Hi everyone, my name is Tobias. I finished my PhD at the University of Oxford almost a year ago now, and I am currently a postdoc at the University of Toronto. I work a lot with Galaxy Zoo classifications, specifically to study bars in galaxies. We’ve just submitted a new paper (which can be found here), but I thought it would be fun to go over some of the main results in this blogpost. I heavily relied on classifications from Galaxy Zoo in order to identify the bars, so this work would not have been possible without your help!

Introduction

There are different ways to classify a bar in a galaxy. One you are all doubtlessly very familiar with, is a classification based on morphology: bars can be classified as either weak or strong. This is what Galaxy Zoo has been asking volunteers to do since GZ DECaLS.

However, another useful approach to classify bars is based on how they move (i.e. the kinematics). Bars will rotate around the centre of their host galaxy with a specific speed, often called the bar pattern speed (). However, and this is crucial, this rotation rate is disconnected from the rotation rate of the disc of the galaxy. This means that we can classify bars depending on whether they rotate ‘slow’ or ‘fast’ relative to the stars in the disc. Slow bars typically rotate much slower than stars in the disc, whereas fast bars rotate with a velocity similar to the disc (well, technically, the bar-end of a fast bar will rotate with a velocity similar to stars in the disc at the same radius, but that’s a detail). These definitions can feel a bit convoluted at first, but I made an interactive tool to help you get a grasp on what this actually means. If you’re interested, this tool can be found here.

In a previous paper, we tried to measure the kinematics of bars in a bunch of galaxies (this paper can be found here). Bar strength has been reasonably well studied over the last few years, but the kinematics of bars hasn’t been studied observationally to the same degree. Even more, the interplay between bar strength and kinematics (i.e. does a strong slow bar behave the same way as a strong fast bar?) has not really been looked at yet in much detail. This is exactly the topic of our new paper. I have been working on this problem for a while now, and I have recently finished! I’ll go over some of the main findings below, but if you want to learn more, please feel free to check out the actual paper itself: https://arxiv.org/abs/2405.05960

Intermezzo: different regions of a barred galaxy

Before we continue, I want to spend a little time to clarify different regions of a bar. The names are relatively intuitive. The picture below shows a famous barred galaxy called NGC 1300. All credits of the original picture go to NASA, ESA and the Hubble Heritage Team. Overlaid on this image are the different regions of the bar that I will talk about. The bar itself is bounded by the dashed box. The centre of the bar typically coincides with the centre of the galaxy. The bar then has two arms that stretch out either side. The bar-end region is where, as the name implies, the bar ends. The bar-end region where the bar sometimes transitions into spiral arms (as is the case here) or a large ring structure, but this is not always necessarily the case.

How do we study bar kinematics?

We want to study how bar strength and bar kinematics affect their host. Specifically, we will look at how bar strength and kinematics affect the distribution of star formation. I won’t go into too much detail on this now, but the amount of star formation is related to a quantity called the equivalent width of H, also written down as EW[H]. So whenever you see EW[H] in the plots and text below, you can simply replace it with “star formation”.

In order to measure how the star formation changes in different locations within the bar, we will create so-called radial plots, which is explained using the Figure below. Panel (1) shows the image of a galaxy. Panel (2) shows the amount of EW[H] (read: star formation) that happens in every pixel. You can already see that there is more star formation along the bar and in the centre of the galaxy. There is also a significant amount of star formation happening in the spiral arm in the top of the plot, but we’ll ignore that for now. Also note the black rectangle drawn over this plot. This is drawn over the bar region, and we will look at the pixels within that box in more detail in Panel (3). Here, we plot the EW[H] against radius from the centre. Every blue dot here is a pixel within the rectangle from the previous panel. To make comparison between different barred galaxies easier later, we normalise this distance to the length of the bar. This means that, in these units, at R = 1, we are at the end of the bar. At R = 0.5, we are in the middle of the bar. Note that, while there is some scatter, there is a pattern here. There is a lot of star formation in the centre of the galaxy and just beyond the ends of the bar. The black line shows the median value at different radius intervals. This line is shown again in Panel (4) in black. All the grey lines in the background are other profiles of other strongly barred galaxies. We can take an average of all these lines (effectively taking an average of averages) to create an overall profile of EW[H] of strongly barred galaxies. This is the so-called radial profile.

