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What’s all the fuss about bars in galaxies?

Since our discovery in 2010 that the red spirals identified by your classifications in the first phase of Galaxy Zoo were twice as likely to host galactic scale bars as normal blue spirals, a lot of our research time has focused on understanding which types of galaxies host bars, and why that might be.

Barred spiral, NCG 1300, observed with the Hubble Space Telescope.

Barred spiral, NCG 1300, observed with the Hubble Space Telescope.

 

Our research with the bars identified by you in the second phase of Galaxy Zoo continues to gives us hints that these structures in galaxies might be involved in the process which quenches star formation in spiral galaxies and through that could be part of the process involved in the reduction of star formation in the universe as a whole.

We’ve also used your classifications as part of Galaxy Zoo Hubble and Galaxy Zoo CANDELS to identify the epoch in the universe when disc galaxies were first stable enough to host a significant number of bars, finding them possibly even earlier in the Universe than was previously thought.

Last Friday I spoke at the monthly “Ordinary Meeting” of the Royal Astronomical Society, giving summary of the evidence we’re collecting on the impact bars have on galaxies thanks to your classifications (a video of my talk will be available at some point). This was the second time I’ve spoken at this meeting about results from Galaxy Zoo, and it’s a delightful mix of professional colleagues, and enthusiastic amateurs – including some Galaxy Zoo volunteers.

Prompted by that I thought it was timely to write on this blog about what these bars really are, what they do to galaxies, and why I think they’re so interesting. I wrote the below some time ago when I had a spare few minutes, and was just looking for the right time to post it.

The thing about galaxies, which is sometimes hard to remember, is that they are simply vast collections of stars, and that those stars are all constantly in motion, orbiting their common centre of mass. The structures that we see in galaxies are just a snapshot of the locations of those stars right now (on a cosmic timescale), and the patterns we see in the positions of the stars reveals patterns in their orbital motions. A stellar bar for example reveals a set of very elongated orbits of stars in the disc of a galaxy.

Another extraordinary thing about a disc galaxy is how thin it is. To put this is perspective I’ll give you a real world example. In the Haus der Astronomie in Heidelberg you can walk around inside a scale model of the Whirlpool galaxy. The whole building was laid out in a design which reflects the spiral arms of this galaxy. However it’s not an exact scale model – to properly represent the thickness of the disc of the Whirlpool galaxy the building (which in actual fact has 3 stories and hosts a fairly large planetarium in its centre) would have to be only 90cm tall…..

The Haus der Astronomie, a building laid out like a spiral galaxy.

The Haus der Astronomie, a building laid out like a spiral galaxy.

Such an incredibly thin disc of stars floating independently in space would be quite unstable dynamically (meaning its own gravity should cause it to buckle and collapse on itself). This instability would immediately manifest in elongated orbits of stars, which would make a stellar bar (as part of this process of collapse). Simple computer models of disks of stars immediately form bars. Of course we now know that galaxy discs are submerged in massive halos of dark matter. So my first favourite little fact about bars is

(1) the fact that not all disc galaxies have bars was put forward as evidence that the discs must be embedded in massive halos before the existence of dark matter was widely accepted.

Now we can model dark matter halos better we discover that even with a dark matter halo, as long as that halo can absorb angular momentum (ie. rotate a bit) all discs will eventually make a bar. So my second favourite little fact is that

(2) we still don’t understand why not all disc galaxies have bars.

M101 - an unbarred spiral galaxy (Credit: ESA/NASA HST).

M101 – an unbarred spiral galaxy (Credit: ESA/NASA HST).

What this second fact means is that perhaps what I should really be doing is studying the galaxies you have identified as not having bars to figure out why it is they haven’t been able to form a bar yet. It should really be the properties of these which are unexpected….. We find that this is more likely to happen in blue, intermediate mass spirals with a significant reservoir of atomic hydrogen (the raw material for future star formation). In fact this last thing may be the most significant. Including realistic interstellar gas in computer simulation of galaxies is very difficult, but people do run what is called “smooth particle hydrodynamic” simulations (basically making “particles” of gas and inserting the appropriate properties). If they add too much gas into these simulations they find that bar formation is either very delayed, or doesn’t happen in the time of the simulation…..

