Great news everybody! The latest Galaxy Zoo 1 paper has been accepted by MNRAS and has appeared on astro-ph: http://arxiv.org/abs/1402.4814
In this paper, we take a look at the most crucial event in the life of a galaxy: the end of star formation. We often call this process “quenching” and many astrophysicists have slightly different definitions of quenching. Galaxies are the place where cosmic gas condenses and, if it gets cold and dense enough, turns into stars. The resulting stars are what we really see as traditional optical astronomers.
Not all stars shine the same way though: stars much more massive than our sun are very bright and shine in a blue light as they are very hot. They’re also very short-lived. Lower mass stars take a more leisurely pace and don’t shine as bright (they’re not as hot). This is why star-forming galaxies are blue, and quiescent galaxies (or “quenched” galaxies) are red: once star formation stops, the bluest stars die first and aren’t replaced with new ones, so they leave behind only the longer-lived red stars for us to observe as the galaxy passively evolves.
Blue Ellipticals & Red Spirals
The received wisdom in galaxy evolution had been that spirals are blue, and ellipticals are red, meaning that spirals form new stars (or rather: convert gas into stars) and ellipticals do not form new stars (they have no gas to convert to stars). Since you’re taking part in Galaxy Zoo, you know that this isn’t entirely true: there are blue (star-forming) ellipticals and red (passive) spirals. It’s those unusual objects that we started Galaxy Zoo for, and in this paper they help us piece together how, why and when galaxies shut down their star formation. You can already conclude from the fact that blue ellipticals and red spirals exist that there is no one-to-one correlation between a galaxy’s morphology and whether or not it’s forming stars.
Blue, Red and…. Green?
A few years back, astronomers noticed that not all galaxies are either blue and star forming or red and dead. There was a smaller population of galaxies in between those two, which they termed the “green valley” (the origin of the term is rather interesting and we talk about it in this Google+ hangout). So how do these “green” galaxies fit in? The natural conclusion was that these “in between” galaxies are the ones who are in the process of shutting down their star formation. They’re the galaxies which are in the process of quenching. Their star formation rate is dropping, which is why they have fewer and fewer young blue stars. With time, star formation should cease entirely and galaxies would become red and dead.
The Green Valley is a Red Herring
Ok, why is this green valley a red herring you ask? Simple: the green valley galaxies aren’t a single population of similar galaxies, but rather two completely different populations doing completely different things! And what’s the biggest evidence that this is the case? Some of them are “green spirals” and others are “green ellipticals”! (Ok, you probably saw that coming from a mile away).
So, we have both green spirals and green ellipticals. First: how do we know they must be doing very different things? If you look at the colour-mass diagram of only spirals and only ellipticals, we start to get some hints. Most ellipticals are red. A small number are blue, and a small number are green. If the blue ellipticals turn green and then red, they must do so quickly, or there would be far more green ellipticals. There would be a traffic jam in the green valley. So we suspect that quenching – the end of star formation – in ellipticals happens quickly.
In the case of spirals, we see lots of blue ones, quite a few green one and then red ones (Karen Masters has written several important Galaxy Zoo papers about these red spirals). If spirals slowly turn red, you’d expect them to start bunching up in the middle: the green “valley” which is revealed to be no such thing amongst spirals.
Galaxy Quenching time scales
We can confirm this difference in quenching time scales by looking at the ultraviolet and optical colours of spirals and ellipticals in the green valley. What we see is that spirals start becoming redder in optical colours as their star formation rate goes down, but they are still blue in the ultraviolet. Why? Because they are still forming at least some baby stars and they are extremely bright and so blue that they emit a LOT of ultraviolet light. So even as the overall population of young stars declines, the galaxy is still blue in the UV.
Ellipticals, on the other hand, are much redder in the UV. This is because their star formation rate isn’t dropping slowly over time like the spirals, but rather goes to zero in a very short time. So, as the stellar populations age and become redder, NO new baby stars are added and the UV colour goes red.
