What energy sources power galaxies?

One of the most significant discoveries of the 1980's was that of ultra-luminous IR galaxies, systems in which some process - perhaps starbursts or accretion by massive black holes - produces enormous infrared luminosities on a scale previously identified only with quasars. IRAS survey results suggest that this energetic activity correlates with galaxy interactions or mergers. Such luminous targets can be traced all the way back to the formation epoch with observations in the 1990's. Comparison of infrared luminosity distributions with those measured from other surveys, particularly in X-ray or ultraviolet radiation, is essential to account for luminous galaxies and quasars otherwise overlooked because of obscuration.

What is the distribution of matter in the Milky Way and nearby galaxies?

The evolution of galaxies is marked by a continuing cycle of birth and death of stars. This leads to the evolution of the elements from nearly pure hydrogen and helium to material with sufficient heavy elements to form earth-like planets. IR studies out to the distance of the Virgo cluster and beyond will measure elemental and chemical abundances of many of the heavy elements, both in the reservoir of the interstellar medium and as newly-formed material ejected from supemovae, novae and red giant stars. The age, composition, and structure for our Galaxy are crucial benchmarks for understanding other galaxies. Our vantage point is immersed in the obscuring dust of the Galactic disk, but infrared observations allow us to penetrate the dust in order to study stars and interstellar matter throughout the Galaxy. IRAS far-IR and COBE near-IR images of the sky toward the Galactic center, together with a visual view, are displayed as the frontispiece of this report. The COBE and IRAS images show the distribution of stars and luminous dust clouds, respectively, in this region of sky - extraordinary demonstrations of the power of infrared observations to reveal the grand design of the Milky Way Galaxy.

The essential questions concerning star and planet formation, processes which are central to our concept of the universe in which we live, remain unanswered. Most of the visible matter in the Universe is in the form of stars, and star formation is central to the formation and evolution of galaxies. Closer to home, the formation of planets and planetary systems is a prerequisite for the formation of life as we know it. Both of these birth processes occur deep within dense clouds of dust and gas opaque at visible wavelengths but transparent in the infrared.

How do stars form, and what conditions lead to protostellar collapse?

Star and planet formation begins with a dense molecular cloud core which collapses to form a protostar embedded in a circumstellar protoplanetary disk. The protostar grows by direct infall of material onto the star and by accretion from the inner boundary of the disk. The gas and dust remaining in the disk is the raw material from which planets form.

The rate at which stars form and the resultant distribution of stellar masses must depend on the physical properties of the molecular cloud; composition, gas density, temperature, velocity field, chemical and ionization state, and magnetic field. In the 1990's we will image molecular clouds with sufficient sensitivity, spatial resolution and spectral resolving power to measure the conditions throughout star-forming clouds, to detect the emission from individual embedded stars, to determine the luminosity function into the substellar range - far below the hydrogen burning limit of about 0.08M_o - and to correlate star formation rates and stellar masses with the cloud properties. The spatial and spectral resolution available at far-infrared and submillimeter wavelengths will enable the detailed study of numerous infalling cores in nearby molecular clouds. It is believed that the infall halts abruptly at an accretion shock, which marks the boundary of the protostar or the protostellar disk. By detecting the IR spectral lines from these dust embedded accretion shocks, and measuring their profiles, the observations will probe the non-spherically symmetric infall onto the protostellar system and reveal how planet-forming disks are assembled.

The infall phase ceases when an outflow from the protostar impacts the infalling material, reversing its direction and sweeping it outward. The outflow is frequently collimated, by an as yet unidentified process, into a bipolar or jet-like flow. In the 1990's, IR imaging and proper motion observations will make it possible to see the jets and outflows as close as 10 AU from the protostar, thereby probing the agent of collimation. By observing the IR emission from the shocks produced when the outflow encounters ambient gas or protostellar disks, we will discover how the outflow evolves, how it inhibits infall, how it affects the disk, and how it interacts with the ambient molecular clouds.

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