Turning Back the Clock: Inferring the History of the Eight O'clock Arc
  We present the results from an optical and near-infrared spectroscopic study
of the ultraviolet-luminous z = 2.73 galaxy, the 8 o'clock arc. Due to
gravitational lensing, this galaxy is magnified by a factor of > 10, allowing
in-depth measurements which are usually unfeasible at such redshifts. In the
optical spectra, we measured the systemic redshift of the galaxy, z = 2.7322
+/- 0.0012, using stellar photospheric lines. This differs from the redshift of
absorption lines in the interstellar medium, z = 2.7302 +/- 0.0006, implying
gas outflows on the order of 160 km/s. With H and K-band near-infrared spectra,
we have measured nebular emission lines of Halpha, Hbeta, Hgamma, [N II] and [O
III], which have a redshift z = 2.7333 +/- 0.0001, consistent with the derived
systemic redshift. From the Balmer decrement, we measured the dust extinction
to be A_5500 = 1.17 +/- 0.36 mag. Correcting Halpha for dust extinction and the
assumed lensing factor, we measure a star-formation rate of ~ 270 Msol/yr,
which is higher than ~ 85% of star-forming galaxies at z ~ 2-3. Using
combinations of all detected emission lines, we find that the 8 o'clock arc has
a gas-phase metallicity of ~ 0.8 Zsol, showing that enrichment at high-redshift
is not rare, even in blue, star-forming galaxies. Studying spectra from two of
the arc components separately, we find that one component dominates the dust
extinction and star-formation rate, although the metallicities between the two
components are similar. We derive the mass via stellar population modeling, and
find that the arc has a total stellar mass of ~ 4 x 10^11 Msol, which falls on
the mass-metallicity relation at z ~ 2. Finally, we estimate the total gas
mass, and find it to be only ~ 12% of the stellar mass, implying that the 8
o'clock arc is likely nearing the end of a starburst.
