Distributions of Atomic and Molecular Hydrogen

We believe that stars form in molecular clouds, in which major component of Hydrogen is ``molecule.'' $M_{\rm H\sc I}$ is larger than $M_{\rm H_2}$ and H$\sc I$ gas distributes rather uniformly than H$_2$ gas. Figure 3.5(a) shows distributions integrated intensities of H$\sc I$ (red) and CO (blue) for Large Magellanic Cloud (Fukui et al. 2009). CO gas is found as GMCs with a mass $M\sim 10^{5-6}M_\odot$, while HI gas is distributed more uniform than that of CO gas.

Figure 3.5(b) shows that line width of HI is much larger than that of CO. The comparison in the phase-space (position VS line-of-sight velocity) indicates us that the HI gas coexisting with H$_2$ gas was overestimated when the comparison is made for the column density $\sigma$. H$_2$ gas associated with GMCs may be a small fraction of HI gas.

Figure 3.6 displays the distributions of HI and CO gas. CO and HI data are divided into pixels whose 3D size is 40pc$\times$ 40pc $\times 0.17{\rm km s^{-1}}$. The number of pixels where HI ($> 7.2$K) and CO ($> 0.21$K) are both detected are plotted by a histogram with dark color. While that of $T_b(H{\sc I})> 7.2$K but $T_b(CO)< 0.21$K (light-shade histogram) shows a different distribution. The ratio of pixels associated with CO increases with $T_b(H{\sc I})$ (Figure 3.6 bottom). This may indicates an accretion of HI gas to form GMCs.

Figure 3.5: (Left) Distributions integrated intensities of H$\sc I$ (red) and CO (blue) for LMC. The H$\sc I$ integrated intensity is obtained from combination of aperture synthesis and single-dish observations. (Kim et al. 03). (Right) Spectra of HI and CO for a direction are shown.

Figure 3.6: (Left, upper) Number of pixels are plotted against the brightness temperature of HI. CO and HI data are divided into pixels whose 3D size is 40pc$\times$ 40pc $\times 0.17{\rm km s^{-1}}$. (Left, bottom) Number of pixels associated with CO emission increases with higher HI brightness temperature (Fukui et al.2009). (Right) Classification of GMCs. Type I GMCs are observed without any indication of massive star formation; Type II's are GMCs with HII regions. However, young clusters are embedded in the GMCs and not observed. Type III are associated with HII regions and young clusters (Kawamura et al.2009)

Another important finding of NANTEN survey is a classification of GMCs from a stand point of massive star formation. Kawamura et al (2009) indicate that ``Type I shows no signature of massive star formation, Type II is associated with relatively small HII region(s), and Type III with both HII region(s) and young stellar cluster(s),'' as shown in Figure 3.6(right). The lifetime of a GMC through Type I to Type III is estimated to be a few $10^7$ yr. The average HI intensity for Types I, II, and III are estimated as $34\pm 16$K, $47\pm 17$K, and $56\pm 19$K. The HI intensity increases with the sequence of star formation.