An Out‐of‐Plane CO ( J = 2–1) Survey of the Milky Way. II. Physical Conditions of Molecular Gas

Abstract

Physical conditions of molecular gas in the Ðrst quadrant of the Galaxy are examined through comparison of the CO J \ 2È1 data of the TokyoÈNobeyama Radio Observatory survey with the CO J \ 1È0 data of the Columbia survey. A gradient of the CO J \ 2È1/J \ 1È0 intensity ratio (4R 2h1@1h0) with Galactocentric distance is reported. The ratio varies from ^0.75 at 4 kpc to ^0.6 at 8 kpc in Galactocentric distance. This conÐrms the early in-plane results reported by Handa et al. We classify molecular gas into three categories in terms of on the basis of a large velocity R 2h1@1h0 gradient model calculation. Very high ratio gas (VHRG ; is either dense, warm, and opti-R 2h1@1h0 [ 1.0) cally thin gas or externally heated, dense gas. High ratio gas (HRG ; is warm and R 2h1@1h0 \ 0.7È1.0) dense gas with high-excitation temperature of the J \ 2È1 transition K), and it is often (T ex Z 10 observed in central regions of giant molecular clouds. Low ratio gas (LRG ; has low-R 2h1@1h0 \ 0.7) excitation temperature of the J \ 2È1 transition K) because of low density or low kinetic tem-(T ex [ 10 perature, or both, and is often observed in dark clouds and outer envelopes of giant molecular clouds. It is shown that the CO J \ 2È1 emission is better characterized as a tracer of dense gas rather than a tracer of warm gas for molecular gas with kinetic temperature higher than 10 K. The observed large-scale decrease in as a function of Galactocentric distance is ascribed to the fractional decrease R 2h1@1h0 of HRG and VHRG from ^40% near 5 kpc to ^20% near the solar circle. The HRG and VHRG are found predominantly along the Sagittarius and Scutum arms, probably in their downstream. This fact and the deÐciency of atomic gas compared with molecular gas in the inner Galaxy indicate that physical conditions of interstellar gas are a †ected by grand-design, nonlinear processes , such as compression by spiral density waves followed by gravitational collapse, and not by disso-ciation of low-density molecular gas by young stars.