion galaxies are merging with the quasar host. The north-eastern companion galaxy is relatively old and very massive, with a luminosity-weighted stellar age of $ 65^{+9}_{-4} $ Myr, stellar mass of $ (3.6^{+0.6}_{-0.3})\times10^{11}\,M_\odot $, and star-formation rate (SFR) of ∼15-30 M_⊙ yr^–1. A bridge of gas connects this companion galaxy and the host, confirming their ongoing interaction. A second merger is occurring between the quasar host and a much younger companion galaxy to the south, with a stellar age of 6.7 ± 1.8 Myr, stellar mass of (1.9 ± 0.4)×10¹⁰M_⊙, and SFR of ∼40-65 M_⊙ yr^–1. There is also another galaxy in the field, likely in the foreground at z = 1.135, which could be gravitationally lensing the quasar with a magnification of 1 < μ < 2 and, thus, < 0.75 mag. Overall, the system is a 'train-wreck' merger of three galaxies, with star formation and extreme quasar activity that were likely triggered by these ongoing interactions.