Effect of bar strength

We can then compare different radial profiles of different groups of galaxies. For example, below, we compare the radial profiles of unbarred (grey), weakly (blue) and strongly barred galaxies (orange). There is a lot going on in these plots, but I’ll walk you through the most important parts. Firstly, the full line is the median radial profile of a group of galaxies (e.g. all strongly barred galaxies), constructed as discussed above. The contours around it show the 33th and 66th percentiles, which can be interpreted as the uncertainty around the median of the radial profile. The bottom part of the figure shows the difference between two radial profiles. Additionally, the size of the dot shows how significant the difference is. A large dot implies that the difference is very significant, while a small dot implies that it is not significantly different.

There are multiple things that we notice. Firstly, the shape of the strongly barred radial profile, when you only look at R < 1.5, is shaped like a “V”: there is a peak in the centre of the galaxy, a minimum in the arms of the bar (at R = 0.5), and a second peak just beyond the bar-end region. This distinct shape is only seen for strongly barred galaxies. The shape of weakly barred and unbarred galaxies are similar to each other: it simply increases slowly with radius. These observations are consistent with the picture that strongly barred galaxies collect gas from the galaxy, and funnel them through the arms of the bar to the centre of the galaxy. This gas is then used in the centre (and to a smaller extent, just beyond the bar-end) to form stars. We can only conclude this for strong bars, not for weak bars.

Effect of bar kinematics

Now that we’re familiar with radial profiles and how to use them, we’ll move to the main point of the paper: what happens when we look into bar kinematics as well? This is done in the Figure below, where we look at the radial profiles of EW[H] for strongly barred galaxies in the left panel and weakly barred galaxies in the right panel. In each panel, we also divide the sample into slow (green) and fast bars (purple). In the left panel, you can see that the radial profile of slow strong bars has higher values of EW[H] across the board than fast strong bars. However, we also see that the uncertainty is quite large and that the observed differences are not significant. One thing to note is that the sample sizes are relatively small. This is because measuring bar kinematics is not straight forward. This is one potential reason for why the differences we see are not significant. I’m currently working on measuring it for more galaxies to hopefully mitigate this problem. On the other hand, the left panel shows that the radial profile of slow weak bars is similar to that of fast weak bars.

So, we see some effect of bar kinematics on the radial profile of strongly barred galaxies, but it is not significantly different, presumably due to low sample sizes. Perhaps we can tease out any potential differences in another way. Remember that the radial profiles were constructed by looking at the area around the barred region, by choosing the pixels that were contained within a box placed parallel on the bar (the red box in the example below). We can compare that to a box placed perpendicular to the bar (the blue box in the example below). By comparing these “on bar” and “off bar” radial profiles with each other we can directly assess the effect the bar has on its host, and perhaps learn more about the importance of bar kinematics.

This is exactly what we did in the Figure below. Here, we show radial profiles for on-bar regions (the red lines) and off bar regions (blue lines). We have done this for four subsets of barred galaxies: slow strong bars (top-right panel), fast strong bars (top-left panel), slow weak bars (bottom-left panel) and fast weak bars (bottom-right panel). There are a lot of different things to look at in this plot, but the main take-away is that the largest differences observed in EW[H] (and thus, star formation), and the only ones that are actually significantly different, are found in slow strong bars. It seems that, a slow bar will affect the star formation in its host significantly, but only if it is also a strong bar. There are some slight differences in some of the other panels (e.g. fast weak bars), but those differences are not significant.

Why would kinematics even matter?

We have shown that slow bars effectively increase the star formation along the bar. But why? To help see why this might be the case, it is helpful to go back to the definitions. As I mentioned above, a slow bar typically moves much slower than the stars in the disc at the bar-end. This means that the difference in velocity between the bar and the stars and gas in the disc is relatively large, as visualised in the left panel of the figure below. The stars and gas in the disc will continuously overtake the bar. This also implies that the bar will come into contact with much more of the gas in the disc. We hypothesise that, because of this difference in velocity, the bar is much more efficient at collecting, funnelling and concentrating gas from the disc into the bar region. This would, in turn, concentrate the resultant star formation along the bar as well and increase the overall values of EW[H] along the bar.

