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.