Tag Archive | Citizen Science

Bayesian View of Galaxy Evolution


The Universe is pretty huge, and to understand it we need to collect vast amounts of data. The Hubble Telescope is just one of many telescopes collecting data from the Universe. Hubble alone produces 17.5 GB of raw science data each week. That means since its launch to low earth orbit in April 1990, it’s collected roughly a block of data equivalent in size to 6 million mp3 songs! With the launch of NASA’s James Webb Telescope  just around the corner – (a tennis court sized space telescope!), the amount of raw data we can collect from the Universe is going to escalate dramatically. In order to decipher what this data is telling us about the Universe we need to use sophisticated statistical techniques. In this post I want to talk a bit about a particular technique I’ve been using called a Markov-Chain-Monte-Carlo (MCMC) simulation to learn about galaxy evolution.

Before we dive in into the statistics let me try and explain what I’m trying to figure out. We can model galaxy evolution by looking at a galaxy’s star formation rate (SFR) over time. Basically we want know to how fast a particular galaxy is making stars at any given time. Typically, a galaxy has an initial constant high SFR then at a time called t quench (tq) it’s SFR decreases exponentially which is characterised by a number called tau. Small tau means the galaxy stops forming stars, or is quenched, more rapidly. So overall for each galaxy we need to determine two numbers tq and tau to figure out how it evolved. Figure 1 shows what this model looks like.


Figure 1: Model of a single galaxy’s SFR over time. Showing an initial high constant SFR, follow by a exponential quench at tq.

To calculate these two numbers, tq and tau, we look at the colour of the galaxy, specifically the UVJ colour I mentioned in my last post. We then compare this to a predicted colour of a galaxy for a specific value of tq and tau. The problem is that there are many different combinations of tq and tau, how to we find the best match for a galaxy? We use a MCMC simulation to do this.

The first MC – Markov-Chain – just means an efficient random walk. We send “walkers” to have a look around for a good tq and tau, but the direction we send them to walk at each step depends on how good the tq and tau they are currently at is. The upshot of this is we quickly home in on a good value of tq and tau. The second MC – Monte Carlo – just picks out random values of tq and tau and tests how good they are by comparing the UVJ colours and our SFR model. Figure 2 shows a gif of a MCMC simulation of a single galaxy. The histograms shows the positions of the walkers searching the tq and tau space, and the blue crosshair shows the best fit value of tq and tau at every step. You can see the walkers homing in and settling down on the best value of tq and tau. I ran this simulation by running a modified version of the starpy code.


Figure 2: MCMC simulation for a single galaxy, pictured in the top right corner. Main plot shows density of walkers. Marginal histograms show 1D projections of walker densities. Blue crosshair shows best fit values of tq and tau at each step.

The maths that underpins this simulation is called Bayesian Statistics, and it’s quite a novel way of thinking about parameters and data. The main difference is that instead of treating unknown parameters as fixed quantities with associated error, they are treated as random variables described by probability distributions. It’s quite a powerful way of looking at the Universe! I’ve left all of the gory maths detail about MCMC out but if you’re interested an article by a DPhil student here at Oxford does are really good job of explaining it here.

So how does this all relate to galaxy morphology, and Galaxy Zoo classifications? I’m currently running the MCMC simulation showing in Figure 2 over the all the galaxies in the COSMOS survey. This is really cool because apart from getting to play with the University of Oxford’s super computer (544 cores!), I can use galaxy zoo morphology to see if the SFR of a galaxy over time is dependent on the galaxy’s shape, and overall learn what the vast amount of data I have says about galaxy evolution.


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.


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.


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.


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.


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.

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.

Galaxy Zoo: Now Available In Chinese (Mandarin)

GZ MandarinWhat follows is a press release from Academia Sinica’s Institute of Astronomy & Astrophysics, regarding the new Mandarin Galaxy Zoo. Below is some context for English speakers and regular Galaxy Zoo users.

