Formation redshift

It is also interesting to note that solar metallicities are reported for z=2, e.g. [22], and that disks of old stars have been found at redshifts as high as z=2.5, see e.g. [23]. These types of findings indicate that indeed the formation of the thick disk in our galaxy might have happened well in the past. 

In the next very few years stellar abundances and kinematics will be available for as many stars as redshifts are now available for galaxies. This abundance of information can, provided we approach the analysis and interpretation with due imagination, advance the astrophysics of galaxy formation as much as Cosmology has advanced over the last few decades. No doubt our image of galaxy evolution will be similarly revolutionised. 

The epoch and mode of galaxy formation are not well known, but both quasars and star-forming galaxies are known with redshifts up to about 7, corresponding to an era when the expanding Universe was only 1/8 of its present size, and the emission-line spectra of quasars indicate a large heavy-element abundance (solar or more Hamann Ferland 1999), suggesting prior stellar activity. The first stars, on the other hand, known as Population IIP, would have been devoid of metals whether they differed from normal stars in other basic characteristics, notably their mass distribution, is not known, since no completely metal-free stars have been... 

Fig. 12.4. Global co-moving star-formation rate density (assuming a Salpeter(O.l) IMF) compiled from surveys by Lilly et al. (1996), Connolly et al. (1997) and Steidel et al. (1999), assuming Einstein-de Sitter cosmology with h = 0.5. The point at zero redshift is based on an Ha survey by Gallego et al. (1995). After Pettini (1999). With kind permission of Springer Science and Business Media. Courtesy Max Pettini.

It is surprising to note that zinc in DLA systems hardly evolves at all over a range of redshifts from 4 to 0.5, whereas star formation, deduced from the changing colour of the galaxies as a function of z, would indicate the opposite effect. 

Chemical Control. The formation of mixed chalcogenide/mixed metal semiconductors is another way of fine tuning optical spectra. For example, exposure of CdS in ArH films to H2Se resulted in a redshifting in the absorbance spectrum of CdS attributed to the formation of CdS,Se species (60). XPS results revealed that after... 

For comparison, the primordial baryon fraction from observations at high redshift is 15%. Presumably these baryons were present initially, when the galaxy formed. Indeed modelling of disk formation requires an initial baryon fraction of 10 — 15% in order for sufficient cooling to have occurred to form the disk. The missing" galactic baryons amount to a baryon fraction comparable to what is observed, namely around 5 — 10% of the dark halo. 

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