Post-starburst galaxies paper submitted!
Today’s blog post is from Ivy Wong:
Hello Zoo-ites! I’m a work colleague of Kevin’s and I just recently submitted a Galaxy Zoo paper too. I just wanted to let you know all about it because I also wanted to thank you all for the great work which you’ve done in classifying so many galaxies. I am quite excited by the results and hope that it will be published soon. My research interests spans from understanding the processes of star formation to the evolution of galaxies and the Universe as we see today.
Ivy’s research assistants
The Galaxy Zoo paper that I just submitted consists of nearby galaxies which appear to be transitioning from being star-forming to passively-evolving galaxies. In particular, I looked at a sample of post-starburst galaxies (PSG). These PSG had a recent burst of star formation but they have since ceased forming stars. Thanks to the compilation of all the morphology classifications and the merger votes produced by the Zoo-ites, we were able to determine that most of these PSG have an indeterminate morphology with a higher fraction of interaction than regular spirals or ellipticals. It is possible that these interactions were responsible for the burst of star formation as well as the disturbed galaxy morphology.
The majority of PSG are low-mass but most of their stellar distribution already resemble those of ellipticals. However, they are still somewhat “green” and will likely turn red once the starlight of the youngest population of stars start to fade. Therefore these nearby PSG will probably end up as redder, low-mass and more passively-evolving galaxies. This result agrees with previous works asserting that the most massive and passively-evolving galaxies were formed at earlier times in the history of the Universe.
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.
Classification tree tweaks
Some of you may have noticed that on Thursday we made a couple of small changes to the flow of questions that are asked for each object in Galaxy Zoo: Hubble. Both of these changes relate to the set of additional questions which we introduced during the switch from Galaxy Zoo 2 to Galaxy Zoo: Hubble. As you will have certainly noticed, the new Hubble Space Telescope images contain many more galaxies with a clumpy appearance. This type of galaxy was very rare in the Sloan Digital Sky Survey images and doesn’t really fit into the classification tree we used for Galaxy Zoo 2. To obtain useful classifications for these objects in Galaxy Zoo: Hubble we therefore decided to add another branch of questions to the “classification tree”.
During the first month or so of Galaxy Zoo: Hubble we have received a great deal of very useful feedback, particularly on the forum. In particular, two features of the new classification tree appeared to cause a fair bit of consternation amongst some of the Zooites. After considering your comments, and much deliberation, we decided to make a few changes.
Both points of contention related to the question asked after an answer had been clicked for ‘How many clumps are there?’. If the answer was anything except ‘one’, then we then asked ‘Do the clumps appear in a straight line, a chain, a cluster or a spiral pattern?’. Now, that’s a hard enough question to answer when there is only three clumps, but doesn’t make much sense at all when there are just two. We were trying to keep things simple but, to be perfectly honest, this wasn’t very sensible on our part. We have now changed the tree so that if the answer given is ‘two’, the question about how they are arranged is skipped.
The second issue was more interesting, because the frustration it caused told us something about the appearance of the clumpy galaxies which we hadn’t properly appreciated when planning the questions. New astrophysical insight before we’ve even collected enough clicks to start analysing! If the answer to ‘How many clumps are there?’ was ‘one’, the classification tree went back to the branch for ‘Smooth’ galaxies and asked ‘How rounded is it?’. Our thinking here was that a galaxy that was mostly just one clump would probably be an elliptical or maybe a bulge within a smooth disk galaxy.
It seems we both underestimated the discriminatory power of the Galaxy Zoo participants and how clearly different clumpy galaxies are from other types, even when there is only one clump. After having seen a few clumpy galaxies, it seems that many Zooites come to recognise that there are subtle features that set them apart from other types of galaxies. This suggests that single-clump galaxies really are a clearly different type of galaxy to the ellipticals and disks that are more common nearby. For single clump galaxies we now carry on asking the usual clumpy galaxy questions, skipping those that don’t make sense for only one clump.
Don’t worry – all your previous classifications of one (and two) clump galaxies are still safely stored away and will be very useful in helping us catalogue the subtle differences between the appearances of all these objects. Thank you, and keep clicking!