在可觀測宇宙散佈千億星系,許多以美麗著稱。光芒閃耀的每個星系裡,都有數十億顆恆星。新推出的「星系動物園」網站中文版,和研究星系大有關聯,不管有沒有天文背景,只要有網路,無論愛上網咖還是宅宅A咖,只花二分鐘也可參與星系分類的Galaxy Zoo計畫,自2007年以來,在英、美、歐地區成為網民科普熱門運動,已經招募87萬名星系分類員(志工),大受歡迎。原來星星可以這樣數。


從”Galaxy Zoo”到「星系動物園」,天文所推廣組表示,「兩年前就想過要做」的這個計畫,今年8月,一經天文所博士後研究Meg Schwamb再次提議,立刻獲得響應,網站中文化水到渠成,也讓台灣在全球天文學界再博得一次「亞洲第一」的小獎勵(註:目前該網站只有英文版和西語版)。推廣組表示,由於星系資料持續新增,分類員在圖像庫中撈到某個從未曾被人見過的星系,或「全球第一人」這樣的說法,確實所言不虛。

來自英國的Galaxy Zoo計畫主持人Chris Lintott表示,在網民科學網站傘狀計畫下的項目還有很多,天文類的譬如行星獵人(Planet Hunters)和火星氣候(Planet Four)。這些都必須靠各位地球人以好眼力來熱情相挺,電腦可幫不上忙。為什麼呢?歡迎上網一探究竟:http://www.galaxyzoo.org/?lang=zh


Last weekend, led by Dr. Meg Schwamb (who is part of the Planet Hunters and Planet Four teams), a team of Taiwanese astronomers helped introduced a Chinese (Mandarin) version a Galaxy Zoo to the public on the Open House Day of Academia Sinica, the highest academic institution in Taiwan.

A big crowd of enthusiastic students and parents, attracted by the long queue itself, visited the ‘Citizen Science: Galaxy Zoo’ booth to try the project hands-on by doing galaxy classifications. They were excited to participate in scientific research and enjoyed it very much.

“Amazing! In just two minutes, we have helped astronomer doing their research, it’s so cool! Also, we learn new astronomical facts we never knew before. It’s a good show.”

The Education Public Outreach team of Academia Sinica’s Institute of Astronomy & Astrophysics (a.k.a. “ASIAA”), has helped translated Galaxy Zoo from English to Chinese (Mandarin). The main translator, Lauren Huang said, “we were keen to do a localized version for Galaxy Zoo since 2010, so when Meg brought up this nice idea again, we acted upon it at once.” In less than six weeks, it was done. The other translator, Chun-Hui, Yang, who contributed to the translation, said that she likes the website’s sleek design very much. “I think the honor is ours, to take part in such a well-designed global team work!” Lauren said.

Talking about the translation process process, Lauren provided an anecdote that she thought about giving “zoo” a very local name, such as “Daguanyuan” (“Grand View Garden”), a term with authentic Chinese cultural flavour, and is from classic Chinese novel Dream of the Red Chamber. She said, “because, my personal experience in browsing the Galaxy Zoo website has been very much just like the character Ganny Liu in the classics novel. Imagine, if one flew into the virtual image database of the universe, which contains all sorts of hidden treasures waiting to be explored, what a privilege, and how little we can offer, to help on such a grandeur design?” However, the zoo is still translated as “Dungwuyuan”, literally, just as “zoo “. Because that’s what some Chinese bloggers have already accustomed to, creating a different term might just be too confusing.

You can check out the Traditional Character Chinese (Mandarin) version of Galaxy Zoo at http://www.galaxyzoo.org/?lang=zh

Quench Boost: A How-To-Guide, Part 4

Now that we’ve been initiated into the cool waters of Tools (Part 1), we’ve compared our *own* galaxies to the rest of the post-quenched sample (Part 2), and we’ve put your classifications to use, looking for what makes post-quench galaxies special compared to the rest of the riff-raff (Part 3), we’re ready for Part 4 of the Quench ‘How-To-Guide’.

This segment is inspired by a post on Quench Talk in response to Part 3 of this guide. One of our esteemed zoo-ite mods noted:

There are more Quench Sample mergers (505) than Control mergers (245)… It seems to suggest mergers have a role to play in quenching star formation as well.

Whoa! That’s a statistically significant difference and will be a really cool result if it holds up under further investigation!

I’ve been thinking about this potential result in the context of the Kaviraj article, summarized by Michael Zevin at http://postquench.blogspot.com/. The articles finds evidence that massive post-quenched galaxies appear to require different quenching mechanisms than lower-mass post-quenched galaxies. I wondered — can our data speak to their result?

Let’s find out!

Step 1: Copy this Dashboard to your Quench Tools environment, as you did in Part 3 of this guide.

  • This starter Dashboard provides a series of tables that have filtered the Control sample data into sources showing merger signatures and those that do not, as well as sources in low, mid, and high mass bins.
  • Mass, in this case, refers to the total stellar mass of each galaxy. You can see what limits I set for each mass bin by looking at the filter statements under the ‘Prompt’ in each Table.

