the halo model structure formation: cosmic capitalism halos: abundances, clustering and evolution...
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The Halo Model
• Structure formation: cosmic capitalism
• Halos: abundances, clustering and evolution
• Galaxies: a nonlinear biased view of dark matters
• Marked correlations: There’s more to the points
Ravi K. Sheth (UPitt/UPenn)
Galaxy Surveys
Galaxy clustering depends on type
Large samples now available to quantify this
Light is a biased tracer
To use galaxies as probes of underlying dark matter distribution, must understand ‘bias’
Center-satellite process requires knowledge of 1) halo abundance; 2) halo clustering; 3) halo profiles; 4) how number of galaxies per halo depends on halo mass.
(Also a simple model of earthquakes and aftershocks!)
Neyman & Scott
•Hypothesis testing (J. Neyman) •Model of ozone•Model of cancer•Model of BCGs (E. Scott)
•Clustering model (Neyman & Scott)
The halo-model of clustering• Two types of pairs: both particles in same halo, or
particles in different halos
• ξdm(r) = ξ1h(r) + ξ2h(r)
• All physics can be decomposed similarly: influences from within halo, versus from outside (Sheth 1996)
Gaussian fluctuations as seeds of subsequent structure formation
Gaussianity simplifies mathematics: logic which follows is general
N-body simulations of
gravitational clustering
in an expanding universe
Cold Dark Matter
• Simulations include gravity only (no gas) • Late-time field retains memory of initial conditions
• Cosmic capitalism
Co-moving volume ~ 100 Mpc/h
It’s a capitalist’s life…
• Most of the action is in the big cities• Newcomers to the city are rapidly stripped
of (almost!) all they have• Encounters generally too high-speed to lead
to long-lasting mergers• Repeated ‘harassment’ can lead to change• Real interactions take place in the outskirts• A network exists to channel resources from
the fields to feed the cities
Spherical evolution model• ‘Collapse’ depends on initial over-density i; same for all initial sizes• Critical density depends on cosmology• Final objects all have same density, whatever their initial sizes•Collapsed objects called halos; ~ 200× denser than background, whatever their mass
(Tormen 1998)
(Figure shows particles at z~2 which, at z~0, are in a cluster)
Spherical evolution model• Initially, Ei = – GM/Ri + (HiRi)2/2
• Shells remain concentric as object evolves; if denser than background, object pulls itself together as background expands around it
• At ‘turnaround’: E = – GM/rmax = Ei
• So – GM/rmax = – GM/Ri + (HiRi)2/2
• Hence (Ri/rmax) = 1 – Hi2Ri
3/2GM
= 1 – (3Hi2 /8G) (4Ri
3/3)/M
= 1 – 1/(1+i) = i/(1+i) ≈ i
Virialization• Final object virializes: −W = 2K• Evir = W+K = W/2 = −GM/2rvir= −GM/rmax
• So rvir = rmax/2: final size, density of object determined by initial overdensity
• To form an object at present time, must have had a critical overdensity initially
• To form objects at high redshift, must have been even more overdense initially
• At any given time, all objects have same density (high-z objects denser)
Virial Motions• (Ri/rvir) ~ f(i): ratio of initial and final
sizes depends on initial overdensity• Mass M ~ (1+i)Ri
3 ~ Ri3 (since initial
overdensity « 1)• So final virial density ~ M/rvir
3 ~ (Ri/rvir)3 ~ function of critical density: hence, at any given time, all virialized objects have the same density, whatever their mass
• V2 ~ GM/rvir ~ M2/3: massive objects have larger internal velocities/temperatures
Spherical evolution model• ‘Collapse’ depends on initial over-density i; same for all initial sizes• Critical density depends on cosmology• Final objects all have same density, whatever their initial sizes•Collapsed objects called halos; ~ 200× denser than background, whatever their mass
(Tormen 1998)
(Figure shows particles at z~2 which, at z~0, are in a cluster)
Initial spatial distribution within patch (at z~1000)...
…stochastic (initial conditions Gaussian random field); study `forest’ of merger history ‘trees’
…encodes information about subsequent ‘merger history’ of object(Mo & White 1996; Sheth 1996)
The Halo Mass
Function•Hierarchical; no massive halos at high-z•Halo abundance evolves strongly •Massive halos (exponentially) rare•Observable → mass difficult
(Reed et al. 2003)
(current parameterizations by Sheth & Tormen 1999; Jenkins et al. 2001)
Universal form?• Spherical evolution
(Press & Schechter 1974;
Bond et al. 1991) • Ellipsoidal evolution
(Sheth & Tormen 1999; Sheth, Mo & Tormen 2001)
• Simplifies analysis of cluster abundances (e.g. ACT)
Jenkins et al. 2001
Most massive
halos populate densest regions
over-dense
under-dense
Key to understand galaxy biasing (Mo & White 1996; Sheth & Tormen 2002)
n(m|) = [1 + b(m)] n(m)
Halo clustering
• Massive halos more strongly clustered
• Clustering of halos different from clustering of mass
Percival et al. 2003
massive halos
dark matter
linear theory
non-
Halo clustering Halo abundances
Clustering is ideal (only?) mass calibrator (Sheth & Tormen 1999)
The halo-model of clustering• Two types of pairs: both particles in same halo, or
particles in different halos
• ξdm(r) = ξ1h(r) + ξ2h(r)
• All physics can be decomposed similarly: influences from within halo, versus from outside
The dark-matter correlation function
ξdm(r) = ξ1h(r) + ξ2h(r)
• ξ1h(r) ~ ∫dm n(m) m2 ξdm(m|r)/2
• n(m): number density of halos• m2: total number of pairs
• ξdm(m|r): fraction of pairs which have separation r; depends on density profile within m-halos
• Need not know spatial distribution of halos! • This term only matters on scales smaller than the
virial radius of a typical M* halo (~ Mpc)
• ξ2h(r) ~ larger scales, depends on halo clustering
Clustering in simulations
ξdm(r) = ξ1h(r) + ξ2h(r)
•Expect (and see) feature on scale of transition from one- halo to two-halo
•Feature in data?
