The Galactic center region
• Concentrated stars and interstellar matter• High energy density (gravity, MHD, kinetic)• Strong magnetic field : B ~ mG
High external pressure: P/k 5 x 106 cm–3 K
Yusef-Zadeh, Morris, AJ (1987)
Hot and Diffuse Gas near the Galactic Center
Probed by Metastable H3+
Thomas R. Geballe Gemini Observatory
Miwa Goto Max-Planck-Institut für Astronomie
Tomonori Usuda Subaru Telescope
Benjamin J. McCall University of Illinois, Urbana-Champaign
Columbus Symposium June 23, 2005
Takeshi Oka
Department of Chemistry and Department of Astronomy and Astrophysics
The Enrico Fermi Institute, The University of Chicago
The Central Molecular Zone (CMZ)
M ~ 5 ×107 M⊙
Dominantly molecularDense clouds, n(H2) > 104 cm-4
Volume filling factor 0.1Hot gas T ~ 300 KHigh velocity dispersionLow dust temperature T ~ 20-30 K
Mezger, Duschl, Zylka, A&A Rev. (1996)
Morris, Serabyn, ARA&A (1996)
CMZ
Sightlines toward the CMZ
Visual extiction AV ~ 25 – 40
Cotera et al. ApJS, 129, 123 (2000)
AV(dense) : AV(diffuse) ~ 1 : 2
Mezger, Duschl, Zylka, A&A Rev. (1996)
J
K32.9 K
361 K
(J, K)
The ideal energy levels of H3+
+ortho I = 3/2para I = 1/2
(3, 3) metastable level
(1, 1) ground level
Δk = ± 3 forbidden transitions
27.2 days (2, 2) unstable level
Watson, JMS (1971)
(1, 0)
Oka & Epp, ApJ 613, 349 (2004) 0 ±1 ±2 ±3 k
Ncrit ~ 200 cm-3
H3+ + H2 → (H5
+)* → H3+ + H2
Discovery of metastable H3+
Goto, McCall, Geballe, Usuda,Kobayashi, Terada, OkaPASJ, 54, 951 (2002)
(3, 3) metastable
The Quintuplet
Okuda et al. ApJ 351, 89 (1990)
(1, 1) ground levelCMZ and spiral arms
(3, 3) metastable level All CMZ
(2, 2) unstable level Neither
CO Mostly spiral arms
High temperature
Low density
-100 km/s 0 km/s
-50 km/s
The Expanding Molecular RingKaifu, Kato, Iguchi, Nature (1972)Scoville, ApJ (1972)
Whiteoak, Gardner, MNRAS (1979)
)2,2(
)3,3(
N
N
Discovery of high temperature (T ~ 250 K), low density (n ≤ 100 cm-3) clouds
)1,1(
)3,3(
N
N
Oka & Epp, ApJ 613,349 (2004)
Clouds vLSR range N( H3+) Temperature Density
(km s-1) (1014 cm-2) (K) (cm-3)
-100 km s-1 -140 → -74 15.7 ± 1.7 270 ± 70 ≤ 50
-50 km s-1 -74 → -40 6.6 ± 1.3 250 ± 100 ≤100
0 km s-1 -40 → +32 8.4 ± 1.6 130 ± 100 ≤ 200
Total (CMZ) 30.7 ± 2.7 Spiral arms 12.3
Total 43 ± 3
Clouds toward GCS 3-2
Other probes: H, OH, H2CO, CO, CS, HCN, NH3, HCO+, H2
Oka, Geballe, Goto, Usuda, McCall, ApJ in press
Main Results so far• Vast amount of gas• High temperature: T ~ 250 K• Low Density: n ~ 100 cm-3
• H3+ total column density: 4.3 1015 cm-2
• ¾ in CMZ, ¼ in intervening spiral arms
• ½ of H3+ in CMZ in the Expanding Mol. Ring
• Pathlength ~ 50 pc• Ionization rate ~ 5 10-15 s-1
Hot and Diffuse Gas, Questions abound
What is the heating mechanism?
How is the pressure balanced?
How are they related to molecular clouds and H II regions?
What is the relation to X-ray (-ray) sources?
What is their role in the strong magnetohydrodynamic effects?
Galactic plane
Quintuplet
Telescopes and Spectrometers
UKIRT 3.8 m CGS4 8 km s-1 Mauna Kea
Subaru 8 m IRCS 15 km s-1 Mauna Kea
Gemini South 8 m Phoenix 5 km s-1 Cerro Pachon
1. Sightline crosses spiral arms.2. Sightline passes though a more diffuse gas (AV=15-25 mag) than any other in the Galaxy. 3. Sightline passes through Galactic Center clouds - a unique and complex environment.
Inner 300 pc – showing Sgr B2 and Expanding Ring
Radiation Collision Reaction
Rigorous Selection Rules Approximate Rules Random
+ - ΔJ = ± 1, 0
ortho para ΔI = 0 ortho para ΔI = 0 ΔItotal = 0
A
A
B
B
C
A
D
B
Ψ’
ψ
Trad 3 K « Tkin
Rate Equations
1' ' ' '2
' '
( ) exp( )2
J K JK JK J KJK JK
J K
g E Ek C
g kT
2
' '' '
' '
' '' '
' '
( )( ) ( )
( ' ') ( )
( ' ') ( )
( ) ( ) ( ) ( )
J K JKJK J K
J K
J K JKJK J K
J K
e XX
dn JKn H f JK
dt
A n J K A n JK
k n J K k n JK
k n e n JK k n X n JK
Production
Spontaneous emission 4 10-7 s-
1
Collision n(H2) 100 cm-3 10-7 s-1
Destruction 10-9 s-1
10-17 s-1
10-7 cm3 s-1 10-9 cm3 s-1
' '' '
' ' ' '
exp( )J KJK JK JK J KJKJ K J K
k g E E
k g kT
H3+ + H2 (H5
+)* H3+ + H2
Detailed balance
Cosmic ray ionization
Dissociative recombination Proton hop reaction
0 =
Steady State
Oka, Epp ApJ, 613, 349 (2004)