Transcript The Dark Matter Problem astrophysical perspectives 陈学雷 中国科学院国家天文台

The Dark Matter Problem
astrophysical perspectives
陈学雷
中国科学院国家天文台
What can we learn from astrophysics?
The data
evidence of DM
abundance of DM
distribution of DM
The questions
Nature of DM
property of DM (mass, interaction, ...)
role of DM in cosmic history
origin of DM, and relation with DE
Outline
What we have learned
evidence of DM and its abundance
DM is not baryonic
DM is not hot
WIMP: the classic CDM
What we are learning
cuspy halos and missing satellites
alternative models of DM
CDM strikes back: the mundane answers
The role of DM (an example)
DM decay and reionization
Evidence of DM
galaxy rotation curve
dynamics of galaxy cluster
Virial theorem
U=2K
K =  mi vi2
U ~ GM2/R
Coma
cluster
mass to light ratio (B)
typical cluster: 100/h-300/h Sun
stellar pop: 1-10 Sun
critical: 1390 h +- 35%
X-ray cluster
hydrostatic equilibrium
beta model:
Strong Gravitational Lensing
Weak Lensing mass reconstruction
Image ellipticity -> shear->
invert the equation
RXJ1347.5-1145
(Bradac et al 2005)
DM Abundance
• mass to light ratio x light density
• cluster baryon fraction/BBN baryon abundance
• cluster mass function
• evolution of cluster mass function
Bahcall:
m=0.2
Blanchard: m=1.0
WMAP result
Spergel et al 2003
WMAP Combined fit:
mh2=0.135+-0.009
m=0.27+-0.04
Results depend on
Supernovae and Hubble
constant data.
Can DM be baryons?
If all DM is baryonic, it is in
conflict with Big Bang
Nucleonsynthesis and
Cosmic Microwave
Background anisotropy.
MAssive COmpact Halo Objects
(MACHO)
LMC
The result of MACHO experiment
(Alcock et al 1996):
20% of halo can be due to MACHO
Abundance of DM: WIMP?
dark -- weakly interacting?
In early Universe, even weak interaction is effective, abundance
given by
freeze out when H = n <Av>, the dark matter abundance is
comparable to weak interaction
Collisional Damping and Free Streaming
Kinetic decoupling at T ~ 1 MeV (Chen, Kamionkowski,
Zhang 2001)
Initial density perturbation is damped by the free
streaming of the particles before radiation-matter
equality
perturbations on scales smaller than rFS is smoothed out.
Structure Formation
at freeze-out
at freeze-out
if weakly interacting
hot dark matter relativistic
m< 1 keV
warm dark matter quasi-relativistic 1 keV < m < 10 keV
cold dark matter non-relativistic m > 10 keV
The failure of HDM: clusters form before galaxy, can not
account small scale structures.
The first dark halos
Diemand, Moore, Stadel 2005
Due to collisional damping and free-streaming, the smallest halo (no
sub-structure) is 10-6 solar mass (earth mass) for neutralino. Dection
of such halo may probe the nature of DM.
substructure of DM halo
B. Moore et al
missing satellites?
simulated Local Group mass system
Dark matter halo profile
simulation (Navarro, Frenk,
white 1996): cusp
observation: core
NFW96, rotation curve
Alternatives to CDM
WDM: reduce the small scale power
Self-Interacting Dark Matter (Spergel & Steinhardt 2000)
Strongly Interacting Massive Particle
Annihilating DM
Decaying DM
Fuzzy DM
WDM
From Jing 2000
SIDM
DM strongly interact with itself, but no EM
interaction can create an core in hierachical scenario
(eventually core collapse -> isothermal profile)
Interaction strength: comparable to neutron-neutron
Difficulty: make spherical clusters: against lensing
SIMP
Motivation:
• SIDM may have QCD interaction but not EM
• Not detectable in WIMP search, blocked.
CMB & LSS constraint:
Before decoupling, photons and baryons are tightly coupled, interaction
with baryon will cause additional damping of perturbation
Test DM interaction
with CMB and LSS
Chen, Hannestad, Scherrer 2002
missing satellites: CDM solution
• satellites do exist, but star formation suppressed (after
reionization?)
• satellites orbit do not bring them to close interaction with
disk, so they will not heat up the disk.
• Local Group dwarf velocity dispersion underestimated
• halo substructure may be probed by lensing (still
controversial)
• galaxy may not follow dwarf
Rotation curve
• Is density profile really universal? scatter in
concentration
• What is the real slope
NFW: 1.0 Moore 1.5, ..., Power et al, Diemand et al, 1.2
• Observation
beam smearing? 21cm vs H
some agree w/ cusp, but most dwarf slope 0.2
• Cusp cheat as core
vg != vc , because of inclination, effect of bulge and bar, gas
supported by pressure, star orbit in triaxial halo, ... (Rhee et al
2004, Hayashi etal 2003)
active DM: decaying particle
Reionization
Rephaeli & Szalay 1981; Salati & Wallet 1984;
Ionization of Reynolds layer, ISM, IGM
Sciama 1982-1996; Melott, 1984; but see Bowyer et al 1999
Resolve the conflict between SCDM model and =0.3
Gelmini, Schram & Valle, 1984; Turner, Steigman, Krauss, 1984; Doroshkevich, Khlopov, 1984
If decay early, can affect BBN
Audouze, Lindley, Silk 1985; Starkman 1988, Dimopoulos et al 1988
If decaying particle heavy, may give Ultra High Energy Cosmic Rays
Frampton & Glashow 1980; Ellis, Steigman, Gaisser 1981; Berezinsky, Kachelriess & Vilenkin 1997;
Birkel & Sarkar 1998
Decaying dark matter
& small scale crisis
Cen 2001
Candidates of decaying DM
active neutrino, sterile neutrino, unstable susy particle,
crypton, super heavy dark matter, R-violating gravitino,
moduli, axino, SWIMP, quintessino, Q-ball, topological
defect, primodial black hole ...
decaying DM & reionization
standard picture of
reionization
surprise from WMAP:
early reionization
Thermal History with Decaying DM
Long lived particle
short lived particle
Constraints on decaying DM
Summary
• Observations are in general agreement with LCDM, most data consistent
with low DM density (0.2-0.3), but there are different voices.
• Small scale crisis: the problem is complicated, explanations inside/outside
LCDM paradigm are available
• Many properties of DM can be studied with astrophysical observation
• Some observations unexplained in simplest version of LCDM (tight TullyFisher relation, downsizing in galaxy formation, ...)
• Open questions: role of DM in cosmic evolution? relation with DE?