Step 2: Compare the mass histogram for the Control galaxies with merger signatures with the mass histogram for the total sample of Control galaxies.

  • Click ‘Tools’ and choose ‘Histogram’ in the pop-up options.
  • Choose ‘Control’ as the ‘Data Source’.
  • Choose ‘log_mass’ as the x-axis, and limit the range from 6 to 12.
  • Repeat the above, but choose ‘Control – Merging’ as the ‘Data Source’.

The result will look similar to the figure below. Can you tell by eye if there’s a trend with mass in terms of the fraction of Control galaxies with merger signatures?


It’s subtle to see it in this visualization. Instead, let’s look at the fractions themselves.

Step 3: Letting the numbers guide us… Is there a higher fraction of Control galaxies with merger signatures at the low-mass end? At the high-mass end? Neither?

To answer this question, we need to know, for each mass bin, the fraction of Control galaxies that show merger signatures. I.e.,


Luckily, Tools can give us this information.

  • Click on the ‘Control – Low Mass’ Table and scroll to its lower right.
  • You’ll see the words ‘1527 Total Items’.
  • There are 1527 Control galaxies in the low mass bin.
  • Similarly, if you look in the lower right of the ‘Control – Merging – Low Mass’ Table, you’ll see that there are 131 galaxies in this category.
  • This means that the merger fraction for the low mass bin is 131/1527 or 8.6%.
  • Find the fraction for the middle and high mass bins.

Does the fraction increase or decrease with mass?

Step 4: Repeat the above steps but for the post-quenched galaxy sample.

You may want to open a new Dashboard to keep your window from getting too cluttered.

Step 5: How do the results compare for our post-quenched galaxies versus our Control galaxies? How can we best visualize these results?

  • In thinking about the answer to this question, you might want to make a plot of mass (on the x-axis) versus merger fraction (on the y-axis) for the Control galaxies.
  • On that same graph, you’d also show the results for the post-quenched galaxies.
  • To determine what mass value to use, consider taking the median mass value for each mass bin.
  • Determine this by clicking on ‘Tools’, choosing ‘Statistics’ in the pop-up options, selecting ‘Control – Low Mass’ as your ‘Data Source’, and selecting ‘Log Mass’ as the ‘Field’.
  • This ‘Statistics’ Tool gives you the mean, median, mode, and other values.
  • You could plot the results with pen on paper, use Google spreadsheets, or whatever plotting software you prefer. Unfortunately Tools, at this point, doesn’t provide this functionality.

It’d be awesome if you posted an image of your results here or at Quench Talk. We can then compare results, identify the best way to visualize this for the article, and build on what we’ve found.

You might also consider repeating the above but testing for the effect of choosing different, wider, or narrower mass bins. Does that change the results? It’d be really useful to know if it does.

Happy Tooling!

Galaxy Zoo in this week's Guardian

Just a quick note to let you know that there’s a very nice story about Galaxy Zoo, it’s beginnings and citizen science in general in the Guardian: http://www.guardian.co.uk/science/2012/mar/18/galaxy-zoo-crowdsourcing-citizen-scientists

It features, amongst others, our very own Alice (@penguingalaxy) and Hanny (@hannyvanarkel). For more late-breaking Galaxy Zoo news, why not follow us on Twitter (@galaxyzoo)?

How to Navigate the Astro Literature, Part 1

So you want to learn about current astrophysics research? You’re in luck! Not only are there many excellent blogs, pretty much all of the peer reviewed literature is out there accessible for free. In many areas of science, the actual papers are behind paywalls and very expensive to access. Astrophysics, like a few other areas of physics and mathematics, puts most papers on the arxiv.org preprint server where they are all available for download form anywhere. In addition, we have a very powerful search tool in the form of the NASA Astrophysics Data System which allows you to perform complex searches and queries across the literature.

Suppose you wanted to learn more about the green peas, one of our citizen science-led discoveries. Your first stop could be the ADS:

ADS, like any search engine, will now scour the literature for papers with the words “green peas”, “green” and “peas” in it, and return the results:

As you can see, the discovery paper of the peas, “Cardamone et al. (2009)” is not the first hit. That’s because in the meantime there has been another paper with “green peas” in the title. You can click on Cardamone et al. and find out more about the paper:

This is just the top of the page but it already contains a ton of information. Most importantly, the page has a link to the arxiv (or astro-ph) e-print (highlighted). Clicking there will get you to the arxiv page of the paper where you can get the full paper PDF.

