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 Subaru 8.2 m IRCS 15 km s -1 Mauna Kea Gemini S 8 m Phoenix 5 km s -1 Cerro Pachon UKIRT Subaru Gemini South

Why observe H 3 + (or any molecule) toward the Galactic center ? Expanding Molecular Ring C entral Molecular Zone 1.Sightline crosses spiral arms containing “dense clouds” (opaque to UV) 2.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 1.Massive (3x 10 6 M sun ) black hole 2.Densest cluster of stars in the Galaxy (~10 6 M sun 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 reaches us) by dust mixed with the gas within and outside of the center X 2  m images from Gemini (with adaptive optics) 2 pc ~ 6 l.y.

Expanding Molecular Ring and Central Molecular Zone CMZ lies within the EMR R ~ 180 pc M gas ~ 5 × 10 7 M ⊙ (thought to be dominantly molecular) Clouds have n(H 2 ) > 10 4 cm -3 Volume filling factor % of total ISM of Galaxy Warm gas T ~ 300 K Low dust temperature EMR: A chain of clouds at R~180 pc expanding at ~160 km/s, rotating at 60 km/s and containing 1x10 7 M sun of gas. Vertical extent of ring is about +/- 50 pc (Sofue & Yoshiaki 1995)

History of H 3 + toward the Galactic Center Discovery (and first indication of high abundance of H 3 + in diffuse i.s.m.) - McCall et al. 1998, Geballe et al.1999 Discovery of rotationally excited metastable H Goto et al 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! GCS 3-2 GC IRS 3

The metastable (3,3) level of H astrophysical significance (5,5) metastable (3,3) metastable 4 hrs 16 hrs 8 hrs 27 days (n crit ~ 200 cm -3 ) 20 days ortho para para 1.Except for the GC, absorption lines of H 3 + have been detected only from the lowest (J=1) ortho and para levels. 2.(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. (2,2) unstable

Needed: (1) Higher resolution and higher sensitivity spectra of H 3 + (2) Spectra of CO lines What are the physical conditions of the clouds containing H 3 + in the ground and excited levels? ==> requires more detailed observations of H 3 + lines from J=1, 2, 3 Is the H 3 + in diffuse clouds or dense clouds ==> very helpful to have more detailed observations of CO lines. (H 3 + 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 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). Expanding molecular ring 4.5 kpc arm 3 kpc arm Local? 20 km/s cloud

H 3 + and CO spectra of GCS3-2 (1,1): 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 H 3 + R(3,3) u line. No broad absorption trough. N(H 3 + ) ~ 4.2 x cm -2 3/4 of this is in the CMZ and EMR !!

C-R Ionization Rate in the Central Molecular Zone For Galactic (cold) diffuse clouds, and latest k e and  = 1.2 x s -1 from  Per: n diff (H 3 + )  1 x 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 k e is 3X lower than in cold diff clouds. ==> destruction rate is 1-3X faster. What is  in the GC ?? Will diff. cloud value work?? N EMR (H 3 + ) =3 x cm -2 For same value of , L= 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 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 H 3 + is populated. Most (3/4) of the H 3 + 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 H 2 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: (z=0.0582) Geballe, Goto, Usuda, Oka, & McCall. - submitted to ApJ IRAS d = 250 Mpc a glimpse of the future … Sloan Digital Sky Survey (optical)