CHANDRA, COLD FRONTS, AND ICM PHYSICS: THE IMPORTANCE

OF MAGNETIC FIELDSJohn ZuHone, MIT Kavli Institute

withMatthew Kunz (Princeton), Maxim Markevitch (NASA/GSFC),

James Stone (Princeton), Veronica Biffi (SISSA)

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 COLD FRONTS

200 kpc

A 2 1 4 2

200 kpc

A 3 6 6 7

Fig. 3. ChandraX-ray images of clusters with the first discovered cold fronts, A2142 and A3667. InA2142, at least two sharp brightness edges are seen, between blue and black in the NW and betweenpurple and blue south of center. In A3667, there is a prominent edge SE of center. Unrelated compactX-ray sources are not removed. (Images are created from the recent long Chandra exposures.)

10

2.1 Cold fronts in mergers

a

b

c

d

e

Fig. 4. Cold fronts in A2142 (reproduced from M00). (a) X-ray image with red overlays showingregions used for derivation of temperature profiles (panel b). In panels (b-e), the southern edgeis shown in the left plot and the northwestern edge is in the right plot. Panel (c) shows X-raybrightness profiles across the edges in the same sectors. The red histogram is the brightness modelthat corresponds to the best-fit gas density model shown in panel (d). Panel (e) shows pressureprofiles obtained from the temperature and density profiles. Error bars are 90%; vertical dashedlines show the positions of the density jumps.

of such a shape are shown in panel (d), and their projections are overlaid on the data ashistograms in panel (c) — they provide a very good fit. Since there is no way of knowingthe exact three-dimensional geometry of the edge, for such fits we have to assume that thecurvature of the discontinuity surface along the line of sight is the same as in the sky plane.To ensure the consistency with this assumption, it is important that the radial profiles andthe three-dimensional model for the gas inside the discontinuity are centered at the centerof curvature of the front, which is often offset from the cluster center. At the same time, themodel of the outer, “undisturbed” gas may need to be centered elsewhere (e.g., the clustercentroid).

Panel (b) in Fig. 4 shows the gas temperature profiles across the edges. For a shock dis-continuity, the Rankine–Hugoniot jump conditions directly relate the gas density jump,

11

2.1 Cold fronts in mergers

a

b

c

d

e

Fig. 4. Cold fronts in A2142 (reproduced from M00). (a) X-ray image with red overlays showingregions used for derivation of temperature profiles (panel b). In panels (b-e), the southern edgeis shown in the left plot and the northwestern edge is in the right plot. Panel (c) shows X-raybrightness profiles across the edges in the same sectors. The red histogram is the brightness modelthat corresponds to the best-fit gas density model shown in panel (d). Panel (e) shows pressureprofiles obtained from the temperature and density profiles. Error bars are 90%; vertical dashedlines show the positions of the density jumps.

of such a shape are shown in panel (d), and their projections are overlaid on the data ashistograms in panel (c) — they provide a very good fit. Since there is no way of knowingthe exact three-dimensional geometry of the edge, for such fits we have to assume that thecurvature of the discontinuity surface along the line of sight is the same as in the sky plane.To ensure the consistency with this assumption, it is important that the radial profiles andthe three-dimensional model for the gas inside the discontinuity are centered at the centerof curvature of the front, which is often offset from the cluster center. At the same time, themodel of the outer, “undisturbed” gas may need to be centered elsewhere (e.g., the clustercentroid).

Panel (b) in Fig. 4 shows the gas temperature profiles across the edges. For a shock dis-continuity, the Rankine–Hugoniot jump conditions directly relate the gas density jump,

11

2.1 Cold fronts in mergers

a

b

c

d

e

Fig. 4. Cold fronts in A2142 (reproduced from M00). (a) X-ray image with red overlays showingregions used for derivation of temperature profiles (panel b). In panels (b-e), the southern edgeis shown in the left plot and the northwestern edge is in the right plot. Panel (c) shows X-raybrightness profiles across the edges in the same sectors. The red histogram is the brightness modelthat corresponds to the best-fit gas density model shown in panel (d). Panel (e) shows pressureprofiles obtained from the temperature and density profiles. Error bars are 90%; vertical dashedlines show the positions of the density jumps.

of such a shape are shown in panel (d), and their projections are overlaid on the data ashistograms in panel (c) — they provide a very good fit. Since there is no way of knowingthe exact three-dimensional geometry of the edge, for such fits we have to assume that thecurvature of the discontinuity surface along the line of sight is the same as in the sky plane.To ensure the consistency with this assumption, it is important that the radial profiles andthe three-dimensional model for the gas inside the discontinuity are centered at the centerof curvature of the front, which is often offset from the cluster center. At the same time, themodel of the outer, “undisturbed” gas may need to be centered elsewhere (e.g., the clustercentroid).

Panel (b) in Fig. 4 shows the gas temperature profiles across the edges. For a shock dis-continuity, the Rankine–Hugoniot jump conditions directly relate the gas density jump,

11

Markevitch & Vikhlinin 2007, Markevitch et al 2000

T (keV) B (G)

• 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”

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?

SLOSHING WITH MAGNETIC FIELDS

T (keV)No Fields With Fields

VISCOSITY AND COLD FRONTS

Roediger et al 2013

10% Spitzer 1% Spitzer

0.1% Spitzer Inviscid

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

VISCOSITY AND COLD FRONTS

ZuHone et al 2014, arXiv:1406.4031

VISCOSITY AND COLD FRONTS

ZuHone et al 2014, arXiv:1406.4031

VISCOSITY AND COLD FRONTS

ZuHone et al 2014, arXiv:1406.4031

similar

A WORD OF CAUTION

dissimilar

ZuHone et al 2014, arXiv:1406.4031

THERMAL CONDUCTION

(also see ZuHone et al 2013)

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 situ!in Near-Earth Solar Wind

qe ~ Sp

itzer

electron mfp x (dlnT/dr)

Bale

+20

13

heat

flux

Measured via !3rd moment of!

the electron!distribution!

function!!

β ~ 1 on avg!!

Clusters in!Regime where!Spitzer is Valid!

! !

Bale et al 2013 (taken from a talk by E. Quataert)

Heat Flux Measured in situin near-Earth solar wind

VISCOSITY AND COLD FRONTS

Line Shift Line Width

line broadening without turbulence ZuHone et al 2014, arXiv:1406.4031

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!