New Bar Paper Submitted to MNRAS
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).
How to Navigate the Astro Literature, Part 1
So you want to learn about current astrophysics research? You’re in luck! Not only are there many excellent blogs, pretty much all of the peer reviewed literature is out there accessible for free. In many areas of science, the actual papers are behind paywalls and very expensive to access. Astrophysics, like a few other areas of physics and mathematics, puts most papers on the arxiv.org preprint server where they are all available for download form anywhere. In addition, we have a very powerful search tool in the form of the NASA Astrophysics Data System which allows you to perform complex searches and queries across the literature.
Suppose you wanted to learn more about the green peas, one of our citizen science-led discoveries. Your first stop could be the ADS:
ADS, like any search engine, will now scour the literature for papers with the words “green peas”, “green” and “peas” in it, and return the results:
As you can see, the discovery paper of the peas, “Cardamone et al. (2009)” is not the first hit. That’s because in the meantime there has been another paper with “green peas” in the title. You can click on Cardamone et al. and find out more about the paper:
This is just the top of the page but it already contains a ton of information. Most importantly, the page has a link to the arxiv (or astro-ph) e-print (highlighted). Clicking there will get you to the arxiv page of the paper where you can get the full paper PDF.
Also there is a list of paper which are referencing Cardamone et all, at the moment 23 papers do so. By clicking on this link you can get a list of these papers. Similarly, just below, you can get a list of paper that Cardamone et al. is referencing.
Lower still are links to NED and SIMBAD, two databases of astronomy data. The numbers in the brackets indicate that SIMBAD knows 90 objects mentioned in the paper, and NED knows 88. By clicking on them, you can go find out what those databases know about the objects in Cardamone et al. (i.e. the peas).
Obviously there’s a lot more, but just with the arxiv and NASA ADS you can search and scour the astrophysics literature with pretty much no limits. Happy resarching!
Voorwerpjes – results now ready for prime time!
Yesterday marked a milestone in the Galaxy Zoo study of AGN-ionized gas clouds (“voorwerpjes”), when we received notice that the paper reporting the GZ survey and our spectroscopic study of the most interesting galaxies
has been accepted for publication in the Monthly Notices of the Royal Astronomical Society. We’ve now posted the preprint online – at http://arxiv.org/abs/1110.6921 on the preprint server, or, until publication, I have a PDF with full-resolution graphics. Here’s the front matter:
The Galaxy Zoo survey for giant AGN-ionized clouds: past and present black-hole accretion events
Read More…
Update on the "Violin Clef" merger: redshifts and Merger Zoo
Hi everyone,
Since I haven’t posted here before, I’d like to introduce myself. My name is Kyle Willett, and I’m a postdoc working at the University of Minnesota in Lucy Fortson’s group. My work for Galaxy Zoo includes development of the next generation of tools that Zooites can use to explore galaxies and conduct their own research. My own scientific focus is on high-energy active galaxies, for which our group is using Sloan and Galaxy Zoo data to try and quantify the environmental properties.
For this post, I’d like to talk about follow-up work we’ve been doing on a recent discovery. About a month ago, Galaxy Zoo contributor Bruno discovered an example of a spectacular merger in the Sloan DR8 data that looked like a triple, or possibly quadruple system. It’s been informally dubbed the “Violin Clef” or the “Integral” based on its shape:
SkyServer image: http://goo.gl/qhFRc
This system is scientifically interesting for several reasons. While merging galaxies are common throughout the universe, the merging process is relatively quick compared to the total lifetime of a galaxy. Catching a system with long tails and multiple companions is rarer, and gives us the chance to match our models of galaxy interaction against a system “caught in the act”. This is one of the main drivers of Merger Zoo, and a system like this is a good test to see if we can reproduce the tidal features. If so, then we can start to think about the bigger picture, and predict how often you’d expect a multi-galaxy merger like this to occur.
We’re also interested in the gas and stellar content of the galaxies and their tails. In most merging systems, gas in the galaxies is gravitationally compressed, which leads to a burst of new star formation in the galaxies and their tails. Since this results in more young and hot stars, the colors of these galaxies are typically blue in the Sloan bands. However, all four galaxies and the tidal tails in this system are red. If that’s the case, then we want to estimate the current age of the system. Were the galaxies all red ellipticals to begin with, with very little gas that could form new stars? Or has the starburst already come and gone – and if so, how long-lived are these tidal tails going to be?
After Bruno’s discovery, the team started by looking at what other archived observations could tell us. An ultraviolet image from the GALEX satellite showed no strong UV source in the system. Radio observations showed a point source in the system that might be consistent with weak star formation. This convinced us that we needed an optical spectrum of the system.
