A Hubble Space Telescope lensing survey of X-ray luminous galaxy clusters - III. A multiply imaged extremely red galaxy at z=1.6

We present near-infrared spectroscopy and Hubble Space Telescope (HST) imaging of galaxy J003707+0909.5, the brightest of three gravitationally lensed images of an extremely red object (ERO) at z=1.6, in the field of the massive cluster A68 (z=0.255). We exploit the superlative resolution of our HST data and the enhanced spatial resolution and sensitivity afforded by the lens amplification to reconstruct the source-plane properties of this ERO. Our morphological and photometric analysis reveals that EROJ003707 is an L^* early-type disc galaxy and we estimate that ~10 per cent of galaxies with (R-K)>=5.3 and K<=21 may have similar properties. The unique association of passive EROs with elliptical galaxies therefore appears to be too simplistic. We speculate on the evolution of EROJ003707: if gas continues to cool on to this galaxy in the manner predicted by hierarchical galaxy formation models, then by the present day, EROJ003707 could evolve into a very luminous spiral galaxy.