RECOILING BLACK HOLES IN GALACTIC CENTERS

Michael Boylan-Kolchin, Chung-Pei Ma, and Eliot Quataert (UC Berkeley)

astro-ph/0407488

Outline

• supermassive black hole binary formation and coalescence

• gravitational radiation recoil

• effects of recoil on stellar distributions

• comparison with early-type galaxies

Supermassive Black Holes and LCDM

• hierarchical cosmology + SMBH=black hole binaries

• tdf << tH only for major mergers

• BH coalescence rate determined by both cosmological and galactic physics: galaxy merger rate BH merger rate!

Why 1 parsec should matter to a cosmologist

if ab shrinks by a factor of ~150, gravitational wave emission causes rapid coalescence

)2010( MProblem: need mass of stars

…but loss cone only contains enough stars to reduce

ab by a factor of ~10 (i.e. M)

How? gravitational slingshot

Gravitational Radiation Recoil

• Anisotropic emission of gravitational waves gives a “kick” to the newly-formed BH

• Recoil velocity depends on BH mass ratio, BH spins, and spin alignment

• Recoil velocity can reach 100-500 km/s (Favata et al. 2004)

• Many consequences - Merritt et al.; Madau & Quataert; Haiman (all 2004)

Does radiation recoil have observable effects on elliptical galaxies?

• Plan: use purely gravity N-Body experiments (GADGET) to study the effects of gravitational radiation recoil

• simulate a kicked black hole, and follow the evolution of the stellar density and velocity profiles and trajectory of the black hole

E8

1)E(

E

02

2

2

d

d

df

r

GM

ar

GMr BH*)(

Initial ConditionsUse the equilibrium distribution function to set up the particles’ phase space coordinates:

33*

)/1(

1

2)(

arr

a

a

Mr

MBH=0

MBH=M*/300

Effects on the Stellar Density

M*=1010 Msun, a=1 kpc:

vesc=293 km/s=2.82 vcirc

tdyn=26 Myr

rh=0.089 a=89 pc

Long-term evolution:

tdyn=26 Myr

v<vesc

v>vesc

No dynamical friction

dynamical friction

Dynamical friction enhances core formation

Additional Effects

• flattened density profile core in surface brightness profile

• small change of the inner velocity dispersion

• effects should be largest in galaxies with smallest vcirc(a)/vesc and for largest MBH/M

Faber et al. (1997)

So why do “power-law” ellipticals (without central cores) exist?

• power-law galaxies are typically less massive than “core” ellipticals, so the effect of a kick should be more pronounced

• power-law galaxies seem to host central black holes

Does gas play a role?

• Faber et al. (1997): gas-rich mergers could lead to power-law galaxies

• problem: requires that starburst duration is long enough to counteract both binary coalescence effects and radiation recoil effects

• solution: can gas accelerate the coalescence process?

Escala et al. (2004)

Conclusions

• supermassive BHs + hierarchical cosmology = binary black holes

• radiation recoil can lead to cores in stellar systems analogous to those seen in some early type galaxies

• gas may play an important role in enabling binary BHs to coalesce; in turn, this may help explain the existence of power-law early-type galaxies that form hierarchically

Why 1 parsec should matter to a cosmologist

BH binary must eject ~

for ab to shrink by a factor of ~150

Mq

q

1)2010(

Problem:

loss cone only contains enough stars to reduce

ab by a factor of ~5-10