Extragalactic Color Magnitude Diagrams
Prerequisite – This activity is written assuming you have had experience classifying galaxies in the Galaxy Zoo and understand the concepts of magnitude and color in astronomy.
Recommended – Preflight – Magnitude and Launch-Color
When looking for patterns in astronomical data, one of the favorite tools of astronomers who study galaxies is a graph called the color magnitude diagram. When we plot the distribution of color verses absolute brightness (as magnitude) for hundreds of thousands of galaxies, we observe some interesting patterns. In this activity, we explore this graph and see how it can be used to compare populations of galaxies.
Begin in the Galaxy Zoo
• Go to Galaxy Zoo.
• Click on the icon in the upper left to reveal the login header.
• Log in. You will need to classify at least a few galaxies before beginning this activity. Add several to your Favourites. You will need them later. The Favourites button is available from the classification page.
Open One of Your Galaxy Favourites in Zoo Tools
Before we get to know the color magnitude diagram for galaxies, we need to find it. The steps that follow show you one way to open a dashboard and get started in Zoo Tools. If you need more help, the video shows you how it it’s done.
• Go to the Profile menu in Galaxy Zoo.
• Choose a Favourite that you would like to see in Zoo Tools.
• Click your choice to view the details of that object in Examine.
• Find and click the Discuss This link. This takes you to Talk in a new tab.
• Click the Open in Tools button near the upper right hand side of the page.
• A Zoo Tools dashboard opens in a new tab and generates three linked tools to get you started.
Load Galaxy Data into the Color Magnitude Tool
Once you have data imported into the Zoo Tools, you can analyze it using any one of the tools in the Tools tab at the top of the page. We will be using the Color Magnitude option. Later, we will use these same steps to import larger collections of galaxies.
• From the Tools tab, click the Color Magnitude button to open the tool.
• The diagram opens without any data loaded. To plot data, choose a data set from the Data Source drop down menu. In most cases, the source data that you imported is labeled Zooniverse – 0.
• The diagram reloads with the location of your Favourite plotted on the graph.
Explore the Color Magnitude Diagram
As with any graph, your first step is to observe and understand how the axes are labeled. Color is labeled on the Y axis. Color in astronomy is the measurement that results from subtracting the magnitude measurement for the object in two different filters, in this case u-r. As you move up the scale, the objects become more red. Remember, as the value of r goes down, it appears brighter in that filter.
Absolute magnitude is plotted on the X axis. This number is calculated by the SDSS. The measured magnitude of the galaxy and the estimated distance to that galaxy are used to determine how bright the galaxies would appear if they were all the same distance from Earth.
The relationship of color to absolute magnitude is plotted for all the galaxies that were part of Galaxy Zoo II. This represents a collection of approximately 300,000 galaxies from the Sloan Digital Sky Survey that are relatively nearby.
The colors you see on the graph are a result of a binning process that counts the number of points that fall within a particular range. Bins with a lot of galaxies are the darkest red while bins with fewer galaxies are blue or white. The colors used to create the graph do not reflect the actual colors of the galaxies.
You can observe a definite pattern to the distribution of the data which is common to all color magnitude diagrams of galaxies. Astronomers name the dense region of red toward the upper left of this graph as the red sequence and the less populated region below and to the right as the blue cloud.
This tool has some built-in features that allow us to explore this graph more closely:
• Return to the color magnitude diagram that you opened previously.
• Place your cursor over any one of the colored bins in the area of the red sequence.
• As you mouse over any bin on the graph, the tool loads one or more samples to the right of the graph.
• Make careful observations. Try other areas of the graph as well. What do you observe?
One last thing to note about this graph is the Rank display at the bottom. As you mouse over different bins on the graph, the display changes. The most common bin of the 3505 bins on the page receives a rank of 0/3505. You also see a percentile for that rank. In the case of 0/3505 the percentile is 100. That means that zero percent of the bins on the graph contain more galaxies than that one. If the percentile for a galaxy is 85 then only 15 percent of the bins on the graph would contain more galaxies. The percentile for each rank is calculated using each bin’s rank and the overall spread of the numbers.
When you load data from individual galaxies onto the diagram, you can see how they compare to a much larger population of galaxies. Let’s look at how that works next.
Ready, Aim, Collect
To get a real sense of the information on this graph and how we can use it, you are going to target a place on the diagram that interests you and then return to Galaxy Zoo Talk to create a collection of galaxies you think might fall into the region. Once you have a collection, you will send it to Zoo Tools, view the data in the Color Magnitude diagram and see how close you come to your target. Follow these steps.
Choose a region on the color magnitude diagram as your target.
When you choose a region of the color magnitude diagram to explore, remember, if you choose a region where there are few representative galaxies in the population as a whole, you may find it more challenging to create a collection. Choose one bin as your center point and record its rank.
Study representative galaxies around your center point.
Use the mouse-over feature of the diagram tool to observe characteristics of galaxies within several bins of your center point. Create a description you can use later to search for similar images.
Return to Galaxy Zoo Talk and create a collection of galaxies.
You want the collection of galaxies you create to match as closely as possible the characteristics you described in the previous step. There are many ways to go about identifying images to add to a collection. If you have never created a collection before try using the search feature to target the characteristics you identified. For more help download Working with Collections in Talk. WARNING – Only SDSS images can be plotted on the Color Magnitude Diagram in Zoo Tools.
Open Your Collection in Zoo Tools
It’s time to find out how well you did targeting your collection to a particular region of the color magnitude diagram. Follow the same basic steps you used above to open a single object in Zoo Tools. This time open the entire collection you made.
• If you are beginning in Galaxy Zoo, log in and go to Discuss, then Talk.
• Click the Profile link at the top of the page.
• Open the collection you want to use.
• Find the Open in Tools button in the upper right hand side of the page. When you click this button your entire collection opens in Zoo Tools.
• Three tools are activated automatically when you import objects or collections from Talk. Arrange them in the window so that you can see them at the same time.
Review Your Collection – Delete Objects If Needed
Notice that when you can select a line in the data window (usually labeled as Zooniverse-0), the line in the table tool is automatically highlighted. At the same time the image in the Subject Viewer snaps to that image. This is very helpful for reviewing your collection one last time before opening it in the color magnitude diagram tool. If you find objects you no longer want as part of your collection simply select that object in the data table. Click the “X” to remove it from the list. It will be removed from any tool that was previously created from this data set. The help video for this section demonstrates how this works.
Load Your Collection Data into the Color Magnitude Diagram
Just as you did previously with a single galaxy, use the Color Magnitude tool to plot your data:
• From your Collections page click the Open in Tools button. Zoo Tools open automatically.
• Click the tool tab at the top of the Zoo Tools page.
• Select the color magnitude tool.
• Select the data source from the drop down menu. Although you can use any previously created tool as your data source, in this case Zooniverse-0 contains the data you need.
• The data points for your collection load automatically.