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)