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Chapter 9
Summary and Future Work
Here we brie y summarize the major ndings of the previous chapters and discuss
projects for future study.
9.1 Summary
This thesis presents data from the H i Southern Galactic Plane Survey. We have
surveyed the H i and 21-cm continuum emission in the Galactic plane between l =
253 ф 358 ф and b = 1 ф at an angular resolution of 2 0 and spectral resolution of
0:8 km s 1 . The survey constitutes over an order of magnitude improvement in angular
resolution and sensitivity over previous surveys of this region. The general goals of
the survey are to understand the structure, dynamics and thermal distribution of the
neutral hydrogen component of the ISM. We have full polarization information that
we are using to study the magnetic eld structure of Milky Way and magneto-ionic
medium. The SGPS has taken its place among three very large surveys of the Galactic
Plane that form the International Galactic Plane Survey. This collaboration seeks to
survey the entire Galactic plane at an angular resolution of 1 arcminute, a spectral
resolution of 1 km s 1 and to a sensitivity limit of 1 K.
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In this work we have focused on presenting the survey goals, observations and anal-
ysis procedures and some of the results of the survey. In particular, we have studied
H i shells in the Galaxy. In Chapter 1 we described the survey goals and outlined some
of the immediate projects underway with these data. In Chapter 2 we explained the
Parkes portion of the survey. We described the observing method and the data reduc-
tion strategy. Because some characteristics of the data reduction are non-standard, in
particular our treatment of frequency-switched data, we explained these in detail. The
chapter summarizes the tools developed and fully explains how to use them. We con-
cluded the chapter with a summary of the ten 14 ф 22 ф data cubes that were produced
and will be made public.
In Chapters 3 and 4 we carefully examined two pairs of large H i shells discov-
ered in the SGPS Parkes data. Chapter 3 presents the outer Galaxy chimneys, GSH
277+00+36 and GSH 280+00+59. Both shells are extremely large, R sh  305 pc and
215 pc, respectively, and appear to be chimneys that have opened to the Galactic halo.
These shells are located on the edge of the Sagittarius-Carina spiral arm. In Chapter 4
we presented GSH 304-00-12 and GSH 305+01+24, two shells behind the Coalsack
nebula. GSH 304-00-12 is particularly large in angular diameter, covering 29 ф  20 ф ,
and expanding from the Sagittarius-Carina spiral arm into the interarm region. Using
a simple numerical model of a stellar wind-blown bubble, we show that GSH 305+01-24
is associated with the Centaurus OB1 association.
Chapter 5 is a catalog of large shells in the SGPS region. We presented fteen
additional new shells and discussed their individual properties. We found that one,
GSH 337+00-05 is associated with an OB association, Ara OB1, that another, GSH
298-01+35 is a chimney candidate, and that many shells are joined. We discussed the
radius and expansion velocity distribution and compared those with catalogs compiled
from other galaxies. We also began to explore H i shells at high resolution. We showed
that all shells have very thin, dense, walls and suggest that these may help to distinguish

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pressure-driven shells from shells caused by gravitational instabilities (Wada et al. 2000).
These high resolution observations open the door to studies of instabilities in shells,
and will hence allow us to probe the physical conditions of shell break-out much more
accurately.
Chapter 6 began our discussion of the ATCA survey. In this chapter we described
the observing technique for this survey, the largest mosaicing survey completed to date.
We explain the process of calibration and solutions developed for problems with survey
data. We elaborate on techniques for combining single dish and interferometer data
and nally present selected images from data cubes of the full survey region.
In Chapter 7 we presented an analysis of the SGPS Test Region (325: ф 5  l  333: ф 5;
0: ф 5  b  3: ф 5). The objective of this chapter was to elucidate possible uses of
the SGPS data, particularly distance estimates from continuum absorption and direct
comparisons of the continuum and H i emission. The analysis focused on the interaction
of massive stars with the ISM. In particular, we examined the ISM around H ii regions,
supernova remnants (SNR) and small H i shells. We presented absorption spectra
towards two SNR that provided new distances to these remnants. These results serve
to show that a wealth of information about the morphology of H ii regions and SNRs
can be obtained from observations of the surrounding medium.
Finally, in Chapter 8, we presented the distribution of H i shells in the Galaxy, in
particular with respect to spiral structure. By examining the distribution of SGPS shells
in l-v space and with respect to the Taylor & Cordes (1993) model for the spiral arms
we nd that many large shells are in interarm regions. We included the Heiles (1979,
1984) shells and found that many of those shells are also between the spiral arms. In
addition, we conrmed that the largest shells are at large galactocentric radius. Using
spiral density wave theory we showed that shells expanding into interarm regions may
experience exaggerated expansion down a density gradient that extends from the arm
into the interarm region. Additionally, the timescale and several generations of massive