Anyway I hope this has given you a flavour of what I find interesting about bars in galaxies. I think it’s fascinating that they give us a morphological way to identify a process which is so dynamical in nature. And it’s a very complex process, even though the basic physics (just orbits of stars) is very simple and well understood. Finally, I have become convinced though tests of the bars identified by you in Galaxy Zoo compared to bars identified by other methods, that if you want a clean sample of very large bars in galaxies that multiple independent human eyes will give you the best result. You are much less easy to trick that automated methods for finding galactic bars.

So thanks again for the classifications, and keep clicking. :)

Here’s a link to all blog posted tagged with “bars”.

Hubble science results on Voorwerpjes – episode 1

After two rounds of comments and questions from the journal referee, the first paper discussing the detailed results of the Hubble observations of the giant ionized clouds we’ve come to call Voorwerpjes has been accepted for publication in the Astronomical Journal. (In the meantime, and freely accessible, the final accepted version is available at http://arxiv.org/abs/1408.5159 ) We pretty much always complain about the refereeing process, but this time the referee did prod us into putting a couple of broad statements on much more quantitively supported bases. Trying to be complete on the properties of the host galaxies of these nuclei and on the origin of the ionized gas, the paper runs to about 35 pages, so I’ll just hit some main points here.

Montage of Hubble images of Voorwerpjes

Montage of Hubble images of Voorwerpjes

These are all in interacting galaxies, including merger remnants. This holds as well for possibly all the “parent” sample including AGN which are clearly powerful enough to light up the surrounding gas. Signs include tidal tails of star as well as gas, and dust lanes which are chaotic and twisted. These twists can be modeled one the assumption that they started in the orbital plane of a former (now assimilated) companion galaxy, which gives merger ages around 1.5 billion years for the two galaxies where there are large enough dust lanes to use this approach. In 6 of 8 galaxies we studied, the central bulge is dominant – one is an S0 with large bulge, and only one is a mostly normal barred spiral (with a tidal tail).<?p>

Numerical model of precessing disk of gas from a disrupted companion of NGC 5972

Numerical model of precessing disk of gas from a disrupted companion of NGC 5972

Incorporating spectroscopic information on both internal Doppler shifts and chemical makeup of the gas we can start to distinguish smaller areas affected by outflow from the active nuclei and the larger surrounding regions where the gas is in orderly orbits around the galaxies (as in tidal tails). We have especially powerful synergy by adding complete velocity maps made by Alexei Moiseev using the 6-meter Russian telescope (BTA). In undisturbed tidal tails, the abundances of heavy elements are typically half or less of what we see in the Sun, while in material transported outward from the nuclei, these fractions may be above what the solar reference level. There is a broad match between disturbed motions indicating outward flows and heavy-element fractions. (By “transported” above, I meant “blasted outwards at hundreds of kilometers per second”). Seeing only a minor role for these outflows puts our sample in contrast to the extended gas around some quasars with strong radio sources, which is dominated by gas blasted out at thousands of kilometers per second. We’re seeing either a different process or a different stage in its development (one which we pretty much didn’t know about before following up this set of Galaxy Zoo finds.) We looked for evidence of recent star formation in these galaxies, using both the emission-line data to look for H-alpha emission from such regions and seeking bright star clusters. Unlike Hanny’s Voorwerp, we see only the most marginal evidence that these galaxies in general trigger starbirth with their outflows. Sometimes the Universe plays tricks. One detail we learned from our new spectra and the mid-infared data from NASA’s WISE survey satellite is that giant Voorwerpje UGC 7342 has been photobombed. A galaxy that originally looked as if it night be an interacting companion is in fact a background starburst galaxy, whose infrared emission was blended with that from the AGN in longer-wavelength IR data. So that means the “real” second galaxy has already merged, and the AGN luminosity has dropped more than we first thought. (The background galaxy has in the meantime also been observed by SDSS, and can be found in DR12).

