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Report of Galactic Structure SessionSession Leader: Bruce ElmegreenNuclear RegionInfrared line observations (C II, O I, CO) of the nuclear region of the Milky Way (Lugten et al. 1986; Genzel et al. 1990; Poglitsch et al. 1991; Nakagawa et al. 1995) have shown a rich structure with clouds, arched filaments associated with ratio continuum structures, and a small tilted disk. The C II is probably at the interface between dense clouds and bright uv sources, such as OB stars. Some of the material may be accreting to the Sgr A core. Submillimeter continuum emission from cool dust in this region shows sources associated with molecular clouds and a ridge between them (Dent et al. 1993). A ten meter sub-mm wave telescope at the South Pole can map the nuclear region in C I emission at 370 and 600 microns, N II emission at 205 microns, and in the high J lines of CO and other molecules. The C I map can be compared with the N II map and a C II map from ISO to determine variations in the PDRs and diffuse ionization structure. The brightness of a PDR indicates the relative location of the neutral gas and the source of ionization. This could be used to map the relative positions of the dense clouds and uv sources in the nuclear region. The CO lines can be used to give gas densities in nuclear molecular clouds. The densities in these clouds are known to be very high, so sub-mm J transitions are necessary to determine their values and the corresponding pressures. Extensive polarization maps of the nuclear region and filaments at sub-mm wavelengths will reveal the magnetic field orientation and structure with unprecedented detail. Since much of the theoretical work on nuclear gas processes and nuclear jets involves magnetic forcing, these observations should stimulate future developments of the theory. C I maps of the extended nuclear region will include the Galactic Bar and possible evidence for streaming motions along the leading edge of the bar, as observed in other galaxies.Polarized Dust Emission from other LocationsInfrared polarization from aligned dust emission provides a rich source of information on magnetic field orientation in a wide variety of regions in the Galaxy (Hildebrand 1988; Barvainis, et al. 1988; Flett & Murray 1991). Extensive maps of polarized dust should be made at 350 microns for every possible environment: in high latitude loops, dense clouds, suspected shock fronts near supernovae remnants, continuous (C-type) shock fronts in molecular clouds, diffuse clouds, connections between clouds, and so on. At larger distances from the Sun, chimneys or vents of hot gas to the halo might be found in regions where the polarization becomes perpendicular to the Galactic plane. Variations in the ratio of sub-mm polarization to continuum emission should also be mapped to help understand the alignment of grains and how it depends on gas density, temperature, and velocity field. Emission line polarization is a new field. Many discoveries about the origin and dynamics of Galactic magnetic fields and about dust alignment should come from these studies.Local CloudsThe intricate "fractal" structure of local and other clouds, particularly at high latitude, was originally observed in warm dust continuum emission by IRAS (Low et al. 1974). A similar structure is inferred for unresolved molecular clouds because of the ease of penetration by uv light, as indicated by pervasive emission from C II (Stutzki et al. 1988; Howe et al. 1991; Jaffe et al. 1994; Boreiko & Betz 1995) and other fine structure lines (Meixner 1992; Stacey et al. 1993). Penetrating C II has also been observed from diffuse clouds at high Galactic latitude (Bock 1993), and from the rho Oph cloud (Yui et al. 1993). Other evidence for fractal structure comes from perimeter-area relations in CO or IRAS maps (Dickman, et al. 1990; Falgarone et al. 1991), and from the cloud size distribution and the mass-size relation (Elmegreen & Falgarone 1996). Theoretical studies of interstellar gas dynamics and star formation depend our knowledge of the detail structure of gas and its motions. Most likely, the observed structure and motion is related to turbulence, but it could also be affected by regular magnetic waves, localized stellar or pre-main-sequence winds, and large scale flows, as in a spiral wave. The ten meter telescope will be the most important groundbased instrument for mapping the detailed structure of neutral gas (C I; Phillips & Huggins 1981) and dust emission in local and other Galactic clouds. This should clarify the suspected fractal or filamentary structure like no other experiment. The angular resolution will be much higher than for IRAS or H I studies at 21 cm, and C I is pervasive for dense diffuse gas (n > 300 cm-3). Comparisons between the continuum or C I structures and the magnetic field orientation obtained with the same telescope will give unprecedented clarity to many processes relevant to interstellar gas dynamics and cloud formation. From the South Pole, regions that can be studied include diffuse clouds in the outer Galactic flare and warp, the Galactic center and Carina spiral arm, the Gum nebula, eta Carina and other important nebulae and molecular clouds, much of Lindblad's ring and local high latitude clouds, and possibly the Magellanic stream and other high velocity clouds. Studies using both continuum from dust and line emission from C I and N II should be made.The Galactic PlaneDiffuse emission from C II has been observed along the galactic plane from photodissociation regions near and inside known molecular clouds and possibly from the diffuse ionized medium (Shibai et al. 1991). Emission from this and many other FIR fine structure lines, including C I and N II, was observed by COBE (Wright et al. 1991). Similar emission was mapped for the nearby galaxy NGC 6946 (Madden 1993). The theory of diffuse line FIR emission was considered by Wolfire et al. (1995), not including the C I. The ten meter telescope should be used to survey the Southern Galactic plane in C I and N II. The N II emission comes from H II regions and will be a much better tracer of distant ionization than H alpha because there is no dust extinction for N II. The C I comes from dense gas, including both atomic and molecular regions. C I absorption against background dust continuum might also be observed. The Southern molecular ring in our Galaxy should be mapped in detail. Comparison between C I and CO should reveal the extensive diffuse cloud structure around molecular clouds, and the ratio of these two line strengths should be a good measure of the molecular fraction, which is known to vary with radius in the Galaxy. C I should also trace galactic dynamics as well as CO, but it should also include slightly lower density regions, thereby covering a larger volume filling factor than CO or other molecules.Stellar and Gaseous Distance TracersFIR emission from stars, whether from lines or the continuum, or from variable or steady sources, may eventually be found to contain enough information to determine the stellar distance independent of other information, such as velocity. Such a discovery, like a period- luminosity relation for Cepheids, would allow distance determinations in the infrared, which has the advantage over optical methods of begin free from dust extinction. Possible sources for such a distance calibrator might be Carbon stars, Carbon rich WR stars, LGB stars, or protostars. For example, one might search for a correlation between intrinsic IR luminosity and IR spectral linewidth, as might arise in a wind or from associated disk rotation, or one might search for an IR period-luminosity relation in evolved stars. If such a stellar IR distance indicator is found, then it might be possible to map the far side of the galactic disk, completing the spiral arcs that are found on the near side. IR distances to nearby galaxies might be possible too, using bright stars that are rare enough to have no similar stars in the same field of view. A distance indicator might be present in the gas as well, using a size- linewidth relation, for example. If IR fine structure lines are found to show a linewidth correlation with the size of the mapped cloud, as is the case for CO lines in nearby molecular clouds, then maps of the far- side galactic plane at high spatial resolution (high enough to resolve the scale height), can be decomposed into discrete clouds with measurable linewidths and sizes. This should give the 3D structure of the Milky Way when the observed sizes are converted into distances using the intrinsic sizes from the linewidths. C I would seem to be a good choice for such an emission map because of its clear association with molecular and other neutral gas clouds. Preliminary calibrations on local clouds would be necessary.References
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