This is a guest post by Freya Pentz, who has spent much of this summer doing research with Galaxy Zoo.
Hi Galaxy Zoo volunteers!
I’m a summer student at the Zooniverse. I’m at university studying natural sciences about to go into my second year and for the past 5 weeks I’ve been working at the Zooniverse office here in Oxford. I wanted to let you know what I’ve been doing during that time.
I’ve been using data from the Galaxy Zoo: Bar Lengths project, writing code to process the information and making sure it looks sensible. Before I started working at the Zooniverse, I had done very little computing so I had to learn a lot! For those of you who are interested, I’ve been using python to extract the measurements you did on the galaxies and plotting graphs with all the data. Learning how to use python was like learning another language but it was definitely worth it.
The first thing I did was to find out how many of the galaxies that you’ve classified have bars. That meant looking at the answers to the first question about the galaxy in the Bar Lengths project ‘Does this galaxy have a bar’ and seeing for each galaxy if most people answered ‘Yes’ or if most people answered ‘No’.
Luckily, the code could do that for me; otherwise I would have had to look at over 66000 answers! So far, 4960 galaxies have been classified out of a total of 8612 in the project. Your classifications show that 700 of these have a bar, meaning that the fraction of classified galaxies with a bar is around 14%. This is similar to the 10% bar fraction referred to in the study recently done by the Galaxy Zoo and CANDELS teams on bar fractions out to z=2 (blog post & paper). This number will probably change a little bit as more galaxies get classified, but it’s good that it is similar to known values so far.
The next thing was of course to find the lengths and widths of the bars. When you draw lines on the galaxy to mark the length and width, the database records this as coordinates. Each line has four coordinates, 2 x coordinates and 2 y coordinates. Once you have the coordinates, it’s fairly simple to turn them into lengths. All you need is some Pythagoras. When plotting a histogram of the lengths, the shape was a Gaussian distribution, or a bell curve. This shows that most of the galaxies have lengths between certain limits (5-15 kpc) and then as you go beyond these limits, the number of galaxies decreases.
During my time here, I found some interesting galaxies. When I first looked at the redshifts, there was a galaxy with a redshift of 4.25. I mentioned this to a couple of people on the Zooniverse team and they all said there wouldn’t be a galaxy with such a high redshift in the sample. I checked it out and this is the galaxy in question:
You can see that there is a bright blue smudge in the top left of the galaxy. When I first saw this, I thought it was a lens. It looks like one, and you can just see a small bit of blue on the other side of the galaxy’s core, suggesting a lens even more. According to the experts in the Zooniverse however, this is probably not a lens, as the galaxy does not look massive enough to lens light. Also, the blue curve is well inside the galaxy, instead of being around the outside. Usually, all the mass of the galaxy is needed to lens an object so the light would appear around the edge. The blue curve is most likely an unusual feature of the galaxy itself, which can explain why the reported redshift is so high. The redshift for this galaxy was measured photometrically. This is where astronomers use galaxy colours across a wide range of wavelengths to predict the likely redshift. This method of measuring redshift is much more prone to error than spectrometry (where the absorption lines for certain elements in a galaxy are observed and the shift of these lines is measured) so the blue smudge could have easily made the telescope think the redshift was higher than it is. This redshift is therefore almost definitely a mistake. We also know this from the high resolution of the image. You normally wouldn’t be able to see a galaxy with even a redshift of 1 this well!
The reason telescopes have to use photometric redshifts sometimes even though they are often wrong is that there is not enough time to take a spectrum of every galaxy when you are conducting a large survey of the sky. Telescope time is expensive and photometric measurements allow you to get a bit of information about lots of galaxies which can sometimes be more useful that getting a lot of information about a few.
When running into problems like this it was really useful to be able to look at a picture of the galaxy on the Galaxy Zoo: Bar Lengths website. Looking at the galaxies and seeing in real life what the data on the graphs was telling me was probably my favourite part of my time at the Zooniverse. It’s so amazing that thanks to the Sloan Digital Sky Survey, the Hubble Space Telescope projects and other mass surveys of the universe, we can actually look at pictures of thousands of galaxies easily.