BTA Doppler maps of Voorwerpjes

BTA Doppler maps of Voorwerpjes


Now we’re on to polishing the next paper analyzing this rich data set, moving on to what some colleagues find more interesting – what the gas properties are telling us about the last 100,000 years of history of these nuclei, and how their radiation correlates (or indeed anti-correlates) with material being blasted outward into the galaxy from the nucleus. Once again, stay tuned!

First Results from Galaxy Zoo: CANDELS

I am very happy to present the results from the first published paper based on your classifications of the HST-CANDELS Images.

Galaxy Zoo: CANDELS combined optical and infrared imaging from the Hubble Space Telescope, which allows us to probe galaxies back to when the universe was only around 3 billion years old (early than we could do with optical HST images alone). So we are looking at galaxies whose light has taken over 10 billion years to reach us!

Our first area of research with this data is to look at disk and barred disk galaxies, as the title suggests…….

title

This work is based on an initial sample of 876 disk galaxies, which are from the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey (CANDELS). We want to explore what happens to barred disk galaxies beyond eight billion years ago, building on our work looking at the evolving bar fraction with Galaxy Zoo: Hubble.

When we began this work, we were unsure what we would find when looking so far back. From our Galaxy Zoo: Hubble work we had identified that 10% of disk galaxies hosted a galactic bar eight billion years ago, but beyond this our knowledge of disks was limited to a single simulation of disk galaxies. This simulation predicted that bars in disk galaxies were very rare beyond the epoch we had observed to, as the Universe would be to young for disk galaxies to
have settled down enough to form barred structures.

candels

Figure 1: The bar fraction of GZ: CANDELS galaxies (top), and the absolute magnitudes of all the disk and barred disk galaxies in the sample (bottom) (Figure 5 in the paper).

As Figure 1 shows, we actually find that roughly 10% of all disk galaxies host a bar, even back to when the Universe was only 3 billion years old! This is a very exciting result, as it shows that disk galaxies were able to settle at much earlier times than originally believed.

What we need to understand now is how do these disk galaxies form their bars? Could they be completely settled disk galaxies which have naturally formed bars, even during this epoch of violent galaxy evolution where galaxy mergers are more frequent? Or were these bars formed by a galaxy-galaxy interaction, as seen by some simulations? The answer could be one or the other, or most likely a combination of these two theories. Either way, we hope to explore this population of barred disk galaxies in greater detail over the coming months!

candels_galaxies

Figure2: Examples of disk (top row) and barred disk (bottom row) galaxies from Galaxy Zoo: CANDELS (Figure 3 in the paper).

So there is a summary of the first Galaxy Zoo: CANDELS paper. If you would like to see this in more detail, please take a look at the paper here, and why not check out the RAS press release too! Thank you all for your hard work, and keep classifying!

Posted on behalf of Tom Melvin. 

Fibers and Voorwerpjes

While preparing for more observations of the Galaxy Zoo giant AGN clouds (Voorwerpjes), this is a good time to introduce more complete ways of obtaining astronomical spectra. Traditionally, we’ve put a long slit in front of spectrographs, so we can measure everything along that line without worries about overlapping spectra of different objects or pieces of sky. In some cases, as with the optical fibers used by the Sloan Digital Sky Survey, we get the light summed within a circular aperture on the sky (with Sloan, from hundreds of different objects at each pointing of the telescope). But many of the things we want to understand are large and oddly shaped, so these approaches limit us to a very partial view (or to making many observations to cover everything of interest). Enter the Integral-Field Unit (IFU), which is any kind of device that lets us get the spectrum of every point in some region of the sky. They often use fiber optics to rearrange light from the object, so each small region of it comes out at a different place on what would otherwise be the spectrograph slit. After that it all becomes a software problem.

