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Light Detectives Using WISE Data to Identify Brown Dwarfs and ULIRGs

Figure 1: An artist's rendition comparing stars, brown dwarfs, and planets to the same size scale. From left to right is the limb of the Sun, a very low mass star, a pair of brown dwarfs, and the planet Jupiter. These objects have masses ranging from 1000 times that of Jupiter

(for the Sun) through 75, 65, 30, and 1 Jupiter mass, respectively. Overview: If you have ever looked at the sky through a telescope on a dark night, stars were probably not the only celestial bodies within your field of view. Space is filled not only with stars, but planets, asteroids, giant clouds of dust and gas (nebulae), white dwarfs, brown dwarfs, and entire galaxies. Astronomers are able to distinguish between objects and learn about their properties by the observing and analyzing the light they emit. Every type of light, or electromagnetic radiation, is associated with a wavelength and energy. The unique characteristics of the light emitted by an astronomical body can provide clues about things like its composition, temperature, size, mass, and how it interacts with its environment.1 The goal of the WISE mission is to take a detailed look at space in infrared light. In this activity, students will learn to identify two classes of objects that have very distinct features when observed in this part of the electromagnetic spectrum. Brown Dwarfs A brown dwarf is an astronomical object that, when it was formed, did not quite have the mass necessary to become a star. On the inside of stars that are similar in size or bigger than the Sun, hydrogen atoms join together and become helium, releