1. The effect of magnetic fields on the survival of high-velocity clouds
Asger Grønnow
PhD Student
Supervisors:
Joss Bland-Hawthorn
Thorsten Tepper-García

2. High Velocity Clouds (HVCs)
HI clouds with LSR velocities exceeding 90 km s-1
Several origins
Typical properties:
L~1 kpc, n~0.1 cm-3, d~10 kpc
Total mass in MW ~4×108 M☉
Important source of gas
for star formation
Smith cloud in HI, Nichols et al. (2014)
Map of Milky Way HVCs, Putman et al. (2012)

3. Measurements of magnetic fields in HVCs
Based on Faraday rotation
Strong (≳5 µG) fields
HVC in the Leading Arm (McClure-Griffiths et al. 2010)
Smith Cloud (Hill et al. 2013)

4. Open questions relating to magnetic fields in and around HVCs
Are the magnetic fields internal or external?
Where could internal fields come from and how would they be sustained?
Are the fields organised and amplified by compression?
Do magnetic fields hinder or accelerate HVC destruction?

5. Lifetime of HVCs
𝜒= 𝜌 𝑐 𝜌 ℎ
HVCs should be destroyed within a couple of “cloud crushing times” but some are much older
Plausible mechanisms are dark matter confinement and magnetic fields
𝑡 𝑐𝑐 ~ 𝜒 1/2 𝑣 −1 𝑅 𝑐
Schneider & Robertson (2016)

6. HVC simulation using magnetohydrodynamics (MHD)
Grid based code PLUTO
Outflow boundary conditions
Assume infinite conductivity (ideal MHD)
Ensure that magnetic field stays divergence-free B v
Schematic of “wind tunnel” simulation

7. Previous simulations with magnetic fields
Gregori et al. (1999)
Magnetic fields can accelerate the destruction of a cloud
McCourt et al. (2015)
Magnetic fields can hinder the destruction of a cloud

8. Strategy Probe the parameter space Add complications one at a time
Compare to equivalent simulations without magnetic fields
Check convergence

9. Parameter space Various BB: Banda-Barragan et al. 2015
Velocity
Density
Subsonic
Transonic
Supersonic
χ~10 MC SH
χ~100 GR
χ~1000 DP BB
Cloud
External
No field
Perpendicular
Parallel
Oblique
Tangled
Various DP
BB, SH,GR MC
BB, SH
BB: Banda-Barragan et al. 2015
SH: Shin et al. 2008
MC: McCourt et al. 2015
DP: Dursi & Pfrommer 2008
GR: Gregori et al. 1999

10. Effect of the magnetic field

11. Preliminary findings
A uniform magnetic field can affect cloud survival – but the orientation of the field matters
Less mixing in simulation with transverse field
Initially weak transverse magnetic field quickly becomes significantly amplified

12. “Draping” of the magnetic field

13. Future work
Examine different magnetic configurations, in particular differing external/internal fields
Add more physics, e.g. cooling
Clouds with fractal density distributions

14. Summary
There are many unresolved questions about the origin and effect of magnetic fields in HVCs
HVCs survive longer than can be explained by hydrodynamics alone
Magnetic fields are a plausible method to prolong the lifetime of HVCs
The configurations of the magnetic fields matter

15. 2D density projections

16. Convergence testing

17. Stability of initial conditions
Initial conditions cannot be completely stable when there is a magnetic field
Ram pressure can help confine internal magnetic fields