Lyman-break galaxies at z ~ 3 represent both a large jump in redshift and a substantially different detection/selection technique than has been used previously in galaxy studies. While we do not yet have enough empirical evidence to allow us delineate an evolutionary scenario to link this population to galaxies observed at lower redshifts or in the present-day universe, several important traits of this population can already be traced:
The Lyman-break galaxies
form a substantial population of star formers already
at 
15% of the cosmic
age. They have assembled at least ~ 10-15% of all the stars
observed in the local universe, but the true fraction is very likely
higher by a factor of
several due to the presence of dust obscuration in the observed
rest-frame far-UV light of
these galaxies. These stars seem unavoidably linked to the oldest
stellar systems observed
in the present-day universe, i.e., the elliptical galaxies and the
bulges of spiral galaxies.
The rest-frame UV spectra of
these galaxies bear close resemblance to those of local
starburst galaxies. While the ISM of these galaxies has certainly
undergone some degree
of metal enrichment, as indicated by the numerous metal interstellar
absorption lines, it
is hard to make quantitative statements at this time, because the
features are heavily
saturated. Despite the high star-formation rates and the abundance of
ionizing photons,
the Ly
line is often weak or
absent, very likely due to the presence of dust in the ISM
coupled with resonant scattering. The kinematics of the absorption and
emission lines
shows the existence of large outflows of gas, with velocity in excess of
~ 400-500 km s-1,
very likely due to stellar winds and supernova explosions. This stresses
the importance
of understanding the effects of feedback in the process of star (and
galaxy) formation.
Both the UV colors and
optical nebular emission lines directly point to the presence
of dust obscuration. Correcting factors very likely range between 3x and
7x, but
unfortunately the exact numbers depend quite strongly on the shape of
the extinction law,
which is poorly constrained in these galaxies. The unobscured star
formation rates of
the individual galaxies, if sustained for ~ 109 yr, have
produced stellar masses that span
the range observed in the local universe, with a typical value around
M*, or 1011
M
.
HST high-resolution
imaging of the UV morphology of the Lyman-break galaxies
shows a dispersion of properties. The brighter systems have compact
morphologies with
light profiles characteristic of collapsed systems. They appear
characterized by a
relatively high degree of spherical symmetry, and have half-light radii
that compare with
those of present-day bulges and elliptical galaxies of intermediate
luminosity. At the
fainter end of the luminosity distribution the morphology seems to
become more irregular
and fragmented. A preliminary inspection of rest-frame optical images
obtained with
HST + NICMOS seems to show morphologies that are rather similar
to the UV ones.
The Lyman-break galaxies
have clustering properties expected from massive dark
halos. As a consequence, they are also heavily biased tracers of the
mass. Values of the
mass of the dark halos that host Lyman-break galaxies at the bright end
of the luminosity distribution are M ~ 1012
M. Large
concentrations of these systems with total mass
typical of rich clusters (i.e., ~ M ~ 1015
M) are also observed
already at redshift z ~ 3.
The Lyman-break galaxies are characterized by strong spatial clustering,
with comoving
correlation length r0 = 2.1 (3.3)
h-1 Mpc, only a factor of
3 (2) of the present-day
correlation length. The slope of the correlation function is
= -1.9, very
close to the
one observed in the local universe. If the biasing parameter is only
mildly varying as
a function of the spatial scale, this disfavors standard CDM models and
points towards
an open universe. We are currently studying the presence of clustering
segregation with
the UV luminosity (i.e., the dependence of the star-formation activity
on the mass of the
hosting halos) using the HDF sample of Lyman-break galaxies
(Giavalisco et al. 1988, in preparation).
We would like to thank Charles Steidel, Mark Dickinson, Max Pettini, Kurt Adelberger and Mindy Kellogg for many useful discussions and for allowing us to use unpublished material in this paper. We would also like to thank the organizers of this ST ScI Symposium for the excellent meeting. We gratefully acknowledge support from the Hubble Fellowship program through grant HF-01071.O1-94A awarded by the Space Telescope Science Institution, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.