The Zooniverse is such a cool organisation and I’m lucky to have worked for them this summer. The great thing about them is that you can get involved too! I know from my work with Bar Lengths that even if a few people log on and classify in any of the projects, it can be really helpful. None of the science can be done without you providing the data.
Measure some galaxies here:
Or have a look at some of the other projects here:
We’re extremely excited to announce the launch of two new image sets today on Galaxy Zoo. Working with some new scientific collaborators over the past few months, we’ve been able to access data from two new sources. This blogpost will go into more details on where the images come from, what you might expect to see, and what scientific questions your classifications will help us answer. (See Part 1 of this post to learn about the other new images from the DECaLS survey).
The second set of new data comes from the Illustris Project. Illustris is a state-of-the-art simulation of the Universe, led by a large team of researchers in the US, UK, and Germany. Large-scale cosmological simulations are a critical tool in astronomy; since we don’t have laboratories where we can replicate the conditions of processes like galaxy formation, we use computer simulations to investigate them instead. Such simulations start with what we believe conditions in the very early Universe were like (which we infer from the cosmic microwave background), and can include both dark matter and baryons (particles like protons and neutrons that eventually form the stars, dust and gas in galaxies). The simulation then tracks what happens to the matter and energy over billions of years as the Universe expands, evolving according to the laws of physics that are programmed into the simulation. This includes relations like the law of gravity, which dominates how dark matter moves, and hydrodynamics, which describe the motions of the gas. It’s truly amazing – scientists can watch galaxies form and evolve over huge scales of distance and time, and compare the results to real observations to test if the physics of the simulations are correct. Illustris is one of the largest and most detailed simulations ever run, taking more than 19 million CPU hours to run on powerful supercomputers.
This comparison to real data is the key feature that sparked the collaboration between Illustris and Galaxy Zoo. Once the simulation is run, astronomers analyze the results to see if their galaxies match the properties of those seen in the real Universe. This includes measurements like the total number of stars formed, the ratio of stars to dark matter, and the distribution of galaxies of different masses and luminosities. Another critical parameter we want to compare is galaxy morphology; measuring the ratio of ellipticals to spirals, for example, is an important test of whether the galaxy merger rate is correct, and if the simulation codes for star formation and gravitational interaction are correct.
The Illustris scientists have created images of the galaxies from their simulation that GZ volunteers will classify by their morphology. Our comparison data set for this will be the SDSS results from Galaxy Zoo 2, and the images are designed to match the Sloan images as closely as possible. This includes the same set of filters for the telescope, sizing the images so that the galaxies look like they’re at cosmic distance from the Milky Way, and setting them against backgrounds of stars and other galaxies. The quality of the simulations and images are amazing – these look to me like real galaxies in every way. It’s something that astronomers definitely couldn’t do ten years ago.
Although these images aren’t of “real” galaxies, we want to emphasize again how much your classifications will help scientists to do astronomical research. Simulations like Illustris are the only way that we can probe galaxy formation and evolution as it happens. Your classifications, both from Galaxy Zoo 2 and from the new Illustris data, provide vital tests for the output and will be fed back to the science teams in order to improve future versions of these sims.
If you have questions or want to discuss anything you see in the new images, please join the discussion with scientists and volunteers on Talk. The Illustris Project also has some amazing online tools if you want to learn more, including an interactive explorer of the simulation and videos of the evolving Universe. You also can explore specific galaxies you’ve classified via GZ:Examine. As always, thanks to everyone for your help!
We’re extremely excited to announce the launch of two new image sets today on Galaxy Zoo. Working with some new scientific collaborators over the past few months, we’ve been able to access data from two new sources. This blogpost will go into more details on where the images come from, what you might expect to see, and what scientific questions your classifications will help us answer. Part 2 of this post will discuss the other set of new images from the Illustris simulation.
