Discovery of molecular gas fueling galaxy growth in a protocluster at z=1.7
Abstract
Based on ALMA Band 3 observations of the CO(2-1) line transition, we report the discovery of three new gas-rich (MH2 ~ 1.5-4.8 x 1010 Msun, SFRs in the range ~5-100 Msun/yr) 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 Dz = 0.012. Under simple assumptions, we estimate that the system has a total mass of >= 3-6 x 1013 Msun, and show that it will likely evolve into a >~ 1014 Msun 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 (MH2 ~ 2 x 1011 Msun). We fitted the FRII resolved CO emission with a 2-D Gaussian model with major (minor) axis of ~ 27 (~ 17) kpc, that 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 towards the nucleus of ~ 5.5 x 1023 cm-2. Such a dense ISM may then contribute significantly to the total nuclear obscuration measured in the X-rays (N(H,X) ~ 1.5 x 1024 cm-2) in addition to a small, pc-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. 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 structure's velocity dispersion. All this, coupled with the large amount of gas around the FRII, its stellar mass of ~ 3 x 1011 Msun, SFR of ~ 200-600 Msun/yr, and powerful radio-to-X-ray emission, suggests that this source is the likely progenitor of the future brightest cluster galaxy.