Galaxy Zoo: Do spiral or elliptical galaxies tend to be closer to other galaxies?
For our last project as Citizen Science Ambassadors, we were tasked with experimenting using data from Galaxy Zoo.
To create our experiment, we first developed testable questions about the Galaxy Zoo data, using information like axial ratios, local cluster density, distance to the nearest galaxy cluster, and many other components. Our team decided to focus our question by using the local cluster density, and also the galaxy’s mass. For reference, local density refers to how many nearby galaxies there are within a certain distance. Ultimately, we decided that we wanted to test if spiral or ellipticals galaxies tended to be physically closer to other galaxies. We hypothesized that spiral galaxies will be closer to other galaxies than elliptical galaxies since spirals extend outwards because of their arms (see above image) and are not self-contained like ellipticals.
Before we started manipulating the data, we had to sort and filter our data. Since we only needed information about the type of galaxy, the galaxy’s mass, and the local density, we filtered until all the only columns of data we had were ‘probability of elliptical galaxy,’ probability of spiral galaxy,’ the log of the mass, and the log of the local density. Then, we divided the data into two categories, using the probability of each galaxy being either type. We also disregarded galaxies that had less than a 80% probability of being either a spiral or elliptical in order to clarify our results.
The first step of our experiment was to compare all of the ellipticals to all of the spirals. When we did this, however, we noticed that the masses of the two types of galaxies were distributed differently. This would eventually lead us to dividing the data further. After we compared the mass distribution, we created histograms for the log of the local density for both elliptical and spiral galaxies. Since we were already planning to sub-categorize, we didn’t spend a lot of time focusing on this graphs, but we did notice that the graph of the ellipticals seemed to have two ‘humps’, instead of the more Normal, one ‘hump’ graph we expected.
In order to subcategorize the data, we found the minimum, maximum, median, and 1st and 3rd percentile for the mass of elliptical galaxies and for the mass of the spiral galaxies. We used these values to divided the data for each type into four groups: minimum to 25th percentile, 25th percentile to median, median to 75th percentile, and 75th percentile to the maximum. Then, we found the summary data for each of these groups and compared ellipticals and spirals in the same grouping. Interestingly enough, elliptical galaxies were- on average- closer to their neighbors than spiral galaxies for all four quartiles.
As such, the data we found contradicted our original hypothesis, since we found that elliptical galaxies actually tend to be closer to other galaxies. We came up with several possible explanations for this; we thought it might have something to do with either the age of the galaxies or the galaxy cluster. When galaxy clusters are formed, there are a lot of small, young galaxies packed together. As they grow older, the cluster condenses into more distinct points. Elliptical galaxies are younger, and spiral galaxies are older. A connection to age would explain why the density appears to decrease for spiral galaxies. Also, with age comes expansion, which could create distance between the galaxies within the cluster.
Working on this project was a great experience. My group was a lot of fun to work with, and I think we had a polished final project. Even though we all had different backgrounds and widely different strengths, I believe that we each did our part to contribute in whatever way we could. I worked mostly with the data. I filtered it, created the graphs, and tried my best to lead my group in our exploration. Sometimes, it was a little difficult to stay on task, and we were down a team member one of the days, but we finished everything on time. One thing I was especially proud of was the overall aesthetic of our presentation. I felt like our use of colors, graphics, and charts combined well with abbreviated bullet points so that our presentation was attractive and engaging.
Overall, this program as a whole was a great experience; I absolutely recommend it. I met so many amazing people, and I got the chance to work on some really cool projects. Spending my summer at the Adler was really a lot of fun, and I feel that I’ve learned a lot.