Archive by Author | Brooke Simmons

Galaxy Zoo CANDELS

screen-shot-2016-10-11-at-12-39-47-pm

We submitted the Galaxy Zoo CANDELS paper in May. Now, after some discussion with a very helpful referee, the paper is accepted! I hope our volunteers are as thrilled as I was to get the news. It happened within days of the Galaxy Zoo: Hubble paper acceptance. Hurray!

gzc_title_abstract

Spot the typo! (No, just kidding.) (Well, sort of. There is one, but it’s not easy to find and it’s pretty inconsequential.) This is not quite the longest paper I’ve ever written, but it is the longest author list I’ve ever been at the top of. It includes both Galaxy Zoo and CANDELS scientists. And the volunteers are acknowledged too, in that first footnote. A lot of people did a lot of work to bring this together.

If you’d like to read the paper, it’s publicly available as a pre-print now and will be published at some point soon in the Monthly Notices of the Royal Astronomical Society. The pre-print version is the accepted version, so it should only differ from the eventual published paper by a tiny bit (I’m sure the proof editor will catch some typos and so on).

The paper may be a little long for a casual read, so here’s an overview:

  • We collected 2,149,206 classifications of 52,073 subjects, from 41,552 registered volunteers and 53,714 web browser sessions where the classifier didn’t log in. In the analysis we assumed each of those unique browser sessions was a separate volunteer.
screen-shot-2016-10-11-at-12-39-23-pm

Most subjects have 40 classifications apiece, although some were retired early from active classification and others were classified further, until about 80 volunteers per galaxy had told us what they thought.

  • The raw consensus classifications are definitely useful, but we also weighted the classifications using a combination of “gold standard” data and consensus-based weighting. That is, classifiers were up- or down-weighted according to whether they could tell a galaxy apart from a star most of the time, and then the rest of the weighting proceeded in the same way it has for every other GZ dataset. No surprise: the majority of volunteers are excellent classifiers.
  • 6% of the raw classifications were from 86 classifiers who both classified a lot and gave the same answer (usually “star or artifact”) at least 98% of the time, no matter what images they saw. We have some bots, but they’re quite easy to spot.
  • Even with a pretty generous definition of what counts as “featured”, less than 15% of galaxies in the relatively young Universe that this data examines have clear signs of features. Most galaxies in the data set are relatively smooth and featureless.
  • Galaxy Zoo compares well with visual classifications of the same galaxies done by members of the CANDELS team, despite the fact that the comparison is sometimes hard because the questions they asked weren’t the same as what we did. This is, of course, a classic problem when comparing data sets of any kind: to some extent it’s always apples-vs-oranges, and the devil is in the details.
screen-shot-2016-10-11-at-12-45-06-pm

We devote an entire section of the paper to comparing with the CANDELS-team classifications (from Kartaltepe et al. 2015, which we abbreviate to K15 in the paper). The bottom line: the classifications generally agree, and where they don’t we understand why. Sometimes it’s because there’s interesting science there, like mergers versus overlaps. The greyscale shading is a 2-D histogram; the difference in the blue versus red points is in which axis was used to separate the galaxy into bins so that the average trends could be computed.

  • By combining Galaxy Zoo classifications with multi-wavelength light profile fitting — where we fit a 2D equation to the distribution of light in a galaxy, the properties of which correlate pretty well with whether a galaxy has a strong disk component — we’ve identified a population of likely disk-dominated galaxies that also completely lack the features that are common in disk galaxies in the nearby, more evolved Universe. These disks don’t have spiral arms, they don’t have bars, they don’t have clumps. They’re smooth, but they are disks, not ellipticals. They tend to be a bit more compact than disk galaxies that do have features, even though they’re at the same luminosities. They’re also hard to identify using color alone (which echoes what we’ve seen in past Galaxy Zoo studies of various different kinds of galaxies). You really need both kinds of morphological information to reliably find these.
  • The data is available for download for those who would like to study it: data.galaxyzoo.org.

With the data releases of Galaxy Zoo: Hubble and Galaxy Zoo CANDELS added to the existing Galaxy Zoo releases, your combined classifications of over a million galaxies near and far are now public. We’ve already done some science together with these classifications, but there’s so much more to do. Thanks again for enabling us to learn about the Universe. This wouldn’t have been possible without you.

