Abstract
Lyman-alpha blobs (LABs) are spatially extended nebulae of emission in the Lyman-alpha (Ly alpha) line of hydrogen, seen at high redshifts(1,2), and most commonly found in the dense environment of star-forming galaxies(3,4). A recent study showed that nearly 100% of the sky is covered by Ly alpha emission around high-redshift galaxies(5-7). The origin of Ly alpha emission in the LABs is still unclear and under debate(8). It may be powered by photoionization involving galactic superwinds/outflows, resonant scattering of Ly alpha photons from starbursts or active galactic nuclei(9-16), or by cooling radiation from cold streams of gas accreting onto galaxies(17,18), as demonstrated by recent simulations(19). 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 Ly alpha 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 Ly alpha emission, but it hints at another mechanism to explain the origin of the extended Ly alpha emission around young galaxies.
The gas kinematics within a Lyman-alpha blob show clear evidence of infall, consistent with the release of gravitational accretion energy as cold streams radiate Lyman-alpha photons. These cold streams are apparently responsible for feeding central galaxies.