GH2005Gas Dynamics in Clusters III

Craig SarazinDept. of Astronomy

University of Virginia

A85 Chandra (X-ray)Cluster Merger

Simulation

• Heat and compress ICM• Increase entropy of gas• Boost X-ray luminosity, temperature, SZ

effect• Mix gas• Disrupt cool cores• Produce turbulence• Provide diagnostics of merger kinematics

Thermal Effects of Mergers

Merger with mass ratio of 3:1, offset merger (Ricker & Sarazin)

Cosmological simulation, temperature (Tittley)

• Typical shock velocity 2000 km/s

• E (shock) ~ 3 x 1063 ergs

• Main heating mechanism of intracluster gas

Merger Shocks

Ricker & Sarazin

Merger Shocks (cont.)• Although energetic, mainly weak shocks as

cluster gas is already hot

• Mach numbers ℳ ≡ vs / cs ~ 1.5-2

Merger Shocks (cont.)

Shocks are hard to see in X-rays in clusters• Shocks → more heating than compression• Weak shocks → small compressions, not very bright in X-

rays• Projection effects

Merger Boosts to LX & TX

Ricker & Sarazin

•Mergers temporarily boost

•X-ray luminosity (factor of ≲ 10)

•Temperature (factor of ≲ 3)

•Are the most luminous, hottest clusters mainly mergers?

Merger Boosts (cont.)

Randall et al.

The most luminous, hottest clusters should mainly be mergers. Affects values of ΩM and σ8 from luminous clusters.

Merger Boosts to LX & TX

Boost → ΩM underestimated by ~20%

σ8 overestimated by ~20%

Merger Boosts of SZ EffectMergers compress, heat gas, increase pressure

→ increase SZ effect

rSZ = characteristic radius

Pdldlncm

kTy eT

e

2

AAyPdVydAY area large

SZ Merger Boosts (Cont.)y

0 or

Y

y0

Y

Merger Boosts (cont.)

(Meneghetti et al., Randall et al.)

Lensing studies of masses of most luminous, hottest clusters confirm merger boosts LX & Tx

Mergers boost S-Z effect (Wik et al.)

Mergers also appear to boost probability of strong lensing

Smith et al.

•Merger simulations → turbulence in post merger shock regions of clusters

•Eturb ≈ 20% Etherm , decays following merger (Ricker & Sarazin)

•Explains radio halos in merging clusters?

•Astro-E2 should detect turbulence in merging, radio halo clusters

Turbulence and Mergers

•High spectral resolution, nondispersive X-ray spectra

•Directly measure Doppler shifts in X-rays due to gas motions in clusters

•Line widths → turbulence in cluster

•Launch in summer 2005

Astro-E2 and Mergers

Observed anticorrelation between mergers and cool cores

Not due to shocks – density gradient too big

Mainly due to ram pressure, changes in potential, instabilities, & mixing

Mergers Disrupt Cool Cores

Ricker

Cool cores → high density gas at bottom of deep potential well

Cool cores can survive for some time in mergers

Almost like a solid object moving through cluster

High density → prominent in X-ray images

Sharp front edge → very prominent in Chandra images

Cool Cores in Mergers

Ricker

Chandra: “Cold Fronts” in Mergers

Merger shocks?

No: Dense gas is cooler, lower entropy, same pressure as lower density gas

Abell 2142Markevitch et

al.

Contact discontinuity, cool cluster cores plowing through hot shocked gas

Abell 3667

Vikhlinin et al.

Cold Fronts (cont.)

Cold Fronts (cont.)

Markevitch et al.

1E0657-56 Abell 2034 Abell 85 South

Kempner et al. 2002,2003

Cold Front with Merger Bow Shock

1E0657-56

Markevitch et al.

Cool Trails Behind Cold Fronts

Cold fronts with cool trails behind (due to ram pressure)

Abell 1644 (Reiprich et al.)

Image XMM-Newton HardnessSimulation

Tittley

Give merger Mach number ℳ• Rankine-Hugoniot shock jump conditions• Mach cone angle• Stagnation condition at cold front• Stand-off distance of bow shock from cold

front

Merger Kinematics

Give merger Mach number ℳ• Rankine-Hugoniot shock jump conditions

r2/ r1 = (γ + 1) ℳ 2 /[(γ - 1) ℳ 2 + 2]

P2/P1 = [2γℳ 2 - (γ - 1)]/(γ + 1)g = 5/3

Merger Kinematics

Cold Front with Merger Bow Shock

1E0657-56

Markevitch et al.

Give merger Mach number ℳ• Rankine-Hugoniot shock jump conditions

r2/ r1 = (γ + 1) ℳ 2 /[(γ - 1) ℳ 2 + 2]

P2/P1 = [2γℳ 2 - (γ - 1)]/(γ + 1)g = 5/3

Merger Kinematics

Give merger Mach number ℳ• Rankine-Hugoniot shock jump conditions• Mach cone angle

Merger Kinematics

Cold Front with Merger Bow Shock

1E0657-56

Markevitch et al.

Give merger Mach number ℳ• Rankine-Hugoniot shock jump conditions• Mach cone angle• Stagnation condition at cold front• Stand-off distance of bow shock from cold

front

Merger Kinematics

Merger Kinematic Diagnostics

Give merger Mach number ℳ• Rankine-Hugoniot shock jump conditions• Mach cone angle• Stagnation condition at cold front

Merger Kinematics

Give merger Mach number ℳ• Rankine-Hugoniot shock jump conditions• Mach cone angle• Stagnation condition at cold front• Stand-off distance of bow shock from cold

front

Merger Kinematics

Shock stand-off distance (Sarazin 2002)

Give merger Mach number ℳ• Rankine-Hugoniot shock jump conditions• Mach cone angle• Stagnation condition at cold front• Stand-off distance of bow shock from cold

frontFind ℳ ≈ 1.5-2, shock velocity ≈ 2000

km/s

Merger Kinematics

Merger Kinematics – Abell 85

ℳ = 1.4, v = 2150 km/sD = 1.23 Mpc, qv = 71o, qd = 144o, fv = -36o, fd = 194o, y = 52oShock velocity = expected from infall →

Shocks effectively thermalize kinetic energy

Kempner et al

• Temperature changes by 5x in ≲ 5 kpc < mfp

• Thermal conduction suppressed by ~ 100 x

• Kelvin-Helmholtz instabilities suppressed

• Due to transverse or tangled magnetic field?Is conduction generally suppressed in clusters?

Transport Processes – Thermal Conduction

Ettori & Fabian; Vikhlinin et al.