PSA: Bars Kill Galaxies

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Many spiral galaxies are known to harbor bars. Not the sort in which liquor is served as a social lubricant, but rather, the kind in which gas is served to the central regions of a galaxy. But just as recent studies have identified alcohol as one of the most risky drugs, a new study using results from the Galaxy Zoo 2 project have indicated galactic bars may be associated with dead galaxies as well.

The Galaxy Zoo 2 project is the continuation of the original Galaxy Zoo. Whereas the original project asked participants to categorize galaxies into Hubble Classifications, the continuation adds the additional layer of prompting users to provide further classification including whether or not the nearly quarter of a million galaxies showed the presence of a bar. While relying on only quickly trained volunteers may seem like a risky venture, the percentage of galaxies reported to have bars (about 30%) was in good agreement with previous studies using more rigorous methods.

The new study, led by Karen Masters of the Institute of Cosmology and Gravitation at the University of Portsmouth, analyzed the presence or lack of bars in relation to other variables, such as “colour, luminosity, and estimates of the bulge size, or prominence.” When looking to see if the percent of galaxies with bars evolved over the redshifts observed, the team found no evidence that this had changed in the sample (the GZ2 project contains galaxies to a lookback time of ~6 billion years).

When comparing the fraction with bars to the overall color of the galaxy, the team saw strong trends. In blue galaxies (which have more ongoing star formation) only about 20% of galaxies contained bars. Meanwhile, red galaxies (which contain more older stars) had as many as 50% of their members hosting bars. Even more striking, when the sample was further broken down into grouping by overall galaxy brightness, the team found that dimmer red galaxies were even more likely to harbor bars, peaking at ~70%!

Before considering the possible implications, the team stopped to consider whether or not there was some inherent biasing in the selection based on color. Perhaps bars just stood out more in red galaxies and the ongoing star formation in blue galaxies managed to hide their presence? The team referenced previous studies that determined visual identification for the presence of bars was not hindered in the wavelengths presented and only dipped in the ultraviolet regime which was not presented. Thus, the conclusion was deemed safe.

While the findings don’t establish a causal relationship, the connection is still apparent: If a galaxy has a bar, it is more likely to lack ongoing star formation. This discovery could help astronomers understand how bars form in the first place. Given both structure, such as bars and spiral arms, and star formation are associated with galactic interactions, the expectation would be that we should observe more bars in galaxies in which interactions have caused them to form as well as triggering star formation. As such, this study helps to constrain modes of bar formation. Another possible connection is the ability of bars to assist in movement of gas, potentially shuttling and shielding it from being accessible for formation. As Masters states, “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.”