It’s all about gas
Galaxies form stars because they have gas. This gas comes in from their cosmological surroundings, cools down into a disk and then turns into stars. Galaxies thus have a cosmological supply and a reservoir of gas (the disk). We also know observationally that gas turns into stars according to a specific recipe, the Schmidt-Kennicutt law. Basically that law says that in any dynamical time (the characteristic time scale of the gas disk), a small fraction (around 2%) of that gas turns into stars. Star formation is a rather inefficient process. With this in mind, we can explain the behaviour of ellipticals and spirals in terms of what happens to their gas.
Spirals are like Zombies
Spirals quench their star formation slowly over maybe a billion years or more. This can be explained by simply shutting off the cosmological supply of gas. The spiral is still left with its gas reservoir in the disk to form stars with. As time goes on, more and more of the gas is used up, and the star formation rate drops. Eventually, almost no gas is left and the originally blue spiral bursting with blue young stars has fewer and fewer young stars and so turns green and eventually red. That means spirals are a bit like zombies. Something shuts off their supply of gas. They’re already dead. But they have their gas reservoir, so they keep moving, moving not knowing that they’re already doomed.
Ellipticals life fast, die young
The ellipticals on the other hand quench their star formation really fast. That means it’s not enough to just shut off the gas supply, you also have to remove the gas reservoir in the galaxy. How do you do that? We’re not really sure, but it’s suspicious that most blue ellipticals look like they recently experienced a major galaxy merger. There are also hints that their black holes are feeding, so it’s possible an energetic outburst from their central black holes heated and ejected their gas reservoir in a short episode. But we don’t know for sure…
So that’s the general summary for the paper. Got questions? Ping me on twitter at @kevinschawinski
Note: this is a post by Galaxy Zoo science team member Edmond Cheung. He is a graduate student in astronomy at UC Santa Cruz, and his first Galaxy Zoo paper was accepted to the Astrophysical Journal last week. Below, Edmond discusses in more depth the new discoveries we’ve made using the Galaxy Zoo 2 data.
Observations show that bars – linear structures of stars in the centers of disk galaxies – have been present in galaxies since z ~ 1, about 8 billion years ago. In addition, more and more galaxies are becoming barred over time. In the present-day Universe, roughly two-thirds of all disk galaxies appear to have bars. Observations have also shown that there is a connection between the presence of a bar and the properties of its galaxy, including morphology, star formation, chemical abundance gradients, and nuclear activity. Both observations and simulations argue that bars are important influences on galaxy evolution. In particular, this is what we call secular evolution: changes in galaxies taking place over very long periods of time. This is opposed to processes like galaxy mergers, which effect changes in the galaxy extremely quickly.
To date, there hasn’t been much evidence of secular evolution driven by bars. In part, this is due to a lack of data – samples of disk galaxies have been relatively small and are confined to the local Universe at z ~ 0. This is mainly due to the difficulty of identifying bars in an automated manner. With Galaxy Zoo, however, the identification of bars is done with ~ 84,000 pairs of human eyes. Citizen scientists have created the largest-ever sample of galaxies with bar identifications in the history of astronomy. The Galaxy Zoo 2 project represents a revolution to the bar community in that it allows, for the first time, statistical studies of barred galaxies over multiple disciplines of galaxy evolution research, and over long periods of cosmic time.
In this paper, we took the first steps toward establishing that bars are important drivers of galaxy evolution. We studied the relationship of bar properties to the inner galactic structure in the nearby Universe. We used the bar identifications and bar length measurements from Galaxy Zoo 2, with images from the Sloan Digital Sky Survey (SDSS). The central finding was a strong correlation between these bar properties and the masses of the stars in the innermost regions of these galaxies (see plot).
We compared these results to state-of-the-art simulations and found that these trends are consistent with bar-driven secular evolution. According to the simulations, bars grow with time, becoming stronger (they exert more torque) and longer. During this growth, bars drive an increasing amount of material in towards the centers of galaxies, resulting in the creation and growth of dense central components, known as “disky pseudobulges”. Thus our findings match the predictions of bar-driven secular evolution. We argue that our work represents the best evidence of bar-driven secular evolution yet, implying that bars are not stagnant structures within disk galaxies, but are instead a critical evolutionary driver of their host galaxies.