Also there is a list of paper which are referencing Cardamone et all, at the moment 23 papers do so. By clicking on this link you can get a list of these papers. Similarly, just below, you can get a list of paper that Cardamone et al. is referencing.

Lower still are links to NED and SIMBAD, two databases of astronomy data. The numbers in the brackets indicate that SIMBAD knows 90 objects mentioned in the paper, and NED knows 88. By clicking on them, you can go find out what those databases know about the objects in Cardamone et al. (i.e. the peas).

Obviously there’s a lot more, but just with the arxiv and NASA ADS you can search and scour the astrophysics literature with pretty much no limits. Happy resarching!

Spheroidal Post Merger Systems at the AAS

I think Chris said it best – any session which is ended by a guy in a bowtie went well. And for our AAS Galaxy Zoo session, that guy in a bowtie was Alfredo Carpineti from UCL, who talked about his work on the properties of spheroidal post-merger systems selected with the help of the Galaxy Zoo merger classifications, and using a control sample of non-merging spheroidals (or ellipticals) also selected from Galaxy Zoo.

Alfredo provided me with the below description, and his slides are available to download at Carpineti_AAS218talk.pdf.

In this talk we discuss the properties subset of galaxies from the GZ mergers catalogue that are spheroidal ‘post-mergers’, where a single remnant is in the final stages of relaxation after the collision and shows evidence for a dominant bulge, making them plausible progenitors of early-type galaxies.

What is a Galaxy?

What is a Galaxy?

Any of the numerous large groups of stars and other matter that exist in space as independent systems.” (OED)

A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter.” (Wikipedia)

I’ll know one when I see one” (Prof. Simon White, Unveiling the Mass of Galaxies, Canada, June 2009)

The question of “What is a galaxy” is being debated online at the moment, after it was posed by two astronomers – Duncan Forbes and Pavel Kroup in a paper posted on the arXiv last week. It’s an article written for professional astronomers, so doesn’t shirk the technical language in the suggestions for definitions, but in a very “zoo like” fashion (and following the model of the IAU vote on the definition of a planet which took place in 2006) invites the readers of the paper to vote on the definition of a galaxy. This has been reported in a few places (for example Science, New Scientist) and everyone is invited to get involved in the debate.

In fact Galaxy Zoo is cited in the press release about the work as one of the inspirations to bring this debate to a vote.

So what’s all the fuss about? Well it all started because of some very tiny galaxies which have been found in the last few years. There has been a debate raging in the scientific literature over whether or not they differ from star clusters, and where the line between large star clusters and small galaxies should be drawn. It used to be there was quite a separation between the properties of globular clusters (which are spherical collections of stars found orbiting galaxies – the Milky Way has a collection of about 150-160 of them) and the smallest known galaxies.

The globular cluster Omega Centauri. Credit: ESO

The globular cluster Omega Centauri. Credit: ESO

For example globular clusters all have sizes of a few parsecs (remember 1 parsec is about 3 light years), and the smallest known galaxies used to all have sizes of 100pc or larger. Then these things called ‘ultra compact dwarfs’ were found (in 1999), which as you might guess are dwarf galaxies which are very compact. They have sizes in the 10s of parsec range, getting pretty close to globular cluster scales.

UCDs in the Fornax Cluster. The background image was taken by Dr Michael Hilker of the University of Bonn using the 2.5-metre Du Pont telescope, part of the Las Campanas Observatory in Chile. The two boxes show close-ups of two UCD galaxies in the Hilker image. (Credit: These images were made using the Hubble Space Telescope by a team led by Professor Michael Drinkwater of the University of Queensland.)

UCDs in the Fornax Cluster. The background image was taken by Dr Michael Hilker of the University of Bonn using the 2.5-metre Du Pont telescope, part of the Las Campanas Observatory in Chile. The two boxes show close-ups of two UCD galaxies in the Hilker image. (Credit: These images were made using the Hubble Space Telescope by a team led by Professor Michael Drinkwater of the University of Queensland.)

Such objects begin to blur the line between star clusters and galaxies.

And there are things which have been called ‘ultra-faint dwarf spheroidal (dSph) galaxies’. These look nothing like the kind of galaxies you’re used to in SDSS images, although they were found in SDSS data – but perhaps not how you might expect. Researchers colour coded the stars in SDSS by their distance, and looked for overdensities or clumps of stars. So far several concentrations of stars at the same distance have been found. Some were new star clusters, but some look a bit like galaxies. If these are galaxies they are the smallest know, with only 100s of stars. Some are so faint that they would be outshone by a single massive bright star.

