This chapter has considered the issue of the distribution of baryons in the Universe. The major conclusions which can be drawn from the data are the following:
1. Although there is some evidence for the presence of an Intergalactic Medium (IGM), the total population of baryons that are in the IGM is at least an order of magnitude less than the baryons which are located in bound potentials. This indicates that dark matter potentials in the early universe were fairly efficient at sweeping up and collecting baryonic material. At high redshift, many of the smaller potentials could be easily ionized by QSOs thus producing the seed population of "clouds" that are observed to produce QSO absorption lines. The high-redshift universe may well have most of the baryons contained in these "clouds" which constitute a warm IGM composed of discrete sources (see also Weinberg et al. 1997).
2. Exotic scenarios that appeal to the production of ionizing photons from the late decay of massive particles do not appear to be viable making it unlikely that such massive particles exist and dominate the mass density of the Universe.
3. The isotropic distribution of Gamma Ray Bursters remains the most astrophysically puzzling of all cosmological observations. While it seems likely that the production originates in some compact object (center of a galaxy, dense globular cluster), the overall space density of these sources is unknown. Potentially, such objects can provide a significant source of energy feedback to the IGM at high redshifts.
4. The most recent nucleosynthesis constraints (Copi et al. 1995) combined with the most probable range for H0 (e.g., 70-90) leads to
The baryonic inventory cited earlier in Chapter 3 by Persic and Salucci (1992) yields a total b = (2.2 ± 0.06) x 10-3 h-3/2. For our range of H0, this means that most baryons must be dark. LSB galaxies are not included in the Persic and Salucci census. The discovery of LSB galaxies has increased the faint end slope of the GLF. In addition, LSBs contain substantial amounts of dark matter. Under the assumption, needed to explain the Tully-Fisher relation, that baryons scale with dark matter, the contribution of LSB galaxies to b can be as high as 0.013 (Impey and Bothun 1997) to 0.025 (Bristow and Phillips). Hence, LSB galaxies can easily be the sites of much of the "missing" baryonic material in the Universe.
5. Deep galaxy surveys have uncovered an enigmatic population of faint blue galaxies (FBGs). Redshift surveys (e.g., Lilly et al. 1995) now provide evidence of luminosity evolution in this population. Thus, many of the FBGs could have faded by z = 0 to produce the red population of LSBs now detected in the survey of O'Neil et al. (1997). However, most of the LSB population detected to data is quite blue. This is best understood if these galaxies have long dynamical timescales (owing to low matter density) and hence delayed formation times. As such, LSB galaxies may represent the more smoothly distributed and more numerous 1-2 peaks in the initial Gaussian density distribution. Therefore they are fair tracers of the mass distribution on large scale. On small scales, LSBs are fairly isolated which likely reflects the requirement that only low density potentials which are isolated can collapse. Those that are not isolated are assimiated into nearby, denser potentials.
6. LSB galaxies offer a window into galaxy evolution which is different from that which has been traditionally used. Surveys to date have detected galaxies with µ0 as low as 27.0 mag arcsec-2 in the blue, implying a factor of 100 less volume density than a typical spiral galaxy. LSBs are very much a manifestation of a different sequence of galaxy formation and evolution. They may be pre-destined to take this different evolutionary route due to lower density dark matter halos. These haloes account for the extended rotation curves recently measured in some LSB systems.
7. The space density of galaxies as a function of surface brightness appears to be flat for µ0 fainter than 22.0 mag arcsec-2. The space density of LSB disks is several orders of magnitude higher than would be predicted by extrapolation of Freeman's law. This, of course, is why they are significant contributors to the baryon density. These LSB disks are relatively inefficient at converting gas into stars, likely because their interstellar medium conditions do not permit the formation of molecular clouds which subsequently foster widespread star formation. As a result, LSB galaxies remain relatively unevolved. Since there is no hint of a drop-off in the space density we expect future surveys to find yet more galaxies of even lower surface brigthtness. The formation of these very diffuse systems, and the existence of any stars in them at all, will provide a severe challenge to galaxy formation that now must explain why there is, if ellipticals are included, approximately a factor of 1000 range in the volume mass density of galaxies.