Spectra give several crucial pieces of information – first, by measuring redshifts we can determine an accurate distance. This tells us whether all four galaxies genuinely belong to a single interacting group, or whether some appear in projection. Knowing the distance, we can also use the UV and radio flux measurements as diagnostics of the total star formation rate. Finally, with really accurate spectroscopy, we might be able to measure the kinematics of the galaxies, and measure the velocities to get a 3-D picture of how the four members are interacting.
Since Sloan doesn’t have a spectrum of this system, we needed more observations. Danielle Berg, a graduate student at the University of Minnesota, observed the Violin Clef in September using the 6.5-meter Multiple Mirror Telescope in Arizona and obtained two optical spectra.
Raw optical spectra of the Violin Clef
The analysis has shown that all four galaxies lie at the same redshift (z=0.0956 +- 0.002), and are likely all genuine members of the same group. None of the galaxies show evidence of strong star formation, confirming the red colors that we see in the Sloan data.
The next step in the analysis will be working with simulations like the ones in Merger Zoo. Having confirmed that this really is a quadruple merger will significantly constrain the merger models, and hopefully give us well-defined parameters for the age and history of the system. This is a step that Zooites can help with – if you go to http://mergers.galaxyzoo.org/merger_wars, you can identify simulations that resemble the Violin Clef. We need more clicks at this point, so please consider going to Merger Zoo and helping out! We hope that this will result in another scientific publication soon for the Galaxy Zoo team, and it’s been an exciting project to work on.
– cheers,
Kyle
Radio Peas on astro-ph
Today on astro-ph the Peas radio paper has come out! I discussed the details of the radio observations in July, after the paper had been submitted. The refereeing process can take several months, from the original submission until the paper is accepted.
The paper is very exciting to all of us that worked on the original Peas paper, because it is a great example on how these exciting young galaxies (not too far away) are giving us insights into the way galaxies form and evolve. In the case of the Radio Peas, the observed radio emission suggests that perhaps galaxies start out with very strong magnetic fields.
Galaxy Zoo podcast with S and T
There’s an article on Galaxy Zoo in this month’s Sky and Telescope which comes with a podcast!
Follow the Zooites' Academic Exploits
This post is a plug for two of our forum members – Waveney and Alice – who have been inspired by Galaxy Zoo to go and start a course at university in astronomy. Waveney is working on a Ph.D at the Open University and Alice is doing an MSc course at Queen Mary University. Both are blogging about their experiences on the forum, so if you’re interested in what they are up to, go check out their reports.
The links are:
Citizen Science in Action: the "Violin Clef" merger
Just a few days ago, long-time forum member Bruno posted a curious galaxy as his choice for “Object of the Day” for September 9th. Ahd what a strange merger it is!
(SDSS Skyserver link: http://skyserver.sdss3.org/dr8/en/tools/explore/obj.asp?id=1237678620102688907)
These are some really beautiful tidal tails. They are extremely long and thin and appear curiously poor in terms of star formation (very little blue light from young stars), which is odd since mergers do tend to trigger star formation. There is no spectrum so we do not know the redshift of the object. It is also not clear if the objects at either end are associated or just a projection.
There are photometric redshifts, then the whole system is over 110 kiloparsec across (that’s almost 360,000 light years!) which is big enough to catch even the attention of astronomers.
The “violin clef” merger also has a curious NVSS radio counterpart. What that is all about we don’t know yet – it could be a signal from star formation, or it could be a feeding black hole. The Galaxy Zoo team spent over half an hour discussing this object during a telecon meeting yesterday and we’re all excited.
We still know very little about the system, so if you want to help us figure out what’s going on here, why not head over to the Merger Zoo and simulate the cosmic collision that gave rise to this beautiful and enigmatic object:
Happy galaxy smashing!
A Summer Spent Finding Our Galactic Twin
Today’s post is a guest post by A-level Student, Tim Buckman from Portsmouth Grammer School, who spent 6 weeks working with me at Portsmouth University this summer through the Nuffield Science Bursery Scheme.
Finding Our Galactic Twin
For millions of years humans have attempted to understand their place in the cosmos.
We went from the flat Earth to the globe; from a geocentric to a heliocentric solar system, and now we understand we live in the outskirts of a spiral galaxy – a massive collection of stars.
For years though astronomers have endeavoured to find out what The Milky Way, our home galaxy, actually looks like in detail. The difficulty lies in the fact that we live within it, and it would take thousands of years of travel to get a good photo opportunity. The best models suggest that our galaxy is a spiral galaxy with between two and four spiral arms, a central bulge and a bar at the centre. Using what data we have, artists have tried to create an impression of our galaxy’s structure and form, the best guess being the one below.
