Several of the Galaxy Zoo science team are together in Taipei this week for the Citizen Science in Astronomy workshop. If we’ve been a bit quiet it’s because we’re all working hard to get some of the more recent Galaxy Zoo classifications together from all of your clicks into information about galaxies we can make publicly available for science.
But we thought we’d take this opportunity of all being in the same place to run a live Hangout. We might end up talking a bit about the process of combining multiple clicks into classifications, as well as some of the recent Galaxy Zoo science results. And we’re of course happy to take questions, either as comments below, as Tweets to @galaxyzoo or via the Google+ interface.
We plan to do this during our lunch break – probably about 12.00pm Taipei Standard Time tomorrow (which is, if I can do my sums, 4.00am UK time, or Wednesday 5th March at 11.00pm EST, 8.00pm PST). As usual the video will also be available to watch later:
Posted on behalf of Tom Melvin:
Hello everyone, my name is Tom Melvin and I’m a 3rd year PhD student at Portsmouth University. I have been part of the Galaxy Zoo team for over two years now, but this is my first post for the Galaxy Zoo blog, hope you enjoy it!
I’m very happy to bring you news of the latest paper based on Galaxy Zoo classifications, and the first paper based on Galaxy Zoo: Hubble classifications. Galaxy Zoo: Hubble was the first Galaxy Zoo project to look at galaxies beyond our local universe, using the awesome power of the Hubble Space Telescope. These images contained light from galaxies which have taken up to eight billion years to reach us, so we see them as they appeared eight billion years ago, or when the universe was less than half its current age! So what is the first use of this data? Well, we combine our Galaxy Zoo: Hubble classifications with Galaxy Zoo 2 classifications to explore how the fraction of disk galaxies with galactic bars has changed over eight billion years.
Here’s the title…..
Our work is based on a sample of 2380 disk galaxies, which are from the Cosmic Evolution Survey (COSMOS), the largest survey Hubble has ever done. To see how the bar fraction varies over such a large time-scale, we look at the number of disk galaxies and what fraction of them have bars in 0.3 Gyr (300 million year) time steps. In Figure 1 we show that eight billion years ago only 11% of disk galaxies had bars. By 4 billion years ago this fraction had doubled, and today at least one third of disk galaxies have a bar.
We know that bars tend to only form in disk galaxies which have low amounts of atomic gas and are in a relaxed state, or what we call ‘mature’. Combining this knowledge with our observations, we can say that, as the Universe gets older, the disk galaxy population as a whole is maturing. To see whether this is true for all disk galaxies, we split our sample up into three stellar mass bins, allowing us to look at the evolving bar fraction trends for low, intermediate and high mass disk galaxies.
The results for this are shown in Figure 2, where we observe an intriguing result. The bar fraction increases at a much steeper rate with time for the most massive galaxies (red), compared to the lower mass galaxies (blue). From this we can say that the population of disk galaxies is maturing across the whole stellar mass range we explore, but it is predominantly the most massive galaxies which drive the overall time evolution of the bar fraction we observe in Figure 1.
At the end of the paper we offer an explanation as to why the time evolution of the bar fraction differs for varying stellar mass bins. We can make the reasonable assumption that, by eight billion years ago, the majority of massive disk galaxies have formed, and have been, and continue to form bars up to the present day – hence the steeply increasing bar fraction we observe. However, the same assumption is not true for the low mass galaxies. There are some which are ‘mature’ disk galaxies eight billion years ago, but not all are ‘mature’ enough to be classified as disks. As with the most massive galaxies, these low mass disks are forming bars at a similar rate up to the present day, but the difference with this low mass sample is that there are still low mass disks forming up to the present day as well – leading to the much shallower increase in the bar fraction with time we observe.
In addition to these results, we are also able to present an interesting subset of disk galaxies. Your visual classifications has allowed our work to include a sub-sample of ‘red’ spiral galaxies (like those found from Galaxy Zoo 2 classifications). This sub-sample is generally omitted from other works that have explored this topic, as their way of identifying disks is based on galaxy colours. This means that these ‘red’ galaxies would have been classified as elliptical galaxies! Figure 3 shows a few of these ‘red’ disk galaxies (with the full sample of 98 here), so why don’t you take a look and decide for yourself! Not only is it very cool that you are able to identify these ‘red’ disks, but they also influence the results we observe. Just like in our local universe, these ‘red’ disks have a high bar fraction, with 45% of them having a bar! Could this be a further sign that bars ‘kill’ galaxies, even at high redshifts?
So that is a summary of the first results from Galaxy Zoo: Hubble. If you want more detail have a read of the paper in full here and take a look at the press release too! Thanks for all your hard work and help in classifying these galaxies!
Posted on behalf of Tom Melvin.
