Archive by Author | Ross Hart

New papers investigate Galactic Rings in Galaxy Zoo

Hello Galaxy Zoo volunteers!

The fact that galaxies can host a multitude of different structures is no news to the volunteers: recent publications have looked at the spiral arms and bars in galaxies. There has now been some more recent work into a different, altogether rarer phenomenon that are called galactic rings. Galactic rings are objects that are commonly observed in the most strongly barred galaxies — it is theorised that these ring structures grow with the bars in galaxies. However, there are multiple theories that can explain these rare but significant features in galaxies, and the theories that claim to explain them remain purely theoretical.

To get an idea of exactly what is going on in ring galaxies requires a sample of galaxies which have galactic rings. Previous studies have relied on a handful of galaxies to study. However, Galaxy Zoo has proven to be the dataset of choice for any astronomer looking for the rarest of morphological phenomena, and Galaxy Zoo has once again provided the dataset for finding galactic rings in galaxies. Galaxy Zoo has always prided itself on not being just from the public but also for the public and for anyone the field of astronomy: all of the Galaxy Zoo data is available online for any astronomer to use. It is exactly the dataset that has allowed for the publication of two papers on galactic rings, by astronomer and long time friend of Galaxy Zoo, Ronald Buta, a Professor of Astrophysics at the University of Alabama. The first paper (here) used the public Galaxy Zoo 2 classifications to successfully define a sample of 3,962 ringed galaxies in Galaxy Zoo, and some examples are given in Fig.1.

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Figure 1: Examples of various ring morphologies identified with the help of Galaxy Zoo 2.

A direct follow-up paper investigating fifty of these ringed galaxies in more detail was then published, again by Ronald Buta. In the second paper of this Galactic Rings revisited series, the main aim was to check how the rings in galaxies are related to the dynamics of the disc: theory suggests that rings are the result of resonances between the motions of the various rotating materials in galaxy discs. A resonance is an amplification caused when two oscillations match at just the right frequency: think of pushing someone on a swing. If you give a push at the top of the swing, then the amplitude (how high up the swing goes) will increase. The work described in this paper shows that galactic rings are likely to be exactly that: resonances between the bar+spiral pattern and rotating stars, and that various ring morphologies can be caused by different resonances in the outer discs of galaxies.

This work really showcases one of the key strengths of Galaxy Zoo: finding rare classes of objects in samples impossible to be classified by professional astronomers. So once again we’d like to say thank you, on behalf of astronomers around the world in the Galaxy Zoo team and those who have gone on to use the data from your hard work in other ways.

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.

A new paper about spiral galaxies in Galaxy Zoo

I’m happy to announce that my first Galaxy Zoo paper has been published! You may be aware that I recently posted about spiral galaxies in Galaxy Zoo (the blog post can be found here). The first results from these studies have now made it to publication, where we discuss a new method for removing bias in galaxy classifications, as well as comparing the properties of different spiral galaxies.

spirals

Some example 1, 2, 3, 4 and 5+ armed spiral galaxies classified in Galaxy Zoo.

As discussed in my earlier blog post, spiral galaxies are some of the most interesting galaxies in the local Universe. However, studies of these objects have been limited, due to the fact that galaxies need to be visually classified. It is therefore thanks to all of the volunteers in Galaxy Zoo that a paper like this can be published, where we have thousands of spiral galaxies to compare. Thanks to these classifications, we have been able to find some interesting preliminary results: we find that many-armed spiral galaxies are bluer in colour than two-armed spiral galaxies. This suggests that many-armed spirals are sites of significantly enhanced activity in the Universe, where high levels of star formation activity are taking place.

cc1

Colours of spiral galaxies by arm number. Lower u-r  and r-z mean galaxies are bluer in colour (many-armed galaxies show bluer colours).

However, these results are only a hint at what we can achieve in the future. So watch this space and we’ll keep you all informed about any developments in our work on spiral galaxies!

If you are interested in reading more, the full article can be found here.

 

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.

m51_ngc_1232

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.