Galaxy formation
• Gas cools in virialized dark matter ‘halos’. Physics of halos is nonlinear, but primarily gravitational
• Complicated gastrophysics (star formation, supernovae enrichment, etc.) mainly determined by local environment (i.e., by parent halo), not by surrounding halos
(Cole et al. 2000)
Kauffmann, Diaferio, Colberg & White 1999
Also Cole et al., Benson et al., Somerville & Primack,Colin et al.
Colors indicate age
Halo-model of galaxy clustering• Two types of pairs: only difference from dark matter
is that now, number of pairs in m-halo is not m2
• ξdm(r) = ξ1h(r) + ξ2h(r)
• Spatial distribution within halos is small-scale detail
The galaxy correlation function
ξdm(r) = ξ1h(r) + ξ2h(r)
• ξ1h(r) ~ ∫dm n(m) g2(m) ξdm(m|r)/2
• n(m): number density of halos
• g2(m): total number of galaxy pairs
• ξdm(m|r): fraction of pairs which have separation r; depends on density profile within m-halos
• Need not know spatial distribution of halos! • This term only matters on scales smaller than the
virial radius of a typical M* halo (~ Mpc)
• ξ2h(r) ~ larger scales, depends on halo clustering
Type-dependent clustering: Why?
populate massive halos
populate lower mass halos
Spatial distribution within halos second order effect (on >100 kpc)
Comparison with simulations
• Halo model calculation of (r)
• Red galaxies• Dark matter• Blue galaxies• Note inflection at scale
of transition from 1-halo term to 2-halo term
• Bias constant at large r
1h›2h
1h‹2h →
Sheth et al. 2001
steeper
shallower
A Nonlinear and Biased View• Observations of galaxy clustering on large
scales provide information about cosmology (because clustering on large scales is still in the ‘linear’ regime)
• Observations of small scale galaxy clustering provide a nonlinear, biased view of the dark matter density field, but they do contain a wealth of information about galaxy formation
• g(m) characterizes this information and so can inform galaxy formation models
Summary• Hierarchical clustering = cosmic capitalism: Many
models (percolation, coagulation, random walks) give equivalent descriptions
• All models separate cosmology/dynamics from statistics P(k)
• Gastrophysics determined by mass of parent halo• All effects of density (environment) arise through halo
bias (massive halos populate densest regions)• Description quite detailed; language of halo model
also useful for other ‘biased’ observables
Halo Model • Describes spatial statistics well• Describes velocity statistics well• Since Momentum ~ mv, Temp ~ v2 ~ m2/3,
and Pressure ~ Density ×Temp Halo Model useful language for interpreting
Kinematic and Thermal SZ effects, various secondary contributions to CMB, and gravitational lensing (see Cooray & Sheth 2002 review)
• Open problem: Describe Ly- forest
Marked correlation functions
Weight galaxies by some observable (e.g. luminosity, color, SFR) when computing clustering statistics (standard analysis weights by zero or one)
There’s more to the point(s)
• Multi-band photometry becoming the norm• CCDs provide accurate photometry; possible
to exploit more than just spatial information • How to estimate clustering of observables,
over and above correlations which are due to spatial clustering?
• Do galaxy properties depend on environment? Standard model says only dependence comes from parent halos…
Luminosity as a mark
•Mr from SDSS•BIK from semi-analytic model
•Little B-band light associated with close pairs; more B-band light in ‘field’ than ‘clusters’•Vice-versa in K
•Feature at 3/h Mpc in all bands: Same physical process the cause? e.g. galaxies form in groups at the outskirts of clusters
Colors and star formation•Close pairs tend to be redder•Scale on which feature appears smaller at higher z: clusters smaller at high-z?•Amplitude drops at lower z: close red pairs merged?
•Close pairs have small star formation rates; scale similar to that for color even though curves show opposite trends!
•Same physics drives both color and SFR?
Stellar mass
•Circles show M*, crosses show LK
•Similar bumps, wiggles in both: offset related to rms M*, L•Evolution with time: M* grows more rapidly in dense regions
Halo-model of marked correlations
Again, write in terms of two components:
W1gal(r) ~ ∫dm n(m) g2(m) ‹W|m›2 ξdm(m|r)/gal2
W2gal(r) ≈ [∫dm n(m) g1(m) ‹W|m› b(m)/gal]2 ξdm(r)
So, on large scales, expect
M(r) = 1+W(r)1+ξ(r)
= 1 + BW ξdm(r) 1 + bgal ξdm(r)
Conclusions (mark these words!)
• Marked correlations represent efficient use of information in new high-quality multi-band datasets (there’s more to the points…)– No ad hoc division into cluster/field, bright/faint, etc.
• Comparison of SDSS/SAMs ongoing– test Ngalaxies(m);
– then test if rank ordering OK;
– finally test actual values
• Halo-model is natural language to interpret/model
Halo-model calculations
• Type-dependent (n-pt) clustering
• ISW and tracer population
• SZ effect and halo shapes/motions
• Weak gravitational lensing
• Absorption line systems
• Marked correlations
} Review in Cooray &Sheth 2002
} Work in progress
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