Galaxy Zoo 2 - Top Photo

The Science

Galaxy Zoo 1 has already produced lots of brand new science — have a look at The Story So Far section for details of what we've done with all the clicks on the website. For Galaxy Zoo 2, we're after something a little bit different — detailed classifications of about a quarter of a million galaxies that the SDSS captured.

What we want to know

Even with all the new information we have from Galaxy Zoo, there are still many fascinating unanswered questions about galaxies that we want to investigate. Indeed, the studies from the original Galaxy Zoo have led to many new questions, which we now want to answer.

In the original Galaxy Zoo, users were simply asked to divide spirals from ellipticals. This worked pretty well, but it left us confused about a particular type of galaxy called an "S0". These are a cross between spirals and ellipticals, and are notoriously difficult to find. As a result, their true nature is poorly understood — but the Galaxy Zoo 2 questions should help us to spot and then investigate them.

Even for galaxies which are obviously spiral or elliptical, there is still lots of useful information that we need. The shapes of ellipticals contain information about their past, and many spiral galaxies have bars across their centres, including our own Milky Way galaxy. How these bars formed, how long they exist, and what their connection is to galaxy evolution is also a currently debated topic.

Then there are other questions to ask about a galaxy, such as: What fraction of galaxies have two, three, or more arms? How tightly wound are the spiral arms? Does the galaxy have a "boxy" or a "rounded" bulge? How many galaxies are there with "irregular" morphologies? Answering these questions about every galaxy, one galaxy at a time, is essential if we're going to understand the fine details of galaxy formation.

We want to know the answer to all these questions, and more. The primary goal of Galaxy Zoo 2 is to construct a database of detailed shape information for the largest sample of galaxies ever assembled. Such a database will have substantial legacy value for the international astronomy community. In short, Galaxy Zoo 2 hopes to find out everything there is to know about the appearance of galaxies!

If you've read the How to Take Part page then you know that we're also asking you to keep a look out for some rarer objects.

Rare Objects

The sharp-eyed visitors to the Galaxy Zoo are very good at spotting the wierd and wonderful — indeed, this is one of the most active areas of the discussion forum. So, we'd like to see if we can help the community be more effective at discovering certain types of rare object. We have several examples in mind for GZ2, based on the kinds of things found by the community so far.

Gravitational Lenses

Gravitational lenses are galaxies and groups of galaxies that are so massive that they bend the path of light from more distant objects towards themselves, distorting the shapes of background galaxies into arcs and rings, and even causing multiple copies of the images of galaxies and quasars to appear in symmetrical patterns around them on the sky. These cosmic alignments are quite rare — only about one in a thousand elliptical galaxies is acting as a lens in this way. In some cases it is possible to find them using clever image analysis software, but the most interesting cases are too complex for this. However, humans seem to be very good at recognising the tell-tale signs of gravitational lensing!

Why do we want to know about more instances of gravitational lensing? The separation of the multiple images allows us to weigh the lens galaxy, something that is typically very hard to do in astronomy. Once we have measured the mass of the lens, we then know how strong a lens it is — and how much magnifying power it has. The lensed images appear typically 10-100 times brighter than they would without the lens: we can use gravitational lenses as cosmic telescopes to observe the very distant universe. And as usual, the more telescopes we have the better!

Galaxy Mergers

Galaxies can grow in two ways: by forming stars, or by merging together. Our current theories of galaxy formation expect there to be a lot of merging happening, and indeed we do see many examples, but it is very difficult to reliably measure how much merging is really going on. We need big samples, and keen eyes — Sounds like a job for Galaxy Zoo!

Galaxy Wars

We want to try and understand why some galaxies have more prominent spiral arms and bars than others. We think it could have something to do with where they live in the universe, or perhaps the presence of a nearby neighbour. However, to determine what is responsible we have to take into account two other effects we already expect.

When one simply classifies galaxies by looking at them one at a time, one issue is that the classification can vary depending on other aspects of the galaxies appearance. This can be due to similar galaxies being at different distances, or due to real difference between galaxies of intrinsically different sizes. For example, a small galaxy might never have sprial arms as prominent as a large galaxy. Sometimes what we really want to know is: "compared to other galaxies of similar brightness and size as this one, when viewed at the same distance, how prominent are the spiral arms?". So, the best way to arrange galaxies by their features is sometimes to just directly compare them to one another. However, to avoid galaxy size, brightness and distance messing up our results we must compare galaxies which have similar properties. That's exactly what Galaxy Wars does. We have divided most (but not all) of the objects in our sample up into groups, each containing 200 galaxies with similar sizes, brightnesses and distances. We are then asking you to compare each galaxy with others in the same group.

That's a lot of comparisons! Fortunately we don't need to compare each galaxy with every other galaxy in its group, just enough that we can put them in order fairly accurately. For example, if we know galaxy A has a more prominent bar than galaxy B, and galaxy B has a more prominent bar than galaxy C, then we know their order of bar prominence must be A-B-C, without needing to compare A with C. With enough comparisons we can then rank similar galaxies in order of how prominent their spiral arms or bars are. This still needs a very large number of comparisons, but is just about possible with your help!

With the results of Galaxy Wars we can then investigate how spiral arm and bar strengths depend on many other things. For example, are a galaxy's spiral arms more prominent when it's overall colour is blue or red? Or when it has a black hole at its centre which is throwing out lots of radiation? Or is a galaxy's bar stronger when it lives close to lots of other galaxies? These questions are usually tricky to answer because the answers are mixed up with how big and bright the galaxy is, and whether that affects how much people notice the spiral arm and bar features. Fortunately, the Galaxy Wars comparisons will make it very easy to answer these questions. With the complete, accurate and unbiased ranking of spiral arms and bars that Galaxy Wars will produce, we will be able to do studies of these galaxy properties that are way beyond anything that has been done before. This will really help us to understand why galaxies look like they do, and how they change during their lives. We can't wait to analyse the results!

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