A Large Scale, MHD Resonant A Large Scale, MHD Resonant Instability - - PowerPoint PPT Presentation

A Large Scale, MHD Resonant A Large Scale, MHD Resonant Instability in a Galactic-Like Disk Instability in a Galactic-Like Disk

Marco A. Martos Marco A. Martos Instituto de Astronomía Instituto de Astronomía Universidad Nacional Autónoma de México Universidad Nacional Autónoma de México Korean Astrophysics Workshop on Dynamics Korean Astrophysics Workshop on Dynamics

• f Disk Galaxies
• f Disk Galaxies

October 21-24, 2013 October 21-24, 2013

Motivation and Plan Motivation and Plan

• 1. Is the galactic gas flow stable?
• 1. Is the galactic gas flow stable?

(Is there a MRI instability operating in the galactic disk?) (Is there a MRI instability operating in the galactic disk?) Scale of MHD turbulence and other phenomenology Scale of MHD turbulence and other phenomenology seem too large for local processes. seem too large for local processes.

• 2. Consider the following experiment:
• 2. Consider the following experiment:

Let a flat, gaseous, magnetized, non-selfgravitating disk Let a flat, gaseous, magnetized, non-selfgravitating disk rotate in a gravitational galactic-like stellar potential rotate in a gravitational galactic-like stellar potential including a 2-arms spiral. including a 2-arms spiral. Use available information for Use available information for galactic parameters. Follow HD and MHD regimes. Keep galactic parameters. Follow HD and MHD regimes. Keep track of the gas response at resonances. track of the gas response at resonances.

The comparison of near-infrared and optical The comparison of near-infrared and optical images of external galaxies reveal images of external galaxies reveal interesting differences interesting differences

FIR: dust (optically thin at 240 FIR: dust (optically thin at 240 μ μ) ) ; NIR: stars (J, K bands) ; NIR: stars (J, K bands) Drimmel (2000); Drimmel and Spergel (2001)

Drimmel (2000); Drimmel and Spergel (2001)

On the left, the 4 arms of HII regions; the K band (stars); the 2 arms model for J, K bands (dashed) and the K-band fit (solid); surface density of dust (right)

Simulations Simulations (Gómez, Pichardo, Martos 2013)

(Gómez, Pichardo, Martos 2013)

What to expect from previous work on resonances What to expect from previous work on resonances

Contotopoulos and collaborators, in a series of papers, (v.g. Patsis, Grosbol and Hiotelis 1997 and references therein)

Patsis, Contopoulos, Grosbol: in teir self-consistncy study of 12 observed galaxies, in Sb and Sc tzpes te best model is a nonlinear one ending at te 4/1 resonance

• The 4:1 resonance generates a bifurcation of the arms and interarm features
• The central family of periodic orbits do not support a spiral pattern beyond the

position of the 4:1 resonance

• Weak spirals can extend their pattern up to corotation
• The radial extent of 2-armed patterns is consistent with the

location of, either: ILR; 4:1 resonance; OLR

• B. and D. Elmegreen (1990): optical structure linked to those and other resonances

(6:1; 1:1) related to outer edge of spiral pattern, rings, spurs, kinks

• S. Chakrabarti, Laughlin and Shu (2003): the role of ultraharmonic resonances,

wave reflections, local gravitational instabilities in the formation of substructure akin to branches, spurs and feathers in a self-gravitating SID; long before, Shu, Milione & Roberts (1973) showed that substructure appears from ultraharmonic resonances

What the stars and gas do (Martos What the stars and gas do (Martos et al MNRAS 2004) et al MNRAS 2004)

Hydrodynamic Flow (Yáñez, Norman, Martos, Hayes Hydrodynamic Flow (Yáñez, Norman, Martos, Hayes 2008) is Stable 2008) is Stable

Facts about Corotation and Spiral Density Facts about Corotation and Spiral Density Waves Waves

☻ ☻ Spiral-Vortex Structure (Contopoulos 1978; Fridman et al 1997 ): Spiral-Vortex Structure (Contopoulos 1978; Fridman et al 1997 ): Vortices are typical for the stellar and gaseous flows in the CR region of barred and Vortices are typical for the stellar and gaseous flows in the CR region of barred and spiral galaxies, due to the presence of stable long period orbits around the stable spiral galaxies, due to the presence of stable long period orbits around the stable Lagrangian points. Gas streamlines are anticyclonic near the CR radius (and near Lagrangian points. Gas streamlines are anticyclonic near the CR radius (and near the LSR). the LSR). ☻ ☻ Linear theory was considered succesfull: each mode/wave in stellar disk Linear theory was considered succesfull: each mode/wave in stellar disk