IFUs are becoming more common on large telescopes. We’ve gotten excellent data on some Voorwerpje systems with the unit on the 8-meter Gemini North telescope. Here’s a sample of raw data on UGC 11185. Each horizontal streak is the spectrum of an area 0.2 arcseconds square. The sampling, sensitivity, and image quality are superb, revealing multiple clouds of gas moving within a total span of almost 1000 km/s.

ugc11185obs17quickblue

On the other hand, if we want to use its whole wavelength range, the Gemini device covers only 3.5×5 arcseconds of sky at once. I’m headed to the 3.5m WIYN telescope on Kitt Peak to use a complementary device called Hexpak, newly commissioned by instrument designer Matt Bershady of the University of Wisconsin (who I’ve been emailing about this since I learned of the project three years ago). This fiber bundle plugs into the multipurpose spectrograph kept in a climate-controlled room below the telescope, and combines small and densely-packed fibers in the middle (for things like galactic nuclei, small and bright with lots of structure) and large fibers near the edges (collecting a lot of signal from large diffuse surrounding material – sound familiar?). Matt and his team were able to get a short exposure through thin clouds of UGC 11185 as a feasibility test – here’s a piece of that raw data frame, showing the small central fiber and the larger surrounding ones (which show brighter night-sky airglow lines as well as more object signal; the bright [O III] lines and H-beta are near the middle, with wavelength increasing to the right for each spectrum). I hope to get a lot more data like this shortly.
UGC11185hextest

Elsewhere, the European Southern Observatory has commissioned an enormous IFU, and the Sloan team has rebuilt their fiber bundles so that each one now makes multiple IFUs which can be placed on many galaxies at a time – this part of the Sloan survey extension is known as MANGA. Then there is the Spanish-led CALIFA project for hundreds of galaxies, which has publicly released data for their first two subsets. Then there are SAURON (whose data ca be tamed in software by GANDALF) and the upgrade of SCORPIO-2 and more… Swimming in data as we sift for knowledge, I am reminded of this anonymous computer error message in haiku form:

Out of memory.
We wish to hold the whole sky
but we never will.

Making Radio Galaxies in a Computer

At a conceptual level the formation of radio galaxies is pretty simple. According to a basic picture first introduced in the 1970s, a supermassive black hole in the center of a galaxy generates a symmetric pair of oppositely directed, high speed jets or beams of hot, ionized gas as a by-product of energy released or stored from matter falling onto the black hole. Those jets drill holes in the atmosphere of the galaxy and then even far beyond, dumping energy, excavating cavities and possibly entraining gas into the jets and cavities along the way. The jets carry magnetic fields and high energy electrons. Those electrons, spiraling in the magnetic fields light up the jets and the cavities they excavate in the radio band through a process called synchrotron emission.

While calculations based on this cartoon picture can correctly predict a few properties of radio galaxies, anyone who has looked at the images in Radio Galaxy Zoo can see that there must be a whole lot more to the story. Radio galaxies at best have only a rough bilateral symmetry with respect to their host galaxies. Furthermore, no two radio galaxies look alike, and most look pretty complicated; some could only be described as messy. In fact, the physics of radio galaxy formation is really very complex for a whole bunch of reasons that range from inherent instabilities in the dynamics of a fast jet, to the reality that the jets are not steady at the source. Furthermore, the surrounding environments are themselves messy, dynamic and sometimes even violent. All of these influences have impact on the appearances of radio galaxies.

The other side of the coin is that, if they can be understood, these complications may improve opportunities to decipher both the formation processes of the jets as well as the conditions that control their development and dissipation as they penetrate their environments. One part of piecing this puzzle together is expanding our awareness of all the things radio galaxies do, as well as when and where they do what they do. That’s what Radio Galaxy Zoo is about.
On the other hand, to go beyond the cartoon picture of what we see we also have to develop much more sophisticated and realistic models of the phenomena. This is very challenging. Because the detailed physics is so complex (messy!), astronomers have come to depend increasingly on large computer simulations that solve equations for gas dynamics with magnetic fields and high energy electrons. Pioneering gas dynamical simulations of jets in the 1980s already played an important role in confirming the value of the jet paradigm and helped to refine it soon after it was introduced.

Those early simulations were, however, seriously limited by available computer power and computational methods. In important ways the structures they made did not really look much like actual radio galaxies. At best they were too grainy. At worst important physics had to be left out, including the processes that actually produce the radio emission. This made it hard to know exactly how to compare the simulations with real radio galaxies. Thankfully, rapid improvements in both of those areas have led recently to much more realistic and detailed simulations that are starting to look more like the real thing and can be used to better pin down what is actually going on.

