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From First Stars to First Galaxies: Simulating Cosmic Reionization
- 1. From First Stars to First Galaxies to the Reioniza3on of the Universe: 20 Years of Computa3onal Progress Michael L Norman Director, San Diego Supercomputer Center Dis8nguished Professor of Physics UC San Diego
- 2. What we have explored computa8onally
- 3. This talk is about the first generation of galaxies aka primeval galaxies aka protogalaxies • How they formed? • When they formed? • What they were like? • Why are they important?
- 4. Don’t we have observational answers? • Yes and no • High-‐z HUDF galaxies are the 8p of the iceberg • What we can see are the biggest and brightest galaxies of their era • We are interested in what came before
- 5. Inferences from Reionization • Observed high-‐z galaxies don’t provide all the ionizing photons needed to explain reioniza8on by z=7 (Robertson+ 2015) • What does the faint end of the LF look like? Bouwens+ 2014
- 6. Protogalaxies: An artist’s impression A. Schaller, STScI Yikes! Can we possibly simulate that?!! Answer: YES!
- 7. Peak Speed: 13.4 Petaflops Total memory: 1.5 Petabyte Processor cores: 362,240 NCSA, University of Illinois
- 8. Bryan et al. 2014, ApJS, 211, 19 hdp://enzo-‐project.org Our analysis/viz tool: Turk et al. 2011, ApJS, 192, 9 hdp://yt-‐project.org Powerful software (open source), developed over 22 years Our simula8on tool:
- 9. Protogalaxies: A big supercomputer simulation with lots of physics • How they form • When they form • What they are like • Whether they contribute the missing photons to reionize the universe (spoiler: YES) With these simula8ons we have discovered
- 10. Plan for the talk 1. Numerical cosmology 101 2. From primordial density fluctua8ons to the first stars 3. From the first stars to the first galaxies 4. Simula8ng the first galaxy popula8on 5. From the first galaxies to cosmic reioniza8on
- 12. The universe at 380,000 yr ABB ini)al condi)ons for my simula)ons Fluctua8ons have a well-‐ measured power spectrum Comoving volume
- 13. Computing the Universe: Numerical Cosmology • Transformation to comoving coordinates x=r/a(t) a(t1) a(t2) a(t3) • Triply-periodic boundary conditions Input observed fluctua8ons Time step the laws of physics in a computer program Cosmic web
- 14. Equations of Cosmological Hydrodynamics Mass cons: Mom cons: Energy cons: Species cons: 2-‐body reac8ons photo-‐dissoc./ioniza8on Mul8species gas dynamics Newton’s law: Poisson eq. Dark mader dynamics Friedmann eq. for scale factor a(t) Metric
- 15. “HALOS” Gravita8onally bound mixtures of baryons and dark mader Where stars and galaxies form Evolution of gas density (c) Brian O'Shea (MSU) and the Enzo Collabora8on, 2015
- 16. FROM LINEAR FLUCTUATIONS TO THE FIRST STARS
- 17. Forming the First Stars (Population III) • The first genera8on of stars condense from pris8ne H and He gas in very small dark mader halos beginning about 100 Myr amer big bang (Abel, Bryan & Norman 2000, 2002; Bromm et al. 2002) • Rota8onal transi8ons in H2 molecule is the dominant radia8ve cooling mechanism that permits the gravita8onally bound cloud to condense to a star
- 18. Key physics: gravitational instability • What T and ρ to use? • Peebles & Dicke (1968) suggested using mean values at recombina8on Jeans mass MJ = 106 Ms Globular cluster
- 19. Key physics: catalytic formation of H2 • Process starts with residual electrons amer recombina8on H + e- è H- + hν H- + H è H2 + e- H + H+ è H2 + + hν H2 + + H è H2 + H Channel 1 Channel 2
- 20. • Solution: Semi-implicit rate solver (Anninos et al. 1997) Computational difficulties and solutions • Difficulty: noneq. primordial gas chemistry
- 21. Computational difficulties and solutions • Difficulty: Vast range of spa8o-‐temporal scales Gas cloud protostar Space: 1010 Time: 1012
