Every star-forming galaxy has a halo of metal-enriched gas that extends out to at least 100 kiloparsecs1,2,3, as revealed by the absorption lines that this gas imprints on the spectra of background quasars4. However, quasars are sparse and typically probe only one narrow beam of emission through the intervening galaxy. Close quasar pairs5,6,7 and gravitationally lensed quasars8,9,10,11 have been used to circumvent this inherently one-dimensional technique, but these objects are rare and the structure of the circumgalactic medium remains poorly constrained. As a result, our understanding of the physical processes that drive the recycling of baryons across the lifetime of a galaxy is limited12,13. Here we report integral-field (tomographic) spectroscopy of an extended background source—a bright, giant gravitational arc. We can thus coherently map the spatial and kinematic distribution of Mg ɪɪ absorption—a standard tracer of enriched gas—in an intervening galaxy system at redshift 0.98 (around 8 billion years ago). Our gravitational-arc tomography unveils a clumpy medium in which the absorption strength decreases with increasing distance from the galaxy system, in good agreement with results for quasars. Furthermore, we find strong evidence that the gas is not distributed isotropically. Interestingly, we detect little kinematic variation over a projected area of approximately 600 square kiloparsecs, with all line-of-sight velocities confined to within a few tens of kilometres per second of each other. These results suggest that the detected absorption originates from entrained recycled material, rather than in a galactic outflow.
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This work has benefitted from discussions with A. Smette, N. Nielsen and G. Kacprzak. S.L. thanks the European Southern Observatory Scientific Visitor Selection Committee for supporting a research stay at the ESO headquarters in Santiago, where part of this work was done. S.L. has been supported by FONDECYT grant number 1140838. This work has also been partially supported by PFB-06 CATA. N.T. acknowledges support from CONICYT PAI/82140055.
The authors declare no competing financial interests.
Reviewer Information Nature thanks H.-W. Chen and the other anonymous reviewer(s) for their contribution to the peer review of this work.
Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Extended data figures and tables
a, Three-dimensional representation of the signal-to-noise ratio (S/N) at the position of Mg ɪɪ absorption in the unbinned data. b–d, Same as a but in two dimensions and for different binnings. The size of each binned spaxel is indicated in arcseconds; the colour scale is the same for all three panels. Note the expected increase in signal-to-noise ratio with binning size.
a, Gaussian fits to the [O ɪɪ] λ = 3,726 Å, λ = 3,729 Å doublets in the MUSE spectra of G1, G2 and G3, the three [O ɪɪ] sources found by our systematic search. The MUSE spatial resolution barely resolves G1 into three [O ɪɪ] clumps (G1-A, G1-B and G1-C), which cluster around zG1 = 0.98235 and have a velocity dispersion of 35 km s−1. b, MUSE image of RCSGA 032727−132623 centred on [O ɪɪ] emission at z = 0.98. The magenta squares indicate the binned spaxels used to map the Mg ɪɪ absorption against the arc. c, HST/WFC3 F814W image zooming into the G1 system. The blue squares indicate the MUSE regions used to extract the [O ɪɪ] spectra. The scale corresponds to the region close to G1 in the absorber plane.
In the absorber plane (dashed rectangle), the spaxel configuration appears shrunked and the de-lensed spatial elements have different shapes and areas across the absorber plane. After de-lensing, the scale in the absorber plane is given by the adopted cosmology: 5″ = 39.85 kpc at z = 0.98. The impact parameter used here is defined as the projected physical distance between a given position and G1 on this plane. For reference, a 5″ scale bar is shown in the image plane. Coordinates (α, right ascension; δ, declination) are in arcseconds relative to the position of G1 in the image plane.
a, Cumulative distribution of absorption strengths for two different binnings. b, Same as a but for velocities.
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Lopez, S., Tejos, N., Ledoux, C. et al. A clumpy and anisotropic galaxy halo at redshift 1 from gravitational-arc tomography. Nature 554, 493–496 (2018). https://doi.org/10.1038/nature25436