The Dark Energy Camera Legacy Survey (DECaLS) is a public optical imaging project that follows up on the enormous, groundbreaking work done by the various versions of the SDSS surveys over the past decade. The aim of DECaLS is to use larger telescopes to get deeper images with significantly better data quality than SDSS, although over a somewhat smaller area. The science goals include studies of how both baryons (stars, gas, dust) and dark matter are distributed in galaxies, and particularly in measuring how those ratios change as a galaxy evolves. By adding morphology from Galaxy Zoo, our joint science teams will explore topics including disk structure in lower mass galaxies, better constraints on the rate at which galaxies merge, and gather more data on how the morphology relates to galaxy color and environment.
DECaLS observations use the Blanco telescope, which is located at CTIO in northern Chile at an altitude of 2200m (7200 ft). The telescope has a 4-m aperture mirror, giving it more than three times the collecting area of the SDSS telescope. The camera used for the survey is named DECam, a large-area and extremely sensitive instrument developed for a separate program called the Dark Energy Survey. The camera has 570 megapixels and covers a 2.2 degree field of view – more than 20 times the apparent size of the full moon! The combination of the exquisite dark-sky observing site, a sensitive wide-field camera, and larger telescope all combine to generate the new images, which will eventually include more than 140 million unique sources on the sky when DECaLS is finished.
The DECaLS images in Galaxy Zoo are a smaller group taken from a catalog called the NASA-Sloan Atlas. We’re focusing on somewhat larger and brighter galaxies from the catalog. The reason is that although many of these galaxies have been classified in GZ already via their Sloan images, we’re particularly interested in measuring details like tidal tails from mergers, seeing fainter spiral structures, and separating galaxies that couldn’t be individually resolved in the Sloan data. Here’s a great example of a single galaxy in both SDSS and DECaLS – check out how much clearer the spiral arms are in the new images!
Almost all of the morphology and classification tasks are the same as they were for the Sloan images, so it should be familiar to most of our users. If you have questions or want to discuss anything you see in the new images, please join the discussion with scientists and volunteers on Talk. As always, thanks for your help!
The below blog post was written by Alex Todd, an Ogden Summer Intern who spent the summer working on Galaxy Zoo related research projects at the University of Portsmouth. Alex is now off to his next adventure – starting his undergraduate degree in Natural Sciences at the University of Bath.
I have been working with the Galaxy Zoo team at the Institute of Cosmology and Gravitation, in Portsmouth, for 8 weeks this summer. I have been analysing the results of Galaxy Zoo 2, and more specifically the region of the sky known as Stripe 82. In this area, the Sloan Digital Sky Survey (SDSS) took many images of the same patch of sky, instead of only one. These images were combined to produce a single, higher quality image, which showed fainter details and objects. Both these deeper images and the standard depth images of stripe 82 were put into galaxy zoo, and I have been comparing the resulting classifications. I learned to code in python, a programming language, and used it to produce graphs from the data I downloaded from the Galaxy Zoo website. I started by comparing the results directly, comparing the number of people who said that the galaxy had features in each of the image depths.
On the graph, each blue dot is a galaxy (there are around 4,000) and the red dashed line shows the overall trend. As you can see from the graph, when the proportion of people who see features is low, there is a good match between the two image depths. However, when the proportion is high, there is a much bigger difference between the two image depths, with the proportion being higher in the deep image. This is because fainter features are visible in the deeper image.
I then plotted graphs of the difference between the proportions (P(Features)) against the brightness of the galaxy. To measure the brightness, I used the apparent magnitude, a measure of how bright the galaxy appears to us (as opposed to how bright it actually is).
