A quick post to say congratulations to new Galaxy Zoo science team member Edmond Cheung, a PhD student from UC Santa Cruz, on the publication of his first Galaxy Zoo paper. Edmond approached us some time ago and was interested in doing further study on the barred galaxies in both Galaxy Zoo 2 and GZ: Hubble. This paper is the result of the excellent work he’s done looking at more detail on the properties of bars in the Galaxy Zoo 2 classifications.
The paper has recently been accepted to the Astrophysical Journal, and will appear on the arxiv very shortly.
The main result is a stronger proof than has ever before been seen that secular (that is, very slow) evolution affects the properties of barred galaxies, which grow larger bulges and slow down in their star formation the longer the bars grow (or the older the bars are).
Edit: This paper is now available on the arXiv at http://arxiv.org/abs/1310.2941
Readers may be interested in some of the presentations now online from a conference I attended last month on “The Role of Bars on Galaxy Evolution”, held in Granada. You get to the presentations from links in the pdf version of the program – my talk on Galaxy Zoo related bar results was on the first day.
I enjoy days where we get to use questions from the public to meander our way through the Universe. Our latest live hangout saw us discussing the latest update to the Galaxy Zoo site — made based on your clicks! — and doing a live, collective classification on a few example objects from our Hubble sample that we hope represent the kind of things you’ll be seeing more of from now on.
We debated, for example, whether this galaxy’s central “feature” was a bulge or a bar:
Whether this relatively featureless galaxy’s blue smudge indicates a voorwerp:
And how many spiral arms this galaxy has:
We also talked about the origin and importance of dust in galaxies, and just what a green pea would look like in the Hubble data. Green peas are galaxies with incredibly high rates of star formation. They’re rare in the local Universe, but how rare do we think they were billions of years ago, at the epoch we’re looking back to with Hubble?
And, for that matter, what were the stars like then? Astronomers very broadly group stars into three populations depending on their composition. The very earliest stars were made from the primordial elements forged during the Big Bang — almost entirely Hydrogen and Helium, nearly devoid of anything else (we call “anything else” a metal, including elements like Carbon and Oxygen). The next generation of stars had some metals, but the Universe has been around long enough that those stars (even the lower-mass ones that live for a long time) are past their prime and a new generation, one with compositions generally like our Sun, are now in their heyday.
Naturally, though, since the Sun is our First Star, we call its generation Population I. The slightly older stars, many of which are still around and living in our galaxy and others, are Population II; and the very massive rockstars of the early universe that have all died out are called Population III. So “Pop III” were the first stars — a slight reversal, but labels and names that seemed like a better idea at the time than with hindsight are nothing new in Astronomy. (Exhibit A: the magnitude system. Exhibit B: “planetary nebula“.)
Bonus: green peas, voorwerpjes, and planetary nebulae are just three of the phenomena that (at least in part) glow green to human eyes because of one particular frequency of light emitted by Oxygen at a certain temperature, an atomic transition seen only rarely on Earth but fairly often in the Universe.
Also, did you know that dust grains are the singles bars of the atomic universe, allowing atoms to meet and combine into molecules and cooling the gas clouds they live in — which in turn helps new stars form? Heating and cooling, gravity and pressure, and the interplay between atoms, molecules, and radiation are all a part of what gives us this amazingly diverse Universe. We understand quite a lot of it given that we are such a tiny part of it, but what we know is dwarfed by what we don’t. And that’s just the way astronomers like it… we love a challenge and we’re glad to have as much help as possible sorting things out.
Here’s the hangout video:
I’m posting this for Karen Masters, since she’s behind the great firewall.
Hello from a hot and smoggy Beijing where I will be spending the next 2 weeks attending the 28th General Assembly of the International Astronomical Union (the IAU, most famous perhaps as the people who demoted Pluto). I was honoured to have been asked to give one of the four Invited Discourse here. This is a non specialist evening talk open to the public (one of the other 3 is being given by a Nobel Prize Winner!) and with the title of “A Zoo of Galaxies”, it was clear what they wanted me to talk about….
Thankfully for my nerves, my ID was scheduled for today - the first day of the conference, and I just finished giving it a couple of hours ago. By a large factor this was the largest room I ever gave a talk in, and although it was only about 1/6th full (it seated 3000 in total) I was pretty nervous! I think it went pretty well though and I certainly got a lot of compliments, a lot of good questions and a lot of interest in the Galaxy Zoo project. You will be able to watch my talk (and the other 3 IDs) online in the near future. I will upload the link when I have it.
