2024A&A...687A..62F

We present ALMA band-7 observations of J2057-0030, a multi-component merger system at z ∼ 4.68 spanning at least 50 kpc in size, using the [CII] λ157.74 μm line and underlying far-infrared (FIR) continuum. We find two main components, the quasar (or QSO) and a dusty star-forming galaxy (DSFG), both detected in [CII] and continuum emission as well as multiple neighboring clumps detected only in [CII]. Three of these clumps form a (tidal) tail that extends from the QSO in a straight direction to the west, covering a projected distance of ∼10 kpc. This perturbed morphology, added to a spatial distance of ∼20 kpc and a velocity offset of Δv = 68 km s^–1 between the QSO and the DSFG, strongly supports a merging scenario. By fitting a spectral energy distribution model to the continuum data, we estimate star formation rates of ≈402 M_⊙ yr^–1 for the QSO host and ≈244 M_⊙ yr^–1 for the DSFG, which locate them on or close to the main sequence of star-forming galaxies. The J2057-0030 QSO was selected for being one of the brightest unobscured quasars at its redshift while presenting a rather modest star formation rate. Based on a commonly accepted paradigm regarding the formation of quasars, this result is expected for a quasar that has already passed an obscured phase of rapid star formation during a major merger. However, we see that the merger event in this system is far from being finished, and it is rather likely somewhere between the first pericenter and subsequent close passages. This is presumably another case of a high-z quasar residing in a high-density environment with a companion obscured galaxy.