How do spiral arms affect star-formation?

Hi everyone! For those unaware, I am a PhD student at the University of Nottingham looking at spiral galaxies in Galaxy Zoo (for an overview, see this blog post). Following the release of my first refereed publication last year, my second refereed publication has now been accepted (woohoo!). As can be seen from my previous post (found here), we found remarkable differences between the spiral galaxies that we observe in the local Universe, simply by comparing galaxies with different numbers of spiral arms. Galaxies with two spiral arms are distinctly redder in colour than many-armed galaxies. However, the exact reasons for these differences was still up for debate. Red galaxies could have very low star-formation rates, or contain a significant amount of dust, blocking the escaping blue light.

irx_beta

The IRX-beta relation for galaxies with different numbers of spiral arms. A higher IRX value means more light is absorbed by dust: two-armed galaxies are more heavily dust-obscured.

With this in mind, we decided to follow-up that paper with panchromatic data from UV and infra-red wavelengths. UV wavelengths bluer than optical probe the very youngest stars, and infra-red wavelengths redder than optical measure dust emission directly. Combining these measurements allowed us to show the following things:

  1. Star-formation rate does not depend on spiral arm number: all spiral galaxies seem to be forming the same number of stars, regardless of what their spiral arms look like.
  2. The amount of blue light being absorbed by dust is significantly greater in two-armed spiral galaxies.

These two striking results have now shown us that spiral arms are not simply a visual pattern. They act to change the conditions of star-formation in local galaxies, making them much more sensitive to dust. Interested readers can find the full paper here.

About Ross Hart

PhD student at the University of Nottingham

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