Post on 30-Dec-2015
description
transcript
Study of Proto-clusters by Cosmological
Simulation
Tamon SUWA, Asao HABE (Hokkaido Univ.)
Kohji YOSHIKAWA (Tokyo Univ.)
Plan of the presentation
Introduction Structure formation in the universe Recent observation of proto-clusters
Numerical method Results
Overdensity of halo and mass at z=5 Large scale structure at z=5
Summary
hot plasma
Neutral hydrogen formation
The present
Big bang
z~ 1000
Cosmic microwave background
Galaxy formation
z=0
The beginning
z<10
1. IntroductionBig bang and Expansion of the universe
Cosmic microwave background
WMAP observation of CMB tell us many cosmological information (Spergel et al. 2003). Flat universe Content of the Universe
73% dark energy 23% cold dark matter 3% baryon
Hubble constant:H0=73km /s /Mpc Age of the universe: 13.7 Gyr There exists small density fluctuation
The initial density fluctuation of Cold Dark Matter
Assuming CDM model, spectrum of initial density fluctuation can be obtained analytically.
Amplitude of fluctuation is large for small scale. Small scale structures
are formed earlier than large ones.
CDM model and hierarchical structure formation
Under the CDM model, hierarchical structure formation is expected.Small structures are formed earlierLarge structures are made from small ones
Recent observations of proto-clusters
Recent observations of Lyα emitters (LAEs) with Subaru, VLT, etc., show candidates of proto-clusters. Shimasaku et al. 2003 Ouchi et al. 2005 z=4.9
z=5.7
Questions
Such proto-clusters are naturally expected or not in CDM universe?
How LAEs are formed in high-z universe? What is reliable indicator to characterize prot
o-clusters? Is number density of LAEs really suitable?
Our study
We investigate cluster formation at high-z universe in CDM universe by cosmological simulation.
We compare our numerical results with observations.
Simulation box is large enough to realize many clusters.
2. Numerical method(N-body/SPH Simulation)
Particle-Particle-Particle-Mesh (P3M) and Smoothed Particle Hydrodynamics (SPH) method
Size of box: (214 Mpc)3 (1pc = 3.26 lyr; periodic boundary)
2563 ( ~ 17 million) particles of dark matter and the same number of gas
Mass of a particle: 2.15×1010MO for DM 2.08×109MO for gas
Softening length: 80kpc
Cosmological Parameters
Density parameter, 0 = 0.3 Hubble constant, H0 = 70km/s/Mpc
Baryon density, b = 0.015h-1
Cosmological constant, 0 = 0.7
Amplitude of initial fluctuation, 8 = 1.0
Mpc
Mp
c
Distribution of dark matter and galaxy clusters at z=0
White circles indicate clusters of galaxies.
Proto-cluster regions
We find 61 clusters (>1014MO) at z=0. We identify dark matter and gas particles
belong to clusters at z=0 and trace back to high-z.
The regions which include the particles are defined “proto-cluster regions”.
z=0present
z=5past
cluster
Trace back to high-z
Proto-cluster
Dark halos in proto-clusters
We investigate dark halos of which masses are >1012MO as galaxies at high-z.
We compare results of our numerical simulation and observed LAEs distributions.
Dark halos (M>1012MO)
3. Results
Mass overdensity (δmass) and halo overdensity (δhalo) of proto-cluster regions
δhalo at high-z and the largest dark halo at z=0 in the same regions
Large scale structure at high-z universe
Indicators of proto-cluster regions
We use following indicators: Halo overdensity:
Mass overdensity:
We obtain halo and mass overdensity for proto-clusters and random selected fields.
Smoothing scale is 25Mpc (comoving unit; typical scale of proto-clusters).
BG
1proto clus
ha
ter
lo
n
n
BG
BG BG
1 proto clumas
sters
Correlation map of δhalo and δma
ss at z=5
δmass
δ ha
lo
red :proto-cluster regionsgreen: random selected regionsblue: random selected regions which overlapped with proto-clusters
No bias
Natural bias
0.6-0.2 0
02
86
4
Bias parameter
The ratio δhalo/δmass is called bias parameter b. No bias: b=1 Analytical prediction (natural bias): b~ 2 (for 1012MO at z=5)
For large δmass, esp. proto-cluster, b is larger than natural bias. In proto-cluster region, galaxies form earlier than ana
lytical prediction. Numerical simulation is necessary to obtain this resul
t because of its non-linearity.
There exist field regions which has large δhalo
What is the reliable threshold of δhalo?
We calculate δhalo in random selected regions at z=5
find the largest dark halo in that region at z=0⇒
δhalo at high-z and the largest dark halo at z=0 in the same regions
Calculate δhaloIs there rich cluster?
Z=5 Z=0
Pickup many (25Mpc)3 regions in simulation box For each region, we obtain
δhalo at z=5 mass of the largest dark halo at z=0
We check whether the halo is rich cluster or not.
Calculate δhaloIs there rich cluster?
Z=5 Z=0
δhalo at high-z and the largest dark halo at z=0 in the same regions (2)
δhalo at high-z and rich cluster (M>1014MO) at z=0
Fra
ctio
n of
reg
ions
whi
ch
incl
ude
rich
clus
ter
at z
=0
δhalo@z=5
Large scale structure at z=5
Large filamentary structure of dark halos ~ several ten Mpc scale are formed at z=5.
Observation of LAEs at z=5.7 by Ouchi et al. (2005) also show large scale structure. 200×200×40 Mpc3
This suggest that LAEs are in massive dark halos (M>1012MO).
Ouchi et al. 2005
Summary
We do P3MSPH simulation in order to investigate property of proto-clusters and large scale structures Large box size:(214Mpc)3
Large # of particles: 17 million×2(DM & SPH) δhalo,δmass of proto-clusters at z=5
Large bias for large δmass (esp. proto-cluster) δhalo at z=5 and rich cluster at z=0
80% of regions contain galaxy cluster if δhalo >3 at z=5.
Large filamentary structure in high-z universe