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Intracluster light is already abundant at redshift beyond unity


Intracluster light (ICL) is diffuse light from stars that are gravitationally bound not to individual member galaxies, but to the halo of galaxy clusters. Leading theories1,2 predict that the ICL fraction, defined by the ratio of the ICL to the total light, rapidly decreases with increasing redshift, to the level of a few per cent at z > 1. However, observational studies have remained inconclusive about the fraction beyond redshift unity because, to date, only two clusters in this redshift regime have been investigated. One shows a much lower fraction than the mean value at low redshift3, whereas the other possesses a fraction similar to the low-redshift value4. Here we report an ICL study of ten galaxy clusters at 1 z 2 based on deep infrared imaging data. Contrary to the leading theories, our study finds that ICL is already abundant at z 1, with a mean ICL fraction of approximately 17%. Moreover, no significant correlation between cluster mass and ICL fraction or between ICL colour and cluster-centric radius is observed. Our findings suggest that gradual stripping can no longer be the dominant mechanism of ICL formation. Instead, our study supports the scenario wherein the dominant ICL production occurs in tandem with the formation and growth of the brightest cluster galaxies and/or through the accretion of preprocessed stray stars.

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Fig. 1: BCG + ICL images of our ten z 1 clusters.
Fig. 2: BCG + ICL radial profiles.
Fig. 3: BCG + ICL radial colour profiles.
Fig. 4: ICL fraction evolution.

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Data availability

The raw HST near-infrared imaging data used for the current study are publicly available. The processed imaging data are available on the github repository at data are provided with this paper.

Code availability

An exhaustive repository of code for our custom data processing and analyses reported in this manuscript are available on the github repository at


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This study is based on observations created with NASA/ESA Hubble Space Telescope and downloaded from the Mikulski Archive for Space Telescope at the Space Telescope Science Institute. The current research is supported by the National Research Foundation of Korea under programme 2022R1A2C1003130 and the Yonsei Future-Leading Research Initiative programme.

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Authors and Affiliations



M.J.J. conceived, designed and supervised the project. M.J.J. and H.J. analysed the Hubble Space Telescope imaging data, developed the pipeline, interpreted the results and wrote the manuscript.

Corresponding author

Correspondence to M. James Jee.

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Nature thanks Emanuele Contini and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 Definition of common sky areas.

(A) Exposure map for the single-frame image. (B) Same as (C) except that it is for the mosaic image. (C) Science image for single frame. (D) Same as (C) except that it is for the mosaic image. The pink circular region in (A) is the region that is observed in common by all contributing frames. (B) shows how this common region is positioned in one of the input frames. As the central region of this circle is likely to be heavily influenced by the ICL, we excluded the central region and instead defined the annulus shown in (C) and (D) to estimate the background level.

Extended Data Fig. 2 Masking size growth and impacts on background level.

(A), (B) and (C) illustrate our scheme for masking size growth from the original to the ce = 2 and ce = 6 cases. Note that we exhaust pixels for ICL measurement at ce = 6. In (D), we show how the background level (green) changes as we vary the masking size using the expansion coefficient for a single exposure. We observe that at ce 6 the measurement converges (red). The black solid line indicates the result when instead we use a 3σ clipping algorithm without considering the diffuse wings of the astronomical objects. The yellow line shows the surface brightness level measured at each ce. (E) is the same as the left except that the measurement is from the final deep stack. Solid lines indicate the median value and shaded regions show the 68% uncertainty. As the image is deeper, the number of pixels discarded (masked out) at the same ce value is much greater.

Extended Data Fig. 3 Schematic diagram of our ICL-oriented data reduction.

Dark grey rectangles show the steps where external packages are used, while light grey rectangles illustrate our custom procedures. Parallelograms represent the input/output data.

Extended Data Fig. 4 Red sequence selection scheme.

Here we display the case for SPT2106. (A) Colour–magnitude diagram. Black dots are all sources detected by SExtractor. The red dots represent the spectroscopic members, whereas the orange dots are our red sequence candidates. The BCG is indicated with a red star. The red dashed line shows the final, best-fit red sequence. The dot-dashed lines bracket the 68% distribution. (B) Distribution of the F105W < 26 object distances from the best-fit red sequence. The green line shows the best-fit double Gaussian models. The yellow line illustrates a single Gaussian component, which represents the distribution of the red sequence candidates.

Extended Data Fig. 5 Comparison between ICL fraction and cluster mass.

The mass comes from weak lensing studies. No significant correlation between ICL fraction and mass is found.

Extended Data Table 1 Target List
Extended Data Table 2 ICL fractions and impact of various systematics
Extended Data Table 3 Weak lensing mass, the number of spectroscopic member galaxies and their references

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Joo, H., Jee, M.J. Intracluster light is already abundant at redshift beyond unity. Nature 613, 37–41 (2023).

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