Intergalactic bubbles have been observed to live up to 100,00,00 years. Since they rise it is expected that the Kelvin-Helmholtz instability should disrupt them after only a few million years. Here we show in numerical simulations that a helical magnetic field contained in the bubbles can stabilize them long enough to explain the observations. With the right amount of magnetic helicity there is no need for a large amount of magnetic energy.

1. Stabilizing Effect of Magnetic Helicity
on Magnetic Cavities
in the Intergalactic Medium
Simon Candelaresi, Fabio Del Sordo
-1.0 0.0 1.0
x
0.0
2.0
4.0
6.0
8.0
z
hydro H = 1 H = 4
0
20
40
60
80
100
E

2. (Fabian et al. 2000)
galactic disc
Intergalactic Bubbles
hot, under-dense bubble
stratified medium
2
Bubbles’ age is several tens of
millions of years.
Bubbles rise buoyantly through
density difference.
Chandra: X-ray, Perseus cluster

3. M87 in radio
3
(Churazov et al. 2001)

4. MS0735.6+7421 cluster
4
(McNamara and Nulsen 2007)
Hubble (visual) + Chandra (X-ray)

5. Kelvin-Helmholtz Instability
5
Bubbles should get disrupted.
What is the reason for their stability?
(GRAHAMUK/Wikimedia Commons)

6. Simulations (hydro)
6
(Brüggen 2003)

7. Simulations (jet inflation)
7
(Sternberg, Soker 2008)

8. Magnetic Helicity
Conservation of magnetic helicity:
magnetic resistivity
Realizability condition:
Magnetic energy is bound from
below by magnetic helicity.
8
Can magnetic helicity stabilize intergalactic cavities?

9. Numerical Experiments
9
Full resistive magnetohydrodynamics simulations
with the PencilCode.
stratified medium
hot, under-dense bubble

10. Numerical Experiments
10
box size
bubble radius
bubble density
bubble temperature
medium density
medium temperature
gravitational acceleration
magnetic field strength
viscosity
magnetic diffusivity
total time

11. Initial Condition: Beltrami Field
11
smoothing function:
inside bubble:
Fix magnetic energy, vary magnetic helicity.

12. Initial Condition: Spheromak
12

13. Thermal Emission
13
-1.0 0.0 1.0
x
0.0
2.0
4.0
6.0
8.0
z
hydro H = 1 H = 4
0
20
40
60
80
100
E

14. Temperature Iso-Surfaces
14
hydro low helicity high helicity

15. Bubble Coherence
15
0 5 10 15 20 25
t
0.6
0.8
1.0
1.2
1.4
d
mean
B = 0
Hm ≈ H0
m/2
Hm ≈ 2H0
m
Bex = 0.8
Bex = 0.2

16. Bubble Coherence
16
Helical magnetic fields can stabilize the bubbles.
0.5 1.0 1.5 2.0 2.5 3.0 3.5
zmean
0.6
0.8
1.0
1.2
1.4
d
mean
B = 0
Hm ≈ H0
m/2
Hm ≈ 2H0
m
Bex = 0.8
Bex = 0.2

17. Conclusions
simon.candelaresi@gmail.com
●
Magnetic helicity as constraint on plasma dynamics.
●
Magnetic helicity leads to stability at small magnetic energy.
●
Mechanism to stabilize intergalactic bubbles.
(Candelaresi and Del Sordo 2020 ApJ 896 86)
simon.candelaresi@gmail.com