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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
What we have explored computa8onally
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?
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
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
Protogalaxies: An artist’s impression
A. Schaller, STScI
Yikes! Can we possibly simulate that?!!
Answer: YES!
Peak Speed: 13.4 Petaflops Total memory: 1.5 Petabyte Processor cores: 362,240
NCSA, University of Illinois
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:
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
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
NUMERICAL COSMOLOGY 101
The universe at 380,000 yr ABB ini)al condi)ons for my simula)ons
Fluctua8ons have a well-‐measured power spectrum
Comoving volume
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
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
“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
FROM LINEAR FLUCTUATIONS TO THE FIRST STARS
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
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
Key physics: catalytic formation of H2
• Process starts with residual electrons amer recombina8on
H + e- è H- + hνH- + H è H2 + e-
H + H+ è H2+ + hν
H2+ + H è H2 + H
Channel 1
Channel 2
• Solution: Semi-implicit rate solver (Anninos et al. 1997)
Computational difficulties and solutions
• Difficulty: noneq. primordial gas chemistry
Computational difficulties and solutions
• Difficulty: Vast range of spa8o-‐temporal scales
Gas cloud
protostar
Space: 1010 Time: 1012
Computational difficulties and solutions
• Solu8on: recursive adap8ve mesh refinement and hierarchical 8me-‐stepping (Bryan & Norman 1997)
Evolution of grid refinements
(c) Brian O'Shea (MSU) and the Enzo Collabora8on, 2015
L/Δxmin=1010
density
temperature
600 pc 60 pc 6 pc
self-gravitating core
Abel, Bryan & Norman 2002
100x mass of sun
924 cita8ons and coun8ng
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
A deluge of papers followed
FROM FIRST STARS TO THE FIRST GALAXIES
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)
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
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
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
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)
Population III Star Formation Fireworks (Cox/Patterson, NCSA)
Birth of a Galaxy Wise et al. 2012a,b; 2014
(c) John Wise (GIT) and the Enzo Collabora8on, 2012
Galaxy counts (luminosity fcn.)
Volume averaged star forma3on history
STATISTICS OF THE FIRST GALAXIES: THE RENAISSANCE SIMULATIONS
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)
Peak Speed: 13.4 Petaflops Total memory: 1.5 Petabyte Processor cores: 362,240
NCSA, University of Illinois
A virtual tour of the Renaissance Simulation (Cox/Patterson, NCSA)
How do the first galaxies form? Ans: Mergers and Acquisi8ons
z=25
First stars
First galaxies
8me
z=15
When did the first galaxies form? Ans: Immediately amer the first stars
What are they like? Sta8s8cs from ~3000 halos
Stellar mass v. halo mass
UV luminosity v. halo mass
O’Shea et al. (2015)
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
FROM FIRST GALAXIES TO COSMIC REIONIZATION
Escape frac3on vs. halo mass
Frac3on of halos ac3vely forming stars vs. halo mass
Xu et al. (2016)
Escape of ionizing photons
A Calibrated Simulation of Reionization
IGM completes reionization at z=7.3
zrei(100%)=7.3
Planck 1σ
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)
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
James Webb Space Telescope Launch 2018
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)
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
RESERVE SLIDES
Cosmological Parameters
Mass in stars and remnants
Star formation rate densities
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
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
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
FLD versus MORAY
M. Norman, Aspen EoR 2016 Norman et al. (2015)
M. Norman, Aspen EoR 2016
FLD versus MORAY
Norman et al. (2015)