The Galactic center region Concentrated stars and interstellar

The Galactic center region • Concentrated stars and interstellar matter • High energy density (gravity, MHD, kinetic) • Strong magnetic field : B ~ m. G 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 H 3+ Takeshi Oka Department of Chemistry and Department of Astronomy and Astrophysics The Enrico Fermi Institute, The University of Chicago Thomas R. Geballe Miwa Goto Tomonori Usuda Benjamin J. Mc. Call Gemini Observatory Max-Planck-Institut für Astronomie Subaru Telescope University of Illinois, Urbana-Champaign Columbus Symposium June 23, 2005

The Central Molecular Zone (CMZ) M ~ 5 × 107 M⊙ Dominantly molecular Dense clouds, n(H 2) > 104 cm-4 Volume filling factor 0. 1 Hot gas T ~ 300 K High velocity dispersion Low dust temperature T ~ 20 -30 K Morris, Serabyn, ARA&A (1996) CMZ Mezger, Duschl, Zylka, A&A Rev. (1996)

Sightlines toward the CMZ Visual extiction AV ~ 25 – 40 Cotera et al. Ap. JS, 129, 123 (2000) AV(dense) : AV(diffuse) ~ 1 : 2 Mezger, Duschl, Zylka, A&A Rev. (1996)

H 3+ more abundant in Diffuse Clouds Dense clouds Mc. Call, Geballe, Hinkle Oka, Ap. J 522, 338(1999) N(H 3+) ~ 3. 6 × 1012 cm-2 AV N(H 3+) ~ 4. 4 × 1013 cm-2 AV Diffuse clouds Per Mc. Call, Hinkle, Geballe, Moriarty-Schieven, Evans, Kawaguchi, Takano, Smith, Oka, Ap. J 567, 391 (2002)

Sightlines toward the CMZ Visual extiction AV ~ 25 – 40 Cotera et al. Ap. JS, 129, 123 (2000) AV(dense) : AV(diffuse) ~ 1 : 2

The ideal energy levels of H 3+ ortho I = 3/2 para I = 1/2 + (J, K) H 3+ + H 2 → (H 5+)* → H 3+ + H 2 J Δk = ± 3 forbidden transitions (3, 3) metastable level Ncrit ~ 200 cm-3 361 K 27. 2 days (1, 0) (1, 1) ground level 32. 9 K 0 ± 1 ± 2 ± 3 (2, 2) unstable level K Watson, JMS (1971) k Oka & Epp, Ap. J 613, 349 (2004)

Discovery of metastable H 3+ (3, 3) metastable Goto, Mc. Call, Geballe, Usuda, Kobayashi, Terada, Oka PASJ, 54, 951 (2002)

The Quintuplet Okuda et al. Ap. J 351, 89 (1990)

(1, 1) ground level CMZ and spiral arms -100 km/s -50 km/s (3, 3) metastable level All CMZ High temperature (2, 2) unstable level Neither Low density CO Mostly spiral arms

The Expanding Molecular Ring Kaifu, Kato, Iguchi, Nature (1972) Scoville, Ap. J (1972) Whiteoak, Gardner, MNRAS (1979)

Discovery of high temperature (T ~ 250 K), low density (n ≤ 100 cm-3) clouds Oka & Epp, Ap. J 613, 349 (2004)

Clouds toward GCS 3 -2 Clouds v. LSR range (km s-1) N( H 3+) (1014 cm-2) Temperature (K) Density (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 -40 → +32 8. 4 ± 1. 6 130 ± 100 ≤ 200 0 km s-1 Total (CMZ) 30. 7 ± 2. 7 Spiral arms 12. 3 Total 43 ± 3 Other probes: H, OH, H 2 CO, CS, HCN, NH 3, HCO+, H 2

Oka, Geballe, Goto, Usuda, Mc. Call, Ap. J in press

Main Results so far • • Vast amount of gas High temperature: T ~ 250 K Low Density: n ~ 100 cm-3 H 3+ total column density: 4. 3 1015 cm-2 ¾ in CMZ, ¼ in intervening spiral arms ½ of H 3+ 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?

Quintuplet Galactic plane

Telescopes and Spectrometers UKIRT 3. 8 m CGS 4 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

Inner 300 pc – showing Sgr B 2 and Expanding Ring 1. 2. 3. Sightline crosses spiral arms. Sightline passes though a more diffuse gas (AV=15 -25 mag) than any other in the Galaxy. Sightline passes through Galactic Center clouds - a unique and complex environment.

Trad 3 K « Tkin Radiation Collision Reaction Ψ’ A B C D ψ A B Rigorous Selection Rules Approximate Rules Random + ΔJ = ± 1, 0 ortho para ΔI = 0 ΔItotal = 0

Cosmic ray ionization Steady State 10 -17 s-1 Rate Equations 0= Production Spontaneous emission 10 -7 cm 3 s-1 Dissociative recombination 4 10 -7 s-1 Collision n(H 2) 100 cm-3 10 -7 s-1 Destruction 10 -9 s-1 10 -9 cm 3 s-1 Proton hop reaction Detailed balance H 3+ + H 2 (H 5+)* H 3+ + H 2 Oka, Epp Ap. J, 613, 349 (2004)