In contrast, fast bars move with similar velocities to the stars and gas in the disc. They are always “in sync” with the disc at the bar-end region. This implies that fast bars do not come into contact with as much gas as their slower counterparts, and therefore will have lower amounts of gas and star formation along their barred regions.

Conclusion

We have talked about a lot of different things in this post. The two main points are summarised below:

1) Bar strength has a clear effect on the star formation distribution. Strong bars have higher amounts of star formation in their centre and beyond the bar-end regions, while they have lower amounts of star formation in the arms of the bar. Meanwhile, a weak bar does not seem to influence the star formation distribution of their host at all.

2) Bar kinematics matter as well. Slow bars increase the amount of star formation along the bar, likely because they come into contact with more of the gas of the galaxy and are more efficient at funnelling and concentrating gas. However, we only see this happen if the bar is also strong.

If you’re interested in learning more about this, please feel free to check out this newly released paper, found here. Of course, none of this work would have been possible without the efforts of citizen scientists, such as yourself. The bar strength classifications used in this work are directly based on your classifications, and we are all very thankful for that!

Cheers,
Tobias Géron

Galaxy Zoo has meant a lot to me over the more than a decade I’ve been involved with the science team. I’ve been proud to work with many wonderful colleagues and students in the Galaxy Zoo science team and work on many investigations into the properties of galaxies enabled by your classifications. 

 So to celebrate the publication* of my latest popular science book “The Astronomers Library”, a beautiful pictorial walk through the history of books about astronomy, I am happy to be able to offer a giveaway to Galaxy Zoo volunteers. If you would like to have a chance to win a free signed copy of the book please sign up via this form. One volunteer will be randomly selected at the end of May 2024, and I’ll be in touch to arrange delivery. 

As astronomers, our job involves many tasks, from data collection, analysis and interpretation, to presenting our work at conferences through talks and scientific posters. But there’s also a lot of writing. It’s often said that professional scientists are also professional writers. Lucky for me I happen to have come to really enjoy writing. I teach a class on writing in the sciences at Haverford College, and in recent years I’ve been delighted to find the opportunity to get involved in writing books intended for a popular audience. This book was particularly fun to work on for how it combines two of my favourite things – astronomy and books, and I’m delighted with the end result.  

Karen (PI of Galaxy Zoo). 

*For full transparency, while I was paid to write this book, I do not receive a commission per sale.

Galaxy Zoo Mobile is back! 

This time, we have two challenges for you; understanding rings and finding gravitational lenses. Both can be accessed through the Galaxy Zoo Mobile project on the Zooniverse app. Dive in and start swiping! 

Get the Zooniverse app on Android iOS and find Galaxy Zoo under ‘All Projects’ or ‘Space’. You can also use your browser if you prefer.

Understanding Rings

You already helped us find ringed galaxies; galaxies with a ring of stars around them. But there are many ways to “have a ring”; detached rings, pseudo rings and winding rings to name a few! Help us divide the ringed galaxies you found into all these different types. 

We’re also on the lookout for anything truly weird, as we know there are bound to be some crazy never-before-seen ring structures out there. The images you will see are the most likely to have rings from a suite of 9 million galaxies – so you’re going to see some truly spectacular shapes and sizes. 

We don’t know how rings work. Help us find out.

Finding Gravitational Lenses

Following on from the success of one of Galaxy Zoo’s latest iterations, Galaxy Zoo: Cosmic Dawn, we want to double-down on identifying all images containing a very rare type of object: gravitational lenses. These are galaxies surrounded by the light of a second galaxy directly behind them, with gravity distorting this light into beautiful arcs and rings. These objects can be used to help shed a light on dark matter and the early Universe, if we can find them!

For some of Galaxy Zoo: Cosmic Dawn, we used AI to help speed up the classification process and let you focus on the most interesting images. But our AI doesn’t understand rare objects like gravitational lenses, many of which may have been missed as a result. So, we ask you to take part in identifying these elusive objects hidden in over 20,000 Hyper-Suprime Cam (HSC) images from the Hawaii Two-0 (H20) Cosmic Dawn survey. Start searching on the Zooniverse app.

