Study of the ∼50 kpc circumgalactic environment around the merger system J2057–0030 at z ∼ 4.6 using ALMA

¹ Departamento de Astronomía, Universidad de Chile, Camino el Observatorio 1515, Las Condes, Casilla 36-D, Santiago, Chile
² International Centre for Radio Astronomy Research (ICRAR), The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia
e-mail: melissa.fuentealbafuentes@research.uwa.edu.au
³ Millennium Nucleus on Transversal Research and Technology to Explore Supermassive Black Holes (TITANS), Camino del Sur 495, Depto 504, 4130654 San Pedro de la Paz, Concepción, Chile
⁴ Institute of Astrophysics, Foundation for Research and Technology–Hellas (FORTH), Heraklion 70013, Greece
⁵ School of Sciences, European University Cyprus, Diogenes Street, Engomi, 1516 Nicosia, Cyprus
⁶ School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
⁷ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, Santiago 763-0355, Chile

Received: 16 January 2024
Accepted: 5 April 2024

Abstract

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.