9 Years of Galaxy Zoo

Last year we had so much fun celebrating all that we (including you) had accomplished over the first 8 years of Galaxy Zoo. This year, for our 9th birthday, we thought we’d hand things directly over to you. We sent out a newsletter asking people about their favorite Galaxy Zoo science. We asked people to rank five choices:

  • Hanny’s Voorwerp & the Voorwerpjes (ionized clouds and active galaxies)
  • Green Peas (highly compact & star-forming galaxies)
  • Red spirals (disk galaxies with no/little star formation)
  • Blue ellipticals (spheroid galaxies with ongoing/retriggered star formation)
  • Bars (the galaxy kind; how this mode of disk galaxies drives galaxy evolution)

We’ve now collected just over 200 responses and combined your rankings. Although the distributions were pretty similar, and all the options had plenty of people choosing it as their favorite, one of the options jumped out as a pretty clear leader (at least in this rather informal poll).

Bars - the galaxy kind!

Bars – the galaxy kind!

Of course, the list we asked people to choose from is by no means complete, especially if you include not just the main Galaxy Zoo but also its related projects. In the “Other” box we had a variety of entries, with some mentioning galaxies found in Radio Galaxy Zoo and others citing those seen in Galaxy Zoo: Bar Lengths. Plenty of people mentioned galaxy mergers, and gravitational lenses got a few mentions too! If we had a complete list the rankings would likely be different, but then again, that would be such a long list I’d be worried many fewer people would want to answer.

We also had a space for people to enter whatever text they wanted at the end of the survey, and the responses were varied, interesting, and a treat to read. Here’s a sample (each paragraph is a separate comment):

I do not spend a lot of time here, but when I have the time, I love it. Thank you!

What a great way to feel like a scientist.

I’ve been an on-and-off participant in the Zooniverse citizen science projects since I was 13 years old – and Galaxy Zoo has been one of my favourites for a while! I just wanted to say thank you for providing the opportunity for an ordinary teenager to feel included in fascinating scientific research – that experience has inspired me to pursue a degree in Physics and Astronomy in the fall.

Go Science!

We were also curious about who, as a group, we were asking these questions of. It turns out that quite a large fraction of people who responded to the survey have been with us since the early days, which is so lovely. And we were also delighted to see people engaging with us who’ve just recently discovered Galaxy Zoo. We are so glad all of you are collaborating with us; here’s to many years to come.

Thank you!

P.S. – The big 10 is coming next year… what would you like to see for the occasion?

Go West, Young (?) Astronomer

Sunset in Southern California

Many bargains must be made in pursuit of an academic career, and chief among them is an openness to a nomadic early-career life in exchange for a better chance at staying permanently put somewhere later. Grad students and postdocs move around. Not only do we travel all over the world sharing and discussing our research, but the relatively short duration of postdocs, and the fact that in astronomy doing at least 2 of them is now the norm, means we regularly pull up roots and dash off to live somewhere else. My friends have collectively done postdocs on all continents, including Antarctica. Including places thousands of miles from friends and family; including places where they can neither read nor speak any of the native languages.

In this context, I am so, so lucky. My first postdoc moved me only a medium distance (across just one ocean), and to a place where I could at least understand the words, even if I didn’t always get every nuance of meaning. At Oxford I made lifelong friends and built great collaborations, and I thought the research itself was pretty good, too.

Turns out NASA agrees with me. Last year I applied for and was awarded an Einstein Fellowship, which is an early-career award lasting 3 years, an independent postdoc that can be taken to any institution in the US. They’re very competitive (I had applied the previous year without success), and I was thrilled to be awarded one at my top-choice host institution. My first day was last week.

Here’s what the 2015 Fellows page has to say about my research plans:

Brooke uses a variety of multi-wavelength data, including highly accurate galaxy morphologies from the Galaxy Zoo project, to research the connection between supermassive black holes and the galaxies that host them. This connection appears to exist over many orders of magnitude in black hole and galaxy mass, but its fundamental origin is still a puzzle. As an Einstein Fellow at the University of California, San Diego, Brooke will investigate supermassive black hole growth in the absence of galaxy mergers, using a rare sample of galaxies which have never had a significant merger yet host growing black holes. These active nuclei, selected because their host galaxies lack the bulges which inevitably result from a galaxy merger, provide powerful leverage to disentangle the complex drivers of black hole growth and determine the origin of observed black hole-galaxy correlations.

