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 galaxies5,6,7. 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 nuclei9,10,11,12,13,14,15,16, 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.

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Fig. 1: ALMA and X-shooter data in LAB6.
Fig. 2: Line profiles from molecular and ionized gas in LAB6.

Data availability

All the raw data used in this study are available at the ALMA Science Archive ( under programme ID 2015.1.00952.S, at the ESO Science Archive Facility ( under programme IDs 082.A-0846 and 297.A-5059, and at the NASA/IPAC Infrared Science Archive ( under programme ID GO-3699. The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

Code availability

To fit the Lyα line profile, we developed our own code by adopting a simple contracting model that inflating gas is uniformly located in a thin shell of neutral gas and that Lyα radiation from the central source is scattered by the hydrogen atom in the shell. Codes used in this study are not publicly released yet but are available from the corresponding author on reasonable request.


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Y.A. acknowledges financial support by NSFC grants 11988101, 11933011 and 11373007. J.E.G. is supported by a Royal Society University Research Fellowship. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2015.1.00952.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. This research was based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme IDs 297.A-5059(A) and 082.A-0846(B). This research has made use of the SVO Filter Profile Service ( with support from the Spanish MINECO through grant AyA2014-55216.

Author information




Y.A. is the principal investigator of the ALMA and VLT/X-shooter observing proposals. Y.A. reduced the data and wrote the initial manuscript. Z.Z. conducted the data analysis with the SED modelling, drafted the interpretation and discussion of the Lyα emission and polished the manuscript. S.N. and Z.Z. performed the Lyα spectrum fitting with an infalling shell model. C.H. helped to polish the manuscript. All authors discussed and commented on the manuscript.

Corresponding author

Correspondence to Yiping Ao.

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Peer review information Nature Astronomy thanks Aaron Smith and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary Figs. 1–5 and Table 1.

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Ao, Y., Zheng, Z., Henkel, C. et al. Infalling gas in a Lyman-α blob. Nat Astron 4, 670–674 (2020).

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