Galaxy clusters, characterized by an over-density of massive elliptical galaxies embedded in an extended hot atmosphere, represent the largest mass concentrations in the Universe. However, their early formation, well before the peak of cosmic star-formation rate density, is shrowded in cosmic history. Since massive galaxies had to assemble their stellar mass rapidly within the first two billion years, these protoellipticals would serve as bright beacons of early structure assembly.

Follow-up observations of South Pole Telescope (SPT)-selected millimeter-wave emitters with the APEX telescope and ALMA 12-m array, revealed a population of high-z dusty star-forming galaxies. Only 10% of these galaxies appear unaltered by strong gravitational lensing, allowing to probe the regime of giant elliptical galaxy and cluster formation. Characterizing these titanic starbursts holds the promise to improve the understanding of the metal enrichment and first heating of the intracluster medium, as well as testing cosmology in the face of the abundace of rare, massive galaxies in the early Universe.

The intrinsically most luminous source of the SPT-sample is SPT2349-56, hosting a starburst of ~6700 solar masses per year within a diameter of 400 kpc. In our most recent work, we combined multiple high-resolution ALMA observations into a single, ultra-deep line map using the FIR fine-structure line transition of ionized carbon ([CII]), emitted at 158µm and redshifted to ~358 GHz. With a deep dust continuum, line intensity, and velocity maps, eleven new galaxies were discovered in the structure, along with two 30–60 kpc-long tidal arms wrapping around a central quartet of dusty star-forming galaxies. This tidal debris further fragments into 10 kpc-scale clumps of highly turbulent yet coherently flowing molecular gas. Importantly, the observed ten-fold boost in [CII] line intensity and morpho-kinematic distribution of stripped gas allows us to reconstruct the recent assembly history of the 40+ galaxies in this protocluster. And therefore offering new insights into the formation pathway of a giant galaxy and its circumgalactic medium.

ApJ in press.
Paper: https://dx.doi.org/10.3847/1538-4357/ae2ff0