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# Galactic bulge preferred over dark matter for the Galactic centre gamma-ray excess

## Abstract

An anomalous gamma-ray excess emission has been found in the Fermi Large Area Telescope data1 covering the centre of the Galaxy2,3. Several theories have been proposed for this ‘Galactic centre excess’. They include self-annihilation of dark-matter particles4, an unresolved population of millisecond pulsars5, an unresolved population of young pulsars6, or a series of burst events7. Here, we report on an analysis that exploits hydrodynamical modelling to register the position of interstellar gas associated with diffuse Galactic gamma-ray emission. We find evidence that the Galactic centre excess gamma rays are statistically better described by the stellar over-density in the Galactic bulge and the nuclear stellar bulge, rather than a spherical excess. Given its non-spherical nature, we argue that the Galactic centre excess is not a dark-matter phenomenon but rather associated with the stellar population of the Galactic bulge and the nuclear bulge.

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## Acknowledgements

R.M.C. was the recipient of an Australian Research Council Future Fellowship (FT110100108). S.H. is supported by the U.S. Department of Energy, Office of Science, under award number de-sc0018327. We thank D. Lang for making available code and data that helped with generating the X-bulge template and both S. Nishiyama and K. Yasui for providing the data for the nuclear bulge template. We acknowledge the use of public data and software from the Fermi data archives (http://fermi.gsfc.nasa.gov/ssc/). Finally, the authors would also like to thank F. Aharonian, A. M. Brown, F. Calore, J.-M. Casandjian, H. T. Cromartie, S. Digel, T. Enßlin, M. Kaplinghat, K. Freeman, O. Gerhard, O. Gnedin, X. Huang, N. McClure-Griffiths, D. Nataf, H. Ploeg, B. Roberts, M. Winter, R. Tuffs and G. Zaharijas for enlightening discussions.

## Author information

Authors

### Contributions

O.M. designed and performed the majority of the data analysis. O.M. also constructed the Fermi bubbles, Sun, Moon, inverse Compton and Loop I templates. C.G. processed the WISE data and derived the mixture distribution formulas. R.M.C. suggested the link with the X-bulge. B.C. processed the hydrodynamical 3D map into annuli density maps. D.P. created the interpolated annuli density maps. C.G. and S.H. assisted with the point source modelling. B.C. and D.P. created the dust maps. M.P. created the 3D HI and CO maps. All authors contributed to the interpretation of the results. The text of the final manuscript was mainly written by O.M. and C.G., but all authors had some contribution.

### Corresponding author

Correspondence to Oscar Macias.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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## Supplementary information

### Supplementary Information

Supplementary Sections 1–3, Supplementary Figures 1–10, Supplementary Tables 1–4

### Supplementary Dataset 1

FITS version of Supplementary Table 1

## Rights and permissions

Reprints and Permissions

Macias, O., Gordon, C., Crocker, R.M. et al. Galactic bulge preferred over dark matter for the Galactic centre gamma-ray excess. Nat Astron 2, 387–392 (2018). https://doi.org/10.1038/s41550-018-0414-3

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• DOI: https://doi.org/10.1038/s41550-018-0414-3

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