The Central Molecular ZoneThe central region of our Galaxy contains a super-massive black hole and a high concentration of stars and interstellar matter (ISM), and shows a variety of extraordinary phenomena due to high energy densities of gravity, magnetic field, X-rays and ultraviolet radiation. The inner region with radius ~ 200 pc, called the Central Molecular Zone (CMZ) harbors a vast amount of ISM where the gas is at higher temperature than typical Galactic clouds and has high velocity dispersion reflecting the turbulent nature of the gas in the area (Morris and Serabyn 1996). Studies of the gas in the CMZ provide information vital for understanding the unusual activity in the central region. Morris and Serabyn, ARA&A 34, 645 (1996)

The CMZ, the Treasure House of H3+

Any sightline toward the CMZ contains H3+ column

density on the order of a few times 1015 cm-2, an

order of magnitude higher than the most H3+-rich

sightline in the Galactic disk.

The Ideal Rotational Energy Level System of H3+

H3+ is tailor-made for observations of warm and diffuse gas in the CMZ. This is

because of the ideal arrangement of its rotational levels. Although there is no electric dipole moment and hence no ordinary rotational transitions because of its equilateral triangle structure, weak transitions are allowed because of a spontaneous breakdown of symmetry. The J = K = 2 level (hereafter (2, 2)) decays to the (1, 1) level with the half life time of 27.2 days, which corresponds to the critical density of ~ 200 cm-3 just about the density of diffuse clouds. Thus spectral line R(2, 2)l starting from the (2, 2) level can be used as densitometer. The spectral line R(3, 3)l starting from the metastable (3, 3) level which is 361 K above the (1, 1) ground level can be used as a thermometer. Because of the spontaneous emissions whose Einstein coefficients increase rapidly for higher rotational levels, practically only four rotational levels are populated, i. e. the (1, 1) ground level, the (1, 0) lowest ortho-level, the (2, 2) unstable level, and the (3, 3) metastable level.

The New Category of Gas Revealed by H3+ is Incompatible

with Previous Picture of the Gas in the CMZ

The Subaru Discovery of Metastable (3, 3) H3+

H3+ in the (3, 3) metastable level is observable only in warm gas since it is 361 K above

the (1, 1) ground rotational level. It was discovered toward GCS3-2 and GC IRS3 (Goto et al. 2002) and observed in the CMZ but has not been observed in the Galactic disk.

Subaru Observations of Warm and Diffuse Gas nearthe Galactic Center Probed by Metastable H3

+

T. Oka1, M. Goto2, T. Usuda3, T. Nagata4, T. R. Geballe5, B. J. McCall6, and C. P. Morong1

1University of Chicago, 2Max-Planck Institute of Astronomy, 3Subaru Telescope, 4Kyoto University, 5Gemini Observatory, 6University of Illinois Urbana-Champaign

Sawada, Hasegawa, Handa, Cohen, MNRAS 349, 1167 (2004)Oka, Hasegawa, Sato, Tsuboi, Miyazaki, ApJS 118, 455 (1998)Scoville, ApJ 175, L127 (1972)

145 km s-1

200 pc 130 pc

100 km s-1

EMR

Inferred face-on view of the CMZThe Expanding Molecular Ring

Metastable

27.2 days Critical Density 200 cm-3

Ortho I = 3/2

Para I = 1/2

J

K

16 hrs8 hrs (3,3)

(2,2)Unstable

(1,1)Ground

361 K

μ

Breaking Symmetry

Interstellar NH3 Oka, Shimizu, Shimizu, Watson, ApJ 165, L15 (1971)

Interstellar H3+ Pan, Oka, ApJ 305, 518 (1986)

Thermalization Oka, Epp, ApJ 613, 349 (2004)

R(1, 1)u R(1, 0)

H3+, the New Probe for the Central Molecular Zone

Protonated molecular hydrogen, H3+, the third pure hydrogenic probe after H and H2,

is observed strongly in the infrared from 3.5 to 4.0 μm in the CMZ and provides fresh astrophysical information. Unlike other molecular species observed by radio emission, H3

+ exists mostly in diffuse clouds and thus provides information on the gas which fills the space between dense clouds. We can obtain the following astrophysical quantities which are not readily measurable by other methods.

(1) The temperature T and density n of the diffuse gas.(2) The product ζL of the (cosmic ray) ionization rate ζ and the dimension L of the gas.

Upper traces: broad and featureless spectrum of H3+

in the (3, 3) metastable rotational level (in red), and the spectrum with sharp components and a trough ofH3

+ in the (1, 1) ground level (in blue). The metastable H3

+ absorption is all from the warm gas in the CMZ while the sharp components of (1, 1) are from colder gas mostly in the intervening spiral arms. GCS 3-2 is the brightest infrared star in the Quintuplet Cluster.