Our group at the University of Minnesota has been involved for some years now in pushing forward the boundaries of what can be learned about radio galaxies from simulations. I illustrate below some of the lessons we have learned from these simulations and some of the complex radio galaxy environments that it is now possible to explore through simulations. Each of these simulations was part of the work carried out by a student as part of their PhD training.

The jets responsible for radio galaxy formation propagate at speeds that can be a significant fraction of the speed of light. They are almost certainly supersonic. These properties lead to several related behaviors that are illustrated in Figure 1. It turns out that the flows within such a jet tend periodically to expand and then to contract. As they do so they form a sequence of shocks along the jet. These are visible in the figure. The jet also creates a sonic boom or bow shock in front as it moves forward. A close look at the jet in this figure also reveals that the jet actually does not remain straight as it moves forward. The end of the jet turns out to be unstable, so soon after launch begins to ‘flap’ or wobble. As a result the end of the jet tends to jump around, enlarging the area of impact on the ambient medium.

Figure 1: Shock waves in a simulated high speed jet propagating from the left end of the box. Since the jet is driving supersonically through its ambient medium, it also has created a bow shock (sonic boom) in the ambient medium on the far right.

Figure 1: Shock waves in a simulated high speed jet propagating from the left end of the box. Since the jet is driving supersonically through its ambient medium, it also has created a bow shock (sonic boom) in the ambient medium on the far right.

Many radio galaxies form inside clusters of galaxies, where the ambient medium is highly non-uniform and stirred up as a result of its own, violent formation. This distorts and bends the radio structures. At the same time the energy and momentum deposited by the jets creates cavities in the cluster gas that lead to dark holes in the thermal X-ray emission of the cluster. Figure 2 illustrates some of these behaviors for a simulated radio galaxy formed at the center of a cluster. Even though the source of the radio galaxy is at rest, there are fast gas motions in the cluster gas that obviously deflect the radio galaxy jets. ‘Mock’ radio images representing synchrotron emission by high energy electrons in the magnetic field carried by the jets are shown on the right in the figure at two times. At the same two times mock images of thermal X-rays are shown to the left. The X-ray images have been processed to exaggerate the dark cavities produced by the jets. Note that each image spans about 700 kpc or 2 million light years.

Figure 2: Simulation of radio galaxy formation in a cluster of galaxies that was extracted from a separate simulation of galaxy cluster formation. Mock obsevations of thermal X-rays (left) and radio synchrotron emission (right) are shown at two times after a pair of oppositely directed jets formed in the cluster center.

Figure 2: Simulation of radio galaxy formation in a cluster of galaxies that was extracted from a separate simulation of galaxy cluster formation. Mock observations of thermal X-rays (left) and radio synchrotron emission (right) are shown at two times after a pair of oppositely directed jets formed in the cluster center.

Quite a few radio galaxies in clusters are not made by galaxies anchored in the cluster center, but are hosted by galaxies moving through the cluster. This is especially common in clusters that are in the process of merging with another cluster. In that case the host galaxy can be moving very fast, and even supersonically with respect to its local, ambient medium. Then the radio jets can be very strongly deflected into ‘tails’ by an effective cross wind and eventually disrupted. Figure 3 illustrates the mock synchrotron emission from such a simulated radio galaxy. The abruptness of jet bending depends on the relative speed of the jet with respect to its internal sound speed and the relative speed of the host galaxy through its ambient medium with respect to the sound speed of that medium. So, when strongly bent jets are seen in a radio galaxy it is a strong clue that the motion of the galaxy is supersonic in relation to its environment. When multiple tailed radio galaxies are found in a given cluster it provides potentially valuable information about the dynamical condition of the cluster, since a relaxed cluster ought not to have many galaxies moving at supersonic speeds through the cluster gas.

Figure 3: Mock synchrotron image of a simulated radio galaxy embedded in a supersonic cross wind. The two jets have bent into downstream, turbulent tails .

Figure 3: Mock synchrotron image of a simulated radio galaxy embedded in a supersonic cross wind. The two jets have bent into downstream, turbulent tails .

