We can see from looking at these fractions that as we go further and further out, and the Universe gets younger and younger, the number of galaxies that don't fall nicely into the Hubble tuning fork scheme (the black line) begins to increase. This tells us that at earlier cosmic times the Universe was a messier place than it is today, and that galaxies are busy forming at these times. One puzzling result from this plot is that galaxies which look like Hubble type disk galaxies (the blue line) seem non-existent when the Universe was younger. Other CANDELS work, which has looked at the morphology of galaxies determined by other parameters (e.g. here), have shown some distant galaxies do have properties similar to the disk population in the local Universe. This seems to be telling us there are no galaxies with visual disk morphologies but galaxies which do have similar properties to disks. We can explain this when we consider results from other studies which show that disks in the distant Universe have clumpy structures. These clumps could cause the galaxy to be classed as a peculiar galaxy even though the underlying galaxy structure is that of a disk. In fact, studies have shown, through a process called Integral field spectroscopy (IFS), that such systems do exist. See this publication for a discussion of how IFS has been used to look at the underlying structure of a galaxy.
The results discussed here are important when considering the visual morphology of galaxies. It could be that disk galaxies in the early Universe are simply ‘hiding’ from us because they have different visual morphologies when compared to classic disks from the Hubble tuning fork. This is telling us that judging a distant galaxy based on our ideas regarding visual morphology in the local Universe may not be the best approach. Fortunately the CANDELS dataset has imaging available which is even deeper than what is used in this study, and will have visual classifications for many of these galaxies. These visual classifications come from projects such as Galaxy Zoo and the CANDELS team wide effort to visually classify all the CANDELS galaxies (this will be discussed in a future post). Data from CANDELS will play a key role in the continuing to investigate what visual morphology really means when looking at distant galaxies.