and Cosmology

9. The Universe at High Redshift
360
ing redshift, the characteristic mass of the halos in which
these objects reside can be determined, as well as their
bias.
The Halo Mass of LBGs. If we consider the spatial distribution
of LBGs, we find a large correlation amplitude.
The (comoving) correlation length of LBGs at redshifts
1.5 z 3.5isr 0 ∼ 4.2h −1 Mpc, i.e., not very different
from the correlation length of L ∗ -galaxies in the present
Universe. Since the bias factor of present-day galaxies
is about unity, implying that they are clustered in a similar
way to the dark matter distribution, this result then
implies that the bias of LBGs at high redshift must
be considerably larger than unity. This conclusion is
based on the fact that the dark matter correlation at high
redshifts was smaller than today by the factor D 2 + (z).
Thus we conclude that LBGs are rare objects and thus
correspond to high-mass dark matter halos. Comparing
the observed correlation length r 0 with numerical simulations,
the characteristic halo mass of LBGs can be
determined, yielding ∼ 3 × 10 11 M ⊙ at redshifts z ∼ 3,
and ∼ 10 12 M ⊙ at z ∼ 2. Furthermore, the correlation
length is observed to increase with the luminosity of the
LBG, indicating that more luminous galaxies are hosted
by more massive halos, which are more strongly biased
than less massive ones. If these results are combined
with the observed correlation functions of galaxies in
the local Universe and at z ∼ 1, and with the help of
numerical simulations, then this indicates that a typical
high-redshift LBG will evolve into an elliptical galaxy
by today.
Proto-Clusters. Furthermore, the clustering of LBGs
shows that the large-scale galaxy distribution was already
in place at high redshifts. In some fields the
observed overdensity in angular position and galaxy
redshift is so large that one presumably observes
galaxies which will later assemble into a galaxy cluster
– hence, we observe some kind of proto-cluster. We
have already shown such a proto-cluster in Fig. 6.47.
Galaxies in such a proto-cluster environment seem to
have about twice the stellar mass of those LBGs outside
such structures, and the age of their stellar population
appears older by a factor of two. This result indicates
that the stellar evolution of galaxies in dense environments
proceeds faster than in low-density regions, in
accordance with expectations from structure formation.
It also reveals a dependence of galaxy properties on the
environment, which we have seen before manifested
in the morphology–density relation (see Sect. 6.2.9).
Proto-clusters of galaxies have also been detected at
higher redshifts up to z ∼ 6, using narrow-band imaging
searches for Lyman-α emission galaxies.
Whereas the clustering of LBGs is well described
by the power law (7.19) over a large range of scales,
the correlation function exhibits a significant deviation
from this power law on very small scales: the angular
correlation function exceeds the power law at Δθ 7 ′′ ,
corresponding to physical length-scales of ∼ 200 kpc.
It thus seems that this scale marks a transition in the
distribution of galaxies. To get an idea of the physical
nature of this transition, we note that this length-scale
is about the virial radius of a dark matter halo with
M ∼ 3 × 10 11 M ⊙ , i.e., the mass of halos which host the
LBGs. On scales below this virial radius, the correlation
function thus no longer describes the correlation
between two distinct dark matter halos. An interpretation
of this fact is provided in terms of merging: when
two galaxies and their dark matter halos merge, the resulting
dark matter halo hosts both galaxies, with the
more massive one close to the center and the other one
as “satellite galaxy”. The correlation function on scales
below the virial radius thus indicates the clustering of
galaxies within the same halo, whereas on larger scales,
where it follows the power-law behavior, it indicates the
correlation between different halos.
Winds of Star-Forming Galaxies. The inferred high
star-formation rates of LBGs implies an accordingly
high rate of supernova explosions. These release part of
their energy in the form of kinetic energy to the interstellar
medium in these galaxies. This process will have
two consequences. First, the ISM in these galaxies will
be heated locally, which slows down (or prevents) further
star formation in these regions. This thus provides
a feedback effect for star formation which prevents all
the gas in a galaxy from turning into stars on a very
short time-scale, and is essential for understanding the
formation and evolution of galaxies, as we shall see in
Sect. 9.6. Second, if the amount of energy transferred
from the SNe to the ISM is large enough, a galactic wind
may be launched which drives part of the ISM out of the
galaxy into its halo. Evidence for such galactic winds
has been found in nearby galaxies, for example from