| Description |
scientific abstract the center 10. (about 20 pc) of the milky way contains several interesting phenomena which are not found elsewhere in the galaxy. very near to the dynamical center, there is a unique non-thermal radio point source, sgr a*, which may well be a massive (10^6 m solar) black hole. small scale (1 pc) radio filaments appear to circulate about sgr a* (the mini-spiral). sgr a* is also enveloped by both a massive and dense stellar cluster and a rotating circumnuclear gas and dust disk (cnd). about 20 pc from sgr a*, there are several straight non-thermal radio filaments which extend about 30 pc perpendicular to the galactic plane. these non-thermal filaments appear to be linked to the galactic center by a series of thermal radio arches. enveloping (and perhaps constraining) these large scale radio structures are several massive molecular clouds. we propose to investigate the physical conditions, dynamics, energetics and ionization structure of many of these phenomena through a series of lws fabry-perot and grating studies. besides being a unique and interesting source in of itself, the center of the milky way also provides a nearby prototype to study and compare to more distant spiral nuclei. observation summary our highest priority is to obtain fully sampled grating spectra over a fully sampled region enclosing the cnd and thermal arc, and non-thermal arc, over the full grating range. our map will be aligned with the cnd major axis and fully sample regions 350x1100 and 200x500 in extent. the integration time (total of 7.5 hours) was chosen to ensure detection of lines at the level of 1e-15 w/m^2 over the entire band, and ten times weaker at the more sensitive wavelengths; the value is 100 times weaker than the currently known oi line in this beam. the aot was crafted to use detector lw4 on this strong source. the observations will provide information about the ionization structure (e.g. nii/niii), density, temperature and uv field strength (e.g. oi 63/oi 145 - cii), shocks vs. uv excitation of the molecular gas (e.g. oh/co). in addition, this set of full spectra will provide an important archival reference on the galactic center. our second goal is a series of deep lws fabry-perot observations of sgr a*. the 100 iso beam will encompass the mini-spiral and much of the inner cnd. at 30 km s^-1 resolution, the spectral lines will be resolved in this mode, permitting kinematic studies and separating close line pairs (e.g. co 162.8, oh 163.1, 163.4). in addition, these observations will better separate line from continuum for particularly weak lines. we have selected five intervals for detailed studies as being particularly rich in important lines - many of which are not observable from kao. in priority order these are: 110-125 um (e.g., nii, oh, co, hd), 145-166 um (e.g., oi, cii, co, oh, nh3, heh+), 50-65 um (e.g., oi, si, niii, oiii), 78-90 um (e.g., oh, co, h2o, oiii), and 178-183 um (e.g., h2o, h3o+). we have chosen the integration time to permit detection of lines as weak as about 1e-15 w/m^2. a total time of 6.4 hours is dedicated to this project. our general third goal is a selected beam map of the filaments/arches region in 6 selected lines with the lws fabry-perot. for these regions, the ionization source is not clear, be it uv flux which has escaped from the sgr a* cavity, ionization from nearby ob stars, or critical ionization from caused by the high velocity movement of molecular clouds across magnetic field lines. the grating scan maps will provide our foundation for the whole region, and we have selected the six specific lines to help address the ionization questions. first, the nii(121.9 um)/niii(57.3 um) line pair traces the hardness of the uv field, and is especially sensitive when the ionization source has a characteristic temperature of 30,000 k - the inferred value from the vicinity of sgr a*. the ratio therefore will indicate the relative importance of local o stars and the sgr a* radiation field to the ionization. we will map this ratio over the full region in 6 sampled pointings. we will also measure the oiii 51.8 and 88.3 um lines to eliminate density as a variable. finally, the oi(63.2 um), si(56.3 um) lines together with the published cii map (polglitsch et al. 1991) will constrain the shock contribution (critical ionization mechanism). table of parameters of sag a: lws01 full grating scans, fully sampled maps: 27511 seconds limit: 1e-15 -> 1e-16 w/m^2 lws04 line scans, 6 lines, 6 points: lws03 line scans: 50-65um, 78-90um, 100-125um, 145-166um, 178-183um. |
