New paper on active black holes affecting star formation rates!
Good news everyone, another Galaxy Zoo paper was published today! This work was led by yours truly (Hi!) and looks at the impact that the central active black holes (active galactic nuclei; AGN) can have on the shape and star formation of their galaxy. It’s available here on astro-ph: http://arxiv.org/abs/1609.00023 and will soon be published in MNRAS.
Turns out, despite the fact that these supermassive black holes are TINY in comparison to their galaxy (300 light years across as opposed to 100,000 light years!) we see that within a population of these AGN galaxies the star formation rates have been recently and rapidly decreased. In a control sample of galaxies that don’t currently have an AGN in their centre, we don’t see the same thing happening. This phenomenon has been seen before in individual galaxies and predicted by simulations but this is the first time its been statistically shown to be happening within a large population. It’s tempting to say then that it’s the AGN that is directly causing this drop in the star formation rate (maybe because the energy thrown out by the active black hole blasts out or heats the gas needed to fuel star formation) but with the data we have we can’t say for definite if the AGN are the cause. It could be that this drop in star formation is being caused by another means entirely, which also coincidentally turns on an AGN in a galaxy.
A random sample of galaxies which host a central active black hole used in this work. The disc vote fraction classification from Galaxy Zoo 2 is shown for each image. Images from SDSS.
These galaxies were also all classified by our wonderful volunteers in Galaxy Zoo 2 which meant that we could also look whether this drop in the star formation rate was dependent on the morphology of the galaxy; turns out not so much! If the drop in the star formation rate is being caused directly by the AGN (and remember we still can’t say for sure!) then the central black hole of a galaxy doesn’t care what shape galaxy it’s in. An AGN will affect all galaxies, regardless of morphology, just the same.
Galaxy Zoo Quench – Experience the Full Scientific Process
Experience Science from Beginning to End! Classify, Analyze, Discuss, and Collaboatively Write an Article!
Galaxy Zoo and other Zooniverse projects have given thousands the opportunity to contribute to scientific research. It’s time to take the role of volunteers to the next level. For the next two months*, this new Galaxy Zoo Quench project provides the opportunity to take part in the ENTIRE scientific process – everything from classifying galaxies to analyzing results to collaborating with astronomers to writing a scientific article!
Galaxy Zoo Quench will examine a sample of galaxies that have recently and abruptly quenched their star formation. These galaxies are aptly named Post-Quenched Galaxies. They provide an ideal laboratory for studying how galaxies evolve from blue, star-forming spiral galaxies to red, non-star-forming elliptical galaxies. Using the more than a million galaxies in the Sloan Digital Sky Survey, we identified ~3000 post-quenched galaxies. By classifying these galaxies and analyzing the results, we will explore the mechanisms that quenched their star formation and investigate the role of post-quenched galaxies in galaxy evolution.
The entire process of classifying, analyzing, discussing, and writing the article will take place over an 8 week period*, beginning July 18th. After classifying the galaxies, volunteers will use the tools available within Zooniverse to plot the data and look for trends. Through reading articles and interaction in Talk, volunteers will gain background information. Throughout, they’ll discuss with the science team their interpretation of the results. At the end of the process, volunteers and the science team will collaboratively write a 4-page Astrophysical Journal article.
What causes the star formation in these galaxies to be quenched? How do interactions impact galaxy evolution? What is the fate of our Milky Way? Join us this Summer (or Winter if you’re below the equator!) in exploring these questions, being a part of the scientific process, and contributing to our understanding of this dynamic phase of galaxy evolution!
CLICK HERE TO PARTICIPATE!
We’ll be sharing more details about this project during the next Galaxy Zoo Hangout, on Monday, July 15th at 14:00 CST / 19:00 GMT / 20:00 BST. Have questions about the project? Post them here or tweet at us (@galaxyzoo). Just before the Hangout starts, we’ll embed the video here so you can watch from the blog.
The best way to send us a comment during the live Hangout is through twitter (@galaxyzoo). You can also leave a comment on this blog post, or on Google Plus, Facebook or YouTube. See you soon!
Update: here’s the hangout (and the mp3 version)!
*Note: science timelines often subject to a factor of two uncertainty. We’ll do our best to keep on track, at the same time expecting the unexpected (all part of the fun of doing science!).
Oh, Sweet Spiral Of Mine
See the video of our latest hangout here (or, if you prefer, click to download the podcast version):
Spiral galaxies are seemingly endless sources of fascination, perhaps because they’re so complex and diverse. But why does spiral structure exist? Why do some spiral galaxies have clearly defined spiral arms and others have flocculent structure that barely seems to hold together? What’s the difference between a 2-arm spiral and a 3-arm spiral? How many kinds of spirals do we actually observe? And what is happening to the stars and gas in spiral galaxy disks?
Clockwise from top right: X-ray, UV, optical, near-IR, mid-IR, far-IR, radio
All of the above questions are related to a question we got right at the end of our last hangout: what is the significance of the number of spiral arms? Determining how many spiral arms a galaxy has is hard, and is often subjective — so why bother?
It’s a good question. Part of the reason spiral arm classification & count is a challenge is that it often depends on the wavelength at which you observe a galaxy. New stars tend to form along the spiral arms, whereas older stars have time to spread out into more uniform orbits. So ultraviolet observations of a galaxy, which tend to pick out the new and bright stars, often highlight the spiral arms much more strongly than longer-wavelength observations, which see more light from older stars.