Thank you, as always, for your time and support.

James P, (also) James D, and Mike, on behalf of the Galaxy Zoo team

I’m thrilled to announce the release of Galaxy Zoo DESI; detailed morphology classifications for all 8.7 million well-resolved galaxies in the DESI Legacy Surveys. This is the largest detailed catalogue ever made (by an order of magnitude). 

Today is the end of a long road. GZ DESI is being released eight years to the day after the first DESI images went live on Galaxy Zoo. It took four ‘generations’ of astronomers – Kyle Willett, Coleman Krawcyzk, myself, and Tobias Geron – to keep the project running. And of course, it took all of you. At least 105,459 people* contributed their time to making GZ DESI happen. Thank you all.

Releasing GZ DESI is also the start of a new journey. Space telescope Euclid is in the sky and taking pictures. The Vera C. Rubin observatory should see first light within a year. Both will find tens of millions more galaxies – even more than DESI. But we’re ready. GZ DESI let us build and test the AI tools that now work alongside you. Those tools mean that every classification you make helps us classify every other galaxy as well. And we’re starting to imagine what new tasks – beyond classification – we might be able to do. But that’s for another blog 😉

On to the next telescope!

Mike, and the Galaxy Zoo team

*(unique logged-in Zooniverse accounts)

Of all the galaxies in the sky, which should you look at first?

You might have noticed that our most recent galaxies, taken by Hyper Suprime-Cam in Hawaii, have often been less detailed than our previous galaxies. This happened because we were very generous when selecting which galaxies to include. We had never classified galaxies from this instrument before and we were curious to see what you would find.

Reviewing the classifications so far, the clear majority of galaxies – as many as 90% – were voted as either “smooth” or “problem”. That suggests we’re showing you too many “blobs” and too few interesting featured galaxies.

To fix this, I’m happy to announce a new Galaxy Zoo feature that will prioritise showing you detailed and unusual galaxies.

We’re doing this with an AI algorithm we affectionately call Zoobot. Zoobot tries to classify galaxies based on what volunteers have said before for similar galaxies. If you’d like to test out Zoobot on some DECaLS galaxies, you can play with it here.

Zoobot is very good at simple classification tasks like recognising a smooth galaxy as smooth. We can therefore avoid showing you galaxies that Zoobot already considers to be very smooth. Specifically, we will now avoid showing galaxies where Zoobot is 90% confident that fewer than 2 out of 10 volunteers would click “featured”.

How will the galaxies you see change? You will see fewer totally smooth galaxies and more of everything else. Below are random examples of galaxies on Galaxy Zoo before (left) and after (right) switching on Zoobot.

Galaxies shown on Galaxy Zoo before activating Zoobot (left) and after (right). There will be fewer totally smooth and “bad zoom” images, and more featured and unusual galaxies.

Zoobot will continue to learn from you as you classify galaxies. It should get better and better at removing extremely smooth galaxies over time. If Zoobot learns that a galaxy might not be smooth after all – that is, if Zoobot changes it’s mind – the galaxy will be shown.

You might remember Zoobot from the “Enhanced” workflow we ran during GZ DECaLS. There, Zoobot tried to prioritise galaxies which, if labelled by you, would most help it learn. This worked well and helped us train the improved version of Zoobot that we’re using now. This new system is similar; we’re showing you the galaxies where volunteer labels are most useful for Zoobot and for science. We’re just using a much more straightforward rule to pick these.

Your time is precious. Galaxy Zoo volunteers can recognise and classify the detailed features of galaxies in ways that Zoobot can’t – and nor can any other algorithm. More than that, humans have a unique ability to spot things that look just a little bit weird. Volunteers talking about strange objects has led to some of our favourite discoveries, including the Voorwerpen. Using Zoobot means you will be much more likely to see more diverse galaxies and come across more weird and wonderful objects. We also have another surprise planned around these – stay tuned…

