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

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.

As some might remember, in our last paper (which can be found here), we studied differences between weak and strong bars. One of our results was that star forming galaxies with stronger bars have significantly higher star formation in their centres compared to galaxies with weaker bars. This might be due to differences in the gas flows induced by the different types of bars. To investigate this, we selected a sample of 21 galaxies from Galaxy Zoo, which we plan to observe over the next couple days. 

The relationship between galactic bars and star formation has long been up for debate. Galactic bars are vast structures of co-orbiting gas, dust and stars that form directly across the galactic nucleus. It is thought that gas flows along the arms of the bar into the centre, increasing the central gas density. As gas density increases, the star formation rate would also. So, that should be the answer then? However, in reality, it is not so simple.

There are many different kinds of bars, with varying characteristics such as strengths and orientations. A galaxy might contain a very strong bar – where it clearly dominates even over the galactic disk – or it could have a very weak bar – where the disk dominates over it. So, we need to ask ourselves further, is the gas flow and resultant star formation higher in galaxies with strong bars? What about weak bars? Does it even change at all if we compare either type of bar to galaxies which have no bars? 

These are the questions we are going to answer at the Isaac Newton Telescope, or INT, on the island of Santa Cruz de la Palma. With a sample of 21 galaxies characterised by Galaxy Zoo – 7 strongly barred, 7 weakly barred and 7 with no bars at all – we are investigating if any relation between star formation rate and bar strength exists. An example of each type of galaxy in our sample is shown below. On the left is a galaxy with no bar at all, while on the right is a galaxy with a strong bar. The strong bar clearly dominates over the disk of the galaxy. The middle panel shows a galaxy with a much weaker bar, where the disk dominates over the disk galaxy.

To investigate this, we must turn to spectroscopy. Rather than utilise images, such as the ones above, we align a spectroscopic slit along and perpendicular to the bar direction on the image. The spectroscope will split the incoming light into a spectrum of wavelengths, where we will be able to find any spectral signatures of elements within the bars themselves. 

There are two chemical signatures of star formation that we are looking for. The first, an indirect measurement, is looking for Hydrogen Alpha, or Hα. If there is a much higher abundance of Hα at the core of a galaxy with a strong bar, weak bar or not, it is very likely that there is a higher gas density. Ergo, there is a higher star formation. The second signature we are looking for is Oxygen III, or O[III]. O[III] is highly ionised only typically exists in areas where there are high rates of the star formation; the newly born stars being the cause of the ionisation. This would be direct evidence of higher star formation.

So, what do we find? Thus far, due to the adverse weather conditions caused by tropical storm Hermine on La Palma, we have set our spectroscopic observations on a single strongly barred galaxy. We have extracted the spectrum, removed any sources of contamination and reduced to only that of the bar and galactic nucleus. The top image is the spectrum from the slit at ninety degrees perpendicular to the bar direction and the bottom is aligned along it.

The top spectrum (perpendicular to the bar) appears to be almost empty, with only noise present. Along the bar, however, we get a very strong emission line at precisely 6562.801Å. Guess which wavelength Hα happens to rest at? Precisely the same! 

This is certainly a promising initial result. If the abundance of Hα is much higher along bars than not, then this is certainly a case for them enhancing star formation! The next steps are to confirm this finding by taking observations for the rest of our sample. Once the weather clears up on La Palma, we will be aiming to finally answer the question, what do galactic bars do for star formation? Enhance, prevent or nothing? Well, it looks like enhancing has won the first point!

We will keep you updated!

Cheers,
David, Tobias and Chris

Hi everyone!

I’m Tobias Géron, a PhD student at Oxford. I have been using the classifications of the Galaxy Zoo DECaLS (GZD) project to study differences between weak and strong bars in the context of galaxy evolution. We have made significant amount of progress and I was able to present some results a couple of weeks ago at a (virtual) conference in the form of a poster, which I would love to share with you here as well.

To summarise: I have been using the classifications from GZD to identify many weakly and strongly barred galaxies. Some example galaxies can be found in the first figure on the poster. As the name already implies, strong bars tend to be longer and more obvious than weak bars. But what exactly does this mean for the galaxy in which they appear?

One of the major properties of a galaxies is whether it is still forming stars. Interestingly, in Figure 2 we observe that strong bars appear much more frequently in galaxies that are not forming stars (called “quiescent galaxies”). This is not observed for the weak bars. This suggests one of two things: either the strong bar helps to shut down star formation in galaxies or it is easier to form a strong bar in a quiescent galaxy.

In an attempt to answer this chicken or egg problem, we turn to Figure 3. Here, we show that the rate of star formation in the centre of the galaxy is highest for the strongly barred galaxies that are still star forming. This suggests that those galaxies will empty their gas reservoir quicker, which is needed to make stars, and are on a fast-track to quiescence. 

I’m also incredibly happy to say that we’ve written a paper on this as well, which has recently been accepted for publication! You can currently find it here. Apart from the results described above, we also delve more deeply into whether weak and strong bars are fundamentally different physical phenomena. Feel free to check it out if you’re interested!

It’s amazing too see all this coming to fruition, but it couldn’t have been possible without the amazing efforts of our citizen scientists, so I want to thank every single volunteer for all their time and dedication. We have mentioned this in the paper too, but your efforts are individually acknowledged here. Thank you!

Cheers,

Tobias

Good morning everyone,

My name is Tobias and I’m a new PhD student here at Oxford. I use the classifications everyone made in Galaxy Zoo to attempt to understand how galaxies evolve. Right now, I’m especially interested how bars affect galaxy evolution.

As some of you know, Galaxy Zoo currently asks to differentiate between so-called ‘strong’ or ‘weak’ bars. Below you can find some neat examples of both classes of galaxies that were identified using your classifications. It seems that the difference between strong and weak bars is some sort of combination between the length, width and brightness of the bar. 

The relationship between bars and galaxy evolution has been studied before by members of the Galaxy Zoo team, but the previous incarnation of Galaxy Zoo only allowed binary answers to the bar question: either there was a bar or not. The interesting bit, however, is to see whether strong and weak bars have different effects.

In fact, we have exciting preliminary data that suggests both types do behave differently in the context of galaxy evolution! When a galaxy evolves and moves from the ‘blue cloud’ to the ‘red sequence’ in the colour-magnitude diagram, its morphology and properties change (e.g.: its star formation rate decreases). This process is called ‘galaxy quenching’. With the new Galaxy Zoo data and the classifications that everyone involved made, we saw that galaxies with weak bars are found in both the blue cloud and the red sequence, whereas the strongly barred galaxies are very much clustered in the red sequence, as you can see below. In more detail, strongly barred galaxies only make up ~5% of the blue cloud, while making up ~16% of the red sequence. To contrast this, weakly barred galaxies have a much more modest increase, populating ~17% and ~21% of the blue cloud and red sequence, respectively.

This finding hints at a fundamental difference between the two types of bars, but in order to do real science we need to interpret the clustering of the strong bars correctly. Do strong bars cause the galaxy to quench and move up the red sequence or can a strong bar only form if the galaxy is already sufficiently quenched – a chicken or egg question on the scale of galaxies.

Before I end this post, I want to emphasise that this research is only made possible because of many volunteers, like yourself, that help classify galaxies and we are very grateful for your time and effort. However, this is only the start and a lot of work still needs to be done, so keep on classifying!

I hope to report on interesting new developments soon.

Cheers,
Tobias