The graph below shows the difference in P(Features) plotted against the apparent magnitude. The blue line is at y=0, and the green line represents the average value of the difference between P(Features). As you can see, there is not much difference between the values of P(Features) when the galaxy is particularly bright (Small apparent magnitude) or when it is particularly dim (large apparent magnitude). However, when the galaxy has an average brightness, the difference is quite substantial. We think this is because in bright galaxies, features can be seen in both images, whilst in dim galaxies they can be seen in neither. In medium brightness galaxies, however, they can only be seen in the deeper image. The fact that there are differences between the classifications means that it would be a good idea to classify deeper images of the rest of the sky, to hopefully improve the accuracy of the classifications.
I have greatly enjoyed my time working on at the ICG on galaxy zoo, and would certainly seize the opportunity to pursue it further.
It’s been a pleasure working with Alex this summer. He really impressed me with the speed at which he picked up programming languages. This information about the differences in perception of morphological features between deeper and shallower images is very useful to us as a science team as we plan for future generations of the Galaxy Zoo project with new, more sensitive images from current and ongoing astronomical surveys.
Next up in our series of eight blog posts celebrating eight years of Galaxy Zoo is this post from Tom Melvin, who was the lead author of the the first publication from Galaxy Zoo: Hubble, which looked at how the fraction of barred disk galaxies has evolved over the last eight billion years. Tom is also the first person to write a PhD thesis substantially based on Galaxy Zoo classifications, which he is in the process of completing final corrections for.
This was the first time the Galaxy Zoo volunteers had been asked to classify galaxies taken by the Hubble Space Telescope, which provided beautiful images of galaxies whose light has taken up to eight billion light years to reach us!
With your classifications, we were able to select a sample of disk and barred disk galaxies, as shown above in Figure 1, and explore how the fraction of disk galaxies that are barred has evolved over the last eight billion years. We found that this bar fraction has been increasing as the Universe has grown older, doubling from 11% eight billion years ago to 22% four billion years ago, which is shown below in Figure 2. We also know from Galaxy Zoo 2 that this continues to increase, with around one third of disks having a bar in our local Universe. We were able to expand on this by showing that it was the most massive disk galaxies that were the driver of this evolution.
As bars tend to only form in disk galaxies that are settled and relaxed, or ‘mature’, our results showing an increasing bar fraction over the last eight billion years tells us that the disk galaxy population has matured as the Universe has aged. As this evolution is being driven by the most massive disk galaxies, we were able to conclude that the most massive disk galaxies become mature sooner than their lower mass counterparts.
In addition to these results, we were able to identify a population of ‘red spiral’ galaxies thanks to your classifications. These red spirals’ would typically be omitted from other disk samples, as they would be classified as elliptical galaxies – but as you can see below, these are clearly beautiful red spiral galaxies! What is interesting about this population of disks is that their bar fraction of 45% is much higher than the bar fraction of the whole disk sample, which is roughly 14%.
So, thanks to your help classifying the amazing images from the Hubble Space Telescope, we were able to track the evolving bar fraction of disk galaxies over the last eight billion years. There is plenty more to be done with this sample of galaxies, so keep an eye out for future results looking at how galaxies have evolved over the past eight billion years!
As we approach the 8th anniversary of the Galaxy Zoo project, it is a great opportunity to look back at one of the most fascinating discoveries of citizen science in Galaxy Zoo – the “Green Pea” galaxies. Volunteers on the forum first noted these galaxies due to their peculiar bright green color and small size. Their discovery was published in our 8th paper: ‘Galaxy Zoo Green Peas: discovery of a class of compact extremely star-forming galaxies’ and is noted on the blog here. But the story doesn’t end with their discovery.
In the years since the publication of their discovery paper by the Galaxy Zoo Science Team, the Green Peas are beginning to fulfill their promise as a living fossil of galaxy evolution. Because they aren’t too far away, they provide a unique local laboratory in which we can investigate processes key to the formation and evolution of galaxies in the early universe. They are living ‘fossils,’ undergoing extraordinary, intense starbursts unlike any other galaxies known in the local universe. Their color is due to a large amount of emission in an oxygen line [OIII]/5007A that made their appearance green in the images.
