A forming disk at z similar to 0.6: collapse of a gaseous disk or major merger remnant?

Context. Local spiral galaxies contain roughly two-thirds of the present-day stellar mass density. However, the formation process of disks is still poorly understood. Aims. We present and analyze observations of J033241.88-274853.9 at z = 0.6679 to understand how its stellar disk was formed. Methods. We combine multi-wavelength EIS, HST/ACS, Spitzer/IRAC, and GALEX imaging with FLAMES/GIRAFFE 3D spectroscopy to study its color-morphology and spatially-resolved kinematics. A spectral energy distribution (SED) is constructed and physical properties extracted using stellar population models. Results. J033241.88-274853.9 is a blue, young (320(-260)(+590) Myr, 90% confidence interval) stellar disk embedded in a very gas-rich (f(gas) = 73-82% with log (M-stellar/M circle dot) = 9.45 +/-(0.28)(0.14)) and turbulent phase that is found to be rotating on large spatial scales. We identified two unusual properties of J033241.88-274853.9. (1) The spatial distributions of the ionized gaseous and young stars show a strong decoupling; while almost no stars can be detected in the southern part down to the very deep detection limit of ACS/UDF images (accounting for the light spread by seeing effects), significant emission from the [OII] ionized gas is detected. (2) We detect an excess of velocity dispersion in the southern part of J033241.88-274853.9 in comparison to expectations from a rotating disk model. Conclusions. We considered two disk formation scenarios, depending on the gaseous phase geometry. In the first one, we examined whether J033241.88-274853.9 could be a young rotating disk that has been recently collapsed from a pre-existing, very gas-rich rotating disk. This scenario requires two (unknown) additional assumptions to explain the decoupling between the distribution of stars and gas and the excess of velocity dispersion in the same region. In a second scenario, we examine whether J033241.88-274853.9 could be a merger remnant of two gas-rich disks. In this case, the asymmetry observed between the gas and star distributions, as well as the excess of velocity dispersion, find a common explanation. Shocks produced during the merger in this region can be ionized easily and heat the gas while preventing star formation. This makes this scenario more satisfactory than the collapse of a pre-existing, gas-rich rotating disk.