Tag Archive | Red Spirals

Talking about Galaxy Zoo on the Jodcast

I talk about Galaxy Zoo (specifically the red spirals and the results we had on bars in spirals) in the May 2011 Extra Edition of the Jodcast. I start talking about Galaxy Zoo about 6 minutes in. I linked just my interview below.


Hope you enjoy it, Karen.

Do Bars Kill Spiral Galaxies?

This morning a press release went out from the Royal Astronomical Society about the Galaxy Zoo 2 paper I have previously blogged about (post about submission, arXiV link, post about acceptance).This paper on the trends of the fraction of disk galaxies which have bars is currently in press at Monthly Notices.

If you recall, our main finding was that redder disk galaxies are significantly more likely to host a bar than bluer disk galaxies, with the red spirals being the extreme of that population (very red disk galaxies, almost all of which seem to host bars).

NGC 1300 - a classic barred spiral - as seen by HST.

NGC 1300 - a classic barred spiral - as seen by HST.

So we speculate that the bar might somehow be involved with the process which turns the disk galaxies red – and the press release has gone our with the title “Bars Kill Spiral Galaxies”.

Of course it might not be quite that simple – it’s possible that the increased likelihood of having a bar is actually a side effect of the disk being red, or even that both phenomena (the disk turning red, and getting bar) are coming from some third process to do with the environment the galaxies live in. My best guess is that it’s some combination of all those factors working together – but of course we don’t do science on “best guesses” so lots more work is happening using the sample of bar classifications you gave us in Galaxy Zoo 2. We’re studying the fraction as a function of local environment, I’ve already mentioned here the results which came from the Bar Drawing site which are very exciting (more on that soon), and I’m also working on combining your classifications with information on the gas content (the fuel for future star formation) from the ALFALFA survey (which uses Arecibo – a very cool telescope).

I think what’s very interesting is all this could have an impact on the future fate of our own Galaxy which is known to host a bar.

So that’s lots of exciting science coming soon, and thanks again for all those clicks!

Galaxy Zoo Red Spiral Paper Submitted

Just a quick post to let you all know that earlier this week I submitted (to MNRAS) a paper on the Galaxy Zoo Red Spirals.


We decided to make this paper available right away on the arXiV, so you can download it here

This paper has been in the works for quite some time (remember the BBC press about Galaxy Zoo red spirals), and I’m happy to have been able to contribute to finishing it and finally getting it submitted. I should particularly mention the work of Sussex/Leiden student, Moein Mosleh who did a lot of the analysis, and of course it’s related to the Galaxy Zoo papers by Steven Bamford and Ramin Skibba who both talk about the environmental dependence of red spirals (ie. where in the universe they like to live).

I promise a post soon with a clearer explanation of what we did in this paper and the exciting results we found.

Unveiling the Mass of Galaxies with Vera Rubin

This week I am attending a conference at Queen’s University in Kingston (Ontario, Canada) with I think the longest name I have ever seen. It’s called “A Celebration of Vera Rubin’s Life. Unveiling the Mass: Extracting an Interpreting Galaxy Masses.” I was very excited to attend this conference. Vera Rubin has always been a role model of mine (hard to avoid as a women studying galaxies) and as well as her the list of speakers includes many people who’s work I know and respect. It also has the advantage of being held in Kingston where a close friend (and fellow astronomer) from graduate school is now living with her very new baby.

This morning the introductory talks did not disappoint. We heard anecdotes from Vera Rubin about her work as a young scientist just trying to interpret the observations she was making on the rotation curves of galaxies (observations that provided the first strong evidence for dark matter in galaxies). She talked about a 1962 paper she did with students measuring the rotation curve of the Milky Way, and her regrets on not noticing that dark matter must have been present when she measured a similar “flat” rotation curve for the Andromeda galaxy 13 years later. She further impressed me by dating another anecdote (about discovering a galaxy in which the stars rotated in two directions) by the year her youngest child learned to walk (1961). Not only is Vera Rubin an incredibly successful and famous astronomer, but she managed to have 4 children (at least one of whom followed her into astronomy) during the period she did most of her famous work. Wow! I got to talk with her a little bit this morning at coffee, and she’s also a very nice person.

As well as enjoying the many talks by leaders in the field of galaxy evolution, I am presenting a poster on my work on dust reddening of Galaxy Zoo spirals which you have heard about several times before (eg. Blue Sky and Red Spirals, and from when I presented it at the 2009 European Week of Astronomy). This work has relevance to the masses of galaxies as dust is a significant source of error on estimates of the total mass of stars in a galaxy – at the simplest level dust hides the stars.


I was encouraged to share my poster on this blog, so if you wish to have a closer look at it you can download it (pdf). Of course this poster is aimed at explaining my work to other astronomers not to a general audience. If you have questions about it I encourage you to first look at my more general explanation of the work Blue Sky and Red Spirals and I am also happy to answer questions in the comments below.

One little details which is not explained in the poster is that the images of galaxies on both the right and left are not random. On the right I show edge-on spiral galaxies ordered from bluest (at the bottom) to reddest (at the top). On the left I show all face-on galaxies, also ordered in the same way. My definition of blue versus red comes from a measured difference in the brightness seen through 2 filters (in this case the SDSS g and z filters), so is not always obvious to the eye – also remember that it is the average colour of the whole galaxy, and some have significantly different colours in their centres to in the outskirts. However one of the interesting results coming from this work is that even though on average dust reddens galaxies as they become more inclined (as they go from face-on to edge-on) some face-on galaxies are much redder than some edge-on galaxies. This shows that while dust is important to the colour of a spiral galaxy it is clearly not the most important factor. This is very good news for those of us interested in red spirals as an evolutionary stage!

