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 (MH2 1:54:8 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 z = 0:012. Under simple assumptions, we estimate that the system has a total mass of 361013 M, and show that it will likely evolve into a 1014 M cluster at z = 0. The overdensity includes a powerful Compton-thick Fanaro-Riley type II (FRII) radio galaxy, around which we discovered a large molecular gas reservoir (MH2 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 cm2. A dense interstellar medium like this may then contribute significantly to the total nuclear obscuration measured in the X-rays (NH;X 1:5 1024 cm2) 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 3 upper limits to the molecular gas mass of MH2 2:84:8 1010 M. We derive star formation rates in the range 5100 M yr1 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 oset 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 31011 M, star formation rate of 200600 M yr1, and powerful radio-to-X-ray emission, suggests that this source is the likely progenitor of the future brightest cluster galaxy. © ESO 2020.