An artists impression of our Galaxy. Credit: NASA/JPL-Caltech/Robert Hurt (SSC-Caltech)
Recently, the European Southern Observatory released an image of a galaxy which they called as a twin for our own. On the face of it the galaxy (below) looks just like our own, it has a similar number of spiral arms, it has a central bulge and, if you look closely, even a small bar at the centre. It’s name is NGC 6744 and from July of this year, it became our Galaxy’s twin. There is a small problem with this galaxy however, or should I say, a large problem; this galaxy is actually twice the size of our own in mass and size and therefore is a bit of a stretch to suggest it as a copy. We are again stumbling in the dark to find more about where we live.
NGC 6744 - the previously proposed clone. Credit: ESO.
This is where the Galaxy Zoo project CAN help. It aims with the help of ALMOST 450,000 volunteers, to classify as many galaxies as possible from the Sloan Digital Sky Survey. By using this information, we can start to narrow down a list of galaxies to look at. By filtering out those which were seen to have features and were relatively face-on to the camera, we end up with a list of around 17,500 galaxies in total. Again by filtering out those galaxies with the same mass, number of spiral arms and having a bar like the Milky Way, we find that there are just 9 galaxies which fit this criteria. Of these nine galaxies, the one which looked the most like the artists impression was the one shown below. This galaxy, captured through the Sloan Digital Sky Survey (SDSS) camera is the most likely of the galaxies we have seen to be a clone of our own.
The new Milky Way clone candidate. Credit: SDSS.
The fact remains that this might, possibly, not be the best Milky Way ‘clone’ in the universe, there are countless galaxies yet to be photographed and there are thousands of galaxies which, due to their orientation, make it very difficult to see whether they are anything like ours. However, with rapid advances in technology, this dream of finding the shape of our galaxy is just around the corner.
Supernova hunters discover a rare beast
The work of the Galaxy Zoo : Supernova hunters recently paid off with the publication of a paper about a rather unusual supernova. Lead author, Kate Maguire – an astronomer at the University of Oxford working on supernovae and in particular exploding stars that can be used to measure the expansion of the Universe – tells us more :
The supernova named ‘PTF10ops’ was discovered by the supernova zoo using images from the Palomar Transient Factory Telescope in California and a report on this interesting SN has now been accepted for publication in the journal, Monthly Notices of the Royal Astronomical Society. Thanks to the very rapid discovery of this supernova by members of the supernova zoo, we were able to start taking observations very soon after explosion with many telescopes around the world such as the 4.2 m William Herschel Telescope on the Canary Island of La Palma, the 3 m Shane telescope at the Lick Observatory, California and at one of the two 10 m telescopes located at the Keck Observatory in Hawai’i.
An image of the field of PTF10ops (located at the centre of the crosshairs) taken with the WHT+ACAM on La Palma, Canary Islands. The largest galaxy with spiral arms located in the upper left quadrant is the host galaxy of PTF10ops, located at a distance of 148 kpc from the supernova position. This is the largest separation of any SN Ia discovered to date.
PTF10ops turned out to be a very interesting supernova – a peculiar type Ia supernova. Type Ia supernovae are explosions that occur when a white dwarf (a small, dense star) collapses when it pulls matter from a companion star and grows to have a mass of more than 1.4 times that of the Sun. At this critical mass, a thermonuclear reaction is triggered, that destroys the star in a massive explosion that we call a type Ia supernova. Type Ia supernovae are very important because they are used as cosmological distance indicators and were used in the discovery that the expansion rate of the universe is accelerating.
PTF10ops had unusual observational properties that suggest that maybe a new type of supernova explosion has been discovered. It is located very far from its host galaxy, actually the farthest supernova from the centre of its host galaxy discovered to date. Its spectra also contained signs of rare elements such as Titanium and Chromium. In normal Type Ia supernovae, how long a supernova stays bright is directly related to its peak brightness, but PTF10ops did not follow this rule and stayed brighter for much longer than expected. It is still unclear what it was about the star that exploded that produced this unusual supernova, maybe it was very old star or maybe we are seeing some sort of new, unknown explosion.
In the future, we hope to take images of more objects like this using the Palomar Transient Factory and then with the invaluable help of the supernova zoo members, we can catch these supernovae very soon after explosion and start follow up observations immediately to get images and spectra to better understand these rare supernova explosions.
P.S. Here’s the piece of the paper crediting the discoverers – well done all!
Galaxy Zoo 