The Large Scale Distribution of Baryons in the Universe and the Nature of the Lyman alpha Forest

Big Bang nucleosynthesis calculations predict that there should be about ten times as much ordinary (a.k.a. baryonic) matter in the universe than we can account for locally by looking at the matter contents of our solar system, Galaxy etc. Where do most of the baryons hide ? (this is different from the so-called "dark matter" problem). Searches for the missing baryonic matter used to assume that the baryons are in compact stellar objects (from planets to supermassive black holes), but none of the different candidate classes has proven to contain more than a small fraction of all matter.

The possibility that most of the ordinary matter may not be in galaxies at all but in the form of gas in intergalactic space received serious attention only in the early 1990s. This was partly because the most sensitive method for detecting baryons, i.e., looking for absorption of the light of background QSOs by the intergalactic matter (the Lyman alpha forest, see above) only shows the neutral fraction of the gas (ionized hydrogen does not absorb light significantly). Unfortunately, only a tiny fraction of all baryons are expected to be neutral under the physical conditions prevailing in most of the cosmic volume. Most of the gas is likely to be at a very low density, and is being kept ionized by the pervasive UV background radiation field. Thus, to estimate the total amount of gas that corresponds to the observed neutral gas one needs to have independent estimates of the density and the intensity of the ionizing radiation field. There are various ways for estimating the radiation intensity to probably within a factor of five so the main uncertainty has always been the density of the gas. If most of the gas seen from its neutral hydrogen absorption in the Lyman alpha forest were very tenuous the ionization could be extremely high, and an enormous amount of matter could be hidden in the form of ionizing gas. Conversely, if the intergalactic medium were quite clumpy most of the gas could be neutral and what you observe is really all there is.

A breakthrough in this subject occurred when several groups (e.g., Smette et al; Bechtold et al, Dinshaw et al) succeeded in measuring the lateral extent of the Lyman alpha clouds in the intergalactic medium, from observations of multiple lines of sight to QSO pairs and lensed QSOs. The cloud sizes (at least at high redshift) turned out to be on the order of hundreds of kiloparsecs up to a Megaparsec. That makes the gas densities in these clouds quite low (within a factors of ten of the mean density of the universe) and the total baryon contents of the intergalactic medium becomes very substantial. In fact, if the clouds were spherical (i.e., as extended along the line of sight as across the sky - the above size estimates measure only the latter) then the clouds would contain more baryons than the universe as a whole. This paradox can be resolved by assuming that the gas clouds typically cannot be spherical but must be highly flattened filamentary or sheet-like structures (Rauch & Haehnelt 1995). This qualitative observational picture is in essential agreement with the results from numerical cosmological simulations (e.g. Cen et al 1994; Miralda-Escude et al 1996). Such models predict that the whole universe is permeated by a "cosmic web" of gas in which the galaxies are embedded. The simulations predict the density distribution in the universe, so the only free parameter left when determining the total baryon contents is the strength of the ionizing radiation field.

At those redshifts where the Lyman alpha forest is most easily observed (z ~ 3) it is thought that QSOs make the dominant contribution to the ionizing background. Adding a radiation field of the requisite strength to the cosmological simulations one can predict the amount of baryons necessary to produce a Lyman alpha forest absorption pattern as strong as the one actually observed. Comparisons between such simulations and actual data (Rauch et al 1997) constitute a direct measurement of the baryon density and show that the intergalactic medium may indeed still contain on the order of 90 percent of all matter at redshift 3.

Our work also showed that the ionization of the intergalactic medium barely changes out to at least redshift 4, whereas the population of the putative sources of ionizing photons, QSOs, dwindles rapidly when going to higher redshift. This discrepancy has led to a search for additional sources of ionizing radiation, most likely young stars in early galaxies, or possibly a hidden population of QSOs.

While the main question has been answered and we can now account for the baryons at high redshift, we cannot as easily get a census of baryons in the present universe. If the baryons are still in the form of gas (which is likely, being predicted by theory and suggested by the lack of success of searches for galactic reservoirs like MACHOs or dark clouds), they will be more compressed and hotter than at high redshift, as the universe's large scale structure has evolved to a state of higher entropy. Hot gas at low densities is very hard to see by any astronomical method, and searches are underway to find a hot baryonic component with space-based UV and x-ray missions in the current universe (practitioners of this field keep referring to this situation as the "missing baryon" problem).

Additional evidence from the cosmic microwave background measurements and large galaxy surveys in conjunction with structure formation models corroborate the general picture of the Lyman alpha forest as the main reservoir of baryonic matter.

Publications:
Rauch, M., Miralda-Escude, J. Sargent, W.L.W., Barlow, T.A., Weinberg, D.H., Hernquist, L., Katz, N. Cen, R., Ostriker, J.P.: The Opacity of the Lyman Alpha Forest and Implications for Omega_baryon and the Ionizing Background, Astrophysical Journal, 489:7-20, 1997

Rauch, M.; Weymann, R. J.; Morris, S. L.: Are Lyman-Alpha Clouds Associated with Low Surface Brightness Galaxies?, Astrophysical Journal, 458:518-523, 1996

Rauch, M., Haehnelt, M.G.: Omega_baryon and the Geometry of Intermediate-Redshift Lyman Alpha Absorption Systems, MNRAS, 275:76-78, 1995