Download The final frontier: Dark matter and smallest galaxies

Effects of baryons on the structure
of massive galaxies and clusters
Oleg Gnedin
University of Michigan
Collisionless N-body simulations predict a nearly universal profile
of dark matter halos. The dissipation of baryons is changing that.
galaxy to an observer
galaxy to a theorist
N-body simulations of halos reproduce large-scale
clustering of galaxies
blue  observations
red  simulations
and predict a universal
halo density profile:
 r2
(recent update: Navarro et al. 2010)
Dissipation and condensation of baryons at the centers
of dark matter halos modifies the halo profile:
radial contraction, rounder shape, isotropic velocity distribution,
higher inner angular momentum, …
without baryon dissipation
with baryon dissipation
Zeldovich was the first to propose it in 1980
(in the context of heavy neutrinos)
After Barnes & White (1984) and
Blumenthal et al. (1986) it became
known as “adiabatic contraction”
M(r) r = const before contraction and after
(assuming spherical symmetry, slow collapse)
This dynamical process is simple
and intuitive
In the 1980s galaxy formation was monolithic collapse
and secondary infall. Is the model still valid in the
modern hierarchical structure formation, with halo
mergers and non-spherical accretion?
Cosmological simulation of a galaxy cluster.
Compare runs with and without gas
dissipation, for the same initial conditions
(OG et al. 2004): adiabatic contraction
indeed approximately works!
In the inner regions where baryons dominate mass,
halo shape turns from triaxial to round
b/a
1
c/a
0
1
0
10-2
Kazantzidis et al. 2004
10-1
r/rvir
1
10-2
10-1
1
r/rvir
Zemp et al. 2012
Anisotropy parameter
Velocity distribution turns from radially-biased to isotropic
B: without gas
dissipation
A: with cooling,
star formation,
stellar feeeback
Other effects:
• inner part of DM halo is aligned with the baryon disk, outer part retains
memory of dissipationless formation (halo may twist with radius)
• a given Lagrangian mass of dark matter approximately conserves its
angular momentum over time
Pseudo-phase space density of DM is not a universal power law
in collisionless case:
/3  r-1.9
extent of stellar disk
10 kpc
Zemp et al. (2012): disk galaxy
formation with H2 chemistry
but weak stellar feedback
with dissipation, /3 is reduced
Measurements of mass of elliptical galaxies
using gravitational lensing support the model:
DM density is enhanced relative to NFW
(75000 SDSS galaxies, Schulz et al. 2010)
However, DM-dominated dwarf galaxies show
cores instead of cusps: violation of the model
Oh et al. 2011
Not always contraction: Strong stellar feedback can remove
enough gas mass to reduce DM density, between z=3 and z=0
(such feedback is needed to alleviate other problems of CDM)
Dwarf galaxy simulation with
blastwave outflow feedback
(Governato et al. 2012)
Repeated outflows trigger potential
fluctuations that heat DM particles and
reduce central density, more than
adiabatic expansion in a single outflow
(Pontzen and Governato 2012)
Core creation: not so fast, not so simple
Core forms gradually at 1 < z < 3:
MW-sized galaxy, with Gasoline,
Maccio et al. 2012
One large burst of star formation
removes more DM than 10 small ones,
but does not create a core: dwarf halo,
with Gadget, Garrison-Kimmel et al. 2013
(also shown by OG & Zhao 2002)
No dark matter core in massive galaxy simulations
MW-sized galaxy, with Arepo,
Marinacci et al. 2014
Eris simulation: MW-sized galaxy,
with Gasoline, Guedes et al. 2011
Summary
• Observational and theoretical evidence for halo contraction
in massive galaxies and clusters of galaxies
• Halos become rounder, may twist, gain angular momentum,
and develop isotropic velocity distribution
• Evidence for central cores in dwarf galaxies, due to stellar
feedback and potential fluctuations
• Knowing detailed halo structure is important for modeling
of galaxies, and direct and indirect detection of dark matter
• Zeldovich was right: initial contraction in all cases, but
galaxy formation is more complex