Hot and Diffuse Gas near the Galactic Center Probed by Metastable H 3 + Thomas R. Geballe Gemini Observatory Miwa Goto Max Planck Institute for Astronomy Tomonori Usuda Subaru Telescope Benjamin J. McCall University of Illinois, Urbana-Champaign EFI lunch seminar May 3, 2005 Takeshi Oka Department of Chemistry and Department of Astronomy and Astrophysics The Enrico Fermi Institute, The University of Chicago

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 + Bill Klemperer 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 Vacuum Laser Spectroscopy Array Detector Large Diameter Telescope H H2H2 H3+H3+

Three Proton System + I = 3/2 ortho I = 1/2 para  ++ = uuu p = uud n = udd

Breaking Symmetry Interstellar NH 3 Oka, Shimizu, Shimizu, Watson, ApJ 165, L15 (1971) Interstellar H 3 + Pan, Oka, ApJ 305, 518 (1986) Metastable 27.2 days 20.4 days μ Critical Density 200 cm -3 ortho para J K Collision H H 2 → (H 5 + ) * → H H 2 16 hrs 8 hrs

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 Center ~ 500 pc

Extraordinary Radio Emission Radio Arc –Arched Filaments –Nonthermal Filaments Sgr A *

The Central Molecular Zone M ~ 5 × 10 7 M ⊙ Dominantly molecular n(H 2 ) > 10 4 cm -4 Volume filling factor 0.01 Hot gas T ~ 300 K High Velocity Dispersion

Galactic plane Quintuplet

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

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

(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

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

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

Gemini South