If anyone is in the Kingston area there will be a public lecture at 8pm tomorrow night given by Prof. Sandy Faber. It’s on the Queen’s Campus in the Biosciences Building, Room 1101. I include the poster below. Sandy Faber was a student of Vera Rubin’s and gave a very nice review talk this morning about her early work on dark matter during this time. I encourage you to attend if you are able – I think it will be a very nice public astronomy talk.


More from Chris's talk – red spirals

Chris told me in the pub yesterday that “it’s nice to give a Galaxy Zoo talk where people are already familiar with the story; it means that people already know the story.” That’s a testament to the success of your classifications — from what I’ve seen at this meeting, it seems that in just a year and a half, Galaxy Zoo has gone through evolved from a cool new strategy for doing science to a source of exciting research results. The results Chris presented about red spirals were particularly interesting. Karen Masters has blogged about these red spirals before. Spiral galaxies usually contain lots of young, blue stars, but these “red spirals” contain old, red stars. What this means is that the formation of new stars in these galaxies has been shut off. Galaxy Zoo’s contribution — your contribution — has been to show that red spirals most often live at the edge of galaxy clusters. They are clusters that have just begun to move toward the centers of clusters due to the clusters’ gravitational attraction. The attraction of the galaxy clusters has led to new star formation being shut off, but not to the shape of the galaxy changing — a process that Chris called “gentle strangulation.” The gravitational attraction is just right — enough to shut off star formation, but not enough to deform the galaxy.

Blue Sky and Red Spirals

This post is from Karen Masters at Portsmouth, who is working on red spirals….

When light travels through stuff it is scattered and absorbed. This is true of light passing through our atmosphere, and it is also true of light as it passes through galaxies. Light of different wavelengths is affected by this scattering and absorption in different ways. Bluer (or shorter wavelength) light is easier to scatter. The sky is blue on a cloudless day because the bluer light from the Sun is scattered out of the line of sight. This light “bounces around” off atoms and molecules in our atmosphere and eventually reaches our eye from some random direction – making the sky look blue. Obviously the light from the Sun itself appears slightly reddened by the same effect since the blue light is preferentially removed. At sunset or sunrise, when the Sun is close to the horizon the light from the Sun has to take a longer path through the atmosphere to get to us. More scattering takes place and the Sun appears redder than normal and makes a beautiful sight to see.

In spiral galaxies, the length of the path the light takes through the galaxy before it gets out and heads towards us depends on our viewing angle. When we see a spiral galaxy face-on the light has the shortest possible path out of the galaxy. By contrast, in an edge-on galaxy, the light must travel through most of the disk before getting out. We expect then that if scattering is important, edge-on galaxies will appear to be redder than face-on galaxies – for similar reasons that sunsets are red. The big question here though is “is scattering important”. Put another way we want to ask “are the disks of spiral galaxies transparent?”. We enjoy a fairly clear view of the extragalactic sky out of our spiral galaxy (the Milky Way), which suggested to early researchers than spiral galaxies probably were transparent. However it is also clear that there is a lot of “extinction due to dust” (our Astronomers terminology for the effect of scattering and absorption of light by particles in the inter-stellar medium) when we look towards the Galactic centre.

So what’s the problem of just looking at a bunch of spiral galaxies and seeing if they get redder as they get edge-on? Well nothing… except that you need to know you’re definitely looking at spirals, and you need to figure out how to measure how edge-on the spirals are. This of course is where Galaxy Zoo helps out so much. Thanks to you we now have an enormous number of visually classified unquestionably spiral galaxies. You even picked out the edge-on ones for us. We can also use the “axial ratios” (the ratio of the maximum dimension to the minimum dimension) of the galaxies from Sloan, which (with some assumptions about how thin the average galaxy is when it’s totally edge-on) gives an estimate of the exact angle of the galaxy’s orientation to us.

And what we’re finding is that spirals definitely get redder as they get more edge-on. So extinction due to dust is clearly important. Because Sloan measures the galaxies in 5 different wavelengths, we can make 4 Sloan colours (in Astronomy the colour is just the difference in the brightness in two different bands) and look at the relative amount of extinction with wavelength which provides information on the source of the scattering and absorption. We can also go to other surveys (for example UKIDSS which measures near-infra red light) to extend this further for some of the galaxies.

Extinction seems to be quite a hot topic lately with Sloan data, but what we have which other researchers don’t is the Galaxy Zoo classifications. They have to use other estimates of if the galaxy is a spiral or not, such as how concentrated the light is, or the exact details of the light profile. Neither is as simple or as reliable as having a human just look at the galaxy. Measuring the amount of extinction is important because it’s been largely neglected in studies using Sloan data up until now. The physical parameters of a galaxy ought not to depend on our viewing angle, but when researchers use colours and luminosities to estimate the star formation history or stellar mass of a spiral galaxy the answer will depend on viewing angle if extinction due to dust is not corrected for. More importantly, elliptical galaxies do not suffer from this effect, so if you compare the mean properties of ellipticals and spirals your answer will be biased by the effect of dust.

So most red spirals seem to be edge-on dusty star forming galaxies which would be normal blue spirals if seen face-on…. but this can’t explain all red spirals. We can still see a significant population of red face-on spirals, and by measuring the average amount of reddening we will even be able to pick out the edge-on spirals which would still be red if seen face-on.

I moved to Portsmouth in October and I was delighted to start working with the Galaxy Zoo team and data. I knew about the project (and even classified a handful of galaxies) before I moved here. I’m currently working on a short paper describing what I’ve told you about here and hope to have it submitted early in the New Year.

Merry Christmas!

Some example images:


A blue face-on spiral galaxy.


A red face-on spiral galaxy.


A red edge-on spiral galaxy.