Even more complex motions between the host galaxies and the ambient gas are possible. Those can sometimes lead to really exotic-looking radio structures. One beautiful example of this is the radio source 3C75 in the merging cluster Abell 400. Evidently two massive galaxies have become gravitationally bound into a binary system with a separation of about 7 kpc. The orbital period should be around 100 Myr. The pair  also appears to be moving together supersonically through the ambient medium. Each of those galaxies has formed radio jets. If it were not for the binary the expected outcome might resemble the situation pictured in Figure 3. However, the binary motions cause each of the two galaxies to oscillate in its motion and this causes the radio jets to develop more complex, twisted shapes before they disrupt into tails. Figure 4 illustrates a preliminary effort to simulate this dynamics. The image on the right shows the real 3C75, where pink is the radio emission (VLA) and blue is thermal X-rays (Chandra). The image on the left traces the distribution of gas expelled by each of the two galaxies in the binary system. This simulation seems to capture the general character of the dynamical situation responsible for 3C75.

Figure 4: Right-the radio galaxy 3C75 as seen in the radio (pink) and associated thermal X-rays from the cluster Abell 400 (blue). Left-Simulated jets from a binary pair of galaxies intended to mimic the dynamics of 3c75.

Figure 4: Right-the radio galaxy 3C75 as seen in the radio (pink) and associated thermal X-rays from the cluster Abell 400 (blue). Left-Simulated jets from a binary pair of galaxies intended to mimic the dynamics of 3c75.

From this short set of simulation results it ought to be clear why many different kinds of radio galaxy structures are expected to form. It also ought to be apparent that we need better catalogs of what behaviors do exist in nature in order to see how to focus our simulation efforts and to establish what are the most important dynamical conditions in radio galaxy formation.

The Hunt Is On

One of our scientists Prof. Ray Norris put the call out to the Radio Galaxy Zoo community for a hunt on spiral galaxies hosting powerful radio sources.  The first known galaxy of this type is 0313-192, a galaxy much like our Milky Way and has left astronomers baffled.

IDL TIFF fileFigure 1: 0313-192 The wrong galaxy from the Astronomy Picture of the Day.  Credit: W. Keel (U. Alabama), M. Ledlow (Gemini Obs), F. Owen (NRAO, AUI, NSF, NASA.

Here is Prof. Norris’ post:

Keep an eye out for any hourglass sources that seem to be hosted by galaxies that look spiral in the infrared. These objects are incredibly rare in the local Universe (only 2 or 3 known) and we may not see any in Radio Galaxy Zoo, but if someone does find one, that would be worth writing a paper about (with the discoverer as co-author, of course). The rarity of radio-loud spirals is thought to be because the radio jets heat up and disrupt the gas in the spiral, switching off star formation, and turning the galaxy into a “red dead” elliptical. But we might find one or two where the jets have only just switched on and haven’t yet destroyed the spiral. See The radio core of the Ultraluminous Infrared Galaxy F00183-7111: watching the birth of a quasar for another example of this process in its very early stage. So keep your eyes peeled and yell out (very loudly) if you find one!

We are pleased to announce that the Radio Galaxy Zoo community has identified over a dozen potential candidates and we are in the process of following these up.

Have you seen any? Head over to Radio Galaxy Zoo to join in on the hunt and let us know what you find.

AAAS Symposium in Feb. 2015: Cutting-Edge Research with 1 Million Citizen Scientists

Some colleagues and I successfully proposed for a symposium on citizen science at the annual meeting of the American Association for the Advancement of Science (AAAS) in San Jose, CA in February 2015. (The AAAS is one of the world’s largest scientific societies and is the publisher of the Science journal.) Our session will be titled “Citizen Science from the Zooniverse: Cutting-Edge Research with 1 Million Scientists.” It refers to the more than one million volunteers participating in a variety of citizen science projects. This milestone was reached in February, and the Guardian and other news outlets reported on it.

