Cosmic Star Formation History
  Over the past two decades, an avalanche of data from multiwavelength imaging
and spectroscopic surveys has revolutionized our view of galaxy formation and
evolution. Here we review the range of complementary techniques and theoretical
tools that allow astronomers to map the cosmic history of star formation, heavy
element production, and reionization of the Universe from the cosmic "dark
ages" to the present epoch. A consistent picture is emerging, whereby the
star-formation rate density peaked approximately 3.5 Gyr after the Big Bang, at
z~1.9, and declined exponentially at later times, with an e-folding timescale
of 3.9 Gyr. Half of the stellar mass observed today was formed before a
redshift z = 1.3. About 25% formed before the peak of the cosmic star-formation
rate density, and another 25% formed after z = 0.7. Less than ~1% of today's
stars formed during the epoch of reionization. Under the assumption of a
universal initial mass function, the global stellar mass density inferred at
any epoch matches reasonably well the time integral of all the preceding
star-formation activity. The comoving rates of star formation and central black
hole accretion follow a similar rise and fall, offering evidence for
co-evolution of black holes and their host galaxies. The rise of the mean
metallicity of the Universe to about 0.001 solar by z = 6, one Gyr after the
Big Bang, appears to have been accompanied by the production of fewer than ten
hydrogen Lyman-continuum photons per baryon, a rather tight budget for
cosmological reionization.