During my fellowship I’m planning on moving forward with the research we first published in 2013 investigating bulgeless galaxies with growing black holes. That is: it’s Galaxy Zoo research.

Galaxy Zoo research brought me to Oxford, and now it has brought me to California. UCSD is a great place, and I’ve already made some really excellent scientists. UCSD is also part of the Southern California Center for Galaxy Evolution and has access to some of the world’s best telescopes, so the future is full of potential.

For now, though: I wouldn’t be here, watching sunsets from my office, without your contributions to Galaxy Zoo over the years. Thank you.

(Galaxy) Bars in the Summer

This is a guest post by Freya Pentz, who has spent much of this summer doing research with Galaxy Zoo.

Hi Galaxy Zoo volunteers!

I’m a summer student at the Zooniverse. I’m at university studying natural sciences about to go into my second year and for the past 5 weeks I’ve been working at the Zooniverse office here in Oxford. I wanted to let you know what I’ve been doing during that time.

I’ve been using data from the Galaxy Zoo: Bar Lengths project, writing code to process the information and making sure it looks sensible. Before I started working at the Zooniverse, I had done very little computing so I had to learn a lot! For those of you who are interested, I’ve been using python to extract the measurements you did on the galaxies and plotting graphs with all the data. Learning how to use python was like learning another language but it was definitely worth it.

The first thing I did was to find out how many of the galaxies that you’ve classified have bars. That meant looking at the answers to the first question about the galaxy in the Bar Lengths project ‘Does this galaxy have a bar’ and seeing for each galaxy if most people answered ‘Yes’ or if most people answered ‘No’.

Does this galaxy have a bar? Zooniverse interface

Remember this question?

Luckily, the code could do that for me; otherwise I would have had to look at over 66000 answers! So far, 4960 galaxies have been classified out of a total of 8612 in the project. Your classifications show that 700 of these have a bar, meaning that the fraction of classified galaxies with a bar is around 14%. This is similar to the 10% bar fraction referred to in the study recently done by the Galaxy Zoo and CANDELS teams on bar fractions out to z=2 (blog post & paper). This number will probably change a little bit as more galaxies get classified, but it’s good that it is similar to known values so far.

The next thing was of course to find the lengths and widths of the bars. When you draw lines on the galaxy to mark the length and width, the database records this as coordinates. Each line has four coordinates, 2 x coordinates and 2 y coordinates. Once you have the coordinates, it’s fairly simple to turn them into lengths. All you need is some Pythagoras. When plotting a histogram of the lengths, the shape was a Gaussian distribution, or a bell curve. This shows that most of the galaxies have lengths between certain limits (5-15 kpc) and then as you go beyond these limits, the number of galaxies decreases.

During my time here, I found some interesting galaxies. When I first looked at the redshifts, there was a galaxy with a redshift of 4.25. I mentioned this to a couple of people on the Zooniverse team and they all said there wouldn’t be a galaxy with such a high redshift in the sample. I checked it out and this is the galaxy in question:

Image of a galaxy with lens-like-but-apparently-not-a-lens structure

The galaxy that fooled the computer into thinking it had a redshift of 4.25

You can see that there is a bright blue smudge in the top left of the galaxy. When I first saw this, I thought it was a lens. It looks like one, and you can just see a small bit of blue on the other side of the galaxy’s core, suggesting a lens even more. According to the experts in the Zooniverse however, this is probably not a lens, as the galaxy does not look massive enough to lens light. Also, the blue curve is well inside the galaxy, instead of being around the outside. Usually, all the mass of the galaxy is needed to lens an object so the light would appear around the edge. The blue curve is most likely an unusual feature of the galaxy itself, which can explain why the reported redshift is so high. The redshift for this galaxy was measured photometrically. This is where astronomers use galaxy colours across a wide range of wavelengths to predict the likely redshift. This method of measuring redshift is much more prone to error than spectrometry (where the absorption lines for certain elements in a galaxy are observed and the shift of these lines is measured) so the blue smudge could have easily made the telescope think the redshift was higher than it is. This redshift is therefore almost definitely a mistake. We also know this from the high resolution of the image. You normally wouldn’t be able to see a galaxy with even a redshift of 1 this well!

