AFGL-5376 - A STRONG, LARGE-SCALE SHOCK NEAR THE GALACTIC-CENTER

We present observations of (CO)-C-12 J = 2-1 emission in a 15' x 15' r egion around AFGL 5376 (l = 359 degrees.5, b = 0 degrees.43), an unusu ally warm (T-c = 100 K) and extended IRAS source located near the Gala ctic center. The new spatial maps made with the Caltech Submillimeter Observatory (CSO) confirm that (1) there is a clear association of hig h-velocity CO emission with the infrared source-the CO emission nearly completely surrounds the infrared peak at a radius of about 4', and ( 2) AFGL 5376 defines the most prominent portion of a strong (similar t o 50 km s(-1)), large-scale (90 pc) shock front. Contour maps produced from the Bell Labs (CO)-C-12 J = 1-0 Galactic center database are als o presented; they show that the forbidden velocity emission surroundin g AFGL 5376 forms a well-defined tongue of gas that extends perpendicu larly from the Galactic plane out to b congruent to degrees.9. The sha pe and location of the CO tongue lead us to suggest that it is the mol ecular counterpart of the western limb of a prominent radio continuum feature, the Galactic center lobe (GCL). Its total H-2 mass is similar to 10(6) M(.). The distributions of relatively low velocity (50-115 k m s(-1)) and high velocity (115-160 km s(-1)) gas are separated across a remarkably well defined and narrow rift which extends latitudinally through the CO tongue for about 0 degrees.6 (about 90 pc, assuming d( gc) = 8.5 kpc), making it perhaps the largest continuous shock observe d near the Galactic center. We hypothesize that the rift bisecting the western edge of the GCL marks the location where a system of gas in n oncircular motion about the Galactic center is colliding with ambient material above the Galactic plane. The absence of CO emission directly toward the peak of AFGL 5376 and along other portions of the shock fr ont is attributed to the shock-induced dissociation or heating of CO; only the shock-heated dust grains have survived to manifest themselves as the infrared source AFGL 5376. The western edge of the radio conti nuum GCL is evidence of shock ionization along a line parallel to the dissociation front. The noncircular gas motions characterizing the gas associated with AFGL 5376 are linked to the large-scale kinematical f eature known as the expanding molecular ring (EMR). The observations p resented here are used to contrast competing theories for the EMR: an explosion-induced ring of expanding gas versus the reaction or orbitin g gas to a bar potential.