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  red  observations simulations and predict a universal halo density profile: (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 1 Kazantzidis et al. 2004 0 1 0 10 -2 10 -1 1 10 -2 10 -1 r/r vir r/r vir Zemp et al. 2012 1

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

10 kpc Zemp et al. (2012): disk galaxy formation with H 2 chemistry but weak stellar feedback extent of stellar disk 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