Lyman-α blobs (LABs) are spatially extended nebulae of emission in the Lyman-α (Lyα) 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 Lyα emission around high-redshift galaxies^5,6,7. 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 nuclei^{9,10,11,12,13,14,15,16}, or by cooling radiation from cold streams of gas accreting onto galaxies^17,18, 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.

Lyman-α斑点（LABs）是氢的Lyman-α（Lyα）线中发射的空间扩展星云，在高红移^1,2处可见，最常见于在恒星形成星系^3,4的稠密环境中。最近的一项研究表明，高红移星系^5,6,7周围的Lyα发射几乎覆盖了100％的天空。LABs中Lyα发射的起源尚不清楚，尚有争议⁸。它可以通过涉及银河系超风/外流的光电离作用，来自星爆或活跃的银河系原子核的Lyα光子的共振散射^{9,10,11,12,13,14,15,16}或通过冷却积聚在气体上的冷气流的辐射来驱动星系^17,18，如最近的模拟所示¹⁹。在这里，我们分析了实验室内的气体运动学，为流入的气体提供了罕见的观察证据。这与冷流辐射Lyα光子时释放重力积聚能量是一致的。它还提供了直接的证据，证明可能有冷水流入中央星系。与恒星形成所消耗的气体质量相比，下降气体的质量并不重要，它也不是Lyα发射的主要动力来源，但它暗示了另一种解释年轻银河系周围扩展Lyα发射起源的机制。 。