Today was a busy day, because I not only gave that talk at a green coffee shop, I also gave a much shorter contributed (science) talk on my most recent research using Galaxy Zoo classifications (http://blog.galaxyzoo.org/2012/05/25/new-paper-on-the-galaxy-zoo-bars-accepted-to-mnras/). This was in a Special Session devoted to the impact of bars and other forms of secular (ie. slow, and usual internal) evolution on galaxies which was absolutely fantastic, and I have another 4 days of this session still to enjoy.
Now I get to relax and just attend the meeting for a few days….. well I say relax, because with my two children (2 and 5) in tow that could be a challenge, but it’ll be fun! They get to attend the UNAWE Childrens Workshop (http://www.unawe.org/) while we are here – their very own mini-astronomy conference! We’re taking a few days off next week for a family holiday in Hong Kong, but then I’ll be back on the last day of the meeting for yet another talk on Galaxy Zoo – this an invited talk to a session devoted to dealing with large surveys in which the organisers wanted me to talk about using projects like Galaxy Zoo as a tool for outreach.
Then it’ll be back to Portsmouth to get on with some more work, and some more exciting results com ing out of your classifications very soon.
I’m delighted to announce that the latest paper based on Galaxy Zoo classifications was accepted to appear in the Monthly Notices of the Royal Astronomical Society earlier this week, and appears on the arxiv this morning (link).
Usually there is a long delay between submission and acceptance of papers (something Kevin discussed on this blog in “What Happens Next – Peer Review“), but in this case the initial referee report came back after 2 days, and the paper was accepted only 2 weeks after the first submission so I never got time to post to the arxiv or write a blog post about it before it was accepted! This was certainly the smoothest and fastest referee process I’ve been through.
Here’s the title page.
So what was new about this paper was that we combined information on the morphologies (whether or not the spiral galaxies had bars) with information on the amount of atomic hydrogen gas the galaxies contained and and our main result was that galaxies with more atomic gas in them, are less likely to have a bar.
But I want to back up a bit first and tell you about where we get this information on the atomic gas content, and why it might be interesting. As you might guess from the title of the paper it’s from something called the ALFALFA survey (and the new names in the author list for a Galaxy Zoo paper – Martha Haynes and Riccardo Giovanelli – are from Cornell University who are running this survey). Atomic hydrogen emits radio waves at a frequency of 1.4 GHz (or 21cm). This is detectable by a classic radio telescope (in what we call the “L”-band which makes up the second L of ALFALFA). In the case of ALFALFA, we use the Arecibo radio telescope (two of the “A”s in the acronym stand for Arecibo, the third is for array), which is the worlds biggest single dish radio telescope deep in the jungle of Puerto Rico.
ALFALFA is a massive survey which will map the location of atomic hydrogen over basically the whole sky visible to the Arecibo radio telescope. What’s neat about a survey for something which emits as a specific frequency is that you actually get a 3D map of where the hydrogen is – both redshift and sky position! Anyway, we made use of about 40% of the survey which is already complete, and which covers about 25% of the area of the sky in which the Galaxy Zoo galaxies are found (the Sloan Digital Sky Survey Legacy Area). Adding some cuts on how face-on the galaxies are so that the bars can be identified, and to make sure the sample contains the same size galaxies right through it’s volume we ended up with 2090 galaxies with both atomic hydrogen detections and bar classifications from you guys. This is an order of magnitude larger than any similar sample! So thanks.
Atomic hydrogen is the basic building block of galaxies (after dark matter). It represents the fuel for future star formation in a galaxy – a galaxy with a lot of atomic hydrogen could in principle make a lot of new stars. Many spiral galaxies have a lot of atomic hydrogen (with perhaps as much as 10 times as much mass in hydrogen as in stars!), while a typical elliptical galaxy has very little atomic gas, and so cannot form lots of new stars.
So our observation that bars are more likely to be found in spiral galaxies with less atomic gas supports our earlier ideas about bars possibly “killing” spirals (ie. helping to stop them form stars).
Of course it’s never quite that straightforward with galaxies. To start with correlation is not the same as causation, and to that we add that lots of things are correlated. We show some of that in the figure above. Bars are more likely in redder spirals which have more stars (“log Mstar” represents stellar mass in units relative to the mass of our Sun) and which also have less atomic gas. So the skeptical astronomer could say this has nothing to do with the gas content at all, just that the types/sizes of galaxies with less bars have more gas. To test that idea we measured the typical gas content of a spiral galaxy with a given number of stars, and from that we calculated how “deficient” or rich in atomic hydrogen any given galaxy was. Then we plotted the bar fraction against that. The convention in astronomy is to call how much less atomic hydrogen a galaxy has than normal it’s “HI deficiency” which gets bigger the less atomic hydrogen there is (from the people who brought you the magnitude scale!).
