Recycled gas feeds a massive galaxy in the early Universe

Streams of intergalactic gas, enriched with elements heavier than helium, encircle and spiral into a massive galaxy observed at redshift 2.3, researchers report. The findings provide evidence of enriched gas recycling during galaxy formation in the early Universe. Galaxies form through the accretion of gas from the circumgalactic medium (CGM) and intergalactic medium (IGM), which subsequently condenses into stars. Simulations and observations have shown that cold stream accretion – the accumulation of pristine intergalactic gas that contains almost no elements heavier than helium – provides fuel for the star formation rate of galaxies in the early Universe. However, stellar processes in these early galaxies, such as supernovae, enrich gas within the galaxy with elements heavier than helium, including carbon. Related processes can even eject some of this material back out into the IGM. Theory predicts that the enriched gas could subsequently be recycled, accreting back into galaxies, providing additional fuel to sustain rapid star formation for longer. However, observations of enriched gas feeding high-redshift galaxies are limited. Shiwu Zhang, Zheng Cai, and colleagues use the Keck II and Subaru telescopes to observe gas surrounding a massive galaxy at redshift 2.3. In addition to helium and hydrogen, spectra of this region reveal emission lines of ionized carbon, indicating that the CGM gas surrounding the galaxy has been enriched in elements heavier than helium. Kinematic modelling of the observations suggest that streams of the enriched gas are spiraling toward the massive galaxy. Based on the findings, Zhang, Cai et al. propose that the observed inflowing enriched gas was recycled from an earlier period of star formation and calculate that it could sustain the observed star formation rate of the galaxy.