It’s not quite that simple, though. As you get to longer and longer wavelengths, you start to pick up the heat radiated by clouds of gas and dust, which are often stellar nurseries — and often trace spiral arms. At a wavelength of 21 centimeters you can detect neutral Hydrogen, which provides raw material for the cooling and condensation of gas into cold, dense molecular clouds that form stars in their densest pockets. Each wavelength you observe at provides a glimpse at a different targeted feature of a spiral galaxy.
A map of neutral Hydrogen in the Milky Way — complete with yellow “you are here” arrow.
Including our own, of course: we live in a spiral galaxy (though how many arms it has, and whether it’s flocculent, is a matter of debate), and it provides the best means of studying star formation up close. When studying other galaxies, it’s easy to get caught up in the race to discover the biggest, the smallest, the farthest and the most extreme, and forget that our own Universal neighborhood is pretty amazing too.
Herschel sees much longer wavelengths than HST, so its resolution isn’t as high even though it has a bigger mirror. (Click to see a larger version.) Credit: ESA/NASA
For example, one of the most famous nebulae in the world was recently imaged by one of the most famous telescopes in the world — again — but this time in the near-infrared. The Horsehead Nebula is a well-known feature in the Orion star-forming complex, and the new Hubble images provide a great opportunity to learn even more about this region that has been studied for hundreds of years. How thick and cold is the gas and dust in the nebula? How long will it take for it to dissipate under the harsh radiation of the bright, young stars near it? What’s going on behind it?
The near-infrared view from HST is sort of the sweet spot in this spectacular space — the wavelengths aren’t so long that the resolution suffers, but they are long enough that you see through a bit more of the clouds than in the optical. So you see more of the structure of the cloud itself, and more of where it’s thin and thick. If you zoom in, you can even see distant galaxies peeking through! And not just on the edges: in some parts you can see galaxies through the middle of the nebula. Wow. This image alone contains spiral galaxy insights big and small, near and far, from the very distant universe and right in our own backyard.
Note: right at the end of the hangout, we again got another great question from a viewer that we didn’t have time to answer. So stay tuned for the next hangout when we just might have a thing or two to say about dark matter, dark energy and new projects!
Two Atoms Populate on a Dust Grain
I enjoy days where we get to use questions from the public to meander our way through the Universe. Our latest live hangout saw us discussing the latest update to the Galaxy Zoo site — made based on your clicks! — and doing a live, collective classification on a few example objects from our Hubble sample that we hope represent the kind of things you’ll be seeing more of from now on.
We debated, for example, whether this galaxy’s central “feature” was a bulge or a bar:
We also discussed whether these galaxies are merging or overlapping.
Whether this relatively featureless galaxy’s blue smudge indicates a voorwerp:
Depending on the redshift, a voorwerp in Hubble could be blue, green or red.
And how many spiral arms this galaxy has:
Also, these two galaxies may have roughly the same proportions between them as between the Milky Way and the Large Magellanic Cloud.
We also talked about the origin and importance of dust in galaxies, and just what a green pea would look like in the Hubble data. Green peas are galaxies with incredibly high rates of star formation. They’re rare in the local Universe, but how rare do we think they were billions of years ago, at the epoch we’re looking back to with Hubble?
Thing is, a “green pea” at redshift z=2 would be bright red.
And, for that matter, what were the stars like then? Astronomers very broadly group stars into three populations depending on their composition. The very earliest stars were made from the primordial elements forged during the Big Bang — almost entirely Hydrogen and Helium, nearly devoid of anything else (we call “anything else” a metal, including elements like Carbon and Oxygen). The next generation of stars had some metals, but the Universe has been around long enough that those stars (even the lower-mass ones that live for a long time) are past their prime and a new generation, one with compositions generally like our Sun, are now in their heyday.
Naturally, though, since the Sun is our First Star, we call its generation Population I. The slightly older stars, many of which are still around and living in our galaxy and others, are Population II; and the very massive rockstars of the early universe that have all died out are called Population III. So “Pop III” were the first stars — a slight reversal, but labels and names that seemed like a better idea at the time than with hindsight are nothing new in Astronomy. (Exhibit A: the magnitude system. Exhibit B: “planetary nebula“.)
Bonus: green peas, voorwerpjes, and planetary nebulae are just three of the phenomena that (at least in part) glow green to human eyes because of one particular frequency of light emitted by Oxygen at a certain temperature, an atomic transition seen only rarely on Earth but fairly often in the Universe.
This VLT image shows the planetary nebula IC 1295 in ghostly green. Image Credit: ESO
Also, did you know that dust grains are the singles bars of the atomic universe, allowing atoms to meet and combine into molecules and cooling the gas clouds they live in — which in turn helps new stars form? Heating and cooling, gravity and pressure, and the interplay between atoms, molecules, and radiation are all a part of what gives us this amazingly diverse Universe. We understand quite a lot of it given that we are such a tiny part of it, but what we know is dwarfed by what we don’t. And that’s just the way astronomers like it… we love a challenge and we’re glad to have as much help as possible sorting things out.
Here’s the hangout video:
Galaxy Zoo 