The reason telescopes have to use photometric redshifts sometimes even though they are often wrong is that there is not enough time to take a spectrum of every galaxy when you are conducting a large survey of the sky. Telescope time is expensive and photometric measurements allow you to get a bit of information about lots of galaxies which can sometimes be more useful that getting a lot of information about a few.

When running into problems like this it was really useful to be able to look at a picture of the galaxy on the Galaxy Zoo: Bar Lengths website. Looking at the galaxies and seeing in real life what the data on the graphs was telling me was probably my favourite part of my time at the Zooniverse. It’s so amazing that thanks to the Sloan Digital Sky Survey, the Hubble Space Telescope projects and other mass surveys of the universe, we can actually look at pictures of thousands of galaxies easily.

A cool barred galaxy you can see in Bar Lengths

A cool barred galaxy you can see in Bar Lengths

The Zooniverse is such a cool organisation and I’m lucky to have worked for them this summer. The great thing about them is that you can get involved too! I know from my work with Bar Lengths that even if a few people log on and classify in any of the projects, it can be really helpful. None of the science can be done without you providing the data.

Measure some galaxies here:

www.zooniverse.org/projects/vrooje/galaxy-zoo-bar-lengths/classify/

Or have a look at some of the other projects here:

www.zooniverse.org

Freya

Eight Years and the 8 Most Talked-About Galaxies in Galaxy Zoo

A montage of mostly runners-up

Continuing the countdown to Galaxy Zoo’s 8th birthday, below are 8 of the most-commented-on galaxies in the active Galaxy Zoo. They range near (in astronomical terms) and far, from gorgeous disks to space-warping groups, and some of them aren’t even galaxies at all!

8. Galaxies Interacting (Arp 112)

image of Arp 112

#merger #arc #g-pair #bulge #tidaltails #ugc #wow #agn #ngc #ngc7806 #arp #markarian #dustlane #available_in_dr7 #spiral #gpair #awesome #tidal #lens #no_lens

A lovely example of the diversity of structures in the Universe. The central galaxy may have been a perfectly symmetric spiral before it was seriously disturbed by the elliptical galaxy on the left side of the shot, and what’s that wispy thing off to the right? Is it a former part of the central galaxy? And what is this all going to look like in a few billion years? Whatever happens, the volunteers made it clear this is a special one to classify and to look at.

7. A Beautiful Lens

Gravitational lens in CANDELS

#lens #lensing #knownlens #arc #lense #interesting

This gorgeous gravitational lens was spotted almost immediately upon the launch of the new Galaxy Zoo within the high-redshift CANDELS data. It generated multiple lively discussions and scientists and volunteers alike weighed in with further information. It turned out in this case that this was one of very few lenses that were already known, but there are likely still unknown lenses buried in the data, waiting to be discovered!

6. Emerald in the sky

image of this mysterious green-gem object

#quasar #edge #lbg #star

Initially identified as a high-redshift star-forming galaxy by one of our seasoned volunteers, a number of people subsequently looked further into the existing scientific literature. There was a lot of debate about this particular point of light, but in the end the volunteers uncovered a later paper confirming that this green gem (which would actually be either very red or nearly invisible to the human eye, as it’s “green” because it only shows up in the infrared filters used for this image) is actually just a star in our galaxy. Bummer, maybe, but this process is also an important part of science.

5. Lord of the Polar Ring

image of polar ring galaxy

#dustlane #polar #polarring #beautiful #polar-ring #elliptical #ring #edgeon #mothership #dust #polaring #question

This spectacular example of a polar ring galaxy couldn’t have been found in the original Galaxy Zoo or Galaxy Zoo 2, because it only made it into the Sloan Digital Sky Survey when the sky coverage was extended.

4. Collisional Ring (Arp 148)

Mayall's Object in the Sloan Digital Sky Survey

#merger #arp148 #arp #available_in_dr7 #lookalike #alphabet #ring

It takes a special kind of galaxy crash to make a collisional ring, and you can see this one in progress. It reminded our volunteers and scientists of the Cartwheel galaxy, another spectacular example of these snapshots of a brief moment in time.

3. Triangular Galaxy!

A disturbed spiral galaxy.

