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 Subaru Telescope, August 3, 2005 Takeshi Oka Department of Astronomy and Astrophysics and Department of Chemistry The Enrico Fermi Institute, The University of Chicago Astro-ph/

The Central Molecular Zone (CMZ) R ~ 200 pc M ~ 5 × 10 7 M ⊙ Dominantly molecular n(H 2 ) > 10 4 cm -3 Volume filling factor % of total ISM Hot gas T ~ 300 K High velocity dispersion Low dust temperature Mezger, Duschl, Zylka, A&A Rev. (1996) Morris, Serabyn, ARA&A (1996) Genzel, Hollenbach, Townes (1994) CMZ OH, H 2 CO, CO, CS, NH 3, HCN H3+H3+

The central 30 pc of the CMZ Yusef-Zadeh, Morris, AJ (1987) Nagata et al. (1990) Okuda et al. (1990) The Quituplet

Salient features of H 3 + Protonated H 2 (P.A.=4.4 eV), simplest polyatomic molecule Equilateral triangle, Infrared active 4μ band Production H 2 → H e H H 2 → H H rate = ζn(H 2 ) DestructionDense cloud H CO → H 2 + HCO + Diffuse cloud H e → H + H + H Simple chemistry n(H 3 + ) constant N(H 3 + ) → L cloud dimension ζ n(H 3 + ) ∽ ζ N(H 3 + ) → ζL ioniozation rate Ubiquity Dense cloud N(H 3 + ) = 3.6 × cm -2 A V Diffuse cloud N(H 3 + ) = 4.4 × cm -2 A V The most abundant molecular ion in the Universe Initiator of interstellar chemistry H O → OH + + H 2 OH + → H 2 O + → H 3 O + → H 2 O

2005 Hot and Diffuse Clouds near GC Probed by Metastable H 3 + Oka, Geballe, Goto, Usuda, McCall, Astro-ph/ – 2002 Ubiquity, Dense and Diffuse Clouds McCall, Geballe, Hinkle, Oka Historical sketch of H 3 + observation Oka, PRL. 45, 531 Laboratory Spectrum1996 Geballe, Oka, Nature 384, 334 Interstellar H 3 +, Discovery H H2H2 H3+H Metastable H 3 +, Discovery Goto, McCall, Geballe, Usuda, Kobayashi, Terada, Oka PASJ, 54,

Dense clouds McCall, Geballe, Hinkle Oka, ApJ 522,338(1999) Diffuse clouds McCall, Hinkle, Geballe, Moriarty-Schieven, Evans, Kawaguchi, Takano, Smith, Oka, ApJ 567, 391 (2002)  Per H 3 + more abundant in Diffuse Clouds N(H 3 + ) ~ 3.6 ×10 12 cm -2 A V N(H 3 + ) ~ 4.4 ×10 13 cm -2 A V

Sightlines toward the CMZ Mezger, Duschl, Zylka (1996) Georgelin, Georgelin (1976) A V = 25 ~ 40 Cotera et al. (2000) Toward Sgr A * A V ~ 30 [A V ] DISM ~ 20, [A V ] MC ~ 10 Whittet et al. (1997) L DISM : L MC ~ 200 : 1 N(H 3 + ) DISM : N(H 3 + ) ~ 20 : 1

Discovery of H 3 + toward GC GC IRS 3 GCS 3-2 Geballe, McCall, Hinkle, Oka, ApJ 510, 215 (1999) 2.5 × cm × cm -2

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 Oka et al. ApJ, (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) The Subaru Telescope

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 S 8 m Phoenix 5 km s -1 Cerro Pachon

ground unstable ground unstable metastable CO T ~ 250 K n ≤ 100 cm -3 EMR

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

Dimension or the hot and diffuse clouds ζ L = 2k e N(H 3 + )(n C /n H ) SV R X /f = 3.7 × 10 4 R X /f cm s -1 R X = (n C /n H ) GC /(n C /n H ) SV ~ 3 – 10, f ~ 1 COBE DIRBE Sodroski et al. ApJ 452, 262 (1995) Arimoto, Sofue, Tsujimoto, PASJ 48, 275 (1996) ζ L = (1 – 4) × 10 5 cm s -1 If L = 20 pc, ζ = (2 – 6) × s -1 Extremely high ionization rate Liszt, A&A 398, 621 (2003)

Nagata, Hyland, Straw, Sato, Kawara, ApJ 406, 501 (1993)

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 Volume filling factor of dense clouds « 0.1 [M ISM ] CMZ /[M ISM ] total « 0.1

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?

Interstellar Space Dense (molecular) clouds Diffuse clouds New class of clouds? Galactic Center Expanding molecular ring Galactic nucleus New class of clouds? Extragalactic Object Obscured AGN Magellanic clouds Circumstellar Space Planetary nebulae protoplanetary nebulae Planet Planets Proto-planet Exo-planet Treasure Hunting

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 (1, 1) ground level CMZ and spiral arms (3, 3) metastable level All CMZ (2, 2) unstable level Neither High temperature Low density -100 km/s 0 km/s -50 km/s

Gemini South

1911 Discovery J. J. Thomson 1916 n(H 3 + ) » n(H 2 + ) Arthur Dempster 1925 H H 2 → H + H 3 + Thorfin Hogness +, prediction 1961 Interstellar H 3 +, prediction Earl McDaniel 1973 H X → H 2 + HX + Herbst, Klemperer, Watson Hot and Diffuse Clouds near GC Probed by Metastable H 3 + Ubiquity Dense and Diffuse Clouds Historical Sketch of H 3 + T.G. B.M. M.G Oka, PRL. 45, 531 Laboratory Spectrum1996 Geballe, Oka, Nature 384, 334 Interstellar H 3 +, Discovery H H2H2 H3+H3+ H 3 + in the (3, 3) metastable level

Galactic plane Quintuplet

(1, 1) ground level CMZ and spiral arms (3, 3) metastable level All CMZ (2, 2) unstable level Neither CO All spiral arms High temperature Low density -100 km/s 0 km/s -50 km/s