Don't Judge A Galaxy By Its Cover

As discussed in previous posts (e.g. here and here), the morphology of a galaxy is a powerful tool when it comes to understanding galaxy evolution. The Hubble Tuning fork diagram, first devised by Edwin Hubble in 1925, shows that galaxies have three different types of visual morphologies: spheroidal (or elliptical), disky (or spiral), and peculiar (or irregular).

The Hubble tuning fork diagram. On the left are spheroidal galaxies, also known as elliptical galaxies. The number which follows the 'E' tell us if the galaxy is circular or flattened. On the right of the diagram are disk galaxies, often called spiral galaxies due to their spiral arm features. The 'B' in the notation for spiral galaxies tell us whether or not a bar feature is present, and the lower case letter tells us how tightly wound the arms of the galaxy are. The final galaxy on the far right is an example of a peculiar or irregular galaxy. 

On the left hand side of the Hubble tuning fork above are the spheroidal galaxies and the disky type galaxies are on the right. An example of a peculiar galaxy is shown on the far right. Not only does this image show us the different types of galaxies, it also shows us clearly that galaxies of different types have different colours. The spheroidal type galaxies are redder, while the disk and peculiar galaxies are blue in colour. If a galaxy is blue it is still young and making new stars. If a galaxy is red it is older and no longer has the fuel to make stars.

Rather helpfully, colour is not the only property that correlates with galaxy morphology. Galaxies that have a greater stellar mass tend to be spheroidal, while the lower mass galaxies have disk and peculiar morphologies. The massive, red, and old spheroidal galaxies are usually found in denser environments than disk and peculiar galaxies. Also, galaxy morphology is often measured using different parameters determined computationally, which in turn relate to the visual morphology of the galaxy.

Galaxies of different visual morphologies. Many of the important features which
allow us to tell these different types of galaxies apart have been revealed
to us thanks to the fantastic CANDELS imaging.

So does this mean that by simply looking at the shape of a galaxy you can tell all these different properties? The answer is not so straight forward. Many of the relations discussed above are what we know to be true in the local Universe, that is to say, they are true for galaxies close to us. As we begin to look at galaxies further away, we are looking at earlier epochs in the life time of the Universe. These galaxies are said to be at higher redshift than their local counterparts and may not have the same properties as those found close to us. Local galaxies have had the entire age of the Universe to form, whereas distant galaxies are younger and therefore are at an earlier stage of formation. Unfortunately, as a galaxy gets further from us, we need increasingly good telescopes to clearly see the visual morphology of a galaxy. A particularly difficult question to answer about distant galaxies has been 'What are the visual morphologies of these galaxies and how do they compare to the Hubble tuning fork?' Now, thanks to the high resolution CANDELS imaging, we can take a look at these distant galaxies and find out about their visual morphologies.

The fraction of galaxies with spheroidal (red), disk (blue) and peculiar (black) morphology as a
function of redshift. As redshift increases the distance away from us increases and the age
of the Universe decreases. This plot shows how the fraction of galaxies changes from
when the Universe was ~6 billion years old to when  the Universe was only 2 billion years old.

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.