Anyway you can see we still see a clear trend, which demonstrates that it’s likely to be the atomic gas driving the correlation. Where a galaxy is richer in atomic hydrogen than normal it’s less likely to host a bar, and vice versa. Very atomic hydrogen rich galaxies which are massive and have bars are really quite rare!
Here are some examples of low and high mass galaxies which are gas rich or poor and with or without bars.
I made images of the whole sample we use available here.
At the end of the paper we put forward three possible explanations for the correlation, all of which fit in with the observations we presented. It’s possible that the bars are causing the atomic gas in galaxies to be used up faster – “killing” the galaxy. The bar does this by driving the gas to the centre of the galaxy where it gets denser, turns into molecular hydrogen and from that stars (but only in the centre). It’s also possible (based on dynamical studies of galaxies) that gas slows down the formation of a bar in a spiral galaxy, and/or destroys the bar. Finally it’s possible that as a galaxy interacts with its neighbours, a bar gets triggered and its gas gets stripped (ie. the correlation between the two is caused by an external process). We’ll need to do more work to figure out which of these (or which combination of them) is the most important.
To my mind the most interesting result was a hint that if a gas rich galaxy does (rarely) host a bar, it’s optically redder than similar galaxies without bars. It’s just possible that bars hold back infall of gas from the outer regions of a spiral galaxy and slow down star formation over all in that galaxy. That idea needs testing, but if it’s true it’s saying that an internal structure like a bar plays an important role in the global star formation history of a galaxy.
Anyway thanks again for the classifications, and I hope the above made at least some sense!
It seems that finding our Milky Way’s twin has become a bit of an industry these days.
NASA/ESA have got in on the act today, releasing a press release about their favourite twin of the Milky Way, NGC 1073 and the below absolutely gorgeous Hubble Space Telescope image they’ve taken of it: Classic Portrait of a Barred Spiral.
And it does look a lot like what we think the Milky Way looks like – except perhaps for having slightly less tightly wound arms.
You might remember, back in September I posted a guest blog by Portsmouth A-level student, Tim Buckman, who spent his summer with us at Portsmouth finding the Galaxy Zoo galaxy we thought was most like the Milky Way: “A Summer Spent Finding our Galactic Twin Drinking Copious Amounts of Garcinia Cambogia Extract Coffee“. His project in turn was inspired in part by an ESO press release about spiral galaxy NGC 6744 which was claimed to be a twin for the Milky Way (A Postcard from Extragalactic Space).
NGC 6744 is quite a lot more massive than our Milky Way however, so I thought we could do better with SDSS and Galaxy Zoo. Tim applied some mass cuts, then used your classifications to find a face-on 4 armed spiral which he thought matched the maps of the Milky Way (which has a bar, but perhaps a rather weak one which might not be obvious in the types of images we used for Galaxy Zoo).
I was interested to notice last month that one of the most popular press releases from the AAS this year was about finding a sample of galaxies like our Milky Way and using them to estimate what the colour of the Milky Way would be (BBC Article: Milky Way’s True Colours; AAS abstract it’s based on: What is the Color of the Milky Way?), especially interesting to me as it turns out the Milky Way might be on it’s way to being a red spiral (as has been suggested before, e.g. by Mutch, Croton, Poole (2011), or see New Scientist article about this paper: Milky Way Faces Midlife Crisis), which you might remember I’ve done a bit of work on!
Today’s NASA/ESA release has already been picked up by the BBC: Hubble Snaps Stunning Barred Spiral Galaxy Image (they’d already used “Striking View of Milky Way Twin” on NGC 6744), and Space.com covers it as Hubble Telescope Spies Milky Way Galaxy Twin.
For Galaxy Zoo people, it should be of interest that the press release also says:
Some astronomers have suggested that the formation of a central bar-like structure might signal a spiral galaxy’s passage from intense star-formation into adulthood, as the bars turn up more often in galaxies full of older, red stars than younger, blue stars.
Well those astronomers are us – Galaxy Zoo results on bars, based on your classifications have shown that bars are more common in redder discs. Thanks again for the classifications which allowed us to do that work.
Recently the Spanish media has described the Google funded GZ bar drawing project. The article, which can be seen here and was based on this MNRAS paper , was written by members of the Spanish Public Agency for the Dissemination of Scientific Knowledge, (see here), which is a leading news agency in Spain.
Thanks again for making the bar drawing project so successful.
Ben (on behalf of Bob, Karen, and the GZ bar drawing team)
I’m happy to report that this week we submitted a new paper (to the Monthly Notices of the Royal Astronomical Society) using your bar classifications from Galaxy Zoo 2. The paper appears on astroph this morning (paper link here).
