TURNING BACK THE CLOCK: INFERRING THE HISTORY OF THE EIGHT O'CLOCK ARC

We present the results from an optical and near-infrared (NIR) 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 mu > 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(-1). With H- and K-band NIR spectra, we have measured nebular emission lines of H alpha, H beta, H gamma, [NII], and [OIII], which have a redshift z = 2.7333 +/- 0.0001, consistent with the derived systemic redshift. From the Balmer decrement, we measured the dust extinction in this galaxy to be A(5500) = 1.17 +/- 36 mag. Correcting the Ha line flux for dust extinction as well as the assumed lensing factor, we measure a star formation rate (SFR) of similar to 270 M(circle dot) yr(-1), which is higher than similar to 85% of star-forming galaxies at z similar to 2-3. Using combinations of all detected emission lines, we find that the 8 o'clock arc has a gas-phase metallicity of similar to 0.8 Z(circle dot), 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 both the dust extinction and SFR, 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 similar to 4.2 x 10(11) M(circle dot), which falls on the mass-metallicity relation at z similar to 2. Finally, we estimate the total gas mass, and find it to be only similar to 12% of the stellar mass, implying that the 8 o'clock arc is likely nearing the end of a starburst.