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H 3 + Toward and Within the Galactic Center. Tom Geballe, Gemini Observatory With thanks to Takeshi Oka, Ben McCall, Miwa Goto, Tomonori Usuda. Telescopes and Spectrometers. telescope instrument resolution location UKIRT 3.8 m CGS4 8 km s -1 Mauna Kea

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H 3 toward and within the galactic center
H3+ Toward and Within the Galactic Center

Tom Geballe, Gemini Observatory

With thanks to Takeshi Oka, Ben McCall, Miwa Goto, Tomonori Usuda

Telescopes and spectrometers
Telescopes and Spectrometers

telescope instrument resolution location

UKIRT 3.8 m CGS4 8 km s-1 Mauna Kea

Subaru 8.2 m IRCS 15 km s-1 Mauna Kea

Gemini S 8 m Phoenix 5 km s-1 Cerro Pachon

Gemini South



Why observe h 3 or any molecule toward the galactic center
Why observe H3+ (or any molecule) toward the Galactic center ?

Expanding Molecular Ring

Central Molecular Zone

  • Sightline crosses spiral arms containing “dense clouds” (opaque to UV)

  • Sightline passes though more “diffuse clouds” (opaque to 912Å) than any other

  • (Where are they located?)

  • 3. Sightline passes through Galactic center clouds - a unique and complex environment.

The galactic center
The Galactic Center

1. Massive (3x 106 Msun) black hole

2. Densest cluster of stars in the Galaxy (~106 Msun pc-3)

3. Large amount of ionized, atomic, and molecular interstellar gas, interacting with gravitational field, stellar winds, and magnetic field.

4.. Several clusters of massive, hot

young stars - one cluster surrounds the b.h., several located ~ 30 pc distant

(We use some of these stars as probes)

5. Obscured from optical view by

30 mag (1 optical photon in 1012

reaches us) by dust mixed with the gas within and outside of the center

2 pc ~ 6 l.y.

2m images from Gemini

(with adaptive optics)


Expanding molecular ring and central molecular zone
Expanding Molecular Ring and Central Molecular Zone

CMZ lies within the EMR

R ~ 180 pc

Mgas ~ 5 ×107 M⊙ (thought to be dominantly molecular)

Clouds have n(H2) > 104 cm-3

Volume filling factor 0.1

10 % of total ISM of Galaxy

Warm gas T ~ 300 K

Low dust temperature


A chain of clouds at R~180 pc expanding at ~160 km/s, rotating at 60 km/s and containing

1x107 Msun of gas. Vertical extent of ring is about +/- 50 pc

(Sofue & Yoshiaki 1995)

History of h 3 toward the galactic center
History of H3+ toward the Galactic Center


GCS 3-2

  • Discovery (and first indication of high abundance of H3+ in diffuse i.s.m.)

    - McCall et al. 1998, Geballe et al.1999

  • Discovery of rotationally excited

    metastable H3+

    - Goto et al. 2002

  • Higher sensitivity and higher resolution spectra, and more detailed analysis, are leading to a better understanding of the nature of the i.s.m. in the Galactic center

    - Oka et al. (2005) - data from 2 clear nights in 3 years!

The metastable 3 3 level of h 3 astrophysical significance
The metastable (3,3) level of H3+ - astrophysical significance

  • Except for the GC, absorption

    lines of H3+have been detected

    only from the lowest (J=1) ortho

    and para levels.

  • (3,3) level is >300 K above the

    lowest ortho and para levels

    It can be populated by collisions

    (or by spontaneous emission from

    higher levels) but it cannot

    radiatively decay. Absorption

    from that level signifies warm

    temperatures but does not provide

    information on the density.

    3. If (3,3) level is populated, the kinetic temperature is high, but the density is unknown. The (2,2) level (~100 K above lowest levels) will be populated in LTE if (3,3) is populated and if n>200 cm -3.





4 hrs

16 hrs

8 hrs



20 days

27 days (ncrit ~ 200 cm-3)

ortho para para ortho para para

Needed 1 higher resolution and higher sensitivity spectra of h 3 2 spectra of co lines
Needed: (1) Higher resolution and higher sensitivity spectra of H3+ (2) Spectra of CO lines

What are the physical conditions of the clouds containing H3+in the ground

and excited levels?

==> requires more detailed observations of H3+ lines from J=1, 2, 3

Is the H3+ in diffuse clouds or dense clouds

==> very helpful to have more detailed observations of CO lines.

(H3+ is found in both dense and diffuse clouds, but CO is found only in dense clouds)

Keys to studying line of sight to galactic center
Keys to studying line of sight to Galactic center spectra of H

20 km/s cloud



molecular ring

4.5 kpc arm

3 kpc arm

1. Absorbing clouds have different radial velocities !!

2. Most of the clouds already well-known from radio astronomy

(mapped in emission lines of various molecules).

H 3 and co spectra of gcs3 2

(1,1) spectra of H: Note similarity to CO, except for the broad absorption trough, on which narrow absorption lines from (dense) clouds in external spiral arms are superimposed.

==> trough is formed in diffuse clouds.

(3,3): Overall absorption profile crudely approximates the (1,1) trough.

==> same gas as R(1,1) trough

==> trough gas is warm (250 K)

No narrow features. Blend of broad absorptions

==> trough gas is in rapid motion.

==> Gas is close to the GC.

(2,2): No absorption

==> non-LTE population distribution / low density (confirms comparison with CO).

CO R(1) (2.34 m): Only narrow absorption features; strengths not correlated with features in the H3+ R(3,3)u line. No broad absorption trough.

H3+ and CO spectra of GCS3-2

N(H3+) ~ 4.2 x 1015 cm-2

3/4 of this is in the CMZ and EMR !!

C r ionization rate in the central molecular zone
C-R Ionization Rate in the Central Molecular Zone spectra of H

For Galactic (cold) diffuse clouds, and latest ke and = 1.2 x 10-15 s-1 from  Per: ndiff(H3+)  1 x 10-5 cm-3

In the GC C/H is 3X-10X larger than solar, so dissociative recombination of 3-10X faster

In the GC T~250K, so ke is 3X lower than in cold diff clouds.

==> destruction rate is 1-3X faster.

What is  in the GC ?? Will diff. cloud value work??

NEMR (H3+) =3 x 1015 cm-2

For same value of , L=100-300 pc, but r(CMZ)~180 pc.

If diffuse cloud filling factor is less than unity,  is higher than in diffuse clouds outside the GC.

(Higher  would not be surprising.)

Conclusions spectra of H

Metastable (3,3) level is populated in a highly turbulent environment over a wide velocity range, which must be in the CMZ

T ~ 250 K and n ~ 100 cm-3 for the gas in which the metastable H3+ is populated.

Most (3/4) of the H3+ observed toward the GC is in hot diffuse gas.

(Note contrast with CO).

The hot diffuse gas exists widely in the CMZ (Oka et al. 2005; Goto’s talk tomorrow) and is the dominant gaseous component in the CMZ. Unsuspected before the discovery of the R(3,3)l line.

Cosmic ray ionization rate of H2 is at least as high, and probably a few times higher than the value in diffuse clouds as deduced for the  Per (and thus two orders of magnitude greater than the previously generally assumed ionization rate in diffuse clouds).

H 3 in a distant galaxy
H spectra of H3+ in a distant galaxy:

IRAS 08572+3915

d = 250 Mpc


Sloan Digital Sky Survey (optical)

a glimpse of the future …

Geballe, Goto, Usuda, Oka, & McCall. - submitted to ApJ