Transcript Formation of Disk Galaxies

Dark Matter Candidates: Particles
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WIMPs, particularly LSPs: mass >GeV.
Neutralinos: New parity associated with
supersymmetry (a way for fermionsbosons).
Axions: Invented to explain why weak force
violates CP, but strong force does not.
10-6<max<10-3eV: Upper limit from SN1987A
cooling; lower from BBN.
Currently neutralinos and axions are best
candidates for dark matter; neither has been
detected or is predicted in Standard Model.
Dark Matter Detection
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To detect,
generally
look for
signatures
of Earth
moving
through DM
fluid
(seasonal).
Really Cold and Collisionless?
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2 problems with CDM halos: Too
cuspy, too much substructure.
Dark matter not cold?
Self-interacting (Spergel & Steinhardt): Must avoid core collapse!
Fuzzy: 10-22eV Bose condensate.
Decaying: ~½ of DM decays into
relativistic particles.
Disappearing: Goes into 5th
dimension via brane.
Fluid: Scalar field with quartic
potential yields “pressure”.
Probably forgotten some…
Modified Newtonian Dynamics (MOND)
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MOND proposes
that on large scales,
F=(GMa0)1/2/r.
Can fit RCs of
galaxies extremely
well.
Can almost fit CMB:
3rd peak is key.
Runs into trouble in
clusters and Ly-a
forest.
MOND+baryonic
DM? Hmm…
Aguirre et al 2001
Bullet Cluster:
Dark Matter is
Collisionless
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Interacting cluster
lensing+X-rays shows
that mass doesn’t trace
baryons.
Exactly as predicted by
CDM: Dark matter
passes thru, gas is
shocked.
Difficult with baryonic
DM because high
velocities would destroy
cold, unseen baryons.
Clowe et al 2006
Galaxy
Mergers
Orders of magnitude
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2x1012 M galaxies colliding @ 300
km/s  1053 J (~108-9 SNe, ~binding energy).
Power (assuming 1Gyr time): 1037 W (1 SN)
Stellar collisions VERY rare: Near center, ~1000
stars/ly2  collision prob ~ 10-11.
OTOH, ISM filling fraction is high, so molecular
cloud collisions common, and highly supersonic
(T~100K, v~300 km/s  M~300). Coronal gas
has T~106, so M~1.
Hence old stellar population reconfigures, but new
stars may be formed via collisional processes.
Early N-body merger
simulations
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Holmberg 1941: 74 light
bulbs and a lot of patience.
Toomre & Toomre 1972:
Mergers cause tidal features.
Barnes & Hernquist 1991:
Remnants look like ellipticals,
with kinematic features.
Toomre & Toomre 1972
Holmberg 1941
Mergers fuel
starbursts & transform
morphologies
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Mihos & Hernquist 1996:
Included SF (Schmidt
Law) in hydro sims.
Gas gets driven into
central regions owing to
dynamical instabilities,
fuels starburst.
Remnant looks something
like an elliptical.
Merger Trees & Semi-analytic models
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CDM is a “bottom-up”
structure formation model.
Dark matter has no known
pressure; it collapses
immediately into small
units (size unknown).
Units merge thru
gravitational instability.
Semi-analytic models
(SAMs): Merger tree +
MMW disks + heuristic
algorithm for how mergers
affect galaxies.
Wechsler et al 2001
Ellipticals: Nature vs. Nurture
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Can ellipticals form
mostly from low-spin
halos?
No! Not enough.
But not totally clear
that mergers alone can
explain it either…
In simulations, gas
reaccretes, E’s  S’s!
Not only must merge
spirals, but also
prevent reaccretion.
Spiral Galaxy Formation
Kinematics of merger remnants
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Can mergers reproduce E
isophotes?
Large E’s boxy, small E’s
disky (Davies et al 83).
Naab etal: Put in merger
tree, try to reproduce
fraction of anisotropic
(non-rot) E’s.
boxy
disky
Rotationsupported
Pressuresupported
Spiral-spiral mergers alone
can’t do it!
Need E-E/E-S mergers…
also needs gas supply
shut-off above some M*.
Naab, Kochfar, Burkert 06
Dry Mergers
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If halos merge late, but stars
are old  dry mergers!
Do dry mergers preserve
tight E properties?
Fundamental plane: Rsa I-b.
Red sequence: Tilted!
Clusters & Galaxy Harassment
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In clusters, scl » sgal 
Direct collisions rare!
But morphologies still
altered due to harassment:
Tidal disturbance from
close passage.
Can help explain why
clusters have ~no spirals.
Moore, Katz, Lake 1997
Spiral + Spiral = Spiral
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If initial systems is gas rich enough, then gas
flung to large radii can reaccrete into a spiral.
So gas fraction is another parameter in
morphological transformations.
To produce late-type galaxies today, need to
prevent growth of bulge  AGN?
Robertson et
al 2006
Mergers  AGN?
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diMatteo, Springel,
Hernquist: Assume some
fraction of inflow at
resolution limit (~100 pc)
reaches central BH.
Add feedback energy, grow
BH.
Significantly suppresses
post-merger SF.
Get red sequence, MBH-s
relation, etc.
Realistic?
Springel, di Matteo, Hernquist 2005