Infalling gas in a Lyman-α blob

Lyman-α blobs (LABs) are spatially extended nebulae of emission in the Lyman-α (Lyα) line of hydrogen, seen at high redshifts1,2, and most commonly found in the dense environment of star-forming galaxies3,4. A recent study showed that nearly 100% of the sky is covered by Lyα emission around high-redshift galaxies5–7. The origin of Lyα emission in the LABs is still unclear and under debate8. It may be powered by photoionization involving galactic superwinds/outflows, resonant scattering of Lyα photons from starbursts or active galactic nuclei9–16, or by cooling radiation from cold streams of gas accreting onto galaxies17,18, as demonstrated by recent simulations19. Here we analyse the gas kinematics within a LAB, providing rare observational evidence for infalling gas. This is consistent with the release of gravitational accretion energy as cold streams radiate Lyα photons. It also provides direct evidence for possible cold streams feeding the central galaxies. The mass of the infalling gas is not important in comparison to the gas mass consumed by star formation, and is also not the major powering source of Lyα emission, but it hints at another mechanism to explain the origin of the extended Lyα emission around young galaxies. The gas kinematics within a Lyman-α blob show clear evidence of infall, consistent with the release of gravitational accretion energy as cold streams radiate Lyman-α photons. These cold streams are apparently responsible for feeding central galaxies.