Low Density Molecular Gas in the Galaxy

Details Low Density Molecular Gas in the Galaxy Low Density Molecular Gas in the Galaxy

Dec 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997phdt.........7u&link_type=abstract

PhD Thesis, Department of Astronomy, University of Tokyo, JAPAN, 1997.

Astronomy and Astrophysics

Astronomy

Scientific paper

In this thesis we investigate physical conditions of low density molecular gas in which CO emission becomes weaker. Two approaches are taken. One is an extensive survey of the outer Galaxy in CO(J = 2-1) emission line with the 60-cm telescopes in both hemispheres, and the other one is an attempt to detect absorption lines in [CI] and C18O toward bright continuum background source in millimeter and submillimeter wavelength. In the emission line studies, we used the 60-cm telescopes which are designed to obtain well-calibrated CO(J = 2-1) data in order to compare the line intensities with the CO(J = 1-0) data. We take the CO(J = 2-1) / CO(J = 1-0) intensity ratio, R2 - 1/1-0, which is a good indicator of excitation temperature. The northern survey for the Perseus arm covers the area l = 108circ.5 -- 155circ.0, b = --2circ.5 -- +2circ.5 on a 0circ.5 grid with a 30' beam. A total of 1034 points are observed. The southern survey for the Carina arm covers the area l = 293circ.0 -- 310circ.875, b = --1circ.125 -- +1circ.125 on a 0circ.125 grid with a 9' beam. A total of 2736 points are observed. Analyses are made in three different scales for the two arms: statistical properties of molecular clouds (~100pc), distribution of molecular and atomic gas along the arms (~1kpc), and radial distributions of molecular gas and R2-1/1-0 (> 1kpc, Galactic scale). Our analyses indicate that in the outer Galaxy, (1) Intensity of CO emission per unit mass is lower, (2) The contrast between the compact components (i.e., molecular clouds) and the diffuse components (i.e., intercloud gas) are higher, (3) Fraction of higher ratio gas (R2-1/1-0 >= 0.7; Tk >~ 20 K, n(H2) >~1 × 103 cm-3) which comes from compact components is higher, and the contribution of lower ratio gas (R2-1/1-0 < 0.7; Tk >~ 10 K, n(H2) < 1 × 103 cm-3) which comes from diffuse component is extremely lower, than those in the inner Galaxy. In the outer Galaxy the CO emission from diffuse component 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. In the absorption line study, we have detected [CI] and C18O lines toward the bright radio continuum sources, Sgr B2(M) and W49A. Towards Sgr B2(M), we observed not only position toward the continuum source (on-continuum position) but also positions adjacent to the continuum source (off-continuum positions). Solving the simultaneous equations for on-continuum and off-continuum positions, we get the optical depths (τ) and excitation temperatures (Tex) independently. Column densities of CI and C18O in the 3-kpc arm are N(CI) = (1--7) × 1017 cm-2, and N(C18O) = (3--5) × 1015 cm-2, respectively. The derived H2 column density for the 3-kpc arm is N(H2) = (4.0 +/- 1.8) × 1022 cm-2. Large Velocity Gradient (LVG) line formation model indicates that absorption lines arise from less dense (n(H2) <~ 500 cm-3) molecular gas. If the molecular gas in the 3-kpc arm were the standard molecular gas commonly detected in CO emission, the derived H2 column density should correspond to a CO integrated intensity of ~102 K km s-1 as we estimate with the CO-to-H2 conversion factor (X factor) for the inner Galaxy. However, the observed CO intensity of the 3-kpc arm is only <= 7 K km s-1. Both of the above two approaches point to a possibility that substantial amount of molecular gas resides in the diffuse component. The low density keeps CO unexcited there. This strongly suggests a serious limitation of the use of the X factor; it can very by up to two orders of magnitude in facial value from a component to another. A versatile tool to estimate the amount of the diffuse molecular component needs to be sought.

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