Where are the missing baryons? Qiang Wang (Brandon) B.Sc. Supervisor: Dr Scott Kay School of Physics and Astronomy, University of Manchester 27/29 June 2018 Image from Millennium Simulation 1
Content I. Introduction II. Simulations III. Results IV. Conclusions 2
I. Introduction The theoretical prediction of baryon production is based on the Big Bang Nucleosynthesis (BBN). The CMB provides another independent measure. Image taken from slide by C.S. Frenk 3
Missing baryon problem Planck 2013 result Shull, Smith & Danforth 2012 A substantial fraction of baryons are missing! 4
Warm-Hot Intergalactic Medium (WHIM) Numerical simulations of the intergalactic medium have shown that at the present epoch, a significant fraction (40% 50%) of the baryonic component should be found in the ( T~10 6 K ) warm-hot intergalactic medium (WHIM). (Cen & Ostriker 2006) 5
II. Simulations Cosmological hydrodynamic simulations were carried out using GADGET-2. Dark matter N-body; Baryons SPH (smoothed particle hydrodynamics). A few simplifications were made i.e. no galaxy formation or complicated astrophysical processes. Simulation L [h 1 Mpc] N [DM + gas] ε [h 1 kpc] m baryon [h 1 M ] Ω m Ω Λ n s Ω b H 0 σ 8 z start S1 50 64 3 + 64 3 781 6.4 10 9 S2 50 128 3 + 128 3 390 8.1 10 8 S3 100 64 3 + 64 3 1562 5.2 10 10 S4 100 128 3 + 128 3 781 6.4 10 9 0.315 0.685 0.960 0.0487 67.3 0.829 50 Planck 2013 results Computing power was provided by Microsoft Azure (4 Intel Xeon E5-2673 v4 processors). S4 run took ~13 hours. 6
GADGET-2 code by Volker Springel https://wwwmpa.mpa-garching.mpg.de/gadget/ From INSPIRE database 7
III. Results Movie I shows the evolution of WHIM colour-coded by density. https://youtu.be/u7hzfxcuc7e Movie II shows the evolution of baryons colourcoded by temperature. https://youtu.be/ti_avc66q4q Both are made from S4 run. 8
3D distribution S1 Run L = 50 h 1 Mpc N = 64 3 DM WHIM Hot Warm Baryon phases Hot: T > 10 7 K WHIM: 10 5 K < T < 10 7 K Warm: T < 10 5 K 9
3D distribution S2 Run L = 50 h 1 Mpc N = 128 3 DM Hot WHIM Warm 10
3D distribution S3 Run L = 100 h 1 Mpc N = 64 3 DM Hot WHIM Warm 11
3D distribution S4 Run L = 100 h 1 Mpc N = 128 3 DM Hot WHIM Warm WHIM follows dark matter and mainly resides in the cosmic web. Cen & Ostriker 2006 12
Evolution of baryons Warm WHIM Galaxies Hot Cen & Ostriker 2006 About half of the baryons exist as WHIM at present day. 13
Formation of WHIM S4 run Cen & Ostriker 2006 Closely connected to the gravitational growth of large-scale structure. Shock waves are produced when two collapsing perturbation meet and cross one another. Shock heating plays the major role in the formation of WHIM. 14
Phase diagram Davé et al. 2001 Cen & Ostriker 2006 It should be easier to observe WHIM at the high-temperature end as it is both hotter and denser. 15
Mass distribution Cen & Ostriker 2006 Most of the mass of WHIM resides in the moderate overdensity range. 16
Observational implications Ultraviolet Observation X-ray Observation Sunyaev-Zel dovich (SZ) effect SZ effect Bregman 2007 17
Observational efforts Graaff et al. 2017. Submitted to Nature. Also a similar paper by Tanimura et al. 2017 with similar conclusion. 18
Newest paper by Nicastro et al. 2018 19
IV. Conclusion The missing baryon problem can be resolved with a cosmologically important component called warm-hot intergalactic medium (WHIM). The existence of WHIM is theoretically firm accounting for about half of the baryonic content at present day. The primary mechanism of formation is shock heating which results in a moderate temperature and overdensity range. The detection of WHIM proposes significant difficulties. However, a bright future can be foreseen with the rapid development of observational techniques. 20
Special thanks to my supervisor Scott Kay. References: Cen, R. & J. P. Ostriker 1999, The Astrophysical Journal, 514, 1. Cen, R. & J. P. Ostriker 2006, The Astrophysical Journal, 650, 560. Davé, R., et al. 2001, The Astrophysical Journal, 552, 473. Fukugita, M., C. J. Hogan & P. J. E. Peebles 1998, The Astrophysical Journal, 503, 518. de Graaff, A., Y.-C. Cai, C. Heymans & J. A. Peacock 2017, arxiv:1709.10378 Nicastro, F., et al. 2005, Nature, 433, 495 498 Nicastro, F., et al. 2018, Nature, 558, 406 409 Planck Collaboration 2014, Astronomy and Astrophysics, 571 Shull, J. M., B. D. Smith & C. W. Danforth 2012, The Astrophysical Journal, 759, 23. Steven, V. P., T. S. John & J. M. Shull 2004, The Astrophysical Journal Supplement Series, 152, 29. Tanimura, H., et al. 2017, ArXiv e-prints, 1709, arxiv:1709.05024. Slides by Carlos Frenk is available at http://starwww.dur.ac.uk/~csf/talks/talks_2017/new_building_opening.pptx GADGET-2 is available at https://wwwmpa.mpa-garching.mpg.de/gadget A special webpage has been created to host all visualisation materials, which is available at http://qwang.org/baryon Image from Millennium Simulation