ALMA [CII] observations reveal how dozens of galaxies rapidly merge in the early Universe

ALMA imaging of [CII] line emission from the protocluster SPT2349-56. Star symbols mark the centres of galaxies, while orange contours highlight the tidal arms around the inner region. These tidally ejected, galaxy-scale gas clumps are found to be ten times brighter than expected. The size of the Milky Way disk is shown at the same scale. Image Credit: N. Sulzenauer, MPIfR.

The existence of massive, elliptical galaxies in the early Universe has puzzled astronomers for two decades. A new study used ALMA data to shed light on this open question of galaxy formation. The team discovered enormous, shock-heated tidal debris around a protocluster at z=4.3, which might represent a hallmark phase of massive galaxy and galaxy cluster assembly in the early Universe.

ALMA [CII] line observations reveal evidence of '[CII] streamers', tidal interactions between galaxies and possible shock-heating of the gas. Like 'beads on a string', the [CII] streamers fragment into a few kpc-spaced and turbulent clumps that have a similar column density as spiral arms in the local Universe. Analysis in phase space indicates that about half of the galaxies in the protocluster core populate the same caustic as the [CII] streamers, suggesting a role of angular momentum dissipation via tidal ejection during the formation of the bright central galaxy of this cluster. 

Duncan MacIntyre and Joel Tsuchitori, two undergraduate students on the team, ran detailed numerical simulations to bridge the observations of this protocluster collapse with previous studies of mature galaxy clusters. The striking match between the different types of objects found at different cosmic times might not only demonstrate the importance of simultaneous major mergers during massive galaxy formation. It may also help to explain how heavy elements (such as carbon) are heated and transported throughout the first galaxy clusters. 

You can read the paper here.

Contributed by Nikolaus Sulzenauer