Transcript Large Scale Distribution of Galaxies and Voids

Large Scale Distribution of
Galaxies and Voids
A Hip-Hop Opera in C#
starring:
Sir Johnathan Bongaarts
James Pogemiller, III
Directed by
Evolution of matter power spectrum
log(t)
Now
z=1
P(k)
k
DM   a  t
P(k)
2/3
CMB
k
P(k)
MRE
DM   const
or   ln(t )
DM   a2  t
P(k)
P(k)
k
P(k)
k
log(rcomov)
log(k)
baryonic oscillations
appear – the P(k0
equivalent of CMB
T power spectrum
k
k
EoIn
high-k small scale
perturbations grow
fast, non-linearly
sub-horizon perturb.
do not grow during
radiation dominated
epoch
Harrison-Zeldovich
spectrum P(k)~k
from inflation
Williams, L.L.R., 2006
Overview
• Large Scale Structure
– Classifications
– Quantifying LSS
– Simulated vs. Observed
• Voids
– Definition
– Halos Near Voids
– Local Void
• What to do now
Large Scale Structure (LSS)
• Baryonic and Dark Matter collapsing under
gravity into a “frothy” structure
•Think: “cosmic sponge” or “soap bubbles”
• Filaments (matter)
• Bubbles (voids)
Z ~ 30
LSS: Structure Classification
• Two-Point Correlation Function
• “Fair-Sample Hypothesis”
– Different regions of the universe are described
by the same physical processes, but can be
taken independently
– Independent regions can then be taken to
represent a statistical collection
LSS: Two-Point Correlation
Function
Two-point correlation function for galaxies:
ξ(r) ≡ < δ(x)δ(x+r)>
(δ is the density perturbation field)
LSS: Two-Point Correlation
Function
Power law model approximation:
ξ(r) = (r0/r)γ
r0 = 5.4 ± 1 h-1Mpc, γ = 1.77 ± 0.04
Power law accuracy?
• ~galactic radii (10kpc) to ~10Mpc (falls below ξ(r))
• at ~25Mpc (rises above ξ(r))
LSS: Analytic Scaling Prediction
 r   r
93n

5 n
• ρ vs. r scalings should describe observed
clustering
• For Harrison-Zel’dovich, n=1 and γ=-2, a good
approximation for small k-modes
• n=1 used by Millennium simulation shown later
LSS: Probability Function
• Differential form:
dP = n2[1+ξ(r)]dV1dV2
Gives the probability that, provided one
galaxy in dV1, another galaxy will be found
in dV2 at a distance r
LSS: Probability Function
dP = n2[1+ξ(r)]dV1dV2
• Assumes:
– dV is small enough that finding two galaxies
within it is negligible
• Redshift to distance distorts the function
LSS: Voids vs Filaments
• Wall Galaxies: exist at the void wall
• Field Galaxies: exist elsewhere
– Typically in filaments
– Can exist in voids provided void definition isn’t
extremely rigid
• Filaments are over-dense regions of space
comprised of Dark and Baryonic Matter
(Field and Wall Galaxies)
LSS: Helpful Picture
•Filaments
•Voids
•Galaxies:
• Wall
• Field (Filament)
• Field (Void)
Simulation data of LSS
LSS: Power Spectrum
•Fourier transform of correlation
function
•Amplitude relates to the
expected amount of structuring
at a given λ
• Data suggests that filaments
occur around 100 to 1000
Mpc/h
LSS: Current Events
• Cosmological Perturbation Theory:
– Clustering will occur due to gravity seeding an
early homogeneous universe
• Accurate on large scales only (must go to N-Body
simulations…)
• N-Body Simulations:
– Accurate on smaller scales
– Clustering occurs due to gravity
LSS: Simulation/Observation
• Simulations
• Provide a guide for checking theoretical models
• Millennium Simulation
• Observations
• Sloan Digital Sky Survey (SDSS) (ongoing)
• 2dF Galaxy Redshift Survey (2dF) (completed, 2004)
• Las Campanas Redshift Survey
LSS: Millennium Simulation
z = 18.3 (t = 0.21 Gyr)
LSS: Millennium Simulation
z = 0 (t = 13.6 Gyr)
LSS: Observations
Data from the 2dF Galaxy Redshit Survey
LSS: Observations
Las Campanas Redshift Survey data
Voids
Not much to talk about…
LSS: Void Definition
• Nutshell: any underdense region of space
with a diameter >1025 Mpc
– ∆ρ/ρ ~ -0.8 to -0.96
– Assuming spherical
region:
• V ~ 8x10^3 Mpc^3
• A ~ 500 Mpc^2
– Less luminous
galaxies
LSS: Other Void Definitions
• “Regions of low density in a suitably smoothed
density field”
• Regions with no objects of a certain types, for
example:
– Rich clusters
– Poor clusters
– Galaxies of various morphological types
– Objects over a specified luminosity limit
Lindner et al, 1997
LSS: Quantifying Voids
• Void probability function
(It gets a little “emotional”)
P0 V   e
 nV
• Under-density probability function
– Probability that an under-dense ( δρ/ρ) region of
volume V will be found at a distance r.
