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Realistic Simulations of the Intergalactic Medium: The Search for Missing Physics Michael Norman James Bordner San Diego Supercomputer Center University of California San Diego Collaborators: David Tytler, Pengfei Chen (UCSD) 6/6/2018 - M. L.

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Realistic Simulations of the Intergalactic Medium: The Search for Missing Physics

Michael Norman James Bordner San Diego Supercomputer Center University of California San Diego

Collaborators: David Tytler, Pengfei Chen (UCSD)

6/6/2018 - M. L. Norman Blue Waters Symposium 2018 1

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Observing the intergalactic medium in quasar absorption line spectra

Source: M. Murphy

Lyman α forest

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The Cosmic Web: Origin of the Ly α forest absorption

quasar Cosmic web formed by gravitational clustering of dark matter and baryons (H and He) Hundreds of millions of lightyears

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Simulation v. observation: good qualitative agreement

bserved

simulated Kirkman & Tytler (1997) Zhang et al. (1997)

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N=10243 L = 80 Mpc

Baryon Overdensity, z=3 Jena et al. (2005)

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A small but persistent discrepancy has emerged between sim. & obs. as higher precision has been achieved

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Possible Explanation Finding Observational systematics NO Simulation not converged NO Simulation box size too small NO Missing IGM physics MAYBE Missing galaxy HI absorption MAYBE

This PRAC project

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Quantifying the discrepancy: Flux PDF

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high transmission low transmission Too many Not enough

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An inverted T-ρ relation in the IGM improves agreement!

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Bolton et al. (2008)

Inverted: lower density gas is hotter

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How to obtain an inverted T-ρ relation?

Treat quasars as time-dependent point sources rather than as a homogeneous UV background

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Standard model

IGM ionized by homogeneous UVB

Quasar model

IGM ionized by quasar point sources UVB

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Enzo+MGFLD RT (Dan Reynolds)
QSOs randomly placed in halos

according to QLF every 45 Myr

5 energy groups 54.4 – 400 eV

z=4 z=3.5 z=3 z=2

The Enzo Simulations

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Range of spectral slopes F=(ν/ν0)-β within obs.

range

Reionization by galaxies handled with UVB
10243 Cartesian grids; 80 & 126 Mpc boxes

z=4 z=3.5 z=3 z=2 temperature f(He III)

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Implicit Multi-Group Flux Limited Diffusion

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Global linear system solves performed with the HYPRE library (LLNL)

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Why Multifrequency RT?

A: QSOs have hard UV spectrum

[54.4,65] eV photoheating [65,75] eV [75,125] eV [125,155] eV [155,400] eV

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Synthetic HI absorption spectra @ z=4

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Synthetic HI absorption spectra @ z=2

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Does it improve agreement with data?

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The flux power spectrum is still discrepant

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Summary and Conclusions

We have carried out simulations of the Lyman alpha

forest modeling QSOs as discrete point sources

Inhomogeneous He II reionization and heating have a

surprisingly small effect on the H I observables, compared to standard model which uses a UVB

Subtle and not-so-subtle discrepancies remain

between models and observations, suggesting there is still some Missing Physics

– Resolved galaxy halos? – New dark matter physics?

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New PRAC

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Rollup

Key Challenges: high fidelity simulation of the IGM including

inhomogeneous quasar ionization and heating

Why it Matters: may explain discrepancy between theory and
bservation
Why Blue Waters: capability and capacity to develop and run

very large, computationally and memory intensive simulations

Accomplishments: high-res run completed and analyzed
Blue Waters team contributions: assistance with topology-

aware scheduling

Broader Impact: drove the development of MGFLD capability

for the Enzo community code

Shared Data: none yet
Products: MGFLD capability in Enzo code, papers in prep.

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RESERVE SLIDES

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Quasar model requires 3D time-dependent

multifrequency radiative transfer

– Very computationally intensive

Scale separation requires very large grids

– Must resolve Ly α forest absorbers (25 kpc) in a box large enough to contain hundreds of quasars (100 Mpc) 40003 grids (target)

Model development requires high throughput

for experimentation at scale

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Why Blue Waters is Needed

Progress requires combination of capacity and capability that only Blue Waters can provide

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186 Mpc Projected He+ fraction

Helium reionization by time-dependent quasars: Enzo MGFLD simulation on Blue Waters

He++ He+

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Redshift Evolution of χ(He++) and T

He++ volume fraction Temperature

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Inhomogeneous heating of the IGM by quasars 10243 Enzo-MGFLD on Blue Waters

118 Mpc z=3.41 Z=3.41 Baryon density Baryon temperature

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118 Mpc He+ fraction Photoheating rate Z=3.41

Inhomogeneous heating of the IGM by quasars 10243 Enzo-MGFLD on Blue Waters

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