on the cusp of the dark matter
DESCRIPTION
On the Cusp of the Dark Matter. Sergey Mashchenko Hugh Couchman James Wadsley McMaster University ( Nature 3/8/06; Science 29/11/07 ). Outline. The problem of “cusps” in standard CDM dark matter haloes Toy model for stellar feedback Self-consistent feedback in live, dwarf haloes - PowerPoint PPT PresentationTRANSCRIPT
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On the Cusp of the
Dark Matter
Sergey MashchenkoHugh CouchmanJames Wadsley
McMaster University
( Nature 3/8/06; Science 29/11/07 )
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Outline
• The problem of “cusps” in standard CDM dark matter haloes
• Toy model for stellar feedback• Self-consistent feedback in live, dwarf haloes
The talk considers the interplay between gas (and the astrophysical processes connected with star formation) and collisionless dark matter in cosmic structure formation
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The Cusp Problem in CDM
• Despite successes of ΛCDM on large and intermediate scales, serious issues remain on smaller, galactic and sub-galactic, scales. In particular:– Theory (simulation) predicts – with a fair
degree of confidence – cuspy inner profiles ~ NFW
– Observations show increasingly strong evidence for flat inner cores ~ Burkert
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de Blok & Bosma, 2002
Battaglia et al., arXiv:0802.4220Kinematic Status and mass content ofThe Sculptor dwarf spheroidal galaxy
“…velocity dispersion profiles are best fitted by a cored dark matter halo with core radiusR_c= 0.5kpc.”
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Proposed solutions• Observational problems
– Beam smearing; non-circular motion etc.• New physics
– WDM; self-interacting DM– Modified gravity
• Solutions within standard ΛCDM (requires “heating” of dark matter)– Rotating bar– Passive evolution of cold lumps (e.g., El Zant et al.,
2001)– Recoiling black holes– AGN– “Maximal stellar feedback”/“blowout”
Ideas have variable traction… propose a mechanism that is a natural consequence of structure formation
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• Bulk gas motions in early dwarf galaxies – driven by supernovae and stellar winds - transfer kinetic energy to “heat” the dark matter– Plausible mechanism that must have been
widespread in early, gas-rich dwarfs– Could likely have achieved significant gas
compression in early (small concentration) haloes
– Observe bulk motions of cold gas in present-day dwarfs that are mildly supersonic, have spatial scale similar to that of z>10 dwarfs (few 100pc) and have velocities similar to dark matter dispersion (~10km/s)
• Note: the naïve impact of cooling baryons is to make the cusp steeper
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Sag DIG Young & Lo (1997)
500pc
3.2kpc
Believed to be bulk motion resulting from star formation:
<v2> ~ (10 km/s)2
If sufficient gas can be concentrated and moved in bulk, the gravitational potential will fluctuate, resulting in the transfer of kinetic energy from baryons to dark matter.– For σgas << σdm, the dark
matter will adjust adiabatically
– For σgas >> σdm, the dark matter moves only in the time-averaged potential of the gas lumps
– Would not expect sensitivity to gas density
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Toy Model• Challenging to do full hydro
simulation of stellar-induced bulk motions in a live dark matter halo, so…
• DM halo: z ~ 10 dwarf galaxy (NFW Mvir=109 M; rvir = 3kpc; rs = 850pc; 106 particles), and
• Model gas bulk motions by forced motion of extended rigid bodies moving through the centre of the halo:– Clumps 40pc; amplitude A=rs/2;
speed 11km/s– For r < A, Mgas ~ Mdm => ~ ½ gas
within r = A
• Simple model allows access to, and control of, key parameters…
N.B: early dwarfs were less concentrated and more gas rich than those at low redshift
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Evolution of the DM density profile
t =40 Myr
t =80 Myr
t =140 Myr
V =11 km s-1
mvir=109 M
DM halo
~ 1 fullperiod in DM halo – highly efficient
Oscillationamplitude
Must happen before halo is subsumed into next level of hierarchy
SN 1051 ergs => 80/Myr at ε=10% =>
0.01 M/yr;
gas depletion in 10 Gyr
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ρ(r<A)140 Myr
600 Myr
h = A/2 M → M/2240 Myr
For M → M/4cusp flattening after ~ 800 Myr
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z01030
Epoch of cusp removal by stellar feedback… phase-space density cannot increase in subsequent merger hierarchy
• mvir < 107 M “blowout” – may contribute to effect;
• mvir > 1010 M rotational support/large σDM, small-scale turbulence
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Z=150
Self-consistent cosmological simulations4
Mpc
(co
-mov
ing)
Constrained cosmological simulations.
Build-up of an isolated dwarf galaxy (~109 M) over z=10…5.
15 million particles (10 million hi-res).
mDM= 1900 M
mgas= 370 M
mstar= 120 M
ε = 12pc
1.1 × 107 dark
4.5 × 106 gas
4.5 × 105 star
Z=5
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Added physics…
• Jeans criterion + low-T metal cooling (10-104 K, from Bromm et al. 2001) for star formation.
• Stochastic stellar feedback; model individual supernovae as point explosions.
• Delayed-cooling feedback (Thacker & Couchman; volume-weighted).
• Pressure (not density) is constant across the SPH smoothing kernel – but only for radiative cooling calculations (~ Ritchie & Thomas 2001).
• 6x105 cpu-hour run
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ISM structureOld New
Critical to model low temperature cooling and to include a Jeans criterion in order to develop (more realistic) spatial star formation inhomogeneity
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DM-only cosmological model
Cosmological simulations of the formation of a dwarf galaxy.
Dark matter only (no gas).
Z=150…5
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Cosmologicalsimulations with gas dynamics and stellar feedback.
Central 1.3 kpcof a formingdwarf galaxy.
z = 9…5
Gas is in blue,stars are in yellow
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Evolution of enclosed gas mass for different radii
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Evolution of the central quantities (r=200 pc)
F =ρ
σ3
Enclosedmass:
Phase spacedensity,
r < 1.6kpc
r < 100pc
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Evolution of enclosed DM mass for different radii
DM onlysimulations
Simulationswith feedback
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Radial profiles
DM core: 400 pc
Stellar core: 300 pc
η =(σr2 – σt
2)/
(σr2 + σt
2)
Isotropic velocity dispersion in core
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Long-lived star clusters
Distance from galacticcentre:
• At birth (z~6.2): σr = 37 pc
• After 200 Myr:
σr = 280 pc
Orbits of “Globular Clusters”
•Stellar feedback also acts on GCs, and•Impact of dynamical friction reduced by flat core (e.g., Fornax)
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Stellar population gradients• Have been observed in most dwarf spheroidal galaxies (in
the Local Group).• Older stars are more dispersed, more metal-poor, and
kinematically warmer.• Our model (gravitational heating by bulk gas motions)
naturally explains the observed gradients:– Stars are born near the galactic center, and then gradually pushed
outwards by the feedback.
Age 0 360 Myr
Radial extent 365 pc 637 pc
Velocity disp. 15.6 km s-1 18.3 km s-1
[Fe/H] -1.33 -1.54
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Conclusions Gravitational resonant heating of matter appears to
be an inevitable consequence of bulk gas motions driven by stellar feedback in early, gas-rich dwarfs. The result is:– Large dark matter cores– Stellar population gradients.– A distribution of long-lived globular clusters.– Low stellar density and a flat-cored distribution of
stars in dSphs.– May also help resolve the “overabundance of
satellites” problem– May be relevant to dark matter detections