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The UV spectral region is rich in information on the astrophysical properties of solar system objects, stars, star forming regions, and galaxies. The UV is uniquely sensitive to hot sources, in particular to the massive stars which are responsible for most "star-formation astrophysics," as well as to certain types of old, highly evolved, stars. This regime is also critical for studies of the metal abundances and surface gravities, two fundamental parameters of stellar astrophysics.  « The Antennae Galaxies (NGC 4038/4039) imaged by WFPC2 » The image shows the richness of star-forming regions (rich in hot, young stars visible in blue) and dust lanes present in this merger of two spiral galaxies. The bulges of the merging spirals contain older, colder stars and are visible in red. 1.1 STELLAR ARCHAEOLOGY (a) Resolved Stellar Populations The study of star formation requires analysis of a large number of stars in wavelengths sensitive to temperature, metalicity, and gravity This will contribute significantly to our understanding of the birth of stellar populations. Old stars may have depleted their hydrogen fuel and begun burning Helium. These stars have surface temperatures above 10,000 K (the sun is near 6,000 K). They are faint in visible wavelengths, but shine brightly in the UV. WFC3 will allow detailed studies of these geriatric stars.
(b) Stellar Populations in Integrated Light Stellar populations at great distances may be too small to be seen individually, but WFC3 can collect the integrated light through different filters to determine much the same information, statistically, for a population as it can an individual star. The UV allows direct detection of the massive stars responsible for most of the ionization, photo-dissociation, kinetic-energy input, and element synthesis in galaxies. These processes are responsible for much of the astrophysics of the universe. 1.2 THE ASSEMBLY OF GALAXIES AT HIGH REDSHIFT To analyze galaxies, we have to contend with redshift. According to Hubble's Law, the further away a galaxy is from us, the faster it is receding from us. The apparent speed of recession is quoted in units of the speed of light, c, so a galaxy with redshift 0.5 appears to be moving away from us at half the speed of light. At very high speeds, the spectra emitted by these galaxies are shifted to higher wavelengths (redder). This "redshift" moves diagnostic lines and makes them accessible to moderate UV detectors. We now suspect that much of the “final assembly” of galaxies, and much of the conversion of primeval gas into stars, occurred at relatively low redshifts in the range z = 1-3. These redshifts correspond to lookback times of half to three-quarters of the present age of the Universe. Star-forming galaxies produce an abundance of light around and below 912 Ĺ (far ultraviolet). For small redshift (z=1-3), the light is shifted redward to 2000-3500 Ĺ. The hydrogen Lyman-alpha emission line (1216 Ĺ in the rest frame), is bright in many distant galaxies. For galaxies with redshifts of z = 0.8-3, this line can be detected in the 2000-5000 Ĺ range. Of special interest are the numerous "sub-galactic clumps," which make up a significant part of the faint blue galaxy population. Through the process of repeated hierarchical merging, it is believed that these clumps came together to form the luminous galaxies we see today, i.e. they are the building blocks of galaxies. If they exist everywhere, they may be used to trace the large scale structure of the Universe.
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Curator: Lori Tyahla Content Manager: Lori Tyahla NASA Official: Randy Kimble Last Updated: March 22, 2007 + Contact WFC3 Webmaster |
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