Zooniverse-300x110

The Zooniverse began with Galaxy Zoo, which recently celebrated its seventh anniversary. Of course, Galaxy Zoo has been very successful, and it led to the development of a variety of citizen science projects coordinated by the Zooniverse in diverse fields such as biology, zoology, climate science, medicine, and astronomy. For example, projects include: Snapshot Serengeti, where people classify different animals caught in millions of camera trap images; Cell Slider, where they classify images of cancerous and ordinary cells and contribute to cancer research; Old Weather, where participants transcribe weather data from log books of Arctic exploration and research ships at sea between 1850 and 1950, thus contributing to climate model projections; and Whale FM, where they categorize the recorded sounds made by killer and pilot whales. And of course, in addition to Galaxy Zoo, there are numerous astronomy-related projects, such as Disk Detective, Planet Hunters, the Milky Way Project, and Space Warps.

Screen+Shot+2012-12-11+at+2.03.18+PM

We haven’t confirmed all of the speakers for our AAAS session yet, but we plan to have six speakers who will introduce and present results from the Zooniverse, Galaxy Zoo, Snapshot Serengeti, Old Weather, Cell Slider, and Space Warps. I’m sure it will be exciting and we’re all looking forward to it!

Explaining clustering statistics we use to study the distribution of Galaxy Zoo galaxies

I’ve used some statistical tools to analyze the spatial distribution of Galaxy Zoo galaxies and to see whether we find galaxies with particular classifications in more dense environments or less dense ones. By “environment” I’m referring to the kinds of regions that these galaxies tend to be found: for example, galaxies in dense environments are usually strongly clustered in groups and clusters of many galaxies. In particular, I’ve used what we call “marked correlation functions,” which I’ve found are very sensitive statistics for identifying and quantifying trends between objects and their environments. This is also important from the perspective of models, since we think that massive clumps of dark matter are in the same regions as massive galaxy groups.

We’ve mainly used them in two papers, where we analyzed the environmental dependence of morphology and color and where we analyzed the environmental dependence of barred galaxies. These papers have been described a bit in this post andthis post. We’ve also had other Galaxy Zoo papers about similar subjects, especially this paper by Steven Bamford and this one by Kevin Casteels.

What I loved about these projects is that we obtained impressive results that nobody else had seen before, and it’s all thanks to the many many classifications that the citizen scientists have contributed. These statistics are useful only when one has large catalogs, and that’s exactly what we had in Galaxy Zoo 1 and 2. We have catalogs with visual classifications and type likelihoods that are ten times as large as ones other astronomers have used.

What are these “marked correlation functions”, you ask? Traditional correlation functions tell us about how objects are clustered relative to random clustering, and we usually write this as 1+ ξ. But we have lots of information about these galaxies, more than just their spatial positions. So we can weight the galaxies by a particular property, such as the elliptical galaxy likelihood, and then measure the clustering signal. We usually write this as 1+W. Then the ratio of (1+W)/(1+ξ), which is the marked correlation function M(r), tells us whether galaxies with high values of the weight are more dense or less dense environments on average. And if 1+W=1+ξ, or in other words M=1, then the weight is not correlated with the environment at all.

Skibbaetal09_Fig2

First, I’ll show you one of our main results from that paper using Galaxy Zoo 1 data. The upper panel shows the clustering of galaxies in the sample we selected, and it’s a function of projected galaxy separation (rp). This is something other people have measured before, and we already knew that galaxies are clustered more than random clustering. But then we weighted the galaxies by the GZ elliptical likelihood (based on the fraction of classifiers identifying the galaxies as ellipticals) and then took the (1+W)/(1+ξ) ratio, which is M(rp), and that’s shown by the red squares in the lower panel. When we use the spiral likelihoods, the blue squares are the result. This means that elliptical galaxies tend to be found in dense environments, since they have a M(rp) ratio that’s greater than 1, and spiral galaxies are in less dense environments than average. When I first ran these measurements, I expected kind of noisy results, but the measurements are very precise and they far exceeded my expectations. Without many visual classifications of every galaxy, this wouldn’t be possible.