- 22. Computational difficulties and solutions • Solu8on: recursive adap8ve mesh refinement and hierarchical 8me-‐stepping (Bryan & Norman 1997)
- 23. Evolution of grid refinements (c) Brian O'Shea (MSU) and the Enzo Collabora8on, 2015
- 24. L/Δxmin=1010 density temperature 600 pc 60 pc 6 pc self-gravitating core Abel, Bryan & Norman 2002 100x mass of sun
- 25. 924 cita8ons and coun8ng
- 26. Findings and Implica8ons Abel, Bryan & Norman (2002; Science) • First stars begin forming about 100 Myr ABB • First stars are massive: >100 x mass of Sun • First stars form in isola8on (one per pregalac8c clump) • They will be extraordinarily luminous but only live for a few million years • They will explode as supernovae, and seed the universe with heavy elements (C, N, O, Ca, Si, Fe…..) • They will produce first stellar mass black holes • èThere should be no first stars around today
- 27. A deluge of papers followed
- 28. FROM FIRST STARS TO THE FIRST GALAXIES
- 29. Wait! You skipped a step • Precise mass of final star is currently not computable • But we can es8mate mass from amount of collapsing gas and accre8on rate • We have good Pop III stellar models which give their life8mes, luminosi8es, and fates as a func8on of mass (Heger & Woosley 2002, Shaerer 2003) • We parameterize our ignorance with a primordial ini8al mass func8on (PIMF)
- 30. Assembly of the first galaxies • First galaxies assemble from gravita8onal merger of lower mass halos that previously contained first stars and were processed by their radia8ve, chemical, and kine8c feedback • A typical “first galaxy” will incorporate O(10-‐100) such systems • Need to simulate a larger volume with “AMR everywhere”, tracking the detailed star forma8on and feedback
- 31. Assembly of the first galaxies • Key physics: hierarchical structure forma8on driven by dark mader clustering • Key computa8onal physics: subgrid models for star forma8on and feedback (Pop III stars and metal-‐enriched star clusters) Lacey & Cole 1993
- 32. Forming a Numerical Star/Star Cluster Wise & Abel 2008; Wise et al. 2012a Z>10-‐4 Metal enriched star cluster Test for collapse Test for metallicity Pop III star PIMF Salpeter Life8mes, yields, endpoints Time-‐dependent feedback yes no Create star par3cle Feedback & Pop III remnants
- 33. Assembly of the first galaxies • Key physics: transport of ionizing and photodissocia8ng radia8on from young massive stars • Key computa8onal physics: adap8ve ray tracing (Abel & Wandelt 2002; Wise & Abel 2011)
- 34. Population III Star Formation Fireworks (Cox/Patterson, NCSA)
- 35. Birth of a Galaxy Wise et al. 2012a,b; 2014 (c) John Wise (GIT) and the Enzo Collabora8on, 2012
- 37. Galaxy counts (luminosity fcn.) Volume averaged star forma3on history
- 38. STATISTICS OF THE FIRST GALAXIES: THE RENAISSANCE SIMULATIONS
- 39. The Renaissance Simulations 40 cMpc Each zoom-‐in region is 200 8mes the volume of the “First Galaxy” simula8on but is simulated at nearly equivalent resolu8on è Massive amounts of computer power required (~50 M cpu-‐hrs)
- 40. Peak Speed: 13.4 Petaflops Total memory: 1.5 Petabyte Processor cores: 362,240 NCSA, University of Illinois
- 41. A virtual tour of the Renaissance Simulation (Cox/Patterson, NCSA)
- 42. How do the first galaxies form? Ans: Mergers and Acquisi8ons z=25 First stars First galaxies 8me z=15
- 43. When did the first galaxies form? Ans: Immediately amer the first stars
- 44. What are they like? Sta8s8cs from ~3000 halos Stellar mass v. halo mass UV luminosity v. halo mass O’Shea et al. (2015)
- 45. How Many Are There? UV Luminosity Func8on of First Galaxies Faintest galaxy Hubble can see Faintest galaxy JWST can see O’Shea et al. (2015) Brighter Fainter Numerous faint galaxies dominate photon budget for reioniza8on