• would be ampified by Swing mechanism at CR
• would be ampified by Swing mechanism at CR
• conserve E, L except at Lindblad resonances
• conserve E, L except at Lindblad resonances
• their radial propagation amplified once reflected at CR, and damped at LR
• their radial propagation amplified once reflected at CR, and damped at LR
• in a cavity between center and CR, Swing amplifies for waves inside CR have
• in a cavity between center and CR, Swing amplifies for waves inside CR have

negative energy inside and positive beyond CR towards OLR. negative energy inside and positive beyond CR towards OLR.

• gas disk: torques only at LR and CR. At CR the torque has the sign of the radial
• gas disk: torques only at LR and CR. At CR the torque has the sign of the radial

gradient of vorticity. External potencial excites density waves near LR and CR. No gradient of vorticity. External potencial excites density waves near LR and CR. No net L is transported from CR. So, the L deposited there accumulates in the gas net L is transported from CR. So, the L deposited there accumulates in the gas (Goldreich and Tremaine 1979). (Goldreich and Tremaine 1979). ☻ ☻ MHD density waves may become unstable near CR (Lou, Yuan and Fan 2001); CR MHD density waves may become unstable near CR (Lou, Yuan and Fan 2001); CR resonance may be split into 2 magnetic resonances (Fu and Lai 2010) resonance may be split into 2 magnetic resonances (Fu and Lai 2010) ☻ ☻ Spiral waves driven by magnetic stresses are now considered (Tagger et al 1989, v.g.) Spiral waves driven by magnetic stresses are now considered (Tagger et al 1989, v.g.)

Verniere and Tagger 2002 Verniere and Tagger 2002

The galactic radio gap (Amores, The galactic radio gap (Amores, Lepine, Mishurov 2009) Lepine, Mishurov 2009)

Lagragian trajectories near Lagragian trajectories near inner resonances inner resonances

Lagrangian trajectories around Lagrangian trajectories around Corotation Corotation

Comparison MHD (top) and HD Comparison MHD (top) and HD

Field Reversals (Moss, Beck, Sokoloff et al 2012, Field Reversals (Moss, Beck, Sokoloff et al 2012, v.g.) v.g.)

Field Reversal and Release of Magnetic Energy Field Reversal and Release of Magnetic Energy (Vainshtein, Rosner & Sagdeev 2002) (Vainshtein, Rosner & Sagdeev 2002)

MHD at 1 Gy (top) and 3 Gy: n, Vrad, Baz MHD at 1 Gy (top) and 3 Gy: n, Vrad, Baz

HD 3 Gy; n (top), Vrad HD 3 Gy; n (top), Vrad

Ranges for growth Ranges for growth

Does not in HD (hot or cold; isothermal or Does not in HD (hot or cold; isothermal or adiabatic; short or long pattern extent; adiabatic; short or long pattern extent; pattern speed slow or fast) pattern speed slow or fast) It does in MHD (pattern speed above 15 It does in MHD (pattern speed above 15 km/kpc/s or so; B local in [.1,6] km/kpc/s or so; B local in [.1,6] microgauss or so (weak limit gives rise to microgauss or so (weak limit gives rise to steady state solution with gap but no steady state solution with gap but no radial widening); hot or cold; short or long; radial widening); hot or cold; short or long; isothermal or adiabatic; different pitch isothermal or adiabatic; different pitch angles. angles.

Provisional Conclusions Provisional Conclusions

Unstable MHD flow at Corotation appears related with: Unstable MHD flow at Corotation appears related with:

• An explosive release of magnetic energy stored from
• An explosive release of magnetic energy stored from
• rbital energy
• rbital energy
• A diluted ring at the corotation circle
• A diluted ring at the corotation circle
• An abrupt truncation of the disk
• An abrupt truncation of the disk
• Large scale turbulence
• Large scale turbulence
• Field reversals and creation of random B
• Field reversals and creation of random B

The mechanism is, most likely, the build up of The mechanism is, most likely, the build up of magnetic pressure on both sides of CR, leading to magnetic pressure on both sides of CR, leading to the breakdown of centrifugal equilibrium from an the breakdown of centrifugal equilibrium from an increase of magnetic tension. increase of magnetic tension.