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Bars Kill Spiral Galaxies

November 9, 2010

With the help of the army of volunteers working on the Galaxy Zoo 2 “Ëœcitizen 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 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 center (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 colors 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 color.

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.

“ËœFor 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 “Ëœclicks’.

“ËœIt’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 “ËœZooites’ 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”.

* Full bibliographic information: “Galaxy Zoo: Bars in Disk Galaxies”, Masters K. L. et al, Monthly Notices of the Royal Astronomical Society, in press. A preprint can be seen at http://arxiv.org/abs/1003.0449

Image 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

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