Next Contents Previous


Of the many questions which confront us about galaxies, it is difficult to discern which ones will lead to the most important advances within the next decade. Here, I have asked several simple questions about galaxies and their environments for which there are no simple answers yet. Even the most basic of questions, ``what is a galaxy?'' is not easily answered when one considers systems as diverse as dwarf irregulars, LSB galaxies, ``normal'' spirals, giant cD's, Arp galaxies, and distant quasars. In spite of this diversity, we still tend to rely heavily on conventional classification systems, the fundamentals of which were devised some 70 years ago to describe bright barred or unbarred spirals, bright ellipticals and a few irregulars. The data are now outgrowing these schemes and demanding a more unified, physically based model. Advances are indeed being made, especially for bright ellipticals, by considering where these galaxies fall in physical parameter space. However, for these and other galaxies, it is still true that the vast majority of accumulated data is optical data, representing only a tiny fraction of the electromagnetic spectrum. Until we obtain a more panchromatic view, our understanding of galaxies will necessarily be limited.

For disk galaxies, galaxy size is best measured in neutral hydrogen, since this is the most extended observable component, at the present time. There is some evidence (e.g from damped Ly alpha absorption lines) for even more extended neutral hydrogen disks around galaxies than is currently observed. However, other considerations (e.g. ionization from the extragalactic radiation field) suggest that the outer HI should truncate fairly abruptly. There is now some observational evidence for truncated HI disks, but the rapid decline in column density appears to occur at levels which are at the limits of currently available instrumentation. More sensitive instruments are required which can detect all appropriate spatial scales in reasonable integration times in order to resolve this issue. The neutral hydrogen extent has relevance beyond determining the scale of the visible galaxy, of course, since mapping a velocity field into the region of declining rotation curve has significant implications for constraining the mass model of the galaxy and, possibly, the nature of the surrounding dark matter, itself.

While the optical galaxy may be just the ``tip of the iceberg'', it is now fairly well established that icebergs collide with each other. Much recent theoretical and observational effort has been directed towards galaxy/galaxy interactions, and the evidence for forming ellipticals from merging spirals is becoming more convincing. What is now needed is more thorough data so that the importance of interactions and their dependence on redshift may be established on a more statistical basis. This may be observationally challenging since it is not clear how the subtle interactions, for which there may be little optical morphological evidence, affect the evolution of the participating galaxies. Rapid, sensitive HI imaging may again be helpful since the neutral gas is a sensitive tracer of interactions. With such data, it may be possible to disentangle the environmental drivers of evolution from those related to initial conditions (nurture/nature). It is also important to consider whether the iceberg is interacting with the ocean. Does an apparently isolated galaxy evolve as a dinstinct system within which internal dynamics (e.g. rotation, galactic ``fountains'') may play an important role, or are galaxies coupled to the intergalactic medium around them (e.g. through inflows, outflows, or interconnecting magnetic field lines)? Detailed observations of nearby galaxies may help to answer these questions.


I am especially grateful to Jim Hesser for suggesting that I write this paper. I am also indebted to Claude Carignan and Terry Bridges for carefully reading and commenting on the manuscript. Thanks to A. Vogler for information about X-ray halos, to A. Broeils for information on the rotation curve of NGC 4138 and to Jacqueline van Gorkom for helpful comments on rotation curves. I would like to thank the following people for providing and/or allowing me to use their images: H. Böhringer, G. Bothun, S. Djorgovski, G. Fabbiano, J. Kamphuis, and R. Perley. This work has made use of the Digitized Sky Survey of the Space Telescope Science Institute. The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc. for the National Aeronautics and Space Administration. The Digitized Sky Survey was produced under Government grant NAG W-2166. Support for this work is from the Natural Sciences and Engineering Research Council of Canada, under grant WFA0156373.

Next Contents Previous