Molecules in G1.6-0.025-"hot" chemistry in the absence of star formation at the periphery of the Galactic Center Region

Abstract

We present molecular line mapping of the Giant Molecular Cloud G1.6-0.025, which is located at the highlongitude end of the Central Molecular Zone (CMZ) of our Galaxy. We assess the degree of star formation activity in that region using several tracers, and find very little. We made a large-scale, medium (2') resolution map in the J = 2-1 transition of SiO for which we find clumpy emission over a 0.8×0. ? 3 sized region stretching along the Galactic plane. Toward selected positions we also took spectra in the easy-to-excite Jk = 2k -1k quartet of CH3OH and the carbon monosulfide (CS) 2-1 line. Throughout the cloud these CH3OH lines are, remarkably, several times stronger than both the CS and the SiO lines. The large widths of all the observed lines, similar to values generally found in the Galactic center, indicate a high degree of turbulence. Several high LSR velocity clumps that have 50-80 km s-1 higher velocities than the bulk of the molecular cloud appear at the same projected position as "normal" velocity material; thismay indicate cloud-cloud collisions. Statistical equilibrium modeling of theCH3OH lines observed by us and others yields relatively high densities and moderate temperatures for a representative dual velocity position.We find 8×104 cm-3/30Kformaterial in the G1.6-0.025 cloud and a higher temperature (190 K), but a 50% lower density in a high-velocity clump projected on the same location. Several scenarios are discussed in which shock chemistry might enhance the CH3OH and SiO abundances in G1.6-0.025 and elsewhere in the CMZ.