Skibbaetal12_Fig7a

Second, using Galaxy Zoo 2 data, we measured the clustering of disc galaxies, and that’s shown in the upper panel of the plot above. Then we weighted the galaxies by their bar likelihoods (based on the fractions of people who classified them as having a stellar bar) and measured the same statistic as before. The result is shown in the lower panel, and it shows that barred disc galaxies tend to be found in denser environments than average disc galaxies! This is a completely new result and had never been seen before. Astronomers had not detected this signal before mainly because their samples were too small, but we were able to do better with the classifications provided by Zooites. We argued that barred galaxies often reside in galaxy groups and that a minor merger or interaction with a neighboring galaxy can trigger disc instabilities that produce bars.

What kinds of science shall we use these great datasets and statistics for next? My next priority with Galaxy Zoo is to develop dark matter halo models of the environmental dependence of galaxy morphology. Our measurements are definitely good enough to tell us how spiral and elliptical morphologies are related to the masses of the dark matter haloes that host the galaxies, and these relations would be an excellent and new way to test models and simulations of galaxy formation. And I’m sure there are many other exciting things we can do too.

…One more thing: if you’re interested, you’re welcome to check out my own blog, where I occasionally write posts about citizen science.

Rocks… In… Spaaaaaace

You know those odd features in some SDSS images that look like intergalactic traffic lights?

examples of asteroids in SDSS images

Stop! Asteroid Time.

They aren’t intergalactic at all: they’re asteroids on the move in our own solar system. They move slowly compared to satellite trails (which look more like #spacelasers), but they often move quickly enough that they’ve shifted noticeably between the red, green, and blue exposures that make up the images in SDSS/Galaxy Zoo. When the images from each filter are aligned and combined, the moving asteroid dots its way colorfully across part of the image.

These objects are a source of intense study for some astronomers and planetary scientists, and the SDSS Moving Object Catalog gives the properties of over 100,000 of them. Planetary astronomer Alex Parker, who studies asteroids, has made a video showing their orbits.

I find their orbits mesmerizing, and there’s quite a lot of science in there too, with the relative sizes illustrated by the point sizes, and colors representing different asteroid compositions and families. There’s more information at the Vimeo page (and thanks to Amanda Bauer for posting the video on her awesome blog).

One of the most common questions we receive about asteroids from Galaxy Zoo volunteers is whether there will ever be a citizen science project to find them. So far, as the catalog linked above shows, the answer has been that computers are pretty good at finding asteroids, so there hasn’t been quite the need for your clicks… yet. There are some asteroids that are a little more difficult to spot, and those we’d really like to spot are quite rare, so stay tuned for a different answer to the question in the future. And in the meantime, enjoy the very cool show provided by all those little traffic lights traversing their way around our solar system.

The Green Valley is a Red Herring

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.

Example images of galaxies classified by you. There are blue, green and red spirals, and blue, green and red ellipticals.

Example images of galaxies classified by you. There are blue, green and red spirals, and blue, green and red ellipticals.

As @penguin galaxy (aka Alice) put it....

As @penguin galaxy (aka Alice) put it….

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.

The colour-mass diagram of galaxies, split by shape. On the right: all galaxies. On the left: just the ellipticals (or early-types) on top and just the spirals (or late-types) on the bottom. On the x-axis is the galaxy mass. On the y-axis is galaxy colour. Bottom is blue (young stars) and top is red (no young stars).

The colour-mass diagram of galaxies, split by shape. On the right: all galaxies. On the left: just the ellipticals (or early-types) on top and just the spirals (or late-types) on the bottom. On the x-axis is the galaxy mass. On the y-axis is galaxy colour. Bottom is blue (young stars) and top is red (no young 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.

We can time how fast a galaxy quenches. On the x-axis is the optical colour, dominated by young-ish stars, while on the y-axis is a UV colour, dominated by the youngest, most short-lived stars.

We can time how fast a galaxy quenches. On the x-axis is the optical colour, dominated by young-ish stars, while on the y-axis is a UV colour, dominated by the youngest, most short-lived stars.

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.

A cartoon version of our picture of how spiral galaxies shut down their star formation.

A cartoon version of our picture of how spiral galaxies shut down their star formation.

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.

A cartoon version of how we think ellipticals shut down their star formation.

A cartoon version of how we think ellipticals shut down their star formation.

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

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