It’s always exciting to see a new Galaxy Zoo paper out, but today’s release of our latest is really exciting. Galaxy Zoo 2: detailed morphological classifications for 304,122 galaxies from the Sloan Digital Sky Survey, now accepted for publication in the Monthly Notices of the Royal Astronomical Society, is the result of a lot of hard work by Kyle Willett and friends.
Galaxy Zoo 2 was the first of our projects to go beyond simply splitting galaxies into ellipticals and spirals, and so these results provide data on bars, on the number of spiral arms and on much more besides. The more complicated project made things more complicated for us in turning raw clicks on the website into scientific calculations – we had to take into account the way the different classifications depended on each other, and still had to worry about the inevitable effect that more distant, fainter or smaller galaxies will be less likely to show features.
We’ve got plenty of science out of the Zoo 2 data set while we were resolving these problems, but the good news is that all of that work is now done, and in addition to the paper we’re making the data available for anyone to use. You can find it alongside data from Zoo 1 at data.galaxyzoo.org. One of the most rewarding things about the project so far has been watching other astronomers make use of the original data set – and now they have much more information about each galaxy to go on.
Just a quick note to point out a new paper based on Galaxy Zoo classifications appeared on the arxiv this morning (and just accepted to MNRAS): The Differing Star Formation Histories of Red and Blue Spirals and Ellipticals, by Rita Tojeiro et al.
In this work we took samples of galaxies split by their morphological classifications (from you all, and actually going back to the original Galaxy Zoo project) as well as by their optical colour. With the help of an Ogden Trust undergraduate summer student (Joshua Richards) we then compiled the average star formation histories of these samples, based on fits of star formation models to the Sloan Digital Sky Survey spectra of the galaxies (previously published and called VESPA, or “VErsatile SPectral Analysis” by Rita).
Our main result was that red spirals differ in their star formation histories from blue spirals only in the last billion years or less. We also find that blue ellipticals have very similar star formation histories to blue spirals. We show some results about the dust and metal (astronomers metal) content of the galaxies as well. I think it’s a nice project and I’m very happy to see it finally finished and published.
Thanks again for the classifications.
As usual when the American Astronomical Society meets, this has been an intense week of research results, comparing notes, and laying plans. Galaxy Zoo has once again been well represented. Here’s Kevin discussing the Green Valley in galaxy colors, making the case that it consists of two completely different populations when Galaxy Zoo morphologies are factored in:
Today we’re presenting first results of the Hubble imaging of Voorwerpje systems. This is what our poster looks like:
(or you can get the full-size 2.8 Mbyte PDF). We didn’t have room to lay out all the features we first had in mind, but these are the main points we make:
They show a wild variety of forms, often with filaments of gas stretching thousands of light-years. These include loops, helical patterns, and less describable forms.
The ionization, traced by the line ratio [O III]/Hα, often shows a two-sided pattern similar to the ionization cones around many AGN. This
fits with illumination by radiation escaping past a crudely torus-like structure. However, there is still less highly-ionized gas outside this whose energy source is not clear.
As in IC 2497, the parent galaxy of Hanny’s Voorwerp, many of these galaxies show loops of ionized gas up to 300 light-years across emerging from the nuclei, a pattern which may suggest that whatever makes the nucleus fade so much in radiation accompanies an increase in the kinetic energy driving outflows from its vicinity.
At the bottom of the poster we illustrate with new clarity a point we knew about in the original paper – for the two Voorwerpje systems with giant double radio sources, they completely break the usual pattern of alignment between the radio and emission-line axis. Mkn 1498 and NGC 5972 are aligned almost perpendicular, which can’t be fixed by changing our viewing angle. We’re speculating among ourselves as to how this could happen; maybe interaction of two massive black holes is twisting an accretion disk. But don’t quote me on that just yet.
The color images here show only the ionized gas, with [O III] in green and Hα in red. Starlight from the galaxies has been subtracted based on filters which don’t show the gas, so we can isolate the gas properties. The false-color insets show the [O III]/Hα ratio. The blank regions are areas whose signal is too low for a useful measurement. Red indicates the highest ionization, fading to deep blue for the lowest.
