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[Abridged] We present and analyze observations of J033241.88-274853.9 atz=0.6679, using multi-wavelength photometry and imaging with FLAMES/GIRAFFE 3Dspectroscopy. J033241.88-274853.9 is found to be a blue, young (~320Myr)stellar disk embedded in a very gas-rich (fgas=73-82% withlog(Mstellar/Mo)=9.45) and turbulent phase that is found to be rotating onlarge spatial scales. We identified two unusual properties ofJ033241.88-274853.9. (1) The spatial distributions of the ionized gaseous andyoung stars show a strong decoupling; while almost no stars can be detected inthe southern part down to the very deep detection limit of ACS/UDF images,significant emission from the [OII] ionized gas is detected. (2) We detect anexcess of velocity dispersion in the southern part of J033241.88-274853.9 incomparison to expectations from a rotating disk model. We considered two diskformation 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 hasbeen recently collapsed from a pre-existing, very gas-rich rotating disk. Thisscenario requires two (unknown) additional assumptions to explain thedecoupling between the distribution of stars and gas and the excess of velocitydispersion in the same region. In a second scenario, we examine whetherJ033241.88-274853.9 could be a merger remnant of two gas-rich disks. In thiscase, the asymmetry observed between the gas and star distributions, as well asthe excess of velocity dispersion, find a common explanation. Shocks producedduring the merger in this region can be ionized easily and heat the gas whilepreventing star formation. This makes this scenario more satisfactory than thecollapse of a pre-existing, gas-rich rotating disk.

A forming disk at z~0.6: Collapse of a gaseous disk or major merger remnant?