8. FURTHER QUESTIONS
Most astronomers would agree that galaxies interact and that gravity
plays the leading role in the dynamics of such encounters. At the
present, however, there is considerable controversy over the
significance of interactions in shaping galaxies. The point of view
adopted in this article is that interactions provide important
examples of galaxy transformations.
Numerous questions remain: How much satellite accretion can disk
galaxies stand? How much has occurred? What signatures exist in our
galaxy? What are realistic parameters for merger progenitors and
remnants? What range of remnant dynamics and orbital structure are
permitted by the merger process? What distributions of axial ratios,
minor-axis rotation, and non-elliptical isophote shapes are expected
of mergers? Does long-term dynamical evolution play a role? How
does gas lose angular momentum during interactions and mergers? How
will a multiphase ISM behave? What effects will star formation have?
What photometric, kinematic, and enrichment signatures of starburst
events should we look for in old merger remnants? What controls the
rate at which a monster is fed? What are the relative contributions
of starbursts and nonthermal sources to IRAS galaxies? To powering
``superwinds''? Can ``buried quasars'' break through? And continue
to shine after? Why were QSOs more common at z 2? Why so
much brighter? Do a few, some, or most E galaxies have
central black holes? What is the origin of the density-morphology
relationship? How often are compact groups formed in evolving loose
groups? How much merging took place in the evolution of rich
clusters? How much substructure survives in regular clusters? Where
do cD galaxies come from? How does the merger rate change over time?
Can galaxies long survive the merging of their halos? What is the
relation between bulges and ellipticals? What determines the specific
frequency of globular clusters?
We are especially grateful to John Kormendy, François Schweizer,
Alan Stockton, and Alar Toomre for their generous encouragement and
for comments on the manuscript. We also thank Halton Arp for the
NGC 3808, and
NGC 520, Victor Blanco and CTIO for
the photograph of the Cartwheel, and François Schweizer for the
photographs of NGC 4038 / 9 and
NGC 7252 and for composing
J.B. thanks the Canadian
Institute for Theoretical Astrophysics for
their hospitality and support during the initial work on this article.
This work was supported in part by the
Alfred P. Sloan Foundation,
NASA Theory Grant NAGW-2422, Grants
CS 79-90 and CS 47-91 from the
Institute, and NSF Grant AST 90-18526.