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!
David, Tobias and Chris
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!
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.