Several lines of evidence suggest that the most active phase of
galaxy evolution, especially in the most massive systems, was
largely completed by $z\sim 1$. This results, e.g., from the
observation that the most massive galaxies at low redshift have
very old stellar populations ($\sim 10$ Gyr) and very little gas to
fuel subsequent star formation. Similarly, active galactic nuclei
(AGN) were more numerous and brighter in the early universe.
Ultimately, the direct observation of high-redshift galaxies will
be the only way to understand which processes shaped the universe
we see today, in spite of the rich ``fossil'' data sets we have of
the Milky Way and neighboring galaxies. Thanks to the new $8-10$ m
telescope class and novel instrumentation, including SPIFFI/SINFONI
on the VLT, individual galaxies at redshifts $z\sim 1-3$ ($2-6$ Gyr
after the Big Bang) are now within the reach of astronomical
spectrographs. Methodologically, this thesis focuses on the
analysis of spectrally and spatially resolved optical emission
lines, first of all \ha\ and [OIII]\lam5007, which are shifted into
the near-infrared. {\sc Spiffi / Sinfoni} is very suited to such a
programme, because it records the spectra of a contiguous field of
view of up to 8\arcsec$\times$8\arcsec. The internal kinematic and
chemical gradients within a galaxy can thus be measured in a single
observation. Galaxies in the early universe had particularly high
star-formation rates, so that many targets are bright optical line
emitters. Internal kinematics are measured through the Doppler
effect, line profiles and widths indicate the presence of an AGN,
galactic ``superwinds'' and the relationship of chaotic to ordered
motion. Star-formation rates are measured from the luminosity of
the Balmer lines, especially \ha. Characteristic line ratios
indicate the presence of an AGN, chemical composition, and electron
densities in the ISM, and they allow to distinguish shocks and
photoionization. This thesis is a pilot study: It comprises 9
galaxies that fulfill a variety of selection criteria: they are
either bright UV or submillimeter emitters, or they are radio-loud.
Perhaps the most fundamental result is that gravity (dominated by
dark matter) is the main driver of early galaxy evolution, but it
is not the only important process. Star formation and AGN cause
hydrodynamical feedback processes, which might be a sign of
self-regulated galaxy evolution. It is found that star-formation
related feedback had similar properties at low and high redshift,
but that AGN-driven gas expulsion might have played a major role in
the high-redshift evolution of galaxies, that is without
low-redshift equivalent. Another important result is the rotation
curve we find in the central kiloparsec of a gravitationally lensed
UV-selected galaxy. Velocity gradients of $\sim 100$ \kms\ have
been observed in many high-redshift galaxies, but the
interpretation as rotation curves is generally not unique. Given
the relatively coarse spatial resolution of high-redshift galaxy
data, two nearby galaxies, maybe interacting or undergoing a
merger, might blend into one smooth velocity gradient. Galaxy
mergers are an important ingredient of the ``hierarchical model'',
the current paradigm of structure formation, and therefore nearby
galaxy pairs were likely more common at high redshift than they are
today. The large similarity of the lensed rotation curve with those
of nearby galaxies might be a first sign that galaxies evolved
inside-out.
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