Mapping the z > 2 Cosmic Web with
3D Lyα Forest Tomography
Khee-Gan (K.G.) Lee
Hubble Fellow, Lawrence Berkeley National Lab
Galaxy Evolution Meeting (Tohoku University)

June 3, 2016

Collaborators: Martin White (UC Berkeley), David Schlegel (LBNL), JoeHennawi (MPIA), Xavier Prochaska (UCSC), R. Michael Rich (UCLA), NaoSuzuki (IPMU), COSMOS collaboration
K.G. LeeLyα Forest Tomography

The Cosmic Web and Cosmologyz = 10z=2z=1z = 0.5

z=0Credit: Anatoly Klypin (NMSU) & Andrei Kravtsov (Chicago)

I Pattern of voids, filaments and nodes in the large-scale distribution ofDM + baryons
I Caused by gravitational evolution of Gaussian random-phase initialconditions from inflation
I Detection of cosmic web in the 1980s was key evidence supporting

inflationary cold dark matter paradigm
I Galaxy formation and evolution is influenced by cosmic web environment
I The evolution over time probes gravity models and the cosmologicalconstant
K.G. LeeLyα Forest Tomography

Cosmic Web from GAMA Redshift Survey
I 100k galaxy redshifts with AAOMega on 4m AAT

Eardley+ 2015

Only feasible in nearby Universe (z < 0.3)!
K.G. LeeLyα Forest Tomography

Why study cosmic web at ‘Cosmic Noon’ ?
Peak of cosmic star-formation + AGN activity occured at z ∼ 2 − 3Bouwens+2011
K.G. LeeLyα Forest Tomography

Cosmic Web with Galaxy Redshifts at z ∼ 2?
D < 100 Mpc Local Universe(∆v = 2000 km s−1 slice)

Courtois et al 2013

K.G. LeeLyα Forest Tomography

Cosmic Web with Galaxy Redshifts at z ∼ 2?
D < 100 Mpc Local Universe(∆v = 2000 km s−1 slice)z = 2.3(COSMOS spectro-z’s)

Courtois et al 2013

COSMOS Collaboration

K.G. LeeLyα Forest Tomography

Lyman-α Forest as Probe of z > 2 Universe

I Seen in quasar spectra in their restframe

λ < 121.6nm wavelengths
I Caused by neutral hydrogen in the IGM
I Absorption is non-linear tracer of underlying LSS

density in mildly overdense regime (ρ/hρi ∼ few),
approximately:T0 −0.7
τ(x) ∝Γ



ρ(x)
hρi

Ellison+2000

2−0.7(γ−1)

I T0 , Γ , γ are parameters governing theastrophysics of the photoionized IGM
I In this talk, I will ignore astrophysics and pretend

the absorption is a direct tracer of density
Credit: AmSci/R. Simcoe

K.G. LeeLyα Forest TomographyLyα Forest Tomography
If the quasars have arcmin (∼ Mpc) separations, can enable tomographic
reconstruction full 3D absorption field (Pichon et al 2001, Caucci et al
2008, Lee et al 2014a)

Credit: Casey Stark (Berkeley)

But quasars (rare!) aren’t enough to pull this off. Need to also target
faint (> 23rd mag) UV-bright star-forming galaxies!
K.G. LeeLyα Forest Tomography

Pilot Tomography Survey in COSMOS

14 Mpc/h-6I Pilot observations in 2014-2015

on COSMOS field (Lee+2014b,Lee+2016)
I LRIS spectrograph on 10.3m
16 Mpc/h

Keck-I telescope, Hawai’i
I Total ∼ 15 hrs on-sky, ∼ 2hr

exposures per pointing
I 49 galaxies+QSOs within bluearea (11.80 × 13.50 )→ ∼ 1100 deg−2 (c.f. ∼ 15 deg−2
in BOSS Lyα)?

K.G. LeeLyα Forest Tomography

Example SpectraFirst systematic use of LBGs for Lyα forest analysis!
K.G. LeeLyα Forest Tomography

Tomographic Reconstruction
Measure Lyα forest transmission δF = F/hFi − 1 (’data’), pixel noise
estimates σF , and [x, y, z] positions. Perform Wiener filtering on these
inputs to estimate the map:
M = CMD · (CDD + N)−1 · D
The noise term provides some noise-weighting to the data. We assume
Gaussian correlation function in the map, where
CDD = CMD = C(r1 , r2 ), and"#(∆rk )2(∆r⊥ )22
C(r1 , r2 ) = σF exp −exp −,
(1)
2L2k
2L2⊥
with L⊥ = 2.5h−1 Mpc and Lk = 2.0 h−1 Mpc, and σF = 0.8 (Noteaverage sightline separation hd⊥ i ≈ 2.5 h−1 Mpc).

