Spiral galaxies (and why Galaxy Zoo is perfect for their study)

My name is Ross Hart, and I am a second year PhD student at the University of Nottingham using Galaxy Zoo data to study spiral galaxies. I am keen to write some blog posts about these galaxies, which are not only elegant in appearance, but also have a lot of interesting physics associated with them. In this first post, I will give a brief overview of spiral galaxies, and why Galaxy Zoo could be an important tool for their study.

Two-armed spiral galaxy M51 (left) and many-armed galaxy NGC 1232 (right). Image credit (M51): X-ray: NASA/CXC/SAO; Optical: Detlef Hartmann; Infrared: NASA/JPL-Caltech Image credit(NGC 1232): X-ray: NASA/CXC/Huntingdon Institute for X-ray Astronomy/G. Garmire; Optical: ESO/VLT

Spiral galaxies are the most numerous type of galaxy in the local Universe, with approximately 2/3 of local galaxies exhibiting spiral arms. Most of the gas and young stars in the disks of spiral galaxies are located in spiral arms, giving the appearance of the beautiful spiral patterns observed in galaxies such as M51 and NGC 1232 (shown above). However, the reason as to why we study these galaxies is not simply because they are pleasing to the eye; most of the star-formation in the local Universe occurs in spiral galaxies, so understanding the physics of spiral galaxies holds the key to understanding how stars form in galaxies.

Despite the relative prominence and importance of spiral galaxies, we still do not have a good understanding of how spiral galaxies form and evolve. Much of the problem as to why this is the case is that ‘spiral’ is actually far too broad a term to describe galaxies with different types of spiral structure. In particular, most galaxies have two spiral arms in a grand design spiral structure, like that of M51. However, some galaxies have many spiral arms, like NGC 1232.  However, the difference between M51 and NGC 1232 are actually thought to be much deeper than simply the number of arms that the two galaxies have; the physical processes that are responsible for their formation is also thought to be very different.

In order to study spiral galaxies in any level of detail, we require a method of classifying galaxies by the type of spiral structure they have, which requires detailed morphological classifications. It is for this reason that Galaxy Zoo provides an exciting opportunity to compare spiral galaxies in a way that simply hasn’t been possible before. Thanks to all of the volunteers who helped to classify galaxies in Galaxy Zoo 2, we have access to a  sample of spiral galaxies classified with unprecedented size. All of these galaxies have been classified by arm number, so we have the ideal tool for investigating how these different spiral structures form in a level of detail that hasn’t been possible before.