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Bars kill spiral galaxies

Last Updated on Tuesday, 09 November 2010 09:45
Published on Tuesday, 09 November 2010 00:01

With the help of the army of volunteers working on the Galaxy Zoo 2 'citizen science' project, an international team of scientists have discovered that the bars found in many spiral galaxies could be helping to kill them off. The researchers present their results in the journal Monthly Notices of the Royal Astronomical Society.

The red spiral on the left has a bar, whilst the blue spiral on the right does not
Images of two spiral galaxies made with the Sloan Digital Sky Survey (SDSS). The red spiral on the left has a bar, whilst the blue spiral on the right does not. The red spiral is catalogued as SDSS J083051.86+425544.8 and lies in the direction of that part of the sky marked by the constellation Lynx, at a distance of 762 million light years. The blue spiral is SDSS J151132.83+093645.0 and is 465 million light years away, in the direction of the constellation Bootes. Credit: SDSS
The overwhelming majority of stars in the universe are found in galaxies like our own Milky Way. These vast stellar assemblies contain anything between a few hundred million and one million billion stars and come in a variety of shapes, from irregular to elliptical (shaped like rugby balls) to spirals, where spiral arms wind out in a disk from a central bulge.

About half of these spiral galaxies have a bar - a linear structure of stars crossing the centre (as shown in the galaxy in the left hand side of the image). Bars are important for the evolution of galaxies as they provide a way to move material in and out in the disk and possibly help to spark star formation in the central regions. They may even help feed the central massive black hole that seems to be present in almost all galaxies. But bars provide us with a great puzzle because we still don't understand why some galaxies have bars and others do not.

The team, led by Dr Karen Masters of the Institute of Cosmology and Gravitation at the University of Portsmouth, drew on the work of the volunteers taking part in Galaxy Zoo 2, the follow on from the highly successful Galaxy Zoo project. In this second phase users were asked to make detailed classifications of the galaxies they looked at, including information on the presence of a bar.

With these data – the largest ever sample of galaxies with visual bar identifications – they have shown that red spirals are about twice as likely to host bars as blue spirals. These colours are significant. Blue galaxies get their hue from the hot young stars they contain, implying that they are forming stars in large numbers. In red galaxies, this star formation has stopped, leaving behind the cooler, long-lived stars that give them their red colour.

The astronomers conclude that bars might help to kill spiral galaxies, although how they do it remains a mystery. But the Milky Way has a bar too, so this discovery may be telling us something about its future.

Dr Masters sings the praises of the Galaxy Zoo 2 volunteers. "I'm really delighted to publish this first science result from Galaxy Zoo 2. Having so many people involved in this research is wonderful, and I feel a great weight of responsibility to make sure good science comes out of all the hard work they put into classifying galaxies.

'For some time data have hinted that spirals with more old stars are more likely to have bars, but with such a large number of bar classifications we're much more confident about our results. And all of this is thanks to the dedication of the volunteers who provide the raw 'clicks'.

'It's not yet clear whether the bars are some side effect of an external process that turns spiral galaxies red, or if they alone can cause this transformation. We should get closer to answering that question with more work on the Galaxy Zoo dataset."

The volunteers or 'Zooites' share her enthusiasm. Eric Hobein adds "It's nice to figure out how we help and be a super-tiny part of it all"; whilst Mike Tracey comments "I had fun doing my bit and my high school students were involved too. It is great to be part of a real life project which can produce real science".

Contacts

Dr Karen Masters
Institute of Cosmology and Gravitation
University of Portsmouth
Tel: +44 (0)2392 843137
Email: This email address is being protected from spambots. You need JavaScript enabled to view it.

Ms Lisa Egan
Press & Public Relations
University of Portsmouth
Tel: +44 (0)23 9284 3748
Mob: +44 (0)7904 688492
Email: This email address is being protected from spambots. You need JavaScript enabled to view it.

Dr Robert Massey
Royal Astronomical Society
Tel: +44 (0)20 7734 3307 x214
Mob: +44 (0)794 124 8035
Email: This email address is being protected from spambots. You need JavaScript enabled to view it.

Further information

The paper "Galaxy Zoo: Bars in Disk Galaxies", Masters K. L. et al, will appear in the journal Monthly Notices of the Royal Astronomical Society. A preprint can be seen at http://arxiv.org/abs/1003.0449

Images and captions

Images are available for download from http://icg.port.ac.uk/~mastersk/galaxyzoo2bars.html

Caption: Images of two spiral galaxies made with the Sloan Digital Sky Survey (SDSS). The red spiral on the left has a bar, whilst the blue spiral on the right does not. The red spiral is catalogued as SDSS J083051.86+425544.8 and lies in the direction of that part of the sky marked by the constellation Lynx, at a distance of 762 million light years. The blue spiral is SDSS J151132.83+093645.0 and is 465 million light years away, in the direction of the constellation Bootes. Credit: SDSS

Notes for editors
Galaxy Zoo 2

Galaxy Zoo 2 is the second phase of the highly successful internet-based Galaxy Zoo project (www.galaxyzoo.org). The original Galaxy Zoo asks volunteer users to make relatively simple decisions on galaxy shapes, deciding whether they are spiral or elliptical. This work is now extended as Hubble Zoo, which lets people classify galaxies in Hubble Space Telescope images that are seen as they were when the universe was only half its present age.

Galaxy Zoo attracted so much support that the organisers realised they could ask the volunteers to do even more. In Galaxy Zoo 2, a quarter of a million of the best and brightest galaxies in the original sample were selected for more detailed classifications.

During the time Galaxy Zoo 2 was live (February 2009 to April 2010), over 300000 volunteers provided over 60 million classifications. The Galaxy Zoo 2 data make up the largest, most detailed census of the galaxy population made to date and this scientific gold mine is helping astronomers better understand how galaxies form and evolve.

The Royal Astronomical Society

The Royal Astronomical Society (RAS, www.ras.org.uk), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organizes scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3500 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.