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Extensive diffuse Lyman-α emission correlated with cosmic structure

Nature Astronomy volume 7, pages 1390–1401 (2023)Cite this article

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Abstract

The intergalactic medium represents the dominant reservoir of baryons at high redshift, traces the architecture of the cosmic web dominated by dark matter and fuels ongoing galaxy evolution. The intergalactic medium has been studied using the absorption lines of quasi-stellar objects, including the Lyman-α forest, but these absorption lines are unable to provide the information that emission maps would give. However, because of the low surface brightness and extended, diffuse distribution, direct detection of an emission equivalent to the absorption Lyman-α forest has not been possible with existing instrumentation and observational approaches. Using a purpose-built instrument, with nod-and-shuffle and dual-field subtraction, we have detected an emission Lyman-α forest. The emission forest is highly extended, shows filamentary morphology with filaments connecting galaxies, exhibits statistics like the absorption Lyman-α forest, displays spectra resembling the absorption forest and is correlated with galaxy-traced overdensities consistent with bias like dark matter. We conclude that the emission Lyman-α forest may provide a new tool for tracing a substantial fraction of the cosmic web of baryons and dark matter.

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Fig. 1: The emission Lyα forest at multiple redshifts in two fields.
Fig. 2: Intensity statistics for combined fields A and B.
Fig. 3: The emission Ly-α forest.
Fig. 4: Spatial galaxy–emission cross-correlation functions in 1D and 2D for combined A and B fields.
Fig. 5: Synoptic view of the IGM in ELAF and ALAF.

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Data availability

The observational data used in this paper are available on the Keck Observatory Archive (https://www2.keck.hawaii.edu/koa/public/koa.php).

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Acknowledgements

Resources supporting this work were provided by NSF AAG Grant 1716907 and the California Institute of Technology. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The observatory was made possible by the generous financial support of the W. M. Keck Foundation. We recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the Indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

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Authors and Affiliations

  1. Cahill Center for Astrophysics, California Institute of Technology, Pasadena, CA, USA

    D. Christopher Martin, Behnam Darvish, Zeren Lin, Mateusz Matuszewski & James D. Neill

  2. Department of Astrophysical Sciences, Princeton University, Princeton, NJ, USA

    Renyue Cen

  3. Jet Propulsion Laboratory, Pasadena, CA, USA

    Patrick Morrissey

  4. Research School of Astronomy and Astrophysics, Australian National University, Canberra, Australian Capital Territory, Australia

    Anna M. Moore

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Contributions

D.C.M. is the principal investigator of the KCWI, conceived of and designed the observing strategy, performed 75% of the data analysis and wrote the main paper. B.D. provided the targeting strategy, and contributed to the observations, paper writing and editing. Z.L. led most of the observations and contributed to the data analysis and paper. M.M. was instrument scientist on the KCWI and provided editing support to the paper. R.C. provided the numerical modelling that supported the observation planning. J.D.N. provided the data reduction pipeline for KCWI. P.M. was the technical lead on the KCWI. A.M.M. was the project manager for the KCWI.

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Correspondence to D. Christopher Martin.

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Extended data

Extended Data Fig. 1 Forwarded modeled numerical simulation of Lyα emission from the general cosmic web at z ~ 3 predicted that the KCWI observation would detect emission.

Upper spectrum is sum of enlarged 2 × 2 arcmin2 difference field (source-background) model emission, middle spectrum is observed spectrum with KCWI, parameters discussed in the text, with 1 LU = 1.2 × 10−22 erg cm−2 s−1 arcsec−2. Four narrow-band images are shown for the colored wavelength intervals, upper shows with no noise, lower is adaptively smoothed image1,2 based on simulated observed KCWI data cube, with full-scale comparable to spectrum. Ordinate is average surface brightness (〈SBλ〉) over displayed image.

Extended Data Fig. 2 Tiling overlay of Fields A (top) and B (bottom) at position angles PA=0∘ (left) and PA=90∘ (right).

An array of 6 overlapping KCWI Large slicers, covering an area of ~ 60 × 60 arcsec2. The image cutouts are from the COSMOS HST/F814w data.

Extended Data Fig. 3 Raw, smoothed, exposure, and variance images.

a. Raw image slice 3910-3920Å, smoothed to 20 pixels or 5.8 arcsec. b. Adaptively smoothed image slice. Note that adaptively smoothing reduces the extended emission levels because bright compact regions above the noise threshold are removed. c. Exposure time (top scale gives seconds, bottom seconds times Δλ. d. Variance cube slice.

Extended Data Fig. 4 Cross correlation between ELAF and ALAF.

Estimated 1D cross-correlation function between Field B ELAF and the ALAF present in the field QSO. Errors estimated by bootstrapping both the absorption and emission line data.

Extended Data Fig. 5 Filament plots.

Six 3Å narrow-band images showing filament detection locations. Note that most galaxies are connected by filaments.

Extended Data Fig. 6 Filament plots vs. threshold.

Five 3Å narrow-band images showing filament detection locations vs. surface brightness threshold. Filament lengths and interconnectivity decrease with increasing surface brightness threshold.

Extended Data Fig. 7 Field A overdensity at z=2.23.

Mosaic of 16 3Å slices covering the z ~ 2.23 overdensity. Each central wavelength and corresponding Lyα redshift is shown, and galaxies are displayed for which Lyα falls within 10Å of the slice. Most of the galaxies are embedded in emission at a corresponding redshift, and the majority have emission spanning multiple galaxies at a given redshift.

Extended Data Fig. 8 Narrow-band slices from LAH Model.

LAHs were placed randomly using simulation following density structures in the cosmic web. Two 8Å slices shown from top to bottom. On left, the input surface brightness distribution. On right, results after adpative smoothing using the observed KCWI variance cube. Bright cores of the brighter LAHs are detected, but only part of the halo of the brightest LAH. There is no signal from smoothed out faint halos, even though they are distributed in overdense filamentary structures. The derived voxel volume distribution is given as the grey line (LAH) in Fig. 5.

Extended Data Fig. 9 Illustration of Continuum Photon Pumping Process.

a. With a single illumination source, IGM cloud absorbs in Lyα for observer A, and emits due to resonance scattering in the Lyα into the field of view of observer B. b. With a distribution of line and continuum illumination sources, observer A again sees absorption, which observer B, with illuminating sources resolved and therefore no illuminating sources in the field of view, sees an emission line. c. With a large number of faint illuminating sources, sufficiently numerous that they occupy the B field of view with the average metagalactic surface brightness, the absorption from the cloud exactly compensates for the scattered emission, and no line is observed. d. With a diffuse, uniform source of line and continuum emission the emission and absorption exactly cancel. In this paper, the scenario in panel b. pertains since the majority of the illuminating sources are resolved out and excluded from the emission measurement.

Extended Data Fig. 10 QSO absorption line spectrum converted to emission line spectrum.

a. Field B QSO absorption line spectrum converted to emission line spectrum following Fig. 3. Panels b-e show emission from individual regions near QSO, with vertical lines indicating features seen in the QSO absorption line spectrum which have counterparts (within 2Å) in the emission spectra. Line colors alternate for clarity.

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Martin, D.C., Darvish, B., Lin, Z. et al. Extensive diffuse Lyman-α emission correlated with cosmic structure. Nat Astron 7, 1390–1401 (2023). https://doi.org/10.1038/s41550-023-02054-1

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