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.
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.
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.
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.
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.
Today’s post is from forum member Waveney who is embarking on his own science project:
Three years ago I stumbled upon the Zoo, started clicking, wandered into the forum and made friends. Then there was a request by Chris on the forum for people to check images for mergers, for which I wrote a website in 4 hours (including leaning SQL). This software with modifications has been used for all three Pea hunts, the Voorwerpje hunting, a few private hunts and for the Irregulars Project.
A couple of years ago Kevin asked for ideas for a student project. Several ideas were made, including the irregulars. After the student went on to do something else (Peas), Jules and I carried on talking. As the Zoo was quiet (between Mergers finishing and Zoo2 starting), we launched our own project to categorise the irregular galaxies in the forum topic. This was endorsed by Chris, who initially acted as the project’s supervisor.
There has been a lot of work on the big impressive ellipticals and spirals, on mergers and detailed studies on a few nearby irregulars. However there has not been a large scale quantitative study of them. The project aims to fill that gap. We are using data originally collected by the Forum, not used by any of the Zookeepers so are not treading on anybody’s toes.
What are irregulars?
These are the galaxies that don’t fit into any other category. They are not elliptical, they are not spiral, they are not involved in a merger. In general they are small, numerous and most are blue with star forming regions. Are they one homogenous group or is their more than one type? Are their old irregulars? Where are they? How do they compare with other galaxies? These are just a few of the questions the project is looking at.
How I started the project:
We started with the galaxies reported in the irregular topic on the forum, added those from a few related topics and started. Alice joined us in a supporting role and Aida came on board finding lots of other candidates, others have contributed lists and now the potential list is nearly 20,000 strong. I have managed the clicking on this in three data sets, the first 3,000 was the early reported – bright ones, then next 2,500 continued with the list in order of reported. However in the third set (nearing completion) I brought forward all those with spectra, so it contains 1500 ordinary candidates and another 2000 with spectra.
The website was setup (Note this has an independent login from the rest of the Zooniverse). It asks 10 questions about each image: is it an irregular, the clarity of the image, bright blue blobs, apparent proximity to other galaxies, appearance of any core, bar, arms or spiral structure.
What has been done so far:
Jules has looked at their 2D distribution across the sky and estimated the number that are too faint for SDSS to have taken their spectra. I have already studied the colour properties of irregular galaxies, their metallicity, masses and star-forming rates, hunted for AGNs (none found so far in irregulars), the equivalent width of the [OIII] 5007Å line in the same way that the Peas were analysed and the non-applicability of Photo Z algorithms for estimating Z without spectra to irregulars. I am currently studying galaxy density around irregulars (in 3D) and am looking for clusters of irregulars. I have not yet used the sub classification of the irregulars, just the “is it an irregular property” – this will come.
The project went rather quiet last year – I was terribly over worked and ended up with no spare time, this has changed recently and I have gone back and started looking again at the work we have done and thinking how to take it further. What is being found should be published – how does one do it? When the project had Chris as a supervisor the route was clear, I need to work with somebody in academia so I can present results, bounce ideas off, get ideas from and work with. Relying on the spare time of Zookeepers is not a satisfactory option – why not pay for the time. Then I thought back to a throwaway comment of Chris a year or so ago that “I had done half a PhD” – I recognise this means I have done 10%, but it got me thinking – why not do it properly. I don’t want to do this full time, I have a very full time job – but could I do it part time. Does the Open University do part time PhDs – a quick web search yes it does…
So I have signed up to take a part time PhD at the Open University on the irregulars. This will take several years, I will fund it myself. I am not totally new to doing formal research, in my professional life I had a lot to do with telecoms research: Proposing, taking part in, managing and cancelling many projects both company and EU funded at research houses and Universities, covering everything from social science through home networks to optical switching. I have even been on the other side of the table reviewing a couple of PhDs.
At this point I have applied, have a supervisor, met them once. Chris and Bill are my referees. The formal interview is in March and the formal start of the PhD is in September. However I am working hard looking at your results, reading, analysing, thinking and writing this.
I have created a blog on the forum to report day to day progress.
P.S. I could use a few more clicks.
For a few days after 25th November, the Galaxy Zoo Forum nominated their favourite galactic images from the thousands gathered over our three and a half years of existence, and voted on 48 of them. Here was the selection – as you see, it was a tough choice!
The winner was the stunning blue spiral, merging with a yellow galaxy so torn apart by gravitational forces that it would be hard to classify!
M51 is 33 million light years away and so bright that it has potential to fry the SDSS camera’s delicate optical instruments – so SDSS avoided looking at it too directly. Therefore, it has no reference number; but you can go into its pages and move up, down, right and left by adjusting the RA and DEC until you can more or less centre in on it.
M51 was discovered by Charles Messier, and put into his collection of objects that he thought were pain-in-the-neck smudges giving him false hopes of having discovered a comet! Jules wrote an Object of the Day about him and some of our other Messier Objects on the forum here. The pair of galaxies are also known as NGC 5194 and 5195. I’ve seen them described as 23 million, 31 million and 33 million light years away. The spiral is large, and famous for its dust lanes and intense star formation. You can resolve it in dark skies with a good pair of binoculars; it’s in the constellation Canes Venatici, though you find it just south-west of the brightest star of the Plough’s “saucepan handle”.
You can see a great deal more of this gorgeous object at Hubblesite, Astrocruise, NOAO and four different views altogether (and probably quite a few more) on APOD! The SDSS Telescope also has it proudly displayed on its home page, with a caption if you zoom in.
It won by only 1 vote; many other galaxies got almost as many. We’ve had plenty of time at Galaxy Zoo now to decide which galaxies we love best . . . and the answer is quite often “all of them”. M51 has never had any special attention on the forum that I recall, though it has of course had its fair share of admiration. I guess there are just too many things there to love!
A galactically happy Christmas to all our zooites from our oldest Zooniverse project.
The Galaxy Zoo forum is a hub of activity in the Zooniverse. they talk about everything from merging galaxies, to supernovae and even tea and biscuits. Compared to these tea-drinking stalwarts of the Zooniverse community, I am a relative newcomer. I often wonder what they talk about for ages amongst those many forum sections and threads. For the Zooniverse Advent Calendar I thought I would find out – in the process I made a Christmas card for the forum!
I have taken every word from every forum posting of 2010 (over 100,000 posts) and given it to the Tagxedo website. The result is this lovely Christmas tree made up of all the most popular words used in the Galaxy Zoo forum. The size of the words shows their popularity, and only the 600 most popular words are shown in the image.
You’ll not surprised to see words like ‘galaxy’, ‘hubble’ and ‘universe’ in there. You might be more amused by the inordinate number of smilies that decorate the tree. I enjoyed the random alignment of the words ‘think different’ – which seemed very appropriate. You can also see the names of many of the forum’s more active members as well as words such as ‘friendship’, ‘welcome’ and ‘love’.
Galaxy Zoo is three years old today. Three years ago, I opened my laptop in the back of a Royal Astronomical Society meeting, connected my laptop to the rather flakey wifi network and noticed the site had crashed under the sheer weight of demand.
Three years on, we’ve produced excellent science, have moved on to the distant Universe, built the Zooniverse and, thanks to the contributions of every single person who has ever classified a galaxy, established that involving the public in research is an excellent way to get things done. Here’s to the next three years.
P.S. You can hear Kevin and I discussing his work on active galaxies on today’s 365 days of astronomy podcast.
P.P.S. Thanks in particular to the forum for their birthday cards and best wishes.