PHYSICAL CONDITIONS OF MOLECULAR GAS IN THE GALAXY

Abstract

ABSTRACT Physical conditions of molecular clouds in the Milky Way are examined through extensive observations of CO (J = 2-1) emission with the University of Tokyo-NRO 60 cm radio telescope. We take two complementary approaches-- large-scale mapping observations of nearby molecular clouds and an out-of-plane survey of the Milky Way. The observed CO (J = 2-1) intensity is compared with the CO (J = 1-0) data taken with the same angular resolution. Through a numerical calculation based on a large velocity gradient approximation, the CO (J = 2-1)/CO (J = 1-0) intensity ratio (equiv R2-1/1-0) is found to be a useful tool for researches of large-scale structure of molecular clouds. Molecular gas is classified in terms of the R}2-1/1-0 value; Very High Ratio Gas (VHRG; R2-1/1-0 >>1), High Ratio Gas (HRG; R_{2-1/1-0 ~0.8), Low Ratio Gas (LRG; R2-1/1-0 ~0.5) and Very Low Ratio Gas (VLRG; R2-1/1-0 <0.4). The VHRG is optically thin, dense and warm gas. The HRG is opaque and dense enough for low-J transitions of CO to be excited to the local thermodynamical equilibrium through collisions, while the LRG is less dense and the excitation of these low-J transitions is inefficient. For the HRG and the LRG, the R2-1/1-0 value is sensitive to the variation of gas density rather than gas kinetic temperature. The VLRG is faint and usually not detected in quick surveys. The apparent discrepancy of the observed line intensity and that predicted by the ratio of these intensities can be used to estimate the degree of beam dilution. Giant molecular clouds show large-scale systematic variation of gas density according to the location in the molecular clouds. Contrary to the widely-accepted model of molecular clouds, peripheral regions of GMCs are faint not because surface filling factor of line-emitting gas clumps is small but because even the low-J transitions of 12 CO are not excited enough due to low gas density in clumps in these regions. Heating and compression by associated young stars are efficient within several parsecs from the stars, and they do not enhance the R2-1/1-0 value in a cloud-scale because of this small range of influence. A systematic variation of the R2-1/1-0 value across the Galactic plane was observed. The ratio varies from ~0.75 at 4 kpc to ~0.6 at 8 kpc in galactocentric distance. The corresponding nT values derived by the one-zone model analysis are 8 X 103 and 4 X 103 cm-3 K, respectively. This trend can be understood by large-scale variation of mixing ratio of HRG and LRG as a function of galactocentric distance. On the other hand, there is little, if any, evidence for the difference of physical conditions between the in-plane gas and the out-of-plane gas. There is concentration of HRG along 'the Sagittarius arm' and 'the Scutum arm' found in l- V diagrams. This supports an idea that strong arms exist in the inner Galaxy and they compress molecular gas to contain much of HRG. Analysis of tangential components indicates that the scale height of molecular gas in interarm regions is larger by the factor of 1.5 than in arm regions. Molecular gas with higher R2-1/1-0 value are concentrated to the central ridge-like condensation of massive cloud, the inner Galaxy and the arms, rather than to the cloud envelopes, the outer Galaxy and the interarm regions, respectively. These tendency indicate that virtually all of the regions with high R2-1/1-0 ratio have one feature in common-- they concentrate to the regions with higher gas pressure. This fact as well as the central deficiency of atomic gas compared to molecular gas indicate that the large-scale properties of molecular gas are dominated by the compression processes such as shock compression by density-waves and gravitational collapse, and not by dissociative stripping of low-density gas due to UV photons. Since the Milky Way is the only galaxy in which we can learn about the effects of heating and compression in various scales on physical conditions of molecular gas, the above knowledge about the dominant mechanism for the compression of molecular gas will be used as a guideline for extragalactic works.