A map of stars in the outer regions of the Milky Way Galaxy, derived from the SDSS images of the northern sky, shown in a Mercator-like projection. The color indicates the distance of the stars, while the intensity indicates the density of stars on the sky. Structures visible in this map include streams of stars torn from the Sagittarius dwarf galaxy, a smaller 'orphan' stream crossing the Sagittarius streams, the 'Monoceros Ring' that encircles the Milky Way disk, trails of stars being stripped from the globular cluster Palomar 5, and excesses of stars found towards the constellations Virgo and Hercules. Circles enclose new Milky Way companions discovered by the SDSS; two of these are faint globular star clusters, while the others are faint dwarf galaxies.  Credit: V. Belokurov and the Sloan Digital Sky Survey.

A map of stars in the outer regions of the Milky Way Galaxy, derived from the SDSS images of the northern sky, shown in a Mercator-like projection. The color indicates the distance of the stars, while the intensity indicates the density of stars on the sky. Structures visible in this map include streams of stars torn from the Sagittarius dwarf galaxy, a smaller 'orphan' stream crossing the Sagittarius streams, the 'Monoceros Ring' that encircles the Milky Way disk, trails of stars being stripped from the globular cluster Palomar 5, and excesses of stars found towards the constellations Virgo and Hercules. Circles enclose new Milky Way companions discovered by the SDSS; two of these are faint globular star clusters, while the others are faint dwarf galaxies. Credit: V. Belokurov and the Sloan Digital Sky Survey.

The paper suggests as a minimum that a galaxy ought to be gravitationally bound, and contain stars. They point out that this definition includes star clusters as well as all galaxies, so suggest some additional criteria might be needed. They make a list of suggestions, which we are invited to vote on. For full details (and if you are technically minded) I refer you to the paper. It’s a lesson in gravitational physics in itself (although as I said it is aimed at professional astronomers). Here is my potted summary of the suggestions they make:

1. Relaxation time is longer than the age of the universe. Basically this means that the system ought to be in a state where the velocity of a star in orbit in it will not change due to the gravitational perturbations from the other stars in a “Hubble time” (astronomer speak for a time roughly as long as the age of the universe).  This will exclude star clusters which are compact enough to have shorter relaxation times, but makes UCDs and faint dSph be galaxies.

2. Size > 100 pc (300 light years). Pretty self explanatory. Sets a minimum limit on the size. This makes the UCDs not be galaxies.

3. Should have stars of different ages. (a “complex stellar population”). The stars in most star clusters are observed to have formed all in one go from one massive cloud of gas. But in more massive systems not all the gas can be turned into stars at one time, so the star formation is more spread out resulting in stars of different ages being present. However there are some (massive) globular clusters which are know to have stars of different ages, so they would become galaxies in this definition.

4. Has dark matter. Globular clusters show no evidence for dark matter (ie. their measured mass from watching how the stars move is the same as the mass estimated by counting stars), while all massive galaxies have clear evidence for dark matter. The problem here is that this is a tricky measurement to make for UCDs and many dSph, so will leave a lot of question marks, and may not be the most practical definition.

5. Hosts satellites. This suggests that all galaxies should have satellite systems. In the case of massive galaxies these are dwarf galaxies (for example the Magellanic clouds around the Milky Way), and many dwarf galaxies have globular clusters in them. But there are some dwarf galaxies with no known globular clusters, and UCDs and dSph do not have any.

The paper finished by describing some of the most uncertain objects and provides the below table to show which would be a galaxy under a given definition. I’ve tried to explain what some of these all are along the way, and here is a short summary:

Omega Cen (first image above) is traditionally thought to be a globular cluster (ie. not a galaxy). Segue 1 is one of the ultra faint dSphs found by counting stars in SDSS images (3rd image). Coma Berenices is a bit brighter than a typical ultra faint dSph, and a bit bigger than a typical UCD. VUCD7 is (the 7th) UCD in the Virgo cluster (not the most imaginative name there!), so simular to the Fornax cluster UCDs shown in the second image. M59cO is a big UCD – almost a normal dwarf galaxy but not quite. BooII (short for Bootes III) and VCC2062 are objects which may possibly be material which has been tidally stripped off another galaxy. Or they might be galaxies.

Galaxy defintion

Anyway if you’re interested you can join in the debate here. And if you read the paper you can vote. I voted,  but I think in the interest of letting you make up your own mind I’m not going to tell you what my decision was.

Oh and I just started a forum topic on it in case you want to debate inside the Zoo before voting.