We were able to feature some new data that came in too late to be printed in the poster (by tacking up a smaller printed panel) – the long-awaited images of UGC 7342, among the largest and most complex clouds we’ve found (or more correctly, so many Galaxy Zoo participants found). Hubble observed it Monday afternoon, and after some frantic file-shuffling and processing, I got the data in the same shape as the others. And here it is:
Click on this one to see it larger. We barely know where to begin. The actual AGN may lie behind a dust lane, and there is a large region of very low-ionization as near it. Another loop near the nucleus, and fantastically twisted filaments winding their way 75,000 light-years each way.
There is still more to come – with Vardha Bennert and Drew Chojnowski, we planned the strategy for several upcoming observing runs at Lick Observatory (one starting only next week). These should include getting data on some of the most promising AGN/companion systems to look for the AGN ionizing gas in companion galaxies, and observation of regions in the Voorwerpjes that we only now see a context for. Additional X-ray and radio observations could fill in some of the blanks in our understanding. And by all means, stay tuned!
Longtime readers of the Galaxy Zoo blog will be familiar with the peer review process from the many posts here describing it. The time elapsed between a paper’s submission and its acceptance (if it is accepted) can be long or short, and papers from the Zoo have sampled the whole spectrum.
The process with our paper on supermassive black holes growing in bulgeless galaxies took about 4 months: we submitted the paper in July, received comments and suggestions from the anonymous referee in August, then modified the paper based on the referee’s report and re-submitted it in October. This week, the paper was accepted by MNRAS.
The initial report from the referee was extremely thorough and constructive, and incorporating his/her comments helped to significantly improve the paper. The referee pointed out, for example, that although the paper emphasized the lack of significant mergers in the evolutionary histories of the sample, the bulgeless nature of the sample excludes not just mergers but any violent evolutionary process that can disrupt a disk to the point where it transfers a significant fraction of its stars from a disk into a bulge or pseudobulge. That was certainly a fair point, so we changed our discussion to include further consideration of the implications of those evolutionary processes being excluded.
And we made some other changes, too, including expanded discussion of why our results differ from some other studies and additional description of how we might be affected by dust in these galaxies (and why we think we aren’t). There were also some very interesting questions that we couldn’t really answer within the scope of this paper, but that we had asked ourselves too and that have already formed the basis for additional projects now underway. Overall, this was a classic example of what the peer review process was meant to be.
The accepted version of the paper will soon be available on the arXiv for anyone to download. In the spirit of openness, I had hoped to include the referee’s report and our response in the additional materials on the arXiv, but the referee did not give permission to do so. That’s fine — it’s anonymous and it’s perfectly acceptable if the referee prefers the exact contents of the report to be private as well. Hopefully he/she approves of my summary!
Note: as soon as it’s published, the paper will also be added to the Zooniverse Publications page, which coincidentally happens to have been released today as the first day of the Zooniverse Advent calendar. Have a look — Galaxy Zoo’s contributions are impressive and we’re joined by many, many others.
I hope you all had clear skies during the Transit of Venus. If not, it’ll be over a hundred years before you get another chance…. and in Zoo-related news, the Transit of Venus is an example of one way we find planets around other stars. We look for a dip in the brightness of the star as a planet moves across it from our point of view. Want to know more? Head over to the Planethunters blog, or put in some clicks looking for transits yourself!
So, in actual Galaxy Zoo news, I am very happy to report that the latest Galaxy Zoo study has been accepted for publication in the Astrophysical Journal. As we blogged a while back, we got Chandra X-ray time to observe a small sample of major mergers found by the Galaxy Zoo to look for double black holes. The idea is to look for the two black holes presumably brought into the merger by the two galaxies and see if we find both of them feeding by looking for them with an X-ray telescope (i.e. Chandra).
The lead author of the paper is Stacy Teng, a NASA postdoctoral fellow at NASA’s Goddard Space Flight Center and an expert on X-ray data analysis. In a sample of 12 merging galaxies, we find just one double active nucleus.
