In the early Universe, just one and a half billion years after the Big Bang, three massive galaxies collided in a small volume of space. Their gravitational dance flung out giant streams of cold gas that stretched for 60 kiloparsecs, a distance twenty times the diameter of our Galaxy. These luminous rivers of ionized carbon (atoms that have lost one electron each) became the main discovery for astronomers working with the ALMA radio telescope array in the Chilean Atacama Desert.

For a long time, it remained unclear how supergiant elliptical galaxies accumulate mass in the first billion years and where the metal in the hot cluster gas comes from. Now it has been found: tidal streams, torn out by gravity during the merger, carry away about half of the gas from one of the galaxies and enrich the surrounding environment with heavy elements. Comparing the kinematics of the gas in the streams with the motion of the galaxies in phase space (coordinates and velocities of objects together) showed that about half of the galaxies and all the clumps of the streams lie on a single caustic, a curve along which they move coherently. This indicates a common origin during the collapse.

The gas in these rivers is heated by shock waves, not by ultraviolet radiation from young stars. The mass-to-luminosity ratio in the [CII] line for the clumps is anomalously low, 2.95 solar masses per solar luminosity, which is ten times lower than typical for galaxies at such redshifts. It is as if a river shone not from lights on the shore but from the internal friction of water against stones. Such outflows are a new channel for enriching the proto-intergalactic medium with metals and heating it.

For now, this is pure knowledge: it refines models of galaxy formation, but practical applications are not yet visible. However, without such observations, it is impossible to understand how the brightest cluster galaxies are born in such a short time. Ahead lie observations in the terahertz range with high angular resolution to directly measure dust in the clumps, and spectra of molecular gas to test the shock-heating hypothesis. Numerical simulations including feedback from active galactic nuclei will help understand how often such outflows occurred in the early Universe.