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.