USE THIS LINK https://js9.si.edu/

ASTR-1115L - Spring 2024 Section Group 3XV

Colors

The above files contain images of galaxies that you will need for this section. THEY WILL NOT BE IN A FORMAT YOUR MACHINE WILL RECOGNIZE. You need to download them onto your machine and save them. right click on the filename (e.g. n4449b.fit) and save it to your machine. BUT DO NOT TRY TO OPEN THEM! In this section, you will upload them into an applet that will recognize their format.

Colors of Galaxies

You may recall from the previous labs that stars have distinct colors. We are now going to demonstrate that galaxies also have different colors. Why care about colors? They are a clue to the history of the galaxy. Remember that short-lived massive stars are rare but very luminous, and, being hot, have a blue color. Old stars are much cooler, and are yellow or white. So the color of a galaxy tells you whether you are looking at an old population of stars, where star formation ceased long ago, or a population with ongoing star formation.

You can measure color quantitatively by measuring how much light you receive from the galaxy when viewed through two colored filters: here red and blue. For this analysis we will examine the elliptical galaxy NGC 2768 and the irregular galaxy NGC 4449. The tool we will use for this analysis is called js9. You will be doing the kind of image analysis astronomers do to reveal galaxy colors.

Click on "Start js9" below to open up js9 on the internet. You will be measuring the amount of light from each galaxy in the blue filter. Then we'll compare with the results for the red filter in order to measure the colors of the galaxies. Go through the following recipe for each of the two files you downloaded.

Start js9Links to an external site.

1. Download the two ".fits" files from this section.
2. Within js9 now click on "File" and choose "Open local file". You can also drag an drop the file from its folder on your computer onto the js9 application and it will load it automatically.Start with "n2768b.fits". This is the blue filter image of NGC 2768.
3. Once you have uploaded the image, use the mouse with the left button held down to change the brightness and contrast of the image. Notice how far the faintest light from the galaxy extends to the edge of the image.
4. We want to determine how much light is coming from each galaxy in each filter, but what is not obvious is that the night sky is giving off light too, all over the image, so we need to correct for that light by subtracting it off. Here's how to do it.
   • Click on Regions, choose Box, and a box will appear on the center of the image. If you click inside the box, small rectangles appear at the edges and corners. You can drag the box around with the mouse's left button and change the size by dragging on the small rectangles. Make the box big enough to cover all the light from the galaxy.
   • You will need to know how big your box is. To see this, click on Regions again, and choose List Regions. The third and fourth numbers on the line starting "physical: box" give the current horizontal and vertical extent of the box in pixels. Write these down.
   • Now click on Analysis and choose Histogram (note this toggles on and off with every click), then click inside your box again to select it for analysis. This will display a graph which lists the mean brightness in the galaxy (next to "mean"), in units we'll call "counts". You will need this number too. Now close the graph. Note that if you need to start again, the Region menu has an option to delete your region.
5. Now choose a box far enough from the galaxy that it has only dark sky brightness in it (this may be only the very corners). Repeat the above procedure to find the mean brightness from the sky in the box. Make a note of this number.
6. Since the sky is about uniformly bright, we can now subtract our mean sky brightness from our mean galaxy brightness to remove the sky background and get the net mean brightness from the galaxy. Make a note of this number. Take the result and multiply it by the area of your galaxy box to give you the total brightness of the galaxy for this filter. This is the last number you will need. To repeat what we have done, we have taken the mean pixel brightness in the galaxy box, which has contributions from both the galaxy and the foreground night sky, and subtracted off the contribution from just the sky, leaving only the light from the galaxy. We then multiplied the mean brightness of the pixels covering the galaxy by the number of pixels covering the galaxy, giving us the total amount of light from the galaxy, now with the sky subtracted.
7. Repeat steps 1-6 above for the other galaxy in the blue filter.
8. The values for the red filter are given in the questions below. For those questions, determine the color ratio of each galaxy by dividing the net brightness in the red filter by the net brightness in the blue filter: i.e. color ratio = net (red) / net (blue). This is a measure of the color of the galaxy.

NGC-4449 File:

n4449b.fit

NGC-2768 File:

n2768b.fit

Question 17

Write down the following for the blue filter image of NGC 2768. Your TA will help you through this.

The mean brightness of the galaxy you found in Step 4, in "counts":

The number of pixels in your box for the galaxy in Step 4:

The mean brightness of the sky background from Step 5, in counts:

The net mean brightness of the galaxy from Step 6, in counts:

The total brightness of the galaxy from Step 6, in counts:

Question 18

Now for the red filter, the total brightness of NGC 2768 is about 4,156,000. What is the red/blue total brightness ratio for NGC 2768?

Question 19

Write down the following for the blue filter image of NGC 4449.

The mean brightness of the galaxy you found in Step 4, in "counts":

The number of pixels in your box for the galaxy in Step 4:

The mean brightness of the sky background from Step 5, in counts:

The net mean brightness of the galaxy from Step 6, in counts:

The total brightness of the galaxy from Step 6, in counts:

Question 20

Now for the red filter, the total brightness of NGC 4449 is 5,925,600. What is the red/blue total brightness ratio for NGC 4449?

Question 21

Explain qualitatively the difference in color in terms of the average temperature of the stars in each galaxy. Which galaxy therefore has hotter stars?

Question 22

Explain qualitatively the difference in color in terms of the average age of the stars in each galaxy (hint: are hot stars more massive or less massive? Now, how does mass relate to lifetime?). Which galaxy therefore has older stars?

Question 23

What do the above results tell you about which galaxy is more likely to be forming stars right now?