LSS: Void Hierarchy
LSS: Void Hierarchy
Similar to galaxies and clusters, but void hierarchy
depends on surrounding structures
LSS: Perspective
• Voids occupy ~90% of total observed
space
• Boötes Void: d ~124 Mpc (assume spherical)
– Volume ~ 10^6 Mpc^3 ~ 10^73 m^3
– Compare: Sloan Great Wall ~10^54 m^3
• Boötes Void is ~10^20 times larger than the Sloan
Great Wall
LSS: Boötes Void
LSS: Getting Involved in the
Community
• Millennium Simulation used to study DM
halos around voids (Brunino et al., 2006)
• Used ΩΛ=0.75, ΩM=0.25, Ωb=0.045,
h=0.73, n=1, and σ8=0.9
• Comoving box of side 500 h-1 Mpc
• Spatial resolution of 5 h-1 kpc
• Sample size of 2932 voids, median radius
of 14 h-1 Mpc
LSS: Getting Involved in the
Community
• DM halos have “frozen in” inertia tensors
and angular momentum vectors from turnaround
• According to Tidal-Torque theory, they
should be preferentially aligned depending
on the neighbors
• Could contaminate weak lensing signals, if
you’re into that kind of stuff
Leaning towards parallel
Parallel to void surface
Perpendicular to void surface
No correlation here…
Brunino et al., 2006
But what if we just look at DM
halos with spirals at the center?
Brunino et al., 2006
LSS: The Northern Local Void
• MW has a component of peculiar velocity
away from the NLV
• Faint galaxy in the center is moving
quickly towards the edge
• Other galaxies to the south do not show
this trend
• Negative gravity?! (Well…no…)
LSS: Local Void
Figure from Tully
LSS: What to do now
• LSS formation is fairly well understood on
bigger scales
• Smaller scales are non-linear and more
difficult to deal with
• Dynamical range is a limitation of
simulations
• Dark matter and baryonic matter
– Baryons trace DM on large scales
– What scales do they separate on?
LSS: References
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Brunino, R., Trujillo, I., Pearce, F.R., Thomas, P.A., 2006, astro-ph/0609629
El-Ad, H., Piran, T., 1997, astro-ph/9702135
Gaite, J., 2005, astro-ph/0510328
Lindner, U., Fricke K.J., Einasto, J., Einasto, M., 1997, astro-ph/9711046
Peebles, P.J.E., 2001, astro-ph/0101127
Peebles, P.J.E., 1993, Principles of Physical Cosmology
Rood, H. J. 1988, ARA&A
Ryden, B.S., 1995, astro-ph/9510108
Tully, R.B., 2006, astro-ph/0611357
Williams, L.L.R., 2006, Private conversations
Williams. L.L.R., 2004, AST 5022 course notes, Sec. 4.2.
Las Campanas Redshift Survey, 1998, http://qold.astro.utoronto.ca/~lin/lcrs.html