7.2. Significant Observations as a Function of Redshift
Below we list some significant observations as a function of redshift that either have been made or are waiting to be made. Each of these observations (or potential observations) bears directly on many of the issues discussed in this book:
z = 1100: The COBE measured quadrapole anisotropy provides excellent confirmation that gravitational instability and the associated Sachs-Wolfe effect, at some level, must have provided the seeds for structure formation. It is particularly re-assuring that the observed level of anisotropy is close to that which can be predicted from very simple arguments.
z 100: The universe is 10-3 of its present age at this redshift. This time interval corresponds to the dynamical timescale of globular clusters. It is thus possible that the first star formation and metal production occurs at this redshift. The photon flux from this putative formation event would be redshifted to wavelengths of 10-100 µ where it would be largely undetectable due to a very weak signal in the presence of a noisy background.
z 20: At this redshift the Universe is approximately 1% of its expansion age which coincides with the dynamical timescale of a typical elliptical galaxy. We thus expect spheroid formation to occur at this redshift but to date there is no observational evidence for this.
z 5: The first luminous signposts appear in the form of QSOs. Since the energy budget of QSOs requires infall of gas into supermassive black holes, then the engines for the QSOs had to form at higher redshifts. If these supermassive black holes have evolved from the gravitational coalescence of stellar remnants in a massive stellar cluster, then the precursor to the engine formation could have been the first era of metal production in the Universe. As stated above, this could have occurred at z 100.
z 4.5: The highest redshift damped Lyman system now observed along with the presence of Lyman emitting galaxies. This indicates that 1) the Universe is not completely ionized at this redshift 2) large column density clouds of H I exist, and 3) star formation in some galaxies is occurring. If the IGM is completely ionized at some post recombination redshift, it would take some time for components to cool at and collapse into high column density neutral hydrogen clouds. While its difficult to estimate the cooling times in the absence of many metals (one has to appeal to complex models of cooling involving molecular hydrogen line emission) the cooling time is not likely to be less than 108 years. This suggests a lower limit for the epoch of complete re-ionization by QSOs to be a 10 (see also the arguments given in Peebles 1993).
z 3.5: Deep imaging and follow-up spectroscopy now has revealed the presence of apparently normal star forming galaxies at this redshift. The inferred star formation and metal production rates are modest. In one case, a galaxy at z 3 appears to have a spectrum consistent with a stellar population that is a few Gigayears old. If true, this again is further evidence of a significant conflict between the expansion age of the Universe and the ages of the oldest stars/galaxies that inhabit it.
z 3.0: Observations of metal-line QSO absorption systems suggest a mean metallicity at this redshift of 0.01 solar.
z 3.0: The amount of gas which is locked up in damped Lyman systems appears to reach a maximum at this redshift. This probably marks the beginning of the formation of the disk components of galaxies which is likely to be a slow process that continues over many epochs of z.
z 1.5: Observations of metal-line QSO absorption systems now suggest a mean metallicity at this redshift of 0.1 solar. This factor of 10 increase in mean metallicity since z 3.0 can be attributed to the first generations of stars which form in galactic disks. Age dating of our own Galaxy via use of the white dwarf cooling curve (Winget et al. 1991) strongly suggests that the oldest stars in our disk formed at z = 1-1.5.
z 1.0: The highest redshift X-ray cluster yet detected (Hattori et al. 1996) This indicates that the virialization of rich clusters did occur by z 1. Observations of high redshift radio galaxies and QSOs are also consistent with some clustering occurring up to z 2.
z = 0.083: The discovery of Malin 1 at this redshift indicates that very large gaseous disks can remain cold and relatively devoid of stars even at very recent epochs. This shows that disk galaxy evolution can indeed be very, very slow and that some baryons are fairly effective at hiding.