#merger #odd #dark #needle #holycow #wow #doublenucleus #tidaldebris #disturbed #rocket #cluster #irregularshape #spaceship #rocketship

Well, this is odd. This galaxy looks like it’s on its own, but it has a rather unusual shape that would usually imply some sort of interaction or collision. Our volunteers discussed what could be causing it – until they viewed a zoomed-out image and it became clear that this galaxy has recently flown by a trio of galaxies, which would be more than enough to disrupt it into this lovely shape.

triangle galaxy and a nearby group of galaxies

2. Hubble Resolves the Distant Universe

edge-on disk galaxy with dust lane

#spiral #overlap #dustlane #starburst #edge-on #edgeon

When a new batch of data taken by the Hubble Space Telescope appeared on the latest Galaxy Zoo, this was one of the first stunners remarked on by several people. Some of the parts of the sky covered by Hubble coincide with the Sloan Digital Sky Survey, and we linked the surveys up via Talk. Our tireless volunteers launched a thread collecting side-by-side images from SDSS and Hubble, showcasing the power of the world’s greatest space telescope. Hubble’s primary mirror is about the same size as that used by the SDSS, so the differences between the images of the same galaxy are due to the blurring effect of the atmosphere.

SDSS (on Earth) at left, HST (in space) at right.

SDSS (on Earth) at left, HST (in space) at right.

And, the most talked about image in the latest Galaxy Zoo is…

1. ……..really? this?

A barely-there galaxy.

It’s always the galaxies you least expect.

Okay, okay… If you saw this and said it looks like there isn’t a lot to talk about here, I wouldn’t blame you. And, indeed, there’s only one “short” comment from one of our volunteers, who used our Examine tools and discovered that this little blotch appears to be a very high-redshift galaxy.

However, that same volunteer also started a discussion with the question: just for fun, what’s the highest redshift you’ve found? Others responded, and thus began a quest to find the galaxy in Galaxy Zoo that is the farthest distance from us. This discussion is Galaxy Zoo at its finest, with new and experienced volunteers using the project as inspiration for their own investigations, scouring the scientific literature, and learning about the very early Universe.

It seems like the most likely known candidate so far is a quasar at a redshift of about 5.5 (at which point the Universe was about 1 billion years old), or, if you don’t think a quasar counts, an extended galaxy at = 4 or so (1.5 billion years old). But there’s just so much science wonderfulness here, all of it from our fantastic volunteers, and it all started with a patchy blob and a sense of curiosity.

Galaxy Zoo started with a million blobs (ish) and a sense of adventure. I think that’s fitting.

A Galaxy Walks Into A Bar…

4 barred galaxies from Galaxy Zoo projects past

We are pleased to announce that a Galaxy Zoo project is one of the first projects built on the new Zooniverse! Several years ago we measured the lengths of galactic bars in relatively nearby galaxies in the Sloan Digital Sky Survey, and Ben Hoyle wrote an excellent paper presenting new an interesting results on how bars, which are a distinct feature caused by a change in the nature of the orbits of some of the stars in a galaxy, relate to other physical properties of the galaxy, such as color (indicative of recent star formation) and the nature of spiral arms or rings. That work showed the power of measurements like these, which are not always easy for computers to get right.

Today, we’re hoping you’ll help us extend that set of detailed galaxy measurements into the distant Universe, with measurements of bars in about 8,000 galaxies from our previous projects using Hubble Space Telescope data, including the AEGIS, CANDELS, COSMOS, GEMS and GOODS surveys.

We’ve deliberately been pretty broad in our selection of galaxies which may have a bar, so the first thing the project asks you is to confirm whether you think the galaxy does indeed have one. There are many examples of barred and not-barred galaxies (including examples of sort-of-looks-like-barred-but-actually-isn’t-and-here’s-why) included in the project, and you can access them anytime by clicking the “Need some help?” button.

Galaxy Zoo Bar Lengths classification interface screen shot

We’ve also zoomed in on the central galaxy to make it easier to classify.

If the galaxy doesn’t have a bar, then you can move on to the next one. If it does, there are some follow-up questions about spiral arms and rings, and then we ask you to draw 2 lines on the image: one for the bar width and one for its length.

You can also join in the discussions after the classifications with our new Talk discussion tool, which is completely separate from the main Galaxy Zoo Talk (just like the rest of the project).