Follow-up studies of the Green Peas have looked in great detail at their abundances of various elements, something that cannot be done in their high redshift analogs. The results of these studies show that they have energetic outflows of gas and lower oxygen abundances than other typical local galaxies with similar masses. They also suggest what might be responsible for ionizing the gas in the galaxies and producing those bright emission lines (e.g., Wolf-Rayet stars). Their clumpy morphologies (or shapes) have been confirmed and suggest that star formation in the peas occurs in several separate knots throughout the galaxy. Their radio emission implies they have strong magnetic fields, larger than that of the Milky Way. All of these results paint a picture of galaxies very similar to those that formed in the early Universe.
Results from studies of these galaxies can provide challenges to commonly accepted models. For example, the strong magnetic fields challenge models that suggest magnetic fields grow slowly over time and observations of the variation in Lyman alpha emission line profiles and strengths challenge models of the dependence of the emission line shape on gas properties in the galaxy. The Green Peas have held up their promise of lending new insights into galaxy evolution by characterizing an active mode of star formation, which contrasts with the typical more passive evolution dominating the local galaxy population. Studies of the Peas have suggested that a galaxy’s evolutionary pathway may depend on stochastic initial conditions, leading insights into our understandings of how galaxies throughout the Universe form.
At Galaxy Zoo we’re really proud of our publication record – 48 papers and counting, just from the team using your classifications. In academic research one of the most important numbers a published paper has is the number which counts how many citations that paper has – simply a count of the number of other academic publications mention your work.
And we’re not only proud of the Galaxy Zoo publication record, but the citation record is becoming impressive too (if we do say so ourselves). For this post in the lead up to the 8th anniversary of the launch of Galaxy Zoo, here are the 8 most cited of our papers:
1. Lintott et al. 2008: “Galaxy Zoo: morphologies derived from visual inspection of galaxies from the Sloan Digital Sky Survey “(with 279 citations)
2. Bamford et al. 2009: “Galaxy Zoo: the dependence of morphology and colour on environment” (219 citations)
3. Lintott et al. 2011: “Galaxy Zoo 1: data release of morphological classifications for nearly 900 000 galaxies” (152 citations)
4. Skibba et al. 2009: “Galaxy Zoo: disentangling the environmental dependence of morphology and colour” (114 citations)
5. Schawinski et al. 2010: “Galaxy Zoo: The Fundamentally Different Co-Evolution of Supermassive Black Holes and Their Early- and Late-Type Host Galaxies” (102 citations)
6. Cardamone et al. 2009: “Galaxy Zoo Green Peas: discovery of a class of compact extremely star-forming galaxies” (101 citations)
7. Darg et al 2010: “Galaxy Zoo: the properties of merging galaxies in the nearby Universe – local environments, colours, masses, star formation rates and AGN activity” (92 citations)
8. Masters et al. 2010: “Galaxy Zoo: passive red spirals” (86 citations)
I’m personally especially proud of paper number 8 on that list, because it is one of the first papers I led making use of Galaxy Zoo classifications (and one of my most cited first author papers in fact). In that paper we explored the properties of the unusually passive (ie. not star forming) red spirals that had been noted in both Bamford et al. 2009 and Skibba et al. 2009. For astronomers this is one of the more well known discoveries from Galaxy Zoo, and these passive red spirals continue to be studied for what they can reveal about the modes of evolution of galaxies in our Universe, and that many spirals must stop forming stars before they lose their spiral structure.
(By the way for academics who might be interested the h-index of Galaxy Zoo is 24).
It’s a week until the 8th anniversary of the launch of Galaxy Zoo.
The Hubble Space Telescope observations of giant ionized Voorwerpje clouds near galaxies with active nuclei, many found for the first time though the effort of Galaxy Zoo participants gives us another 8 – one at the end of a long road of numbers. 16,000 galaxies with known or possible active nuclei, 200 highly-ranked cloud candidates based on input from 185 participants, 50 spectroscopic observations, 19 giant ionized clouds, among which we found 8 with evidence that the nucleus has faded dramatically (and then observed by one Hubble Space Telescope). (You wondered where the numeral 8 would come in by now… and there is another one hidden below.) The first batch of scientific results from analysis of these images was described here, and the NASA/ESA press release with beautiful visualizations of the multi-filter image data can be seen here. As a visual summary, here are the images, with starlight and emission from [O III] and H-alpha shown in roughly true visual color.
