Galactic - Center Arc & Threads: Current sheet model Y. - PowerPoint PPT Presentation

Galactic - Center Arc & Threads: Current sheet model Y. Sofue, H. Kigure & K. Shibata 2005 Nov 17-18 @ MHD WS, Chiba University

� Radio 10 GHz (3cm) Radio 10 GHz (3cm) 6x3 deg, Nobeyama 6x3 deg, Nobeyama 45m 45m � Sofue Sofue et al. 1985 t al. 1985

Vertical Magnetic Field in GC

� Radio 1.4 GHz (VLA Y Radio 1.4 GHz (VLA Yusef-Zadeh,Morris sef-Zadeh,Morris)

� Radio 5 GHz (VLA Yusef-Zadeh,Morris) Radio 5 GHz (VLA Yusef-Zadeh,Morris)

Radio Threads

� Radio 90 cm (VLA RaLosa Radio 90 cm (VLA RaLosa et al.) t al.)

� 90 cm (Anantharamaiah � et al. 1991)

Past models for the radio arc Sofue, Fujimoto 1987; Benford 1988 : B ⊥ x B φ (V) Dahlburg et al. 2002: B x Cloud instability

Interaction of Verticld Magnetic field and Rotating Gas

5 cm polarization (Sofue et al.) � RM>0 � RM<0 3 cm polarization (Tsuboi et al.)

field in GC magnetic Tw isted vertical

Origin of Vertical Origin of Vertical field in GC field in GC

High-velocity rotation and dispersion of Molecular Clouds

Molecular ring/clouds in GC Molecular ring/clouds in GC (CO Line BTL 7m Bally et al. ) (CO Line BTL 7m Bally et al. )

Face-on View Face-on View

� CO line LV � 300 � Diagram 200 � High-velocity � Rotation & 100 � V � Dispersion � km/s 0 � of Gas -100 -200 -300 � 3 2 1 0 -1 deg � Long

and Current Sheet model Magnetic reconnection

Thread properties Local, Isolated Thin/narrow Non-axisymmetric No energy injection (such as jets) High contrast against backgr.

GC MHD circumstance Vertical magnetic field Molecular clouds High-rotation (deep grav.pot) High-velocity dispersion

� Origin of � Threads � B � BH � Cloud � Accretion

� Accretion + � Amplification

� Winding, � Tw isting

� Winding, � Tw isting, � Kinking

� Reconnection

� Thread

� Threads � Intermittent � accretion � of Clouds

MHD Simulation of Threads and Arc Non-axisymmetric 3D (Method: Kigure et al. 2005)

� B + Cloud � 171x100x480 � r,phi,z meshes � Current

90 cm (LaRosa et al. 2000)

High contrast of radio brightness

Flux of released energy along the current sheet (thread) by Bz x B φ term = Poyinting flux � f = B z B φ V/4 π thread � f 0 = B z0 B φ 0 V 0 /4 π ambient Ratio of tread/ambient fluxes � r = f/f 0 = B φ /B φ０ V/V 0 >>1 because B φ０～ 0

Radio brightness Σ = 2k T b / λ 2 = δ f/ ν ~ δ (B φ /B 0 )B 0 2 V / 4 πν δ =Efficiency of Poynting flux to Radio flux (CR accel. Etc) ν～ 1 GHz V ~ 100 km/s, B 0 ~ 100 μ G From simulation B φ /B 0 ~ 0.1 . Hence, Tb ~ 10 -4 δ K. Observation: Tb ~ 0.1 – 1 K δ ~ 10 -4 -10 -5

Thread properties understood by Current sheet model: Local, Isolated, narrow Non-axisymmetric No energy injection High contrast against backgr.