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 \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 \pm 0.0012 , using stellar photospheric lines .
This differs from the redshift of absorption lines in the interstellar medium ( ISM ) , z = 2.7302 \pm 0.0006 , implying gas outflows on the order of 160 km s ^ { -1 } .
With H and K-band near-infrared spectra , we have measured nebular emission lines of H \alpha , H \beta , H \gamma , [ N ii ] and [ O iii ] , which have a redshift z = 2.7333 \pm 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 \pm 0.36 mag .
Correcting the H \alpha line-flux for dust extinction as well as the assumed lensing factor , we measure a star-formation rate of \sim 270 M _ { \odot } yr ^ { -1 } , which is higher than \sim 85 % of star-forming galaxies at z \sim 2–3 .
Using combinations of all detected emission lines , we find that the 8 o ’ clock arc has a gas-phase metallicity of \sim 0.8 Z _ { \odot } , 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 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 \sim 4.2 \times 10 ^ { 11 } M _ { \odot } , which falls on the mass-metallicity relation at z \sim 2 .
Finally , we estimate the total gas mass , and find it to be only \sim 12 % of the stellar mass , implying that the 8 o ’ clock arc is likely nearing the end of a starburst .