Cheers,

Mike, on behalf of the Galaxy Zoo team

The Cosmic Dawn Survey is a multi-wavelength survey aiming to understand the co-evolution of galaxies, the dark matter haloes that host them, and their central black holes over cosmic time, all the way from when galaxies first formed in the early Universe. A major part of this is the H20 survey which has obtained ultra-deep Subaru HSC imaging over large and particularly dark areas of the sky. The H20 survey targets two areas of the sky which, as part of the Cosmic Dawn Survey, were observed by the Spitzer Space Telescope in the largest allocation of observing time ever awarded on the spacecraft. When combined with these infrared data, H20 aims to push the boundaries of extragalactic astronomy by studying galaxy evolution out to around 800 million years after the Big Bang. This incarnation of Galaxy Zoo features images from a portion of the sky called the Euclid Deep Field North (EDF-N), from one of the two areas targeted by the H20 survey.

This effort from Galaxy Zoo will therefore also help prepare for the upcoming launch of the Euclid space telescope by the European Space Agency (ESA) in 2023, with the classifications you make now helping to guide what Euclid will observe in more detail with its even higher resolution imaging in visible light and the infrared.

Compared to previous incarnations of Galaxy Zoo such as those using SDSS and DeCALS images, H20 enables us to see fainter and more distant galaxies from earlier in the Universe’s history, thanks to higher angular resolution of HSC and greater depth of the survey. However, deeper imaging also means we can observe many more distant galaxies in the same patch of sky, so the images you will see may often appear redder and blurrier than you might expect.

The Galaxy Zoo decision tree of questions has been modified so that your classifications can help refine the software used by the H20 team, and perhaps that of future Galaxy Zoo incarnations as well. For example, the “Star or Artifact” question now includes a “Bad Image Zoom” option, while selecting “Non-star Artifact” will allow you to classify the type of any image artifact you come across, such as satellite trails. In the future, Galaxy Zoo will also be running a simplified decision tree of questions for some of the fainter distant galaxies, as their lower resolution prevents many features from being identified.

Also, look out for any galaxies containing bright clumps! We’ve added a question about these, as they can help us understand the period of intense star formation that took place in the early Universe.

Finally, we are also asking volunteers to tag any extremely red objects, or those with a lens or arc features, in the Talk board, using the “Done & Talk” option. These are rare objects that we don’t want to miss, especially for such distant galaxies!

We are excited about the new images and looking forward to seeing what you’ll discover. Join the classification now!

James Pearson, Galaxy Zoo and H2O teams

The story so far: on the first night we were able to observe until 02:00 before the weather forced us to close. The following three nights we were confined to the Residencia, the place where they keep all the astronomers when they are not observing. Much to our surprise, this morning we awoke to a sky only sparsely covered by clouds, instead of being in the middle of one. Maybe we have a chance of observing something tonight?

== 19:30 ==

As the weather has cleared somewhat on Roque de Los Muchachos, La Palma, we have received permission to go to the telescope tonight. We’ve just arrived and started taking the first couple of calibration images with much enthusiasm. 

A bit about the telescope itself: we’re using a telescope called the Isaac Newton Telescope (INT). It is a Cassegrain reflector telescope with a ~2.5m primary mirror that weighs ~4000kg! 

It takes a while for the telescope to take all the calibration images (biases, arcs and flats), so we were able to enjoy the sunset right before -hopefully- a very busy night. 

== 22:00 ==

Unfortunately the weather has taken a turn for the worse. We cannot open the dome of the telescope as the humidity is too high. We’ve had our first cups of coffee and are settling in for the night, while keeping an eye on the humidity sensor.

As tonight will be our last night at the telescope and we’ve had bad luck with the weather the last couple of days, we are very hopeful to observe some galaxies tonight. So far we were able to observe only one galaxy before we had to close on the first night. At this point we will be grateful for any data that comes in, even one more galaxy would double our current sample size!

== 00:45 ==

We’re still not able to open, humidity is at 100% and we cannot see any stars. The highlight of the last couple of hours was exploring the library in the INT and listening to an old cassette tape of Joseph and the technicolour dream coat. None of the other tapes work.

== 04:00 ==

We’ve had a popcorn and pizza break, drank multiple cups of coffee and explored most of the telescope building. However, unfortunately we’ve not been able to use the telescope today – the weather gods seem determined to prevent us from getting any data. With any luck we’ll be back next year to try again.