Lower traces: sharp spectral lines of the 2 – 0 overtone band at 4.2 μm of CO which exists mostly in colder and denser gas in the intervening spiral arms and not much in the CMZ. The CO spectrum is crucial for discriminating H3

+ spectrum from spiral arms and from the CMZ

Goto, McCall, Geballe, Usuda, Kobayashi, Terada, Oka PASJ, 54, 951 (2002)

CO J = 1

(2,2) unstable

(3,3) metastable

(1,1) ground

Warm and Diffuse Gas Toward the Brightest Quintuplet Star GCS 3-2 Observed at Gemini South and UKIRT

Our high resolution spectroscopy using the Phoenix Spectrometer of the Gemini South Observatory and the CGS4 of the UKIRT toward the brightest Quintuplet star GCS 3-2 and analyses have revealed existence of a new category of gas with high temperature (~ 250 K) and low density (≤ 100 cm-3) and high velocity dispersion with a long path.

Quintuplet Cluster

CO J = 1

(2,2) unstable

(3,3) metastable

(1,1) ground

The R(1, 1)l spectrum composed of sharp lines from spiral arms and a broad trough from the CMZ. The latter is shown with the yellow dotted line.

The R(3,3)l spectrum with high velocity dispersion from H3+ in

the CMZ. The intense spectrum from the level, 361 K above the (1, 1) level, demonstrates the high temperature of ~ 250 K.

The R(2, 2)l spectrum. Non-detection of this spectrum demonstrates unpopulated (2, 2) unstable level and thus low density of ≤ 100 cm-3

GCS 3-2GCS3-2

Subaru Observation of Eight Infrared Stars Within 30 pc from the Center: High Volume Filling Factor of the Gas

The ubiquity and high volume filling factor of the newly found gas in the CMZ has been demonstrated by our observation of H3+ toward eight stars within 30 pc from the center.

Information on the gas: T, n, and ζL

Relative populations n(3, 3)/n(1, 1) and n(3, 3)/n(2, 2) determine temperature T and number density n of the gas (Oka and Epp 2004).

)2,2(

)3,3(

N

N)1,1(

)3,3(

N

N

Oka & Epp, ApJ 613,349 (2004)

The observed total H3+ column density gives product ζL of the ionization rate ζ and

pathlength L (the dimension of the cloud).

Oka, Geballe, Goto, Usuda, McCall, ApJ 632, 882 (2005)

ζL = 2 ke N(H3+) (nC/nH)SV RX / f

7.3 ×10-8 cm3/s 1.6 ×10-4 1

3.1 ×1015 cm-2 3 - 10

Sodroski et al ApJ 452, 262 (1995)

ζL = (2.5 – 8.4) ×105 cm/s

ζ L

10-16 s-1 (0.8 - 2.1) k pc 10-15 s-1 80 - 210 pc

COBE DIRBE

High ionization rate ! Long pathlength!Oka, Geballe, Goto, Usuda, McCall, ApJ 632, 882 (2005)

(2,2) densitometer

(3,3) thermometer

(1,1) ground level

(2,2) densitometer

(3,3) thermometer

(1,1) ground level

High H3+ column densities of

N(H3+) = (2.3 - 4.5) × 1015 cm-2 have been

observed for all sightlines giving pathlengthsof 80 – 120 pc on the assumed ionization rateof ζ = 10-15 s-1.

The gas temperatures were from 220 K – 400 Kand the densities are equal or less than 100 cm-3

Lazio & Cordes, ApJ 505, 715 (1998)

10 cm-3

104 cm-3

An example of inferred composition of gases in the CMZ (Lazio and Cordes 1998), including three categories of gases.

1)Ultra-hot plasma (107-8 K) inferred from X-rays.2)Hot electron gas (106 K) from hyper-strong radio scattering3)Cold (102 K) and dense gas observed by CO, CS, HCN etc.

Volume filling factors of these gases (inferred from this picture)are perhaps 0.8, 0.1, and 0.1, respectively.

Out of those three environments, 1) and 2) cannot accommodate the observed H3

+ which will be destroyed immediately. Our revelation of the warm and diffuse gas and its inferred large volume filling suggest that 1) and 2) may not exist or exist with much smaller volume filling factor.

Goto, Usuda, Oka, Nagata, Geballe, McCall, Morongmanuscript in preparation.

The X-ray may be from stars and not from extended gas?

Chandra 20 pc × 20 pc (0.5 – 8) keV Muno et al. ApJ 589, 225 (2003)

Warwick, Sakano, Decourchelle (2006) Revnivtsev et al. (2006)

The hyperstrong radio scattering caused by folded magnetic field?

Folded fields as the source of extreme radio-wave scattering in the Galactic center

Goldreich and Sridhar, ApJ 640, L159 (2006)

RX = (nC/nH)GC/(nC/nH)SV SV: solar vicinity