Deciphering the activity and quiescence of high-redshift cluster environments: ALMA observations of Cl J1449+0856 at z = 2

We present ALMA observations of the 870µm continuum and CO(4-3) line emission in the core of the galaxy cluster ClJ1449+0856 at z=2, a NIR-selected, X-ray detected system in the mass range of typical progenitors of today's massive clusters. The 870µm map reveals six F870µm > 0.5 mJy sources spread over an area of 0.07 arcmin2, giving an overdensity of a factor ~10 (6) with respect to blank field counts down to F870µm > 1 (0.5) mJy. On the other hand, deep CO(4-3) follow-up confirms membership of three of these sources, but suggests that the remaining three, including the brightest 870µm sources in the field (F870µm?2 mJy), are likely interlopers. The measurement of 870µm continuum and CO(4-3) line fluxes at the positions of previously-known cluster members provides a deep probe of dusty star formation occurring in the core of this high-redshift structure, adding up to a total SFR~700±100 M?/yr and yielding an integrated star formation rate density of ~104 M?/yr/Mpc3, five orders of magnitude larger than in the field at the same epoch, due to the concentration of star-forming galaxies in the small volume of the dense cluster core. The combination of these observations with previously available HST imaging highlights the presence in this same volume of a population of galaxies with already suppressed star formation. This diverse composition of galaxy populations in ClJ1449+0856 is especially highlighted at the very cluster center, where a complex assembly of quiescent and star-forming sources is likely forming the future Brightest Cluster Galaxy.