1. Eugenio Ursino on behalf of the UM Astrophysics Group University of Miami, USA Looking for the Missing Baryons

2. OUTLINE  The “missing baryons” problem  The solution from simulations  Observations: the baryon census and difficulties  Our take at the missing baryons  Conclusion 2

3. The high redshift Universe

4. The low redshift Universe

5. Cosmological simulations From isotropy to anisotropy

6. Cosmological simulations At present time about half of the baryons are in the Warm-Hot Intergalactic Medium (WHIM), a highly ionized gas characterized by strong emission lines. Simulations tell us how the baryons evolve.

7. Strategies to find the missing baryons According to thermal models the WHIM emits and absorbs EUV and Soft X-rays.

8. UV observations Danforth et al. 2010 Spectra of distant quasars show UV absorption features from intervening WHIM absorbers (Broad Ly-α, C iv, N v, O vi, Si iv)

9. The Baryon Census Shull et al. 2012

10. X-ray observations O vii and O viii absorbers in the X-ray band should make it possible to trace the remaining missing baryons. There is only a handful of WHIM X-ray absorbers detected. Not enough for reliable estimates. The existing WHIM detections are controversial. The statistical significance is often debated. The absorber could be galaxies and not WHIM filaments. There are very few lines of sight that can be exploited.

11. The Diffuse X-ray Background NH Solar Wind Charge Exchange (SWCX): lines from several ions ~40 AU ~100 pc ~2-50 kpc ~z<1 Local hot Bubble: ~10 6 K plasma Galactic Halo/ CircumGalactic Halo: ~3x10 6 K plasma WHIM Unresolved point sources: power law NH The WHIM contribution to the DXB is less than 10% (in the 0.4-1.0 keV band). It is difficult to isolate.

12. The WHIM and the DXB NH Next generation telescopes will be able to resolve emission lines from WHIM filaments. -WHIM -Galactic foreground and unresolved point sources

13. The WHIM and the DXB NH Existing telescopes cannot resolve emission lines. We need a better strategy to identify filaments.

14. WHIM filaments between Clusters NH Mitsuishi et al. 2012 Existing telescope (like Suzaku) could resolve lines from the strongest filaments, located between clusters. -On-Filament -Offset – 1° -Offset – 4°

15. WHIM filaments between Clusters NH The On-filament excess is due to unresolved point sources. Extra galaxies, identified with the Chandra X-ray telescope, that trace the filament. Ursino et al. 2015

16. The Autocorrelation Function A different approach is to identify the WHIM using statistical methods: the AutoCorrelation Function (AcF). On angular scales typical of the f.o.v. of an X-ray telescope (~30 arcmin), the WHIM is the only DXB component expected to have and AcF that shows angular dependence (<10 arcmin). All other components are constant (with the exception of point sources, but we remove them). Galeazzi et al. 2009

17. The Autocorrelation Function Improved analysis using the XMM-CDFS (3 Ms). With long exposure it is easier to remove point sources. Comparing the CcF in the 0.4- 0.6 keV and 0.7-0.9 keV bands we can infer the average temperature density of the WHIM.

18. The Future Next generation telescopes (Athena +, 2028) will allow us to resolve individual filaments. The SZ effect and the WHIM. With the SZ we can probe higher redshift.

19. Conclusion In the nearby Universe we can see only half of the baryons present at the time of last recombination. According to simulations, the missing baryons in the WHIM. UV observations identified part of the WHIM (the coldest phase). The warmest phase of the WHIM should be detected using X-rays. So far it has been almost impossible to observe it. Using the AcF we observe the WHIM emission. It contributes to ~10% of the DXB in agreement with simulations.

20. The “local” missing baryons