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