Hot and diffuse gas near the Galactic center

probed by metastable H3+

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/0507464

The Central Molecular Zone (CMZ)R ~ 200 pcM ~ 5 ×107 M⊙

Dominantly molecularn(H2) > 104 cm-3

Volume filling factor 0.110 % of total ISMHot gas T ~ 300 KHigh velocity dispersionLow dust temperature

Mezger, Duschl, Zylka, A&A Rev. (1996)

Morris, Serabyn, ARA&A (1996)

Genzel, Hollenbach, Townes (1994)CMZ

OH, H2CO, CO, CS, NH3, HCN 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 H3+

Protonated H2 (P.A.=4.4 eV), simplest polyatomic molecule

Equilateral triangle, Infrared active 4μ band

Production H2 → H2+ + e H2

+ + H2 → H3+ + H rate = ζn(H2)

Destruction Dense cloud H3+ + CO → H2 + HCO+

Diffuse cloud H3+ + e → H + H + H

Simple chemistry

10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

100

101

102

103

104

105

106

n(X

)

10-1

100

101

102

103

104

105

106

n(H)

Dense Clouds Diffuse Clouds

H2

HCO

C+H3

+

n(H3+) constant N(H3

+) → L cloud dimension

ζ

n(H3+) ∽ ζ N(H3

+) → ζL ioniozation rate

Ubiquity Dense cloud N(H3+) = 3.6 × 1012 cm-2 AV

Diffuse cloud N(H3+) = 4.4 × 1013 cm-2 AV

The most abundant molecular ion in the Universe

Initiator of interstellar chemistry H3+ + O → OH+ + H2 OH+ → H2O+ → H3O+ → H2O

1961 1972

2005 Hot and Diffuse Clouds near GC Probed by Metastable H3+

Oka, Geballe, Goto, Usuda, McCall, Astro-ph/0507464

1998 – 2002 Ubiquity, Dense and Diffuse Clouds McCall, Geballe, Hinkle, Oka

Historical sketch of H3+ observation

+

1951

1970

1996

1980 Oka, PRL. 45, 531Laboratory Spectrum

1996Geballe, Oka, Nature 384, 334

Interstellar H3+, Discovery

H

H2

H3+

1989 - 95

2002 Metastable H3+, Discovery

Goto, McCall, Geballe, Usuda, Kobayashi, Terada, Oka PASJ, 54, 951

1975

6

5

4

3

2

1

0

H3+

Col

umn

Den

sity

(10

14cm

-2)

50403020100

E(B-V) (mag)

AFGL2136

W33A

MonR2 IRS 3

AFGL961E

AFGL490

AFGL2591

8

6

4

2

0

H3+

Col

umn

Den

sity

(10

14cm

-2)

6543210

E(B-V) (mag)

OphP Cygni

HD 183143

WR 118

Cyg OB2 12

WR 104

Cyg OB2 5

WR 121

HD 168607

HD 194279

GC IRS 3

2 Ori

HD 20041

Dense clouds

McCall, Geballe, HinkleOka, ApJ 522,338(1999)

Diffuse clouds

McCall, Hinkle, Geballe, Moriarty-Schieven, Evans,Kawaguchi, Takano, Smith, Oka, ApJ 567, 391 (2002)

Per

H3+ more abundant in Diffuse Clouds

N(H3+) ~ 3.6 ×1012 cm-2 AV

N(H3+) ~ 4.4 ×1013 cm-2 AV

Sightlines toward the CMZ

Mezger, Duschl, Zylka (1996)Georgelin, Georgelin (1976)

AV = 25 ~ 40 Cotera et al. (2000)

Toward Sgr A*

AV ~ 30 [AV]DISM ~ 20 , [AV]MC ~ 10

Whittet et al. (1997)

LDISM: LMC ~ 200 : 1

N(H3+)DISM: N(H3

+) ~ 20 : 1

Discovery of H3+ toward GC

GC IRS 3

GCS 3-2

Geballe, McCall, Hinkle, Oka, ApJ 510, 215 (1999)

2.5 × 1015 cm-2

2.8 × 1015 cm-2

J

K32.9 K

361 K

(J, K)

The ideal energy levels of H3+

+ortho I = 3/2para 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

Ncrit ~ 200 cm-3

H3+ + H2 → (H5

+)* → H3+ + H2

Discovery of metastable H3+

Goto, McCall, Geballe, Usuda,Kobayashi, Terada, OkaPASJ, 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

metastable

CO

T ~ 250 K

n ≤ 100 cm-3

EMR

The Expanding Molecular RingKaifu, Kato, Iguchi, Nature (1972)Scoville, ApJ (1972)

Whiteoak, Gardner, MNRAS (1979)

Dimension or the hot and diffuse clouds

ζ L = 2keN(H3+)(nC/nH)SVRX/f

= 3.7× 104 RX/f cm s-1

RX = (nC/nH)GC/(nC/nH)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) × 105 cm s-1

If L = 20 pc, ζ = (2 – 6) × 10-15 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

• 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

Volume filling factor of dense clouds « 0.1

[MISM]CMZ/[MISM] 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 cloudsCircumstellar Space

Planetary nebulae protoplanetary nebulaePlanet

Planets Proto-planet Exo-planet

Treasure Hunting

)2,2(

)3,3(

N

N

Discovery of high temperature (T ~ 250 K), low density (n ≤ 100 cm-3) clouds

)1,1(

)3,3(

N

N

Oka & Epp, ApJ 613,349 (2004)

Clouds vLSR range N( H3+) Temperature Density

(km s-1) (1014 cm-2) (K) (cm-3)

-100 km s-1 -140 → -74 15.7 ± 1.7 270 ± 70 ≤ 50

-50 km s-1 -74 → -40 6.6 ± 1.3 250 ± 100 ≤100

0 km s-1 -40 → +32 8.4 ± 1.6 130 ± 100 ≤ 200

(1, 1) ground levelCMZ 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(H3+) » n(H2

+) Arthur Dempster

1925 H2+ + H2 → H + H3

+ Thorfin Hogness

1961 Interstellar H3+ + , prediction, prediction Earl McDaniel

1973 H3+ + X → H2 + HX+ Herbst, Klemperer, Watson

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

H

H2

H3+

H3+ in the (3, 3) metastable level

Galactic plane

Quintuplet

(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

-100 km/s 0 km/s

-50 km/s