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Star formation history of the universe.

Marcin Sawicki

Doctor of Philosophy 1999
Graduate Department of Astronomy, University of Toronto

The evolution of the comoving luminosity density at ultraviolet wavelengths is, effectively, a measure of the history of star formation in the Universe. Similarly, the infrared luminosity density is an indicator of the existent stellar mass. The evolution of the comoving luminosity density can, therefore, be used to reconstruct the history of star formation in the Universe.

A measurement of the luminosity density can be obtained by integrating the galaxy luminosity function. Using the novel technique of photometric redshifts, the galaxy luminosity function is determined over the redshift range 0 < z < 4. Brightening is seen in both the luminosity function and the luminosity density out to z~3; this is followed by a decline in both at z>3.

Since dust shrouds star-forming regions, the amount of dust must be determined in order to accurately measure the amount of star formation at high redshifts. Broadband spectral energy distributions of z>2 galaxies are combined with spectral synthesis models to show that high-z galaxies (Lyman break objects) are shrouded in enough dust to attenuate their ultraviolet fluxes --- and, hence, star formation rates --- by a factor of more than 10. It is also found that these high-z objects are unlikely to be direct progenitors of present-day massive galaxies.

A color selection technique is used to identify candidates for ultra-luminous z>5 galaxies. If indeed at high-z, then these ``g-band drop-outs'' have extremely high star formation rates (greater than about 50--1500 Msun/yr) and produce comoving luminosity densities comparable to those observed at the present epoch. These objects may represent the formative stages of present-day massive galaxies.

The above luminosity density and attenuation measurements, together with other data, are used to construct a qualitatively robust picture in which star formation peaked at t~0.2t_o and declined more or less exponentially ever since. While qualitatively this picture is robust, different combinations of IMF, amount of high-z dust, and underlying cosmology result in scenarios of cosmic star formation which are quantitatively different from each other.

Reproduced with permission by Marlene Cummins
July 12 1999