This project was an outgrowth of the discovery of Hanny’s Voorwerp, which remains probably the signature discovery of Galaxy Zoo. In astronomy, one is a pet rock, ten is a statistically valid sample – so we wanted to know more about how common such clouds might be, and what they could tell us about quasars more generally. Zoo participants answered this challenge magnificently.
The scientific interest in these objects and their history remains intense, and observations continue. I’ve recently finished processing integral-field spectra from the 8-meter Gemini-North telescope, where we have spectra at every point in a small field of view near the nucleus, and just recently we learned that our proposal for spectra in a few key areas at the high resolution of the Hubble telescope has been approved for the coming year.
Even (or especially) for kinds of objects behind its original statistical goals, Galaxy Zoo has provided an amazing ride these last 8 years. Stay with us – and if you see weirdly colored clouds around galaxies, feel free to flag them in Talk!
We are pleased to announce that a Galaxy Zoo project is one of the first projects built on the new Zooniverse! Several years ago we measured the lengths of galactic bars in relatively nearby galaxies in the Sloan Digital Sky Survey, and Ben Hoyle wrote an excellent paper presenting new an interesting results on how bars, which are a distinct feature caused by a change in the nature of the orbits of some of the stars in a galaxy, relate to other physical properties of the galaxy, such as color (indicative of recent star formation) and the nature of spiral arms or rings. That work showed the power of measurements like these, which are not always easy for computers to get right.
Today, we’re hoping you’ll help us extend that set of detailed galaxy measurements into the distant Universe, with measurements of bars in about 8,000 galaxies from our previous projects using Hubble Space Telescope data, including the AEGIS, CANDELS, COSMOS, GEMS and GOODS surveys.
We’ve deliberately been pretty broad in our selection of galaxies which may have a bar, so the first thing the project asks you is to confirm whether you think the galaxy does indeed have one. There are many examples of barred and not-barred galaxies (including examples of sort-of-looks-like-barred-but-actually-isn’t-and-here’s-why) included in the project, and you can access them anytime by clicking the “Need some help?” button.
If the galaxy doesn’t have a bar, then you can move on to the next one. If it does, there are some follow-up questions about spiral arms and rings, and then we ask you to draw 2 lines on the image: one for the bar width and one for its length.
You can also join in the discussions after the classifications with our new Talk discussion tool, which is completely separate from the main Galaxy Zoo Talk (just like the rest of the project).
On a more personal note, this is a big step forward for the Zooniverse as a whole. The first draft version of this project came together in under 1 hour back in April. Afterward, we shared project links between science team members and iterated back and forth on the right questions to ask and the right data to use. This process would normally take at least 6 months and require a lot of one-on-one time with a Zooniverse developer. Instead, because the Zooniverse development team has done a brilliant job creating a Project Builder that’s flexible, powerful and also easy to use, we were able to create a new project in a way that’s analogous to, well, creating a blog.
In these early days of the new site’s release I’m sure there will be some bugs that need zapping, but even so the new capabilities of the Zooniverse are phenomenal. I suspect this is just the first of many new projects to be spun up in the New Zooniverse. (In fact, there are 3 more projects debuting alongside ours.)
Try it out here: Galaxy Zoo: Bar Lengths
Since our discovery in 2010 that the red spirals identified by your classifications in the first phase of Galaxy Zoo were twice as likely to host galactic scale bars as normal blue spirals, a lot of our research time has focused on understanding which types of galaxies host bars, and why that might be.
