Survey of Molecular Lines Near the Galactic Center. I. 6-Centimeter Formaldehyde Absorption in Sagittarius a, Sagittarius B2, and the Galactic Plane from l^(II) = 359°.4 to l^(II) = 2°.2

Abstract

The 1_(11)→ 1_(10) 4830-MHz transition of has been mapped in Sgr A, Sgr B2, and along the galactic equator from l^(II) = 359°.4 to 112 = 2°.2 with a 6' beam and velocity resolution of 1 km s^(-1). That this line is observed in absorption in all locations indicates a low 6-cm excitation temperature for all molecular clouds. Comparison with 21-cm data indicates that the H_2CO is concentrated in distinct clouds to a much greater extent than atomic hydrogen, with many strong H_2CO features occurring at velocities with weak or missing hydrogen lines. This suggests that most of the mass in these clouds is in the form of molecular hydrogen. There are four dominant clouds occurring at l^(II) = 0°.0, 0°.7, 0°.9, and 1°.7. In contrast with atomic hydrogen, the strongest formaldehyde features occur at [V_(LSR)] > 40 km s^(-1) and are apparently associated with the galactic nucleus. Estimates of the hydrogen density in these clouds, based on the assumptinn of stability and also on a comparison with local dark clouds, indicate a number density in the range of 10^3 - 10^4 cm^(-3) and a mass of approximately 5 X 10^5 M_⊙