On a more personal note, this is a big step forward for the Zooniverse as a whole. The first draft version of this project came together in under 1 hour back in April. Afterward, we shared project links between science team members and iterated back and forth on the right questions to ask and the right data to use. This process would normally take at least 6 months and require a lot of one-on-one time with a Zooniverse developer. Instead, because the Zooniverse development team has done a brilliant job creating a Project Builder that’s flexible, powerful and also easy to use, we were able to create a new project in a way that’s analogous to, well, creating a blog.

In these early days of the new site’s release I’m sure there will be some bugs that need zapping, but even so the new capabilities of the Zooniverse are phenomenal. I suspect this is just the first of many new projects to be spun up in the New Zooniverse. (In fact, there are 3 more projects debuting alongside ours.)

Try it out here: Galaxy Zoo: Bar Lengths


bars written in galaxies

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.

One Million for Zooniverse – and One for Galaxy Zoo!

one million in galaxies

Galaxy Zoo started in 2007 because astronomers had 1,000,000 galaxies that needed to be sorted, classified, and examined. After the incredible response from the public, the zookeepers realized that this kind of problem wasn’t limited to galaxies, nor even just to astronomy, and the Zooniverse was born.

Now, seven actual years, close to 30 projectsmore than 60 publications, and hundreds of years’ worth of human effort later, the Zooniverse has just registered its 1,000,000th volunteer. Given that Galaxy Zoo was the project that led to the creation of the Zooniverse, it seems fitting that its millionth citizen scientist joined to classify galaxies! That volunteer (whose identity we won’t divulge unless s/he gives us permission) joins over 400,000 others who have classified galaxies near and far. That number is 40% of the Zooniverse’s overall total — meaning that, while Galaxy Zoo has a large and vibrant community of volunteers and scientists, most people who join Zooniverse start off contributing to a different project. Many of them try other projects after their first: over on the Zooniverse blog Rob described the additions we’ve made to the Zooniverse Home area so that everyone who brought us to a million can see their own contribution “fingerprint” on the Zooniverse. Here’s what mine currently looks like:

The blue at the left is Galaxy Zoo; the orange is Snapshot Serengeti. #addict

The blue at the left is Galaxy Zoo; the dark orange is Snapshot Serengeti. #addict

Our millionth volunteer gets a cheesy prize (but hopefully useful: a Zooniverse tote bag and mug), and while we’d like to give that same prize to the 999,999 who came before him/her and to everyone who contributes to Galaxy Zoo and all Zooniverse projects, perhaps it’s more fitting that we say to everyone what’s really on our mind right now:

Galactic-Scale Gratitude. You all are awesome.

Galactic-Scale Gratitude. You all are awesome.

RGZ Team Spotlight: Lawrence Rudnick

Meet Larry Rudnick, Project Advisor for Radio Galaxy Zoo. Larry is often seen chatting to volunteers on RGZ Talk; now he’s written a bit more of an introduction.

Lawrence (Larry) RudnickI’m a Distinguished Teaching Professor of Astrophysics at the University of Minnesota. But before I was distinguished, I grew up in Philadelphia, where I decided in 6th grade that I would go into science while I was helping my grandfather pour molten lead into molds to make fishing sinkers. My bachelor’s and PhD degrees are in physics, but astrophysics was what really kept me up thinkng at night. I’ve worked mostly in the radio part of the spectrum, using telescopes all over the world, plus some work in X-rays and infrared. I’ve studied radio galaxies, since the late 70s when Frazer Owen and I introduced a classification system for tailed radio galaxies. Identifications were pretty painful then, taking about an hour each to get the radio and optical photographs lined up. We’ve come a long way! My students and I also spent some years studying the radiation from the supernova remnant Cassiopeia A and others, producing the first 3D image of an explosion. Today, my work focuses on clusters of galaxies and their connections with large scale structure.

The most interesting course I teach is one called “Nothing” where we explore everything from the vacuum, to the number zero, to blind people seeing nothing, to placebos, to King Lear. I’ve done a lot of K-12 work, training teachers in using hands-on science activities, and do a lot of public education, through lectures, radio and TV interviews, and working with our local Planetarium. Radio Galaxy Zoo is my first citizen science project, and I’m really looking forward to how much we’re going to learn.

