The following document lists the file abstract/HSMITH_GCENTER.abs
from catalogue VI/111.
A plain copy of the file
(without headers/trailers) may be downloaded.
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 350"x1100" and 200"x500"
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.