A Zooniverse of Galaxies
A galaxy is a collection of stars – typically 200 billion for a large galaxy. When we look out into the universe we see that these galaxies come in two basic types – “spiral galaxies” (like our home, the Milky Way) and “elliptical galaxies”.
The two main types of galaxies in our universe; an elliptical (left) and spiral (right). Credit: SDSS (http://www.sdss.org/).
There is a long history in astronomy of classifying galaxies and sorting them into different types to try to figure out something about their formation and evolution. Perhaps most famously Edwin Hubble in the 1920s developed a classification scheme for galaxies which arranged these ellipticals and spirals into a “tuning fork” shape (shown below). Hubble’s classifications, with only minor modifications have remained the basis of how most astronomers think about the different types of galaxies.
The Hubble Tuning Fork illustrated with colour SDSS galaxy images like those used in Galaxy Zoo.
In the time of Hubble, astronomers had catalogued a few hundred galaxies – for example he used distances to 29 galaxies to discover the expansion of the universe. But the development of digital photography along with massive improvements in computers has made possible the collection of much larger numbers of galaxy images. The largest of these efforts is the Sloan Digital Sky Survey (www.sdss.org), which has now imaged about 40% of the sky, and in its “Main Galaxy Sample” catalogued 1 million galaxies. In order to visually classify this number of galaxies, astronomers needed help – and thus was born Galaxy Zoo (www.galaxyzoo.org).
The original Galaxy Zoo (2007-2009) simply asked volunteers to identify if galaxies in the Sloan images were spiral or elliptical (or neither). These basic classifications were collected for all 1 million galaxies in SDSS (amazingly, with on average 40 people looking at each galaxy). A second version (“Galaxy Zoo 2”) ran in 2009-2010 asking for more detailed information about the brightest quarter of a million galaxies (things like if there was a bar, how big the bulge was etc). In my research, I use both of these sets of classifications to study galaxy evolution. I combine the Galaxy Zoo information with other data on the galaxies to look for correlations and/or unusual objects.
One of the biggest discoveries from the first phase of Galaxy Zoo was a significant population of spiral galaxies which are as red and dead as most ellipticals (e.g. http://news.bbc.co.uk/1/hi/sci/tech/7746298.stm) but have clear spiral arms. Most spirals are blue, and most ellipticals are red, so these red spirals form an interesting “missing link” between the two types. I have led the study of the properties of these red spirals to search for clues to their origin.
A typical blue spiral (left) and red elliptical (right) with a barred red spiral in the middle. Credit: SDSS.
That most spirals are blue tells us that they are the sites of most star formation in the universe (the brightest young stars are very blue, so galaxies with star formation look blue). That most red galaxies are ellipticals tells us that in most cases, the process which turns off the star formation (turning the galaxy red) also changes the morphology of the galaxy. For a long time it was thought that the main way this happened was through the merger of two spiral galaxies.
A merger between two spiral galaxies. Credit: HST (http://hubblesite.org/).
But the existence of a population of red spirals show that there must be a different way to turn off star formation. In my 2010 paper on the red spirals we looked at the optical properties of these objects to try to tease out what it could be that was turning them red.
Barred spiral NGC1300. Credit: HST
One of the most interesting discoveries we made about the red spirals was that they are much more likely to have a bar than typical blue spirals. This led me to take an early look at the Galaxy Zoo 2 classifications, where I was fascinated to see a strong correlation between the fraction of disk galaxies that have a bar and their colour. This first result from Galaxy Zoo 2 was published in MNRAS earlier this year (submitted to the journal 2 days before my son was born!). RAS put out a press release (http://www.ras.org.uk/news-and-press/157-news2010/1891-bars-kill-spiral-galaxies) on the work which was also covered by Nature Research Highlights (http://www.nature.com/nature/journal/v468/n7322/full/468348b.html). This work suggests that bars are intimately connected with whatever it is which turns spirals red – either driving the process, or perhaps more likely as a side effect, and I’m currently working on looking for more clues to see which it is.
None of this research would be possible with the volunteer classifications provided by members of the public through the Galaxy Zoo website. Surveys of the motivations of the Galaxy Zoo volunteers show that a large fraction of them devote their time to classify galaxies because they want to contribute to original scientific research. I am delighted to work with the project and contribute to its success by leading some of that research.
If you want to help Galaxy Zoo, you can visit www.galaxyzoo.org where the Galaxy Zoo: Hubble project is collecting classifications. Instead of being based on images of nearby galaxies from the Sloan Digital Sky Survey, this version of Galaxy Zoo is using images taken by the Hubble Space Telescope of galaxies much much further away. Because light takes some time to cross the universe, when we look very far away we’re actually looking back in time. So Galaxy Zoo Hubble will give us a way to look at how the types and shapes of galaxies have changed as the universe evolved. That’s very exciting stuff for an astronomer like me!
About the author: Dr. Karen Masters is a Leverhulme Early Career Research Fellow at the Institute of Cosmology and Gravitation at the University of Portsmouth, UK/SEPnet (http://www.sepnet.ac.uk/). Karen blogs about extragalactic astronomy for Galaxy Zoo (http://blogs.zooniverse.org/galaxyzoo/), and also runs the LOFAR-UK blog http://blog.lofar-uk.org/. She tweets regularly about astronomy as @KarenLMasters, and she collects her diverse blog posts and writes on other science and astronomy topics at her personal blog “Beautiful Stars” (http://thebeautifulstars.blogspot.com). She lives in Portsmouth with her husband (who is also an astronomer) and their two small children.