The Curious Lives of Radio Galaxies – Part Two

This is the second half of a description of radio galaxies from Anna Kapinska, Radio Galaxy Zoo science team member.

Last time we discussed the early and mid stages of radio galaxy life that take up the majority of the radio galaxy lifetime. Today we will go much further following paths of aging radio galaxies.

‘Only few of us get here’
As we discussed last time, radio galaxies are typically between tens and hundreds of kilo- parsecs in size (30 thousands – 3 million light-years). However, some of our buddies will grow to enormous sizes. Once a radio galaxy reaches one Mega-parsec in size (3.3 million light-years across) it’s called a giant – that is, a giant radio galaxy. Not every radio galaxy will reach such enormous sizes; only the most powerful ones whose environments are not extremely dense do. We don’t see too many giant radio galaxies. There are two main problems. One is that they are of low radio luminosity, and so our telescopes are not always sensitive enough to detect more than a subset of radio galaxies reaching this stage of their lives. The other problem is that giants are often composed of numerous bright knots spread over a large area and it’s difficult for us to tell which of these knots are associated with the giant and which are from unrelated sources.

Fig5-3C236

Figure 5: 3C 236, the second largest giant radio galaxy, extends nearly 15 million light- years across. It is located at a redshift z = 1.0 and has angular dimensions of over 40 arc-minutes in radio images. Credits: NVSS, WRST, Mack et al. 1996.

Giant radio galaxies are usually hundred of thousands, or more, years old and they are very large and extended. They can tell us a lot about what is going on within the space in between galaxies in groups and clusters, and that’s why radio astronomers cherish these giants! The largest giant radio galaxy known is 4.5 Mega-parsecs across (named J1420-0545), which is almost 15 million light-years! In radio images these radio galaxies extended over 20 or 30 arc-minutes, which means you will normally see only one of their lobes at a time in any of the Radio Galaxy Zoo images we classify. This is also the reason why we would tag these Radio Galaxy Zoo images as #overedge or #giants.

Fading away
But radio galaxies will not grow to infinity, they will eventually die. What happens then is that the radio galaxy starts fading away. Physically, at this point, the supermassive black hole stops providing jets with fresh particles, which means the jets and lobes or radio galaxy are not fed with new material. The electrons in the radio galaxy lobes have a finite amount of energy they can release as light, and so the lobes simply fade away until they are no longer visible with our telescopes. Dying radio galaxies become progressively less powerful, and less pronounced: no bright jet knots nor hotspots are present within the lobes anymore (see Figure 6). Eventually we can’t see these radio galaxies any more with our telescopes. You would typically mark these radio sources as #relics or #extended in Radio Galaxy Zoo images. It takes only ten thousand years for the brightest features of radio galaxy lobes to disappear, which is barely 0.01% of the total lifespan of radio galaxy. Again, just as the birth, it’s a blink of an eye!

Fig6-b20924+30

Figure 6: A prototypical dead (relic) radio galaxy, B2 0924+30. Radio waves are imaged in green, while optical image from SDSS is in yellow-red. The angular dimension of this galaxy is 13.5 arc-minutes, and it lies at a redshift z = 0.027, what translates to 430 kilo-parsec (1.4 million light-years) in size. Credits: Murgia

Rebirth
So is that the end?

Well… not really! Astronomers have seen evidence that radio galaxies re-start. What does that mean? That means radio galaxies sort of resurrect. After switching off, the supermassive black hole is radio silent for a while, but it can become active again; that is the whole cycle of radio galaxy life can start all over again. A single host galaxy can have multiple radio galaxy events. We still don’t know the ratio of how long the galaxy is in quiet, silent stage, to how long is in its active, violent radio galaxy forming stage. We also do not know if all galaxies go through the active, radio galaxy forming stage, or whether it’s just some of them. And we don’t know what exactly is the process that makes the galaxies switch on and off. But details on that… that’s yet another story!

Fig7-b1545-321

Figure 7: Re-started radio galaxy, PKS B1545-321. This radio galaxy has the so-called double-double structure, which consists of two pairs of double lobes. The outer pair of lobes are older and they are slowly fading away. The inner pair of lobes are created after the radio galaxy is born again. This radio galaxy is also a giant, the outer lobes extend to one Mega-parsec (3 million light-years). Credits: Safouris et al 2002.