A brief history of clumpy galaxies

The vast majority of galaxies we see around us today can be grouped into just a few categories of visual appearance, or morphology. There are spirals and lenticulars (barred and not), ellipticals and irregulars. These are described in this recent post and will be looked at more closely in the Galaxies 101 series. Things get a bit more complicated when one goes to faint and small “dwarf” galaxies, but we won’t go into that here. There are also a small fraction of galaxies that are in the process of merging, often creating unusual and spectacular morphologies, but again they will have to wait for a future post.

Example tadpole galaxies in the Hubble Ultra Deep Field.

Studying the morphologies of galaxies was quickly recognised as an interesting thing to do, as it gives us lots of clues as to how galaxies originally formed and how they have interacted with one another and their surroundings over the history of the Universe. However, because of the blurring effect of the atmosphere, and the fact that galaxies, like everything else, appear smaller the further away they are, for a long time it was not possible to see the morphologies of distant galaxies. With big telescopes, though, we could still determine their brightnesses, colours and numbers. From these measurements we knew that far-away galaxies were generally different from those nearby. Remember that the finite speed of light means that we see distant galaxies as they were in the past, when the Universe was younger. This useful fact means that we can directly see how the galaxy population has evolved just by looking further and further away. But while our telescopes were stuck on the ground we couldn’t see what galaxies in the early Universe actually look like.

Example clumpy spiral galaxies in GOODS imaging, from Elmegreen et al. 2009. Each panel includes a bar of length 2 kpc, the object’s redshift and COMBO-17 ID number.

The Hubble Space Telescope (HST), together with its camera WFPC2, solved the problem. Free from the atmosphere, it could see details ten times finer than ground-based telescopes. Finally we could see distant galaxies clearly enough to study their morphology. To demonstrate HST’s power, some of the first HST images were taken by staring at the same patch of the sky for a very long time, producing very deep images. Studies of these images of the distant Universe (e.g., by Cowie, Hu & Songaila in 1995 and van den Bergh and colloborators in 1996) revealed that the galaxy types seen nearby were still present, but generally become “messier” the further back in time one looks. Furthermore, there appeared to be types of distant galaxies that we do not see today. Many of these galaxies comprise knots or clumps. In particular, many galaxies were found with an appearance of several clumps arranged in a line, and were named “chain galaxies”. Galaxies with two clumps were simply named “doubles”. There were also galaxies with the appearance of one clump with a tail, appropriately named “tadpole galaxies”!

Example clumpy galaxies, details as above.

For the next few years, most studies of galaxy morphology with the HST concentrated on galaxies at intermediate distances, where HST provided detail impossible to obtain from the ground, without requiring very long exposure times. Galaxy morphologies are becoming messier at these times, but the clumpy galaxies seen in the deepest surveys were much more distant. However, the field of distant galaxy morphology had a further renaissance with the replacement of the WFPC2 camera with the Advanced Camera for Surveys (ACS). This enabled even deeper, clearer images to be obtained more quickly. Studies of these images (e.g., particularly by the Elmegreens and collaborators) find that clumpy galaxies become extremely common in the early universe. The extra depth of these data has revealed a population of clumpy galaxies that do not appear as chains, but rather more circular groups of clumps. These have been named “clump clusters”. While clump clusters share similarities with modern-day irregular galaxies there are a few important differences. Clump clusters are generally much more massive, and today’s irregulars would look irregular no matter which direction they are viewed from. The similarilty between clump clusters and chain galaxies implies that they are the same kind of object, simply viewed from different directions. This means that the clumps must be irregularly distributed in fairly thin disks, which appear as chains when viewed edge-on.

Examples of clumps in an underlying red galaxy, details as above.

Further studies of clumpy galaxies confirm that they are very young galaxies with lots of star formation occuring in the massive clumps, which may be embedded within a slightly older, smoother distribution of stars. Their prevalence means they are likely to be an early phase in the development of most, if not all, galaxies.

As I mentioned in my previous post, for Galaxy Zoo: Hubble we added a series of questions in order to find out about the appearance of clumpy galaxies. This will provide us with a catalogue of their properties that is larger and more consistent than any before. By analysing this data we hope to learn much more about these galaxies. For example, there appears to be a rough developmental sequence from asymmetric clumpy galaxies, to symmetric clumpy galaxies, to clumpy galaxies dominated by a bright, central clump, and finally to spiral galaxies. Other clumpy galaxies may merge together to form ellipticals. By comparing the numbers and properties of these different types of galaxies we will be able to confirm or refute this picture, and better understand the origins of the galaxy population.