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: http://zebu.uoregon.edu/1997/ph410/l22.html
Дата изменения: Tue Mar 11 02:04:48 1997 Дата индексирования: Tue Oct 2 00:14:12 2012 Кодировка: Поисковые слова: ccd |
Lines of sight toward various distant QSOs often intersect clouds of cold interstellar gas. The number density of these clouds is a strong function of their column density (an indicator of their overall density) as well as redshift (see Hu \etal 1995
There are basically three types of QSO absorption lines:
Comments:
This suggests that proto gaseous disks are in place by z=3 and that high column density gas arranged in a disk configuration occurred within a couple of billion years since recombination. The presence of neutral hydrogen at z \app 4 also demonstrates that the Universe can't have been completely re-ionized (by QSOs) at this redshift.
The Epoch of Galaxy Formation:
Clearly, the best constraint on galaxy formation will come when we
actually observe the process and identify at what redshift galaxies
begun to form. Recent ground-based observations have
now detected galaxies, at redshift z ~ 3
Steidel etal 1996 have unambiguously detected star
forming galaxies at this redshift. The amount of star formation
present at this redshift appears to be 5--10 times less than at
redshift z= 1--1.5.
At redshift z=4, the overall star formation
is down by a factor of 5 relative to z=3. Furthermore, in these high
redshift objects, the star formation seems to be confined to much
smaller spatial scales, centered on the galaxy, than at lower redshift.
Since the production of metals is strongly correlated with the star
formation per unit volume at some epoch,z, then the rise in this
rate should correlate with the rise in the metal abundance of QSO
absorption lines.
In summary, the available data on the properties of high redshift
galaxies and QSO absorption lines suggest the following:
Vogt \etal (1996)
show convincing evidence that objects with normal disk kinematics are
in place by z = 1. The presence of these
high redshift structures severely limits the amount of matter that
can be obtained in any HDM model.
In fact, the origin of these massive
black holes, 1 billion years after the birth of the universe is really
quite interesting. If they are the evolved remnants of massive star
clusters, then they obviously formed much earlier than Z =5.
While such a large
star formation rate has been observed in some Ultraluminous IRAS
galaxies (see Sanders \etal 1988),
which are most likely the merger of two well formed galaxies,
there are no objects at high redshift yet identified that exhibit
this behavior. This is a strong argument that galaxy formation is
not a quick process, marked by a very large star formation rate (and
a very large supernova and metal-enrichment rate ), but perhaps is a
far more quiescent and longer process.
Possibly, this process is
the one physical process that we understand - simple Jeans mass collapse
at high redshift. These (baryonic) sub-units then produce galaxies, via
merging, as they respond to the underlying mass distribution which is
dominated by dark matter. This is a potentially complex physical process
that will challenge our understanding.
Pairwise and Peculiar Velocities:
The final small scale constraint which can be considered is the average velocity and/or spatial separation between two random galaxies. Peculiar velocities that might arise from gravitational interactions between galaxies or between a galaxy and an overdense region such as a cluster cause deviations from Hubble flow but do not alter the position of the galaxy on the plane of the sky.
Thus spatial correlation functions that are performed in physical space which may be isotropic become anisotropic when mapped onto redshift space (see discussion in Kaiser 1987). The amount of anisotropy in redshift space can be measured through the lower order moments of the peculiar velocity distribution.
For galaxy pairs, the first moment of the distribution, v12 is sensitive to the growth of the spatial or two-point correlation function. The second moment, s12 provides a direct measurement of the kinetic energy of any random motions. In the equilibrium gas, s12 balances the gravitational potential and hence can be used to measure the effective mass. This is the situation in a cluster of galaxies in hydrostatic equilibrium.
For standard CDM, normalized to give the observed power on small scales, s12 is predicted to be ~ 1000 km/s. Open models in which OMEGA < 0.2 predict s12 ~ 500 km/s. The most recent determination of s12 is based on a sample of 12,800 galaxies that comprise a well-defined subset of the Northern and Southern Sky Redshift surveys. The results (see Marzke \etal 1995 ) of this analysis are unfortunately ambiguous:
If one uses the observed distribution of cluster velocity dispersions (see Zabludoff \etal 1993) , it is possible to estimate how big a volume must be obtained in order for this "contamination" to not be a dominant effect in the sample. Marzke \etal (1995) estimate the required volume exceeds the volume of the existing redshift sample and therefore no fair sample yet exists to properly measure s12 .