We present near-infrared spectra of “ A2 ” , the primary counter arc to the gravitationally lensed galaxy MS1512-cB58 .
The spectra show redshifted H \alpha , [ NII ] , [ OIII ] , and H \beta at z = 2.728 \pm 0.001 .
We observe the same H \alpha / [ OIII ] ratio as cB58 , which together with the redshift confirms that A2 is indeed another image of a single background galaxy .
Published lensing reconstruction reports that A2 is a magnification of the entire source , while cB58 is an image of only a part .
At marginal significance , A2 shows higher line to continuum ratios than cB58 ( by a factor of \sim 2 ) , suggesting a non-uniform ratio of young to old stars across the galaxy .
We observe a second emission line source in the slit .
This object , “ W5 ” , is predicted to be a lensed image of another galaxy at a redshift similar to cB58 .
W5 is blueshifted from cB58 by \sim 400 km s ^ { -1 } and has a significantly lower H \alpha / [ OIII ] ratio , confirming that it is an image of a different background galaxy in a group with cB58 .
The H \alpha emission line in W5 implies a star formation rate of 6 \mbox { M } _ { \odot } \mbox { yr } ^ { -1 } ( H _ { 0 } = 70 
km s ^ { -1 } Mpc ^ { -1 } , \Omega _ { M } = 0.3 , \Omega _ { \Lambda } = 0.7 ) , after correcting for lensing magnification .