Talk given at the "15 Years of Science with Chandra" Symposium, November 2014, Boston, MA.

1. CHANDRA, COLD FRONTS, AND
ICM PHYSICS: THE IMPORTANCE
OF MAGNETIC FIELDS
John ZuHone, MIT Kavli Institute
with
Matthew Kunz (Princeton), Maxim Markevitch (NASA/GSFC),
James Stone (Princeton), Veronica Biffi (SISSA)

2. COLD FRONTS: A CHANDRA
SUCCESS STORY
• Chandra’s resolution led
to their discovery, and
their puzzling features
• Most are remarkably
smooth, free of K-H
instabilities
• Density and temperature
jumps are very sharp, on
the order of ~kpc
2 COLDFRONTS
200 kpc
A2142
Markevitch & Vikhlinin 2007, Markevitch et al 2000
A3667
Fig. 4. Cold fronts in A2142 (reproduced from M00). a
c
200 kpc
2.1 Cold fronts in mergers
a
b
Fig. 4. Cold fronts in A2142 (reproduced from M00). Fig. 3. Chandra X-ray images of clusters with the first discovered cold fronts, A2142 and A3667. In
a
b

3. • Interactions with small subclusters (Asascibar & Markevitch 2006)
• A passing subcluster accelerates both the gas and dark matter components of the cluster core, but the gas
component experiences a “ram pressure slingshot” effect
• As the ram pressure weakens, the cold core gas falls back into the DM core, but overshoots it and begins to
“slosh”
T (keV) B (G)

4. COLD FRONT STABILITY
• Large velocity shears exist across the cold front; the fronts should
be susceptible to the effects of the Kelvin-Helmholtz instability
• Thermal conduction, if present, should smooth out the
temperature gradient
• What could stabilize the front surfaces against these effects?
• Viscosity?
• Magnetic fields?

5. SLOSHING WITH MAGNETIC
FIELDS
No Fields T (keV) With Fields

6. VISCOSITY AND COLD FRONTS
Roediger et al 2013
10% Spitzer 1% Spitzer
0.1% Spitzer Inviscid

7. ICM MICROPHYSICS
• In the ICM, λmfp ≫ ρL, so
momentum and heat transport
are modified strongly by the
magnetic field and become
anisotropic
• Just how anisotropic depends on
the geometry of the magnetic field
• Tangled fields ~ isotropy with a
suppression factor
• Ordered fields ~ strong anisotropy
Momentum
Heat

8. VISCOSITY AND COLD
FRONTS
ZuHone et al 2014, arXiv:1406.4031

9. VISCOSITY AND COLD
FRONTS
ZuHone et al 2014, arXiv:1406.4031

10. VISCOSITY AND COLD
FRONTS
ZuHone et al 2014, arXiv:1406.4031

11. similar
A WORD OF CAUTION
dissimilar
ZuHone et al 2014, arXiv:1406.4031

12. THERMAL CONDUCTION
(also see ZuHone et al 2013)

13. IMPLICATIONS FOR CONDUCTION
• The inability of the magnetic field to
completely suppress conduction across cold
front surfaces is potentially strong evidence
for suppression of conduction along the field
lines
• Recent measurements of thermal
conductivity in the solar wind indicate
Spitzer-level conduction, including in regions
with β ~ 100 like the ICM
• Something missing from the simulations?
Something different about the physics?
Heat Flux Measured in in Near-Earth Solar Wind
Heat Flux Measured in situ
in near-Earth solar wind
qe ~ Spitzer
electron mfp x (dlnT/dr)
Bale+2013
heat flux
Bale et al 2013 (taken from a talk by E. Quataert)

14. VISCOSITY AND COLD
FRONTS
Line Shift Line Width
line broadening
without turbulence ZuHone et al 2014, arXiv:1406.4031

16. SUMMARY
• Chandra’s unprecedented resolution led to the discovery of cold fronts
with intriguing properties
• What stabilizes them against K-H? Viscosity? Magnetic fields? Both? Can we
tell the difference? Need to include both in our models!
• Cold fronts reveal more about thermal conduction—either some
ingredient is missing from simulations or thermal conductivity in the ICM is
suppressed by many orders of magnitude
• Astro-H and Athena will tell us something about the velocity structure of
the gas around cold fronts—but beware of simple interpretations!