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Multi-filament gas inflows fuelling young star-forming galaxies


Theory suggests that there are two primary modes of accretion through which dark-matter halos acquire the gas to form and fuel galaxies: hot- and cold-flow accretion. In cold-flow accretion, gas streams along cosmic web filaments to the centre of the halo, allowing for the efficient delivery of star-forming fuel. Recently, two quasar-illuminated H i Lyman ɑ (Lyα)-emitting objects were reported to have properties of cold, rotating structures1,2. However, the spatial and spectral resolution available was insufficient to constrain the radial flows associated with connecting filaments. With the Keck Cosmic Web Imager (KCWI)3, we now have eight times the spatial resolution, permitting the detection of these inspiralling flows. To detect these inflows, we introduce a suite of models that incorporate zonal radial flows, demonstrate their performance on a numerical simulation that exhibits cold-flow accretion, and show that they are an excellent match to KCWI velocity maps of two Lyα emitters observed around high-redshift quasars. These multi-filament inflow models kinematically isolate zones of radial inflow that correspond to extended filamentary emission. The derived gas flux and inflow path is sufficient to fuel the inferred central galaxy star-formation rate and angular momentum. Thus, our kinematic emission maps provide strong evidence that the inflow of gas from the cosmic web is building galaxies at the peak of star formation.

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Data availability

KCWI data on CSO38 and UM287 is publicly available. Data on UM287 will be available 18 months after the observation in Oct 2017. 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

KCWI pipeline code is available on the W. M. Keck Observatory website.


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This work was supported by the National Science Foundation, the W. M. Keck Observatory and the California Institute of Technology. The VELA simulations were performed at NASA Advanced Supercomputing at NASA Ames Research Center. D.C. is funded by the ERC Advanced Grant, STARLIGHT: Formation of the First Stars (project number 339177).

Author information

D.C.M. is the Principal Investigator of KCWI, performed the analysis of the simulated galaxy, data and MFI models, and was principal author on the paper. D.O. and D.C.M. led the observations of UM287 and CSO 38. D.O. and M.M. reduced the data. D.O., M.M. and E.H. contributed to the paper writing. P.M., M.M., D.C.M., J.D.N., D.O. and A.M. designed, constructed and operated KCWI. J.D.N., M.M. and D.C.M. developed the KCWI data pipeline and produced the final data cubes. D.C. developed the VELA simulations. A.D. and S.L. provided the simulated galaxy VELA07 and contributed to the writing of the paper. D.C. and J.P. contributed the cosmological simulation. C.S., R.T., S.C. and J.X.P. contributed to the development of KCWI, to Keck data for the two protogalaxies, and to the editing of the paper. L.R. made major contributions to KCWI commissioning and participated in the observations of CSO 38.

Correspondence to D. Christopher Martin.

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Supplementary Figures 1–11, Supplementary Tables 1–8, Supplementary References 1–3.

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Fig. 1: Surface density and velocity maps of VELA07 simulation in face-on projection.
Fig. 2: Narrow-band image, mean velocity map and velocity dispersion maps simulation and data.
Fig. 3: Velocity maps simulation and data compared to MFI models.
Fig. 4: Face-on reconstruction of column density, radial velocity and radial mass flux for simulated and observed objects.