Our research with the bars identified by you in the second phase of Galaxy Zoo continues to gives us hints that these structures in galaxies might be involved in the process which quenches star formation in spiral galaxies and through that could be part of the process involved in the reduction of star formation in the universe as a whole.
We’ve also used your classifications as part of Galaxy Zoo Hubble and Galaxy Zoo CANDELS to identify the epoch in the universe when disc galaxies were first stable enough to host a significant number of bars, finding them possibly even earlier in the Universe than was previously thought.
Last Friday I spoke at the monthly “Ordinary Meeting” of the Royal Astronomical Society, giving summary of the evidence we’re collecting on the impact bars have on galaxies thanks to your classifications (a video of my talk will be available at some point). This was the second time I’ve spoken at this meeting about results from Galaxy Zoo, and it’s a delightful mix of professional colleagues, and enthusiastic amateurs – including some Galaxy Zoo volunteers.
Prompted by that I thought it was timely to write on this blog about what these bars really are, what they do to galaxies, and why I think they’re so interesting. I wrote the below some time ago when I had a spare few minutes, and was just looking for the right time to post it.
The thing about galaxies, which is sometimes hard to remember, is that they are simply vast collections of stars, and that those stars are all constantly in motion, orbiting their common centre of mass. The structures that we see in galaxies are just a snapshot of the locations of those stars right now (on a cosmic timescale), and the patterns we see in the positions of the stars reveals patterns in their orbital motions. A stellar bar for example reveals a set of very elongated orbits of stars in the disc of a galaxy.
Another extraordinary thing about a disc galaxy is how thin it is. To put this is perspective I’ll give you a real world example. In the Haus der Astronomie in Heidelberg you can walk around inside a scale model of the Whirlpool galaxy. The whole building was laid out in a design which reflects the spiral arms of this galaxy. However it’s not an exact scale model – to properly represent the thickness of the disc of the Whirlpool galaxy the building (which in actual fact has 3 stories and hosts a fairly large planetarium in its centre) would have to be only 90cm tall…..
Such an incredibly thin disc of stars floating independently in space would be quite unstable dynamically (meaning its own gravity should cause it to buckle and collapse on itself). This instability would immediately manifest in elongated orbits of stars, which would make a stellar bar (as part of this process of collapse). Simple computer models of disks of stars immediately form bars. Of course we now know that galaxy discs are submerged in massive halos of dark matter. So my first favourite little fact about bars is
(1) the fact that not all disc galaxies have bars was put forward as evidence that the discs must be embedded in massive halos before the existence of dark matter was widely accepted.
Now we can model dark matter halos better we discover that even with a dark matter halo, as long as that halo can absorb angular momentum (ie. rotate a bit) all discs will eventually make a bar. So my second favourite little fact is that
(2) we still don’t understand why not all disc galaxies have bars.
What this second fact means is that perhaps what I should really be doing is studying the galaxies you have identified as not having bars to figure out why it is they haven’t been able to form a bar yet. It should really be the properties of these which are unexpected….. We find that this is more likely to happen in blue, intermediate mass spirals with a significant reservoir of atomic hydrogen (the raw material for future star formation). In fact this last thing may be the most significant. Including realistic interstellar gas in computer simulation of galaxies is very difficult, but people do run what is called “smooth particle hydrodynamic” simulations (basically making “particles” of gas and inserting the appropriate properties). If they add too much gas into these simulations they find that bar formation is either very delayed, or doesn’t happen in the time of the simulation…..
Anyway I hope this has given you a flavour of what I find interesting about bars in galaxies. I think it’s fascinating that they give us a morphological way to identify a process which is so dynamical in nature. And it’s a very complex process, even though the basic physics (just orbits of stars) is very simple and well understood. Finally, I have become convinced though tests of the bars identified by you in Galaxy Zoo compared to bars identified by other methods, that if you want a clean sample of very large bars in galaxies that multiple independent human eyes will give you the best result. You are much less easy to trick that automated methods for finding galactic bars.
So thanks again for the classifications, and keep clicking. :)