K.G. LeeLyα Forest Tomography

3D Map of Cosmic Web at 2.2 < z < 2.5
260 h−1 Mpc along LOS; 14 h−1 Mpc × 16 h−1 Mpc transverse →V = 5.8 × 104 h−3 Mpc3 ∼ (39 h−1 Mpc)3

To watch online: https://youtu.be/KeW1UJOPMYI
K.G. LeeLyα Forest Tomography

Correlations with Foreground Galaxies?
There are some known galaxies with spectroscopic redshifts overlapping themap volume. We can compare locations of 31 MOSDEF galaxies with theoverall map PDF:

Higher densities

Galaxies clearly live in high-density regions of our map!

K.G. LeeLyα Forest Tomography

A Galaxy Protocluster at z = 2.44

I See one large (∼ 20 h−1 Mpc) overdensity

in our absorption map (3σ significance)
I Correlated with z = 2.45 galaxy

protocluster from LBGs and LAEs
(Diener+2015, Chiang+2015)
I Comparison to sims gives descendantmass estimates:
M(z = 0) = (3 ± 1.5) × 1014 h−1 Mpc(∼ Virgo cluster)
I Elongated morphology suggests possible

fragmentation into two z ∼ 0 clusters
HETDEX Pilot LAEs (stars, Chiang+2015); LBGs(squares, Diener+2015); Open circles: sightline positions
K.G. LeeLyα Forest Tomography

CLAMATO Survey(COSMOS Lyman-Alpha Mapping And Tomography Observations)
I Co-PIs: Schlegel & WhiteI Upcoming LMAP proposal targeting ∼ 1 sq deg of COSMOS field
I Require ∼ 240hrs on-sky with Keck-LRIS → ∼ 30 nights over 3 years
I Target ∼ 1000 LBGs at 2.3 . z . 3 for R ∼ 1000 spectroscopy→ hzi ∼ 2.3 LSS map over 106 h−3 Mpc3 ∼ (100 h−1 Mpc)3

I Similar spatial resolution (∼ 3 h−1 Mpc) and volume (∼ 106 h−3 Mpc3 ) toGAMA survey at z < 0.3!

K.G. LeeLyα Forest Tomography

CLAMATO Survey(COSMOS Lyman-Alpha Mapping And Tomography Observations)
I Co-PIs: Schlegel & WhiteI Upcoming LMAP proposal targeting ∼ 1 sq deg of COSMOS field
I Require ∼ 240hrs on-sky with Keck-LRIS → ∼ 30 nights over 3 years
I Target ∼ 1000 LBGs at 2.3 . z . 3 for R ∼ 1000 spectroscopy→ hzi ∼ 2.3 LSS map over 106 h−3 Mpc3 ∼ (100 h−1 Mpc)3

I Similar spatial resolution (∼ 3 h−1 Mpc) and volume (∼ 106 h−3 Mpc3 ) toGAMA survey at z < 0.3!

K.G. LeeLyα Forest Tomography

Voids and Protoclusters
M = 1.1 × 1015 h−1 Mcluster progenitor

R = 11.7 h−1 Mpc cosmic void

Casey Stark (UC Berkeley) studied detectability of z ∼ 2.5 protoclusters andvoids with Lyα forest tomography in sims (Stark+2015a, 2015b)
I L = 256 h−1 Mpc TreePM sim with IGM absorption from FGPA
I Generated DM density field and mock tomographic maps with sightline

sampling + noise consistent with real data
I Protoclusters: Look for 3σ peaks in smoothed map, which gives > 90%

completeness and ∼ 75% purity for M > 3 × 1014 h−1 M progenitors
I Voids: Search for spherical low-density regions in DM field andtomographic map → ∼ 65% volume overlap for ∼ 15% filling factor
K.G. LeeLyα Forest Tomography

Voids and Protoclusters in CLAMATO65 h−1 Mpc-Right: Central part of COSMOS Field
Magenta CLAMATO 0.8 sq deg

6Blue Pilot field (2014-2015)65 h−1 Mpc

Dots Photo-z and spectro-z
z = 2.4 − 3.0 LBG targets

Below: Simulated protoclusters and voids(approx to scale)
Large area (> 1 sq deg) needed to resolve
large structures!

K.G. Lee?

