Hot and Diffuse Gas near the Galactic Center Probed by Metastable H3
+
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 OkaDepartment of Chemistry and Department of Astronomy and
AstrophysicsThe Enrico Fermi Institute, The University of Chicago
1911 Discovery J. J. Thomson
1916 n(H3+) » n(H2
+) Arthur Dempster1925 H2
+ + H2 → H + H3+ Thorfin Hogness
1961 Interstellar H3+ + , prediction, prediction Earl McDaniel
1973 H3+ + X → H2 + HX+ Bill Klemperer
1975
2005 Hot and Diffuse Clouds near GC Probed by Metastable H3+
Ubiquity Dense and Diffuse Clouds
Historical Sketch of H3+
T.G.
B.M.
M.G. +
1951
1970
1996
1980 Oka, PRL. 45, 531Laboratory Spectrum
1996Geballe, Oka, Nature 384, 334
Interstellar H3+, Discovery
Vacuum
Laser Spectroscopy
Array Detector
Large Diameter Telescope
H
H2
H3+
Three Proton System
+
I = 3/2 ortho
I = 1/2 para
++ = uuup = uudn = udd
Breaking Symmetry
Interstellar NH3 Oka, Shimizu, Shimizu, Watson, ApJ 165, L15 (1971)
Interstellar H3+ Pan, Oka, ApJ 305, 518 (1986)
Metastable
27.2 days
20.4 days
μ
Critical Density
200 cm-3
ortho orthopara para
J
K
Collision H3+ + H2 → (H5
+)* → H3+ + H2
16 hrs8 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 106 cm–3 K
Center ~ 500 pc
Extraordinary Radio Emission
• Radio Arc– Arched Filaments– Nonthermal Filaments
• Sgr A*
The Central Molecular Zone
M ~ 5 ×107 M⊙Dominantly molecularn(H2) > 104 cm-4
Volume filling factor 0.01Hot gas T ~ 300 KHigh 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 (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
(1, 1) ground levelCMZ and spiral arms
(3, 3) metastable level All CMZ
(2, 2) unstable level Neither
CO All spiral arms
High temperature
Low density
)2,2()3,3(
NN
Discovery of high temperature (T ~ 250 K), low density (n ≤ 100 cm-3) clouds
)1,1()3,3(
NN
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
• 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
Gemini South
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