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