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stars involved in the formation of a large shell may lead to a spatial oset of the shell
away from the leading edge of the spiral arm.
9.2 Future Work
This dissertation asks as many questions as it answers. But, of course, the Southern
Galactic Plane Survey is just beginning and therefore the scientic opportunities avail-
able through this data set are just now revealing themselves. Some of the questions that
this project was designed to answer remain unanswered: what role do H i shells have in
the energy budget of the Galaxy? what is the porosity of the Galaxy due to H i shells?
Other questions appeared along the way and proved intriguing to solve. We hope that
the SGPS may help us to answer some fundamental questions about the structure and
dynamics: where are the spiral arms? what is the large scale density distribution of the
neutral medium? Perhaps most important is that the SGPS will provide a resource for
astronomers of all varieties.
It will also be particularly interesting to expand the shell catalog throughout the
Galactic plane and include small shells. With a statistically complete Galactic shell
catalog we should be able to determine what percentage of shells are between the spiral
arms and if there is a higher probability of nding small shells in the arms and large
shells in interarm regions. An expanded sample will also allow us to study shells in a
variety of evolutionary states.
In Chapter 5 we initiated a discussion of the high-resolution properties of supershells.
Clearly, we have only just begun to explore these opportunities. Small-scale Rayleigh-
Taylor instabilities will allow us to study the density and pressure structure at the shell
walls and how shells interact with one another. As suggested, the density structure of
the shell walls may allow us to distinguish pressure driven shells from those created
by gravitational instabilities. However, in order to conrm this suggestion we need to

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expand the high resolution Galactic shell catalog and search explicitly for shells created
by gravitational instabilities.
Follow-up observations of the outer Galaxy chimney GSH 277+00+36 are under-
way as part of a project to study how chimneys supply gas to the halo and how they
evolve. We will obtain high resolution ATCA data on the chimney break-out of GSH
277+00+36 and higher sensitivity Parkes data to search for condensations of expelled
interior gas. These data should help us to understand the relationship between super-
shells and chimneys; probe the disk-halo interaction through a study of the process of
chimney formation; and determine whether shells on the trailing edges of spiral arms
experience a break-out process into the inter-spiral arm region which is physically sim-
ilar to break-out away from the plane. In the ATCA data we hope to see evidence of
instabilities at the shell caps, shearing along the chimney walls, dissolving or recom-
bining H i density enhancements in the chimney interiors, and polarization structures
that explore the layers of ionization and magnetic elds in the shocks. All of these
clues will help us understand the external density gradient, the energy requirements
for break-out, and the age of the objects. The role of magnetic elds in the transport
of matter from the disk to the halo may also be signicant. Magnetic elds may form
unstable congurations, such as Parker instabilities, through which energy stored in
the elds may move additional matter out of the disk. Similarly, the role of magnetic
elds in break-out or containment of shells expanding into interarm regions has yet to
be explored. Contemporaneous full polarization continuum observations will allow us
to explore the polarization structure of chimneys at high resolution for the rst time.
The next several years will be an exciting time for Galactic H i studies as the new
surveys are analyzed and their many results revealed. I look forward to being a part of
this major undertaking.