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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Tom Geballe, Gemini Observatory
With thanks to Takeshi Oka, Ben McCall, Miwa Goto, Tomonori Usuda
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
Expanding Molecular Ring
Central Molecular Zone
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.
2m images from Gemini
(with adaptive optics)
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)
GC IRS 3
- McCall et al. 1998, Geballe et al.1999
- Goto et al. 2002
- Oka et al. (2005) - data from 2 clear nights in 3 years!
lines of H3+have been detected
only from the lowest (J=1) ortho
and para levels.
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.
27 days (ncrit ~ 200 cm-3)
ortho para para ortho para para
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)
20 km/s cloud
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).
(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 !!
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.)
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).
d = 250 Mpc
Sloan Digital Sky Survey (optical)
a glimpse of the future …
Geballe, Goto, Usuda, Oka, & McCall. - submitted to ApJ