Discovery of molecular gas fueling galaxy growth in a protocluster at z = 1.7

Based on ALMA Band 3 observations of the CO(2→1) line transition, we report the discovery of three new gas-rich (M_H2 ∼ 1.5 - 4.8 × 10¹⁰ M_⊙) galaxies in an overdense region at z = 1.7 that already contains eight spectroscopically confirmed members. This leads to a total of 11 confirmed overdensity members within a projected distance of ∼1.15 Mpc and in a redshift range of Δz = 0.012. Under simple assumptions, we estimate that the system has a total mass of ≥3 - 6 × 10¹³ M_⊙, and show that it will likely evolve into a ≳10¹⁴ M_⊙ cluster at z = 0. The overdensity includes a powerful Compton-thick Fanaroff-Riley type II (FRII) radio galaxy, around which we discovered a large molecular gas reservoir (M_H2 ∼ 2 × 10¹¹ M_⊙). We fit the FRII resolved CO emission with a 2D Gaussian model with a major (minor) axis of ∼27 (∼17) kpc, which is a factor of ∼3 larger than the optical rest-frame emission. Under the assumption of a simple edge-on disk morphology, we find that the galaxy interstellar medium produces a column density toward the nucleus of ∼5.5 × 10²³ cm^-2. A dense interstellar medium like this may then contribute significantly to the total nuclear obscuration measured in the X-rays (N_{H, X} ∼ 1.5 × 10²⁴ cm^-2) in addition to a small, paresec-scale absorber around the central engine. The velocity map of this source unveils a rotational motion of the gas that is perpendicular to the radio jets. All ALMA sources have a dust-reddened counterpart in deep Hubble Space Telescope images (bands i, z, H), while we do not detect any molecular gas reservoir around the known UV-bright, star-forming members discovered by MUSE. This highlights the capability of ALMA of tracing gas-rich members of the overdensity. For the MUSE sources, we derive 3σ upper limits to the molecular gas mass of M_H2 ≤ 2.8 - 4.8 × 10¹⁰ M_⊙. We derive star formation rates in the range ∼5 - 100 M_⊙ yr^-1 for the three new ALMA sources. The FRII is located at the center of the projected spatial distribution of the structure members, and its velocity offset from the peak of the redshift distribution is well within the velocity dispersion of the structure. All this, coupled with the large amount of gas around the FRII, its stellar mass of ∼3 × 10¹¹ M_⊙, star formation rate of ∼200 - 600 M_⊙ yr^-1, and powerful radio-to-X-ray emission, suggests that this source is the likely progenitor of the future brightest cluster galaxy.