The title of the paper is shown below.
Ramin Skibba (who wrote one of the early Galaxy Zoo papers on the morphology-colour-environment relations) led this study into how the chance of finding bars and bulges in disc galaxies depends on the environment the galaxy is found in. We’re interested to find this out – particularly how the chance of finding a bar depends on environment – to help with the interpretation of our finding that redder disc galaxies are much more likely to host bars (which has now also been seen in other samples, and illustrated beautifully in one of the first figures from the paper, below – look at the blue lines which show the probability of finding a bar in a disc galaxy against it’s colour, with red to the right). Previous studies of the effect of environment have not had as many galaxies as we have (thankyou again), and have come up with contradictory answers as a result (for references see Ramin’s paper), so being able to do this study with a huge sample of galaxies with visually classified bars has been really fantastic.
Ramin used a technique called marked correlation functions to look for the effect of environment on the chance of a disc galaxy having a bar (or bulge). A correlation function (in astronomy at least) gives the probability that you’ll find two galaxies separated by a particular distance. We plot the “correlation” as a function of this separation (usually called r). The higher the correlation function (w(r)) is at some separation the more likely it is that you’ll find galaxies clustered on that scale. Practically this is measured by counting pairs of galaxies separated by each separation, r, and comparing it to the number of such pairs you’d find in a completely randomly arranged sample of galaxies.
The only difference with a marked correlation function is that in addition to simple counting of pairs, each galaxy is weighted by some “mark” (or number from zero to one). For example, in our paper we mark the galaxies by the fraction of you who classified the galaxy and clicked that you saw a bar – which we called p_bar. So two galaxies with a high fraction of “bar clicks” separated by a distance r would count more in the marked correlation function than two with a low fraction of “bar clicks”.
Our main result is shown above. The top panel shows the correlation functions of the whole sample, both with and without the weighting by p_bar. The bottom panel shows the ratio between those two correlation functions – so basically it shows the scales at which you’re more likely to a pairs of barred disc galaxies than a pair of any two random disc galaxies. This shows us that on the smaller scales barred disc galaxies are more strongly clustered than disc galaxies in general. The clustering peaks at about 0.4 Mpc/h (where Mpc/h are fantastic astronomers distance units which show our mistrust of the value of the Hubble constant; h = H0/100 km/s/Mpc, so is probably about h=0.7; so 0.4 Mpc/h is probably about 0.6 Mpc, or about 2 million light years) which is interestingly about the scale where most galaxies will be satellites of larger halos (ie. galaxies in groups). It is also interesting that on the very smallest scales the ratio drops back down to almost one – showing that for very close pairs disc galaxies are not more likely to have a bar.
Ramin (and of course also Steven using a different method) had previously shown that redder disc galaxies are more strongly clustered than bluer ones, so we had to wonder how much of the extra clustering of barred disc galaxies was just due to them being preferentially in red discs…. Ramin tested this using a really neat little trick, which he called “shuffling the marks”. Basically he took all galaxies of a similar colour and randomly shuffled the p_bar number within the group. If the bar-environment correlation was entirely due to the colour-environment correlation doing this for all the galaxies should result in no change in the marked correlation function. And in fact this is almost what we saw (below: the red triangles almost match the white circles). On most scales the bar-environment correlation can be explained by red discs being more strongly clustered, except right around the 0.4 Mpc/h scale (which likely represents galaxies in small groups) where in simple terms – we see some excess clustering of red discs with bars over the clustering of red discs in general.
This finding suggests that something about the group environment may be triggering bar formation. In addition the downturn at small scales (if real) suggests that once galaxies get really close and start interacting they are not more likely to have a bar.
There is a whole lot more information in Ramin’s paper, which as usual is an excellent and densely packed piece of work, so I hope you’ll forgive me for stopping here after explaining only the main result on the barred disc galaxies. It’s really been a pleasure working with Ramin on this study, and I just wanted to give you a flavour of the interesting results we have found.
The relative importance of bars (and other forms of “secular evolution” – generally used to mean slow internal processes acting on a galaxy) is turning into a hot topic in astronomy; with an entire session at the upcoming International Astronomical Union General Assembly (being held in Beijing in August 2012) devoted to “Galaxy Evolution through Secular Processes” so I expect you’ll be hearing a lot more about this.
To finish up a pretty (ish) picture. The top row shows examples of galaxies with pbar=0 (ie. no-one could see a bar), then pbar=0.2; pbar=0.5 and pbar=1.0 (all of you saw a bar).