Distinguishing Between Warm and Cold Dark Matter

Louie Strigari UC Irvine Center for Cosmology Aspen Workshop Neutrinos in Physics & Astrophysics 2/2/2007 Collaborators: James Bullock, Manoj Kaplinghat astro-ph/0701581….. Motivations

 Testing the CDM (WIMP) paradigm

 Predictions for well-motivated dark matter models  Warm Dark Matter: Sterile neutrinos  Dark matter from decays  Early Decays ( 106 sec. )  Late Decays ( >1012 sec. )

 New observational tests of dark matter halo structure

Louie Strigari UC Irvine CDM: Cosmological Consequences

• A nearly universal halo density profile scaling as 1/r in the central regions [Navarro et al 2004, Diemand et al 2004]

• Hundreds of dark matter-dominated Milky Way satellite galaxies [Klypin et al, Moore et al 1999]

• Dark mini-halos abundant in the central regions [Diemand et al 2006] • Concentration of CDM halos set by the density of the universe when the halo formed [Bullock et al 2001]

Louie Strigari UC Irvine CDM Predictions cusp core Simon et al. 2005 WDM: Cosmological Consequences

• Less dense dark matter halos. Expect constant central density cores

• Free streaming: Reduces the number of small halos

• Halos form later in the universe with reduced concentrations

Louie Strigari UC Irvine Phase space densities

Q = ρ/σ3 [Hogan & Dalcanton 2000]

$ m ' 3 / 2 Q " 7 #1014 & cdm ) M pc*3(km /s)*3 CDM CDM % 100GeV ( sun

! % m ( 4 Q " 5 #10$4 ' * M pc$3 (km /s)$3 Neutrino WDM & keV ) sun

3 % #3 ( % z ( 3 Dark matter from decays! #6 10 decay #3 #3 Q "10 ' * ' * Msun pc (km /s) ' $m /m * 1000 (non-thermal) & DM ) & )

Louie Strigari ! UC Irvine Phase space density limits

• Phase space elements mixed, or coarse grained, during halo formation.

3 • For fermions, h fmax = 1/2 [Tremaine-Gunn Bound]

3/4 -3 1/8 1/4 mν > 170 eV (1 kpc/rc) (0.1 ρ/Msolarpc ) gν

• Excess mass function is conserved: Applications to dark matter from decays [Kaplinghat 2005]

• Smallest, most dark matter-dominated galaxies expected to have the largest cores

Louie Strigari UC Irvine Free-streaming

• CDM free streaming: structure down to earth mass scales. Radius of earth mass halos 0.01 pc

• WDM: Free-streaming scale depends on the mass of the sterile neutrino

• Dark matter from decays: Free-streaming length

During radiation domination During matter domination

Louie Strigari UC Irvine Power spectrum with dark matter from decays

Lifetime: 1014 sec. Lifetime: 1012 sec.

Full solution of Boltzmann equation gives: Critical scale: k2c 2 η ≈ 1 Neutrino WDM limits s Radiation-era decays: Q-1/3

Matter-era decays: Q-1/3τ-1/3

Strigari, Kaplinghat, Bullock 2006

Louie Strigari UC Irvine Lyman alpha constraints on WDM

-4 • Narayan et al 2000 claim mWDM > 750 eV (Q > 10 )

-5 • Viel et al 2005 claim mWDM > 550 eV (Q > 5 x 10 )

• New data and analysis. Seljak et al 2006; Viel et al 2006

• For different constraints, take a detailed look at dwarf galaxies

Louie Strigari UC Irvine Milky Way Dwarf Spheroidal Galaxies

 Some of the ‘missing’ CDM galaxies?  Exhibit no rotation  DM dominated  Information on DM halo from line of sight velocities

Louie Strigari UC Irvine Parameters of dwarf galaxies

• Generally there are 5 parameters we must marginalize over

• To complete the mass-modeling, we must know the velocity anisotropy of the stars

• Equilibrium analysis with jeans equations gives:

Louie Strigari UC Irvine Line of sight velocity dispersion

Fornax

Strigari et al. 2006

 Degeneracy with cores and cusps. Cannot distinguish WDM from CDM models

Louie Strigari UC Irvine Next step: Proper motions of stars

• Typical errors on proper motions are about 5 km/s ϕ • At < 100 kpc, this corresponds to accuracy < 10 micro-arcseconds/yr

R

Louie Strigari UC Irvine SIM PlanetQuest (Space Interferometry Mission) m f c . x e d n i Reflex Motion of Sun _ m i from 100pc (axes 100 s SIM Positional / M I Error Circle µas) S / v

o (4µas) g . a s na . l p j .

t Parallactic s e u

q Hipparcos Displacement t e n

a Positional of Galactic l

p . /

/ Error Circle Center : p t

t (0.64 mas) h

: m o r f

d Apparent Gravitational e

t Displacement of a p HST Positional Error Distant Star due to a

d Circle (~1.5 mas) Jupiter 1 degree away A Characterizing constraints on the density profile

Consider:

• Motivation: the velocity dispersion data constrain the slope of the density profile at a characteristic radius

Strigari, Bullock, Kaplinghat ApJL 2007 Louie Strigari UC Irvine Breaking the degeneracy

Louie Strigari UC Irvine Targeting dwarf galaxies

Louie Strigari UC Irvine Redefining the dwarf satellite problem Future prospects

• Improved data from dwarf galaxies, rotation curves

• WDM N-body simulations will more accurately predict the relation between Q and core size

• Improved measurements of the power spectrum from 21cm and Lyman alpha

• Constraints on many interesting sneutrino/KK neutrino models from low energy neutrino background

Louie Strigari UC Irvine