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 10 6 cm –3 K Yusef-Zadeh, Morris, AJ (1987)

Hot and Diffuse Gas near the Galactic Center Probed by Metastable H 3 + 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 × 10 7 M ⊙ Dominantly molecular Dense clouds, n(H 2 ) > 10 4 cm -4 Volume filling factor 0.1 Hot gas T ~ 300 K High velocity dispersion Low dust temperature T ~ K Mezger, Duschl, Zylka, A&A Rev. (1996) Morris, Serabyn, ARA&A (1996) CMZ

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

J K 32.9 K 361 K (J, K) The ideal energy levels of H ortho I = 3/2 para 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 N crit ~ 200 cm -3 H H 2 → (H 5 + )* → H H 2

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

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

(1, 1) ground level CMZ 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 Ring Kaifu, Kato, Iguchi, Nature (1972) Scoville, ApJ (1972) Whiteoak, Gardner, MNRAS (1979)

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

Clouds v LSR range N( H 3 + ) Temperature Density (km s -1 ) (10 14 cm -2 ) (K) (cm -3 ) -100 km s → ± ± 70 ≤ km s → ± ± 100 ≤100 0 km s → ± ± 100 ≤ 200 Total (CMZ) 30.7 ± 2.7 Spiral arms 12.3 Total 43 ± 3 Clouds toward GCS 3-2 Other probes: H, OH, H 2 CO, CO, CS, HCN, NH 3, HCO +, H 2

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 H 3 + total column density: 4.3  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  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 (A V =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 ΔI total = 0 AAAA BBBB CACA DBDB Ψ’ψΨ’ψ T rad  3 K « T kin

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