The Initial stages of the collapse of protogalaxies were taking place when the Universe was a few hundred million years old. The Cosmological redshift corresponding to this epoch is about 10.Considering that the largest redshift observed to date is about four, it doubtful that galaxy formation will be amenable to direct observation in the near future. However, it is useful to anticipate and try determine what we should expect to see.

Most of the action occurs in the first hundred million years when the first generation of stars formed. Evidence about the chemical composition of the objects in our Galaxy strongly indicates that many of these early stars, which created most of the heavier chemical elements, must have been very massive. The theory of stellar evolution predicts that such stars rapidly evolve to explode as supernovae. It is often speculated that the formation of galactic nuclei was accompanied by even more violent events. Let us be optimistic and estimate that a young galaxy can just be observed when it has an apparent luminosity of 24 mag. At the distance coresponding to a redshift of 10, the absolute magnitude must then be —23, or about 25 supernovae radiating at any one time. Considering that the decay time of a supernova flash is about a month, we may expect the young galaxy to be just barely visible if it generates one supernova per day, that is 20000 times as many as our Galaxy produces now. Assuming that all the mass passes once through a supernova phase, and assuming that each supernova involves 50M0, a 1011 Mo protogalaxy could keep up this explosion rate for five million years. Since the collapse time is of the order of a hundred million years, this flash is very brief, so that first sight the chances of observing a galaxy’s birth seem slim

However, the above estimate tacitly assumes that the light is emitted as continuum radiation. This need not be the case, for it may be emitted in emission lines. Let us assume the extreme case that all the supernova radiation is in the hydrogen alpha line, and that the line width corresponds to a random Doppler velocity of 10000kmsec-1. Then the peak intensity of the line is about 200 times that which the continuum would have had, so that the supernova rate needed to make the young galaxy visible is 200 times less than the previous estimate: only one per 200 days. A protogalaxy with a mass of 10nM0 could well sustain this rate for a hundred million years, that is during the entire collapse time.

Although these estimates are rather too optimistic for present-day observational techniques, they show that under favourable circumstances the birth of galaxies is observable with optical telescopes. It is also possible that young galaxies are powerful radio sources, but the formation mechanism is far too uncertain to allow predictions of their strengths and spectra.

When the collapse proper has ended, the rate of star formation is negligible as compared to its previous pace. The more massive stars (above 5M0 or SO) reach the end of their evolution within a few hundred million years and the galaxy looks almost an exactly as it would today.

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