QSO Absorption Lines and Other Cosmological Probes

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:

• Lyman limit systems: These are basically low density clouds of hydrogen which are in some partial or fully ionized state. They are responsible for the copious amounts of Lyman-alpha absorption lines seen in the direction of most distant QSOs. This is known as the Lyman alpha forest as there are literally a forest of absorption lines that come from clouds located at different redshifts along the line of sight.

• Damped Lyman Alpha systems: These are very interesting as they represent relatively large column density systems which are needed to produce the strong damping wings on the observed absorption line profile. As such these systems are thought to be the progenitors of disk galaxies. Indeed, at redshift 3.5 the overall abundance of these systems contain sufficient baryons to account for all the stars observed in galaxies today. Example Damped Lyman-alpha system

• Metal line absorptions. These trace the chemical evolutionary history of these cloud systems, mostly through variations of the strength and abundance of Carbon IV systems.

There is strong evolution with redshift!

Comments:

• Prior to z = 2.5--3, metal line strength is fairly low indicating little processing. In fact, the Carbon IV data goes to zero at z = 3.5.

• A major episode of metal-production, which can plausibly be identified with the formation of the disk components of galaxies seems to be occurring between z = 1.5 --3. During this time the mean metallicity appears to increase from 0.01 solar to 0.1 solar.

• At the moment, the highest redshift damped Lyman-alpha system is at z=4.38 towards a z=4.7 QSO ( Lu \etal 1996). The number of these damped systems per unit redshift interval monotonously increases from z=0 to z=4 with no obvious peak in the distribution. However, the amount of matter contained in these systems does seem to show a peak at z ~ 3.

  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:

The initial stage of galaxy formation ,defined as when the first generation of stars is formed, occurred prior to z=3 and is best identified with the formation of spheroids (either elliptical galaxies or spiral bulges). The formation of extended disks clearly takes a longer time and was apparently very active between z = 1--2.

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.

At z = 5 the universe is 7\% of its present age or .7 -- 1.4 billion years. QSOs have been detected at this redshift so we know that small-scale structure formation can occur on the 1 Gyr time scale. Its possible that these distant QSOs are the manifestation of galaxy formation and the formation of the first generation of stars. To generate the QSO activity requires the presence of a massive black hole. Possibly it is these massive black holes that have acted as the seeds to attract additional baryonic material.

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.

The simple idea that a protogalaxy would form the bulk of its stars during the initial collapse is probably incorrect. Over a dynamical timescale (a few x 10⁸ years for galactic potentials), if most of the gas turns into stars then a star formation rate of 100-1000 M\solar per year would result.

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.

The role of feedback to the galaxy formation process either through supernova or the formation of QSOs is not yet well understood. If the Universe has been completely re-ionized by QSOs, the observations indicate that this occurred at z > 5. Possibly this event served to further delay the general process of galaxy formation.

The observations of Steidel \etal that star formation in galaxies was well in place by z = 3.5 is difficult to understand in CDM models as this implies there was already small scale power by this redshift. Mo and Fukigita 1996 demonstrate that the presence of small scale power at this redshift is greatly aided by non-zero LAMBDA as the time per unit redshift interval is greater in this case.

The morphology of objects in the HDF gives the strong visual impression that galaxy formation is occurring via an assembly line process in which small sub-units are being accreted into a larger entity. However, these sub-units are already composed of gas and stars so some process had to produce them at a much earlier epoch.

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:

• The measured s₁₂ is 540 +/- 180 km/s. While this is larger than the 1983 measurement of 340 +/- 40 km/s (see Davis and Peebles 1983), it still does not effectively discriminate between open and closed CDM models.

• The samples are "contaminated" by the presence of rich clusters where s₁₂ reflects the cluster velocity dispersion which is significantly higher than s₁₂ for field galaxies. This "contamination" is severe. When galaxies which are thought to be members of rich clusters are removed from the sample s₁₂ lowers significantly to 295 +/- 100 km/s.

• The amount of "contamination" depends on the volume of the redshift survey. Local samples are biased against selecting galaxies that are members of rich clusters and hence s₁₂ s biased to low values. This explains the low value originally measured by Davis and Peebles.

  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 .

• Nevertheless, the indications are that s₁₂ is relatively low and the small scale velocity field is therefore mostly quiescent. This favors again, low OMEGA models.

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  The Electronic Universe Project
  e-mail: nuts@moo.uoregon.edu