We submitted the resulting paper to the Astrophysical Journal where it underwent peer review. The reviewer suggested some changes and clarifications and so the paper was accepted for publication.
So what’s next? We submitted a proposal, led by Stacy, for the current Chandra cycle. To do a bigger, more comprehensive search for double black holes in mergers to put some real constraints on their abundance and properties. We hope to hear about whether the proposal is approved some time later this summer, so stay tuned and follow us on Twitter for breaking news!
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! 😉
I was embarrassed to discover today that I never got around to writing a full blog post explaining our work studying the properties of the red spirals, as I promised way back in October 2009. Chris wrote a lovely post about it “Red Spirals at Night, Astronomers Delight“, and in my defense new science results from Zoo2, and a few other small (tiny people) things distracted me.
I won’t go back to explaining the whole thing again now, but one thing missing on the blog is the colour magnitude diagram which demonstrates how we shifted through thousands of galaxies (with your help) to find just 294 truly red, disc dominated and face-on spirals.
A colour magnitude diagram is one of the favourite plots of extragalactic astronomers these days. That’s because galaxies fall into two distinct regions on it which are linked to their evolution. You can see that in the grey scale contours below which is illustrating the location of all of the galaxies we started with from Galaxy Zoo. The plot shows astronomical colour up the y-axis (in this case (g-r) colour), with what astronomers call red being up and blue dow. Along the x-axis is absolute magnitude – or astronomers version of how luminous (how many stars effectively) the galaxy is. Bigger and brighter is to the right.
So you see the greyscale indicating a “red sequence” at the top, and a “blue cloud” at the bottom. In both cases brighter galaxies are redder.
The standard picture before Galaxy Zoo (ie. with small numbers of galaxies with morphological types) was that red sequence galaxies are ellipticals (or at least early-types) and you find spirals in the blue cloud. The coloured dots on this picture show the face-on spirals in the red sequence (above the line which we decided was a lower limit to be considered definitely on the red sequence). The different colours indicate how but the bulge is in the spiral galaxy – in the end we only included in the study the green and blue points which had small bulges, since we know the bulges of spiral galaxies are red. These 294 galaxies represented just 6% of spiral galaxies of their kind.
So this is one of my favourite versions of the colour magnitude diagram.
Wednesday’s session at the Austin meeting of the American Astronomical Society will include new results from the Galaxy Zoo sample of overlapping galaxies. Extending the work in Anna Manning’s Master’s thesis, this marks an extension that helps us look ahead to comparison with the higher-redshift Hubble Zoo overlaps. Specifically, we compared visible-light data with ultraviolet data (from the GALEX satellite or a UV/optical monitor instrument on the European Space Agency’s XMM-Newton) to compare the amounts of optical and ultraviolet absorption in galaxies. This tells us, for example, how much we should correct Hubble measurements for high-redshift galaxies, where visible-light filters sample light which was emitted in the ultraviplet, to compare them with the rich SDSS data which see the visible range emitted by nearby galaxies. This is a key tool in trying to use backlit galaxies to search for changes in the dust content of galaxies over cosmic time, by comparing Hubble and Sloan results. Along the way, we see evidence that a common result – the flat so-called Calzetti extinction law in star-forming galaxies – results from the way dust clumps into regions of larger and smaller extinction that we usually see blurred together, since we see this in regions so far out in some galaxies that internal illumination by the galaxy’s own stars doesn’t matter. Here’s the poster presentation:
(That had to be shrunk to fit the blog size limits but should still be just legible – click for a bigger PNG). NGC 2207 is outside the SDSS footprint but had such good data that gave nice error bars that it wound up featuring a whole image series. Now to go back and apply that new set of analysis routines to more GZ pairs…
In other news, a Canadian astronomer working with NED found a new use for the overlap catalog including the “reject” list – to distinguish galaxies in pairs which are seen moving together or apart, since we often have both redshifts and from the dust we know which one is in front.
And to reiterate what it says at the end of the abstract – we thank all the Zooites who have contributed to the overlap sample and made this work possible!