2025/09/25 by Übler, Hannah, Mazzolari, Giovanni, Maiolino, Roberto +40
#Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences
paper · doi:10.48550/arxiv.2509.21575
We present observational evidence for three massive, accreting black holes in the z=5.0167 galaxy J0148-4214 from JWST/NIRSpec-IFU spectroscopy. The black holes are revealed through broad Hα emission (FWHM = 430-2920 km/s) without a forbidden-line counterpart in the bright [O III] doublet. Channel maps of the asymmetric central Hα profile isolate two spatially distinct broad line regions (BLRs), separated by 190±40 pc, while a third BLR is found in the galaxy outskirts with a projected separation of 1.7 kpc. Using single-epoch virial relations, we estimate black hole masses of log(M_\bullet/M_\odot)=7.9±0.4 (primary central), 5.8±0.5 (secondary central) and 6.3±0.5 (third off-nuclear). We argue that the two central black holes will likely rapidly merge, with a simple dynamical friction time estimate of the order of 700 Myr. Assuming that also the off-nuclear black hole is in the process of sinking towards the centre, it will likely lead to a second merger, and we investigate the detection probability of such mergers with LISA. Alternatively, the third black hole may be the result of previous three-body interaction or a gravitational recoil, where our observations would provide evidence that such black holes may retain their accretion discs and BLRs even in the aftermath of such extreme dynamical interactions. The discovery of a black hole triplet at high redshift, together with other recent results on distant black hole pairs, indicates that multiple massive black hole systems were common in the early Universe. Our results highlight the importance of IFU observations for the detection of massive black hole multiplets in distant galaxies, the progenitors of massive black hole mergers that may be detected with next-generation gravitational wave observatories.