In general, there are two choices, baryons are either trapped inside a potential well, and hence are located in a galaxy, or they are distributed outside of galaxies. This is determined by the efficiency of galaxy formation.
What percentage of the primordial hydrogen and helium gas managed to creep into some pre-existing gravitational potential to ultimately fragment to form stars inside of galaxies?
Theory, regardless of the particular structure formation scenario, does not make a reliable prediction of this efficiency and it has to be determined observationally.
In a compelling paper, Persic and Salucci (1992) compare the baryonic mass from light emitting objects from that expected from primordial nucleosynthesis.
In brief, the observed abundance of light elements in the Universe (see Walker etal 1991) suggests that the baryonic density, OMEGAb is
The inventory of Persic and Salucci yields a total of OMEGAb = 2.2 +/- 0.6 x 10-3h-3/2$. For a reasonable range of h this means that most of the baryons which exist are dark.
There are three possible repositories for baryonic material: 1) Easy to detect galaxies, 2) Hard to detect galaxies and 3) a distributed background that is detected via recombination radiation at some particular wavelength.
Distinguishing between these possibilities entails determining the galaxy luminosity function (GLF) and searching for evolution with look back time as well as trying to detect various radiation processes that are tracers of intergalactic atoms. At the very least, we wish to ascertain if there are more baryons located inside of galaxies compared to outside.
With the exception of the CBR, establishing the existence of electromagnetic backgrounds at other wavelengths has been a difficult chore. Here we summarize, in order of increasing wavelength, the current evidence for different backgrounds and what the possible interpretations are. In general, these backgrounds have been discovered via satellite observations. These observations are of rather low resolution and therefore can not effectively distinguish between backgrounds that represent a uniform distribution of hot or cold gas from those that are due to an aggregate of discrete sources.
The Radio Background
At short wavelengths we see the CBR:
At longer wavelengths we see discrete radio sources that can be identified with cosmologically distant galaxies. If we knew how these sources evolved in luminosity over time, we could use the source number counts as a function of flux to deduce the geometry of the Universe. But we don't know anything about radio source evolution.
The Infrared Background
A cosmological background in this wave band has been difficult to unambiguously establish. An all sky map produced by the IRAS satellite over the range 12-100 microns clearly shows the galaxy emits copius amounts of Far Infrared Radiation, due to heated dust in the range 20-40K. This radiation is an important component of understanding global energy transport in galaxies (the subject of tomorrow's physics colloqium by Dr. Rene Walterbos).
The all-sky map (Figure 6-1) produced at 100 microns by IRAS clearly shows the presence of large regions of emission at high galactic latitude. These sources have been named "Galactic Cirrus" as their structure is fairly cloud-like. The heating sources of this high latitude cirrus aren't completely clear but most of the heating probably comes from stars in the galactic plane, where the optical and UV radiation can escape through regions of low opacity and penetrate to high latitude.
Since heated dust has been determined to be pervasive in galaxies, then the aggregate of all the galaxies in the Universe should produce a redshift-smeared background over the range 10-400 microns. Although each individual "dust spectrum" is that of a blackbody, the redshifted smeared sum of these individual sources will not be characterized by a simple blackbody of some given temperature. However, its possible that there might be a "feature" in the spectrum that would represent high star formation rates at high redshift.
A detection of a possible cosmological infrared background (hereafter the CIB), means detecting an isotropic signal, of unknown spectral signature, against the strong signal of our Galaxy, which, due to high latitude cirrus emission, is nearly isotropic itself. Further difficulty arises when trying to calibrate the absolute strength of the CIB as all the strong foreground sources need to be properly removed.
At the moment, a good galactic model for IR emission is still not available to use as a baseline against which to detect residuals of emission at high latitude. These residuals are seen bu COBE (the DIRBE experiment) but its unclear what their real nature is. In any event, the IR background, like the radio background, would represent the sum of discrete sources which are galaxies. There is no intergalactic component to this, and a cosmological sub-mm excess, once thought to exist, is completely ruled out by the COBE spectrum.
The UV Background
As in the case of the observed Infrared background, the observed diffuse UV background is dominated by dust scattering and interstellar emission in our own Galaxy. However, when the galaxy is subtracted out, a nearly-isotropic residual, with an amplitude of
is apparent over the wavelength range 1300-2500 angstroms (see Henry and Murthy 1993 )
Unfortunately, the calibration of this residual is very difficulty and the quoted amplitude is uncertain by a factor of 2--3. There are three plausible extragalactic sources for the observed residual: 1) diffuse thermal emission that would arise from material between the galaxies - the intergalactic medium (IGM) 2) the aggregate light from discrete sources such as UV bright QSOs and star forming galaxies; 3) radiation associated with the decay of some relatively long lived massive particle.
To make a Long Story short:
The X-ray Background
Again, a HOT IGM X-ray component, which was once thought to exist and possible account for OMEGA = 1, can now be completely ruled out . This again is due to COBE. A diffuse X-ray background would produce spectral distortions in the COBE data which are not seen (see Wright etal 1994 )
The overall spectral shape of the XRB now strongly favors QSOs/AGNs as being the dominant contributor (see Comastri etal 1995 ) and there is essentially no room for additional classes of objects and certainly no room for a hot, diffuse IGM.
The Gamma-Ray Background
All I am going to say is that this is one unusual observation. What are these things?
Read more about it
The Space Density of Galaxies
Determining the Galaxy Luminosity Function:
The Key is the faint end slope parameter. The GLF is usually described by a Schechter (1976) function of the form:
Determining the faint end slope is another cottage industry in extragalactic astronomy. A recent example from Sprayberry etal (1997) is shown below.
The above image shows the following:
When all galaxy types are considered from standard catalogs in which surface brightness selection effects are not properly accounted for, one gets a faint end slope of -1.
However, when one looks only at these kinds of galaxies:
which are low mass irregular galaxies, a very steep faint end slope of -1.9 is seen.
The points represent a survey of field LSB galaxies. When these are coupled with normal galaxies a faint end slope of -1.4 is derived. The Bottom Line:
The discovery of Low Surface Brightness galaxies in large numbers has greatly helped to resolve the "missing baryon" problem. Most of the baryons are either in diffuse galaxies or in low mass irregular galaxies (which also can be diffuse).