Lyα Forest Tomography

Pushing Towards Cosmological Volumes
Lyα tomography will push towards large volumes over the next few years...
I CLAMATO (2016-2018):
I

LRIS Spectrograph on 10.3m Keck-I
FOV: 70 × 50 (∼ 0.01deg2 )I Target ∼ 20 sources per FOV (g 6 24.7)
I Time: ∼ 40 nights to cover 0.8deg2→ 2.3 < z < 2.5 Tomographic map probing∼ 2.5 h−1 Mpc over V ∼ 106 h−3 Mpc

IK.G. LeeLyα Forest Tomography

Pushing Towards Cosmological Volumes
Lyα tomography will push towards large volumes over the next few years...
I CLAMATO (2016-2018):
I

LRIS Spectrograph on 10.3m Keck-I
FOV: 70 × 50 (∼ 0.01deg2 )I Target ∼ 20 sources per FOV (g 6 24.7)
I Time: ∼ 40 nights to cover 0.8deg2→ 2.3 < z < 2.5 Tomographic map probing∼ 2.5 h−1 Mpc over V ∼ 106 h−3 Mpc

I

I IGM Tomography Survey on Subaru-PFS(2019- )?
IPrime Focus Spectrograph on 8.2m Subaru
Telescope
I FOV: 1.2deg2
I Target ∼ 2000 sources per FOV (g 6 24.0)
I Time: 10 nights to cover 15deg2 with 6hrs perpointing→ 2.5 < z < 3.2 Tomographic map probing∼ 5 h−1 Mpc over V ∼ 108 h−3 Mpc

K.G. LeeLyα Forest Tomography

Tomographic Survey Planning for PFS

I Exposure times required to make tomographic maps at variousresolutions. Different curves show different map quality.
I Black dotted line: 3hr exposures from PFS Galaxy Evolution Survey ofi < 24 LBGs (Takada+2013).
I Red dashed line: Additional exposures → 6hrs to build up S/N will enablea good-quality map sampling 5 h−1 Mpc in same volume as PFS galaxy
redshifts!
K.G. LeeLyα Forest Tomography

Classifying the Cosmic Web (Lee & White 2016)
After Zel’dovich 1970 (... Hahn+2007, Forero-Romero+2009, Eardley+2015).
Analyze deformation tensor of density field:
Tij =

∂2 Φ
∂xi ∂xj

Easy to do in Fourier space, since ∇2 Φ̃ = k2 Φ̃ = 4πGδk :T̃ij =

ki kj δkk2

(2)

Compute 3 eigenvalues at each point, and count # above eigenvalue threshold
λth :I No eigenvalue above threshold: voidI 1 eigenvalue above threshold: sheet
I 2 eigenvalues above threshold: filament
I 3 eigenvalues above threshold: node/knot

For Ly-a forest flux field δf = F/hFi − 1, just flip the signs!
K.G. LeeLyα Forest Tomography

Classifying the z = 2.5 Cosmic Web with CLAMATO
Analyzed L = 256 h−1 Mpc FGPA sim to find voids (white), sheets, filament,
nodes

CLAMATO-like: hd⊥ i = 2.5 h−1 Mpc
K.G. LeeLyα Forest Tomography

Classifying the z = 2.5 Cosmic Web with PFS Tomography
Voids (white), sheets, filament, nodes

PFS-like: hd⊥ i = 4.0 h−1 Mpc
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DM Overdensity

Flux-Mass Relationship on 4 h−1 Mpc Scales

6Flux in Mock CLAMATO Map

K.G. LeeLyα Forest Tomography

Halo Abundances in Different Densities
Computed halo multiplicity function as function of different overdensities/flux(Error bars: 1σ scatter from CLAMATO-like subvolumes)f(ln M) dln M =K.G. Lee

M dndln Mρ̄ dln M

Lyα Forest Tomography

Halo Abundances in Different Densities
Computed halo multiplicity function as function of different overdensities/flux(Error bars: 1σ scatter from CLAMATO-like subvolumes)f(ln M) dln M =M dndln Mρ̄ dln M

CLAMATO map will deliver estimates of local density/halo-mass scale at any
point within its volume!
K.G. LeeLyα Forest Tomography

Conclusion
I Observations of z ∼ 2 − 3 QSOs + LBGs at high area densities allow

3D reconstructions of foreground Lyα forest absorption
I Ideal for detecting extended z ∼ 2 structures: voids + protoclusters
I Good recovery of cosmic web sheets and filaments — comparable toz ∼ 0.1 galaxy surveys!
I Lyα absorption is proxy for density → halo mass!
I CLAMATO Survey on Keck-I/LRIS (2015-2019):

Will deliver Lyα absorption maps of 2.2 < z < 2.5 cosmic web withhigh fidelity probing ∼ 3 h−1 Mpc scales
I Cover cosmological volume (V ∼ 106 h−3 Mpc3 ) while
simultaneously
I ∼ 500 coeval galaxies from other surveys will enable studies ofgalaxy evolution in context of Cosmic WebII Subaru-PFS will be very powerful for large-volume surveys ofz = 2.5 − 3.5 IGM, and together with coeval galaxies will reveal theinterplay between the Cosmic Web and galaxy evolution.
K.G. LeeLyα Forest Tomography