- 46. FROM FIRST GALAXIES TO COSMIC REIONIZATION
- 47. Escape frac3on vs. halo mass Frac3on of halos ac3vely forming stars vs. halo mass Xu et al. (2016) Escape of ionizing photons
- 48. A Calibrated Simulation of Reionization
- 49. IGM completes reionization at z=7.3 zrei(100%)=7.3 Planck 1σ
- 50. A Calibrated Simulation of Reionization • Early stages driven by intermident star forma8on in smallest galaxies – Leads to recombining HII regions • Later stages driven by steady star forma8on in more massive galaxies • Reioniza8on starts and ends consistent with observa8ons • τes agrees with Planck data within error bars Chen, Norman & Xu (in prep)
- 51. Predic8ons about the first generation of galaxies Ques3on Predic3on How they formed? From the ashes of Pop III stars When they formed? As early as z=25 (earlier in rare peaks of the density field) What were they like? Like ultra-‐faint dwarfs Why were they important? -‐Galaxy building blocks -‐Contributed to reioniza8on -‐May be ancestors of modern-‐day ultra-‐faint dwarfs
- 52. James Webb Space Telescope Launch 2018
- 53. Ongoing Investigations • When does the last Pop III star form and under what condi8ons? – Proximity to first galaxies (Xu et al. 2016) • How do X-‐rays from accre8ng stellar and supermassive black holes modify this picture? – Preheat/preionize the IGM everywhere (Xu et al. 2013, 2015) • How to test our calibrated reioniza8on model? – 21 cm cosmology (Ahn et al. 2015) • How to connect with proper8es of Local Group dwarfs? (Wise et al. 2014; O’Shea et al. in prep) • Observable proper8es for JWST (Barrow & Wise, in prep)
- 54. Acknowledgements to former students, postdocs, and collaborators • Tom Abel (Stanford) • Kyungin Ahn (Korea) • Marcello Alvarez (CITA) • Peter Anninos (LLNL) • Greg Bryan (Columbia) • James Bordner (UCSD) • Pengfei Chen (UCSD) • Brian O’Shea (MSU) • Dan Reynolds (SMU) • Geoffrey So (Intel) • Bridon Smith (Edinburgh) • Mad Turk (UIUC) • John Wise (GA Tech) • Hao Xu (UCSD) And too many NSF and NASA grants to men8on
- 55. RESERVE SLIDES
- 57. Mass in stars and remnants
- 58. Star formation rate densities
- 63. Renaissance Simula)ons Publica8ons Reference Topic Xu et al. (2013) Pop III stars and remnants Xu et al. (2014) X-‐ray feedback from Pop III black holes Chen et al. (2014) Scaling rela8ons for SAMs Ahn et al. (2015) 21 cm signal from X-‐ray prehea8ng O’Shea et al. (2015) UV luminosity func8on Xu et al. (2016a Late Pop III star forma8on Xu et al. (2016b, submided) Galaxy proper8es and escape frac8ons Xu et al. (2016c, in prep) X-‐ray background from Pop III stars
- 64. Renaissance Simula8ons Fact Sheet Configura3on L_periodic (cMpc) 40 L_refined (cMpc) 6.6 N_p (effec8ve) 40963 m_p (solar mass) 2.9 x 104 AMR levels 12 Δx min (pc) 19/(1+z) z_init 99 Physics Cosmology WMAP7 ICs MUSIC Code ENZO gas dynamics 9-‐species primord. 2 metal fields Chemistry/cooling 9-‐species noneq. metal line Radia8ve transfer EUV, LW Lyman-‐Werner bkgd Yes Pop III SF+FB Wise+ 2012b Pop II SF+FB Wise+ 2012b Simula3ons Runs 7 Core-‐hrs ~100 M Data (TB) ~70 M. Norman, Aspen EoR 2016
- 65. Star Forma8on Prescrip8ons Wise et al. (2012a,b; 2014) Pop III [Z/H] <= -‐4 Par8cle Individual Pop III star Mass IMF w/Mchar=40 Msol thresholds fH2>5x10-‐4, δb>5x105, div(V)<0 Star proper8es Schaerer (2002) SN yields Heger & Woosley (2003) Metal-‐enriched [Z/H] > -‐4 Par8cle Molecular cloud/ star cluster Mass > 1000 Msol thresholds Τ<1000Κ, δb>5x105 div(V)<0 SF efficiency 0.07 fcold inside MC radius Radia8ve FB 6000 γ/baryon over 20 Myr SN FB 6.8x1048 erg/s/Msol amer 4 Myr Mass recycling & enrichment Pop III IMF
- 66. FLD versus MORAY M. Norman, Aspen EoR 2016 Norman et al. (2015)
- 67. M. Norman, Aspen EoR 2016 FLD versus MORAY Norman et al. (2015)