EVOLUTION OF THE INTERSTELLAR MEDIUM IN GALAXIES

The interstellar medium in a galaxy controls the rate of star formation and thus the evolution of the galaxy itself. It is the repository of the heavy elements produced in stars. If star formation becomes too violent, interstellar gas may be ejected from a galaxy into the surrounding intergalactic medium. An understanding of the interstellar medium is necessary if researchers are to address such key questions as the following: What are the physical processes that determine the rate at which stars form in a galaxy? What is the feedback between star formation and the interstellar medium? What is the effect of the extragalactic environment on star formation?

All these issues come into play when the formation of the first galaxies is considered. The first galaxies formed out of enormous clouds of neutral atomic hydrogen. Once the galaxies had formed, the interstellar media of these galaxies remained primarily atomic hydrogen, although with increasing amounts of heavier elements as massive, short-lived stars ejected new elements into the medium. The hydrogen gas should be observable at redshifts above 10 with LOFAR. When the SKA is built, it will be able to map the atomic hydrogen up to redshifts of about 10. Within galaxies, some of the atomic gas will be converted to molecular form on its way to being incorporated into stars.
If the earliest stars have ejected enough carbon and oxygen into the interstellar medium, the broad spectral capabilities of the EVLA will enable observation of carbon monoxide, the most abundant molecule after molecular hydrogen, out to redshifts beyond 10. Newly formed stars ionize some of the gas, producing emission lines detectable by NGST. Supernovae heat large volumes


This optical wavelength picture shows the large spiral galaxy M31 (also known as the Andromeda Galaxy) and its small companions M32, lower center, and M110, to the upper right. Andromeda is the Milky Way’s closest large neighbor at a distance of about 2.2 million light-years, and it is very similar in appearance to, and slightly larger than, the Milky Way. In fact, M31 is visible to the naked eye, although we can see only the bright inner bulge. This image comes from photographic plates taken with the 0.6-m Burrell Schmidt telescope of the Warner and Swasey Observatory of Case Western Reserve University. GSMT will be able to study individual stars near Andromeda’s center, which is a very tightly packed star cluster not visible in this saturated image.

of the interstellar gas to millions of degrees, and x rays from this hot gas will be measured by Constellation-X to determine the temperature, pressure, and elemental abundances in this hot plasma. These same instruments will also permit astronomers to trace the evolution of gas in galaxies through cosmic time, as the universe synthesizes the elements needed to form planets and eventually to enable life.

Structure in the interstellar medium of a galaxy spans a wide range of scales, from much less than 1 light-year for the molecular cores that produce individual stars to 100,000 light-years for the galaxy as a whole. The gaseous galactic halo extends farther; it comprises both gas blown

out of the disk and gas accreting from the intergalactic medium. Much of the mass of interstellar gas in disk galaxies is atomic and molecular gas that is quite cold, with a temperature that is less than 100 degrees above absolute zero. A substantial (but uncertain) fraction of the volume of such galaxies is filled by gas that has been heated to more than a million degrees by supernova explosions. There is also a significant amount of gas at intermediate temperatures that is heated by starlight. All this gas is permeated by cosmic rays, particles moving almost at the speed of light, and by magnetic fields. The primary hindrance to a greater understanding of how the interstellar medium mediates the evolution of galaxies is ignorance of the spatial distribution of these various components of the interstellar medium and how they are interrelated. Surveys of the interstellar medium in nearby galaxies with the recommended radio, infrared, x-ray, and gamma-ray facilities will provide valuable data on these issues. Understanding the complex structure of the interstellar medium and how it interacts with the process of star formation is a daunting theoretical problem for this decade.