Friday 15 May, 1998 at 2:30pm

Kumiko S. Usuda - Subaru Telescope, NAOJ

"Low Density Molecular Gas in the Galaxy"

ABSTRACT:"We investigate physical conditions of low density molecular gas in which CO emission becomes weaker. An extensive survey of the outer Galaxy in the CO (J = 2-1) emission line has been done with the twin 60-cm radio telescopes in Nobeyama, Japan and in La Silla, Chile. The northern survey for the Perseus arm covers the area l = 108.5 deg - 155.0 deg, b = -2.5 - +2.5 deg, and the southern survey for the Carina arm covers the area l = 293.0 deg - 310.875 deg, b = -1.125 deg - +1.125 deg on a 0.125 deg grid with a 9' beam. We compare the data with CO (J = 1-0) data to take the CO (J = 2-1) / CO (J = 1-0) intensity ratio, R(2-1/1-0), which is a good indicator of excitation temperature. In these regions, the fraction of higher ratio gas (R(2-1/1-0) >= 0.7; Tk >~20 K, n(H2) >~ 1x 10^3 cm^-3) which comes from compact components such as molecular clouds is higher than in the inner Galaxy, and the contribution of lower ratio gas (R(2-1/1-0) < 0.7) which comes from the diffuse component such as the intercloud component becomes extremely low. In the outer Galaxy, the CO emission from diffuse components is extremely low or is hardly detected. This result suggests that there is a possibility that substantial amount of molecular gas resides unexcited in CO lines because of low density. Our absorption line studies in [CI] and C18O toward Sgr B2(M) and W49A using the JCMT and the NRO 45-m telescope, and other absorption line observations can support this possibility."