Abstract
Galactic outflows are believed to play a critical role in the evolution of galaxies by regulating their mass build-up and star formation1. Theoretical models assume bipolar shapes for the outflows that extend well into the circumgalactic medium (CGM), up to tens of kiloparsecs (kpc) perpendicular to the galaxies. They have been directly observed in the local Universe in several individual galaxies, for example, around the Milky Way and M82 (refs. 2,3). At higher redshifts, cosmological simulations of galaxy formation predict an increase in the frequency and efficiency of galactic outflows owing to the increasing star-formation activity4. Galactic outflows are usually of low gas density and low surface brightness and therefore difficult to observe in emission towards high redshifts. Here we present an ultra-deep Multi-Unit Spectroscopic Explorer (MUSE) image of the mean Mg II emission surrounding a sample of galaxies at z ≈ 1 that strongly suggests the presence of outflowing gas on physical scales of more than 10 kpc. We find a strong dependence of the detected signal on the inclination of the central galaxy, with edge-on galaxies clearly showing enhanced Mg II emission along the minor axis, whereas face-on galaxies show much weaker and more isotropic emission. We interpret these findings as supporting the idea that outflows typically have a bipolar cone geometry perpendicular to the galactic disk. We demonstrate that this CGM-scale outflow is prevalent among galaxies with stellar mass M* ≳ 109.5M⊙.
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Data availability
This work is mainly based on data release 2 (DR2) of the MUSE Hubble Ultra Deep Field surveys. The reduced MUSE datacubes are available in ref. 22.
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Acknowledgements
Y.G. thanks Z. Xu for helpful discussions. Y.G., R.B. and L.W. acknowledge support from the ANR/DFG grant L-INTENSE (ANR-20-CE92-0015, DFG Wi 1369/31-1). L.W. acknowledges support by the European Research Council (ERC) Advanced Grant SPECMAG-CGM (GA101020943). A.V. and H.K. acknowledge support from the Swiss National Foundation grant PP00P2_211023. S.C. gratefully acknowledges support from the ERC under the European Union’s Horizon 2020 Research and Innovation programme grant agreement no. 864361. L.B. acknowledges support by ERC AdG grant 740246 (Cosmic-Gas). J.Brinchmann acknowledges financial support from the Fundação para a Ciência e a Tecnologia (FCT) through research grants UIDB/04434/2020 and UIDP/04434/2020, national funds PTDC/FIS-AST/4862/2020 and work contract 2020.03379.CEECIND. N.F.B. acknowledges support from the ANR grant 3DGasFlows (ANR-17-CE31-0017).
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Y.G. conceived the project. R.B. led the MUSE data acquisition and data reduction. All authors participated in the observation and/or data reduction of the MUSE Hubble Ultra Deep Field surveys. Y.G. performed the sample selection and analysed the data. Y.G., R.B., N.F.B., L.W., J.S., J.Blaizot and S.C. worked on the interpretation of the results. Y.G. wrote the manuscript and produced the figures, with R.B., N.F.B., L.W. and J.Blaizot contributing to their design. All co-authors provided critical feedback on the text and helped shape the manuscript.
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Extended data figures and tables
Extended Data Fig. 1 Distribution of the redshifts and stellar masses in the MUSE sample.
The parent sample is shown by grey bars and the edge-on and face-on subsamples are shown in black and red, respectively.
Extended Data Fig. 2 HST images of all the face-on galaxies.
Each thumbnail has the same size as in Fig. 1.
Extended Data Fig. 3 HST images of all the edge-on galaxies.
Each thumbnail has the same size as in Fig. 1. The dashed and dotted lines show the major and minor axes of the galaxies, respectively.
Extended Data Fig. 4 The surface brightness of the Mg II outflow from each edge-on galaxy.
The distributions of surface brightness (a) and S/N (b). The signals are extracted in 1″-diameter apertures above and below the edge-on galaxies, at a distance to the galactic plane of 1″. The distribution of surface-brightness signals skews towards positive values, despite most of the signals being of low S/N. The negative S/N values correspond to the negative signals in the left panel.
Extended Data Fig. 5 The spectra of the extended Mg II emission.
The spectra in each panel are extracted from the grid cell at the corresponding position in Fig. 1. The black and red spectra denote the edge-on and face-on galaxy samples, respectively. In each panel, the coloured shading represents the 1σ error range of the corresponding spectra. The panels for which the peak of the Mg II 2,796 Å line in the black spectra falls below the 2σ threshold are marked with a lighter colour.
Extended Data Fig. 6 The continuum-subtracted spectrum of the ‘ring’ in face-on galaxies.
The red line denotes the spectrum extracted from the ‘ring’ region of the stacked face-on galaxy sample. For comparison, we also show the continuum-subtracted spectrum from the ‘outflow’ region of the edge-on galaxy. The coloured shadings represent the 1σ error range of the corresponding spectra. The two vertical dashed lines indicate the wavelength of Mg II doublets. The horizontal shadow shows the noise level. The EWs of the Mg II 2,796 Å line for the red and black spectra are −21 ± 15 Å and −56 ± 22 Å, respectively.
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Guo, Y., Bacon, R., Bouché, N.F. et al. Bipolar outflows out to 10 kpc for massive galaxies at redshift z ≈ 1. Nature 624, 53–56 (2023). https://doi.org/10.1038/s41586-023-06718-w
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DOI: https://doi.org/10.1038/s41586-023-06718-w
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