I’m really excited to be able to post that galaxies selected with the help of Galaxy Zoo classifications are being observed at the VLA (Very Large Array) in New Mexico, possibly right now.
The funny thing about observing at the VLA is that you do all of the work for the actual observations in advance.
The VLA runs in queue mode – as an observer you have to submit very (very) detailed information about what you want the telescope to do during your session (called a “scheduling block”) and a set of constraints about when it’s OK to run that (for example you tell them when the galaxy is actually up in the sky above the telescope!). Then the telescope operators pick from the available pool of scheduling blocks at any time to make best use of the array.
This means after you submit the scheduling blocks you just have to sit and wait until you start getting notifications from VLA that your galaxies have been observed. The observing semester for the B-array configuration started on 4th October (had a pause for the US shutdown) and runs until the 13th January 2014. I’m happy to report that we started getting notifications in late November of the first of our 2 hour scheduling blocks having been observed. At the time of writing four of our galaxies have each been observed at least once (we need six repeat visits to each one to get the depth of data we’d like) for a total of 16 hours of VLA time. I’ve been getting notifications every couple of days – which means that as I write this the VLA could be observing one of our galaxies!
Since making these very detailed observation files is the observing prodecure at the VLA – it takes the length of time you’d expect given that…..
So, in September in-between a crazy travel schedule, and with a lot of help from our collaborator Kelley Hess at Cape Town, I spent a lot of time scheduling VLA observations of some very interesting very gas rich and very strongly barred galaxies we identified in the Galaxy Zoo 2 sample (the bit which overlaps with the ALFALFA survey which measures total HI gas in each galaxy).
We have been granted time to observe up to 7 of these fascinating objects (depending on scheduling constraints at the VLA) which I think may reveal some really interesting physics about how bars drive gas around in the discs of galaxies.
You might notice from the picture (and the name) that the VLA is not a “normal telescope”. It’s what astronomers call a radio interferometer. Signals are collected from 27 separate antennas and combined in a computer. This means that as well as observing sources for flux calibration (so we can link how bright our target is through the telescope with physical units) we also have to observe, roughly every 20 minutes or so a “phase calibrator” to be able to know how to correctly add the signals together from each of the antennae (to add them “in phase”).
So a single scheduling block lasting 2 hours for one of our sources comprises:
1. Information to tell the VLA where to slew initially and what instrumentation to use (how to “tune” it to the frequency we know the HI in the galaxy will emit at).
2. A short observation of a known bright source for flux calibration.
Then there’s a loop of
a. Phase calibration
b. Source observation
c. Phase calibration
d. Source observation
and so on – ending with a Phase calibration (on Kelley’s advice we’ll do 5 source observations, and 6 phase calibrations). We have a total of 6 of these blocks for each galaxy, that makes 12 hours of telescope resulting in about 10 hours of collecting 21cm photons per galaxy.
We have to check which times all these sources are visible to the VLA, and set durations for each part which give enough slew time and on source time wherever the sources are on the sky. And this all has to add up exactly to 2 hours to fit the scheduling block.
The benefit of this though is a telescope which acts like it’s much larger than you could ever physically build. We’re trying to detect emission from atomic hydrogen in these galaxies which emits at 21cm. So we need a really large telescope to get a sharp picture.
And just to end, because they’re lovely, here are the four galaxies the VLA has observed so far in the Sloan Digital Sky Survey visible light images.
Thanks again for your help finding these rare and interesting galaxies. They’re rare, because they’re so gas rich and strongly barred – we have previously posted about how we showed strong bars are rare in galaxies with lots of atomic hydrogen. Hopefully we’ll have some exciting results to share once we’ve analysed these data.
(PS. That takes a lot of time too – it’ll be almost 1TB of data to process in total!).
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
Just a quick note to say that the Astronomy & Geophysics article some of us wrote to review the Specialist Discussion we ran at the Royal Astronomical Society in May is now posted on the arxiv. A&G is the magazine of the RAS (so I get a copy, like all RAS members), but also makes some articles free to read for all (Free editors choice articles) – and in this case the entire magazine was made open access.
Here’s the lovely cover art to finish off the post.
Well it was a very close fought battle, but the winner of our fun vote to pick the cover image for the October A&G was:
Apr 142 (aka The Penguin Galaxy):
I include below a screen shot of the poll from today, which confirms that choice. We have now sent this choice to the cover editor, so we won’t count any more votes.
As Kyle posted yesterday, you can now download detailed classifications from Galaxy Zoo 2 for more than 300,000 galaxies via the Sloan Digital Sky Survey’s “CasJobs” – which is a flexible SQL-based interface to the databases. I thought it might be helpful to provide some example queries to the data base for selecting various samples from Galaxy Zoo.
This example will download what we call a volume limited sample of Galaxy Zoo 2. Basically what this means is that we attempt to select all galaxies down to a fixed brightness in a fixed volume of space. This avoids biases which can be introduced because we can see brighter galaxies at larger distances in a apparent brightness limited sample like Galaxy Zoo (which is complete to an r-band magnitude of 17 mag if anyone wants the gory details).
So here it is. To use this you need to go to CasJobs (make sure it’s the SDSS-III CasJobs and not the one for SDSS-I and SDSS-II which is a separate page and only includes SDSS data up to Data Release 7), sign up for a (free) account, and paste these code bits into the “Query” tab. I’ve included comments in the code which explain what each bit does.
-- Select a volume limited sample from the Galaxy Zoo 2 data set (which is complete to r=17 mag). -- Also calculates an estimate of the stellar mass based on the g-r colours. -- Uses DR7 photometry for easier cross matching with the GZ2 sample which was selected from DR7. -- This bit of code tells casjobs what columns to download from what tables. -- It also renames the columns to be more user friendly and does some maths -- to calculate absolute magnitudes and stellar masses. -- For absolute magnitudes we use M = m - 5logcz - 15 + 5logh, with h=0.7. -- For stellar masses we use the Zibetti et al. (2009) estimate of -- M/L = -0.963+1.032*(g-i) for L in the i-band, -- and then convert to magnitude using a solar absolute magnitude of 4.52. select g.dr7objid, g.ra, g.dec, g.total_classifications as Nclass, g.t01_smooth_or_features_a01_smooth_debiased as psmooth, g.t01_smooth_or_features_a02_features_or_disk_debiased as pfeatures, g.t01_smooth_or_features_a03_star_or_artifact_debiased as pstar, s.z as redshift, s.dered_u as u, s.dered_g as g, s.dered_r as r, s.dered_i as i, s.dered_z as z, s.petromag_r, s.petromag_r - 5*log10(3e5*s.z) - 15.0 - 0.7745 as rAbs, s.dered_u-s.dered_r as ur, s.dered_g-s.dered_r as gr, (4.52-(s.petromag_i- 5*log10(3e5*s.z) - 15.0 - 0.7745))/2.5 + (-0.963 +1.032*(s.dered_g-s.dered_i)) as Mstar -- This tells casjobs which tables to select from. from DR10.zoo2MainSpecz g, DR7.SpecPhotoAll s -- This tells casjobs how to match the entries in the two tables where g.dr7objid = s.objid and -- This is the volume limit selection of 0.01<z<0.06 and Mr < -20.15 s.z < 0.06 and s.z > 0.01 and (s.petromag_r - 5*log10(3e5*s.z) - 15 - 0.7745) < -20.15 --This tells casjobs to put the output into a file in your MyDB called gz2volumelimit into MyDB.gz2volumelimit
Once you have this file in your MyDB, you can go into it and make plots right in the browser. Click on the file name, then the “plot” tab, and then pick what to plot. Colour-magnitude diagrams are interesting – to make one, you would plot “rabs” on the X-axis and “ur” (or “gr”) on the yaxis. There will be some extreme outliers in the colour, so put in limits (for u-r a range of 1-3 will work well). The resulting plot (which you will have to wait a couple of minutes to be able to download) should look something like this:
Or if you want to explore the GZ classifications, how about plotting “psmooth” (which is approximately the fraction of people viewing a galaxy who thought it was smooth) against the colour.
That plot would look something like this:
Which reveals the well known relationship between colour and morphology – that redder galaxies are much more likely to be ellipticals (or “smooth” in the GZ2 language) than blue ones.
You can learn more about SQL and the many things you could do with CasJobs at the Help Page (and then come back and tell me how simple my query example was!).
This example only downloads the very first answer from the GZ2 classification tree – there’s obviously a lot more in there to explore.
(Note that at the time of posting the DR10 server seemed to be struggling – perhaps over demand. I’m sure it will be fixed soon and this will then work.)
We are pleased to announce an open vote for the cover image of the October 2013 issue of “Astronomy and Geophysics” (the magazine of the Royal Astronomical Society).
A write-up of the Specialist Discussion the science team ran at the Royal Astronomical Society in May on “Morphology in the Era of Large Surveys” is going to appear in the October issue of A&G , so we were asked to nominate a cover image for this issue.
Rather than just have the science team pick our favourite image, we thought it would be nice to open up the choice to our volunteers. After all it’s you that make Galaxy Zoo special, and a unique way of dealing with morphology in the era of large surveys.
So we have put together five images for you to vote on. These are images which would make good covers for the magazine, and which the science team think either have a special connection to the Galaxy Zoo project, or illustrate something special about Galaxy Zoo and its contributions to the understanding of galaxy evolution. Subject to final approval by the editors, the image that wins this vote will appear on the cover of A&G for October 2013.
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.