Usually a galaxy’s shape corresponds with its color as well as its age. Spiral galaxies appear blue because they are still dynamically forming hot young stars. Elliptical galaxies, on the other hand, are mostly old, dead, and red. But surprisingly, two international collaborations have both separately identified a population of unusual red spiral galaxies. Galaxy Zoo, which uses volunteers from the general public to classify galaxies and the Space Telescope Galaxy Evolution Survey (STAGES) project have found that many of the red galaxies in crowded regions of galaxy clusters are actually spiral galaxies, bucking the trend for red galaxies to be elliptical in shape. However, these galaxies might be in transition, going from young to old, slowly having the life ‘choked’ out of them. “We think what we’re seeing is galaxies that have been gently strangled, so to speak,” said Chris Lintott, Galaxy Zoo team leader from University of Oxford, “where somehow the gas supply for star formation has been cut off, but that they’ve been strangled so gently that the arms are still there.” Sounds like a CSI murder mystery, but in this case astronomers are studying the scene of the crime.
The STAGES team used the Hubble Space Telescope to look in detail at one particular very large cluster of galaxies, known as the A901/902 and found a surprisingly large number of red spiral galaxies. But they didn’t know if what they were seeing was unusual to only that cluster or even that particular region of space. However the Galaxy Zoo team had found red spirals all across the universe. “STAGES had more detailed observations,” said Linott, “but we showed this is something that happens across the population of galaxies. So it’s nice when you can attack it from both ends at once.”
The two teams have concluded that both the mass of a galaxy as well as its local environment are important in determining when and how quickly its star formation is shut down. The red spirals are found on the outskirts of clusters in crowded regions of space where galaxies cluster together. As a blue galaxy is drawn in by gravity from the rural regions to the suburbs, some sort of an interaction with its environment causes a slow-down in star formation. The closer in a galaxy is, the more it is affected.
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Also, the red spirals were also the more massive galaxies, presumably because the smaller ones are transformed more quickly.
So what causes the transformation? The two teams don’t think it was any type of violent interaction.
“Usually, anything disruptive enough to get rid of the gas that fuels the star formation would have disrupted the spiral arms,” said Lintott. “But not so, at least particularly on the outskirts of clusters.” said Lintott.
The cause might be just the galaxy’s gentle interaction with the cluster. “The kind of thing we’re imagining,” explained Lintott, “as the galaxy moves into a denser environment, there’s lot of gas in clusters as well as galaxies, and it’s possible the gas from the galaxy just gets stripped off by the denser medium it’s plowing into.”
The teams have ruled out that a huge quantity of dust in these galaxies is masking a large amount of blue star formation. “Whenever you show a red object to an astronomer they immediately blame dust,” said Lintott, “because dust can absorb light and re-scatter it at a longer wavelength. So the temptation is to say they are just normal galaxies with just lots of dust. But there’s something else going on here, because then you’d expect the effect to be stronger if the spiral was face-on rather than edge- on because you’re looking at more of the galaxy. But we see the red spirals even in galaxies that are face-on. So we know it’s not just the effect of dust and inclination. So the obvious explanation is gone.”
Galaxy Zoo’s armchair astronomers’ biggest contribution wasn’t just the amount of galaxies they could look at (although the 100,000 plus they classified from the Sloan Digital Sky Survey was key) but more importantly, they are classifying galaxies purely by shape. “Since most elliptical galaxies are red and most spirals are blue,” said Lintott, ” if you’re sorting galaxies in an automated way, it’s very tempting to use color to sort them. But because we have people doing this we could separate the two out. So first we found all the spirals, and then we looked at their colors.”
This work is just the beginning of more investigations into these unusual red spiral galaxies. “This paper is our first look at all of this,” Lintott said. “One thing we’re particularly excited about that we’re working on now is looking to see what the black holes in these galaxies are doing. Whenever you see star formation being turned off, we like to blame the black hole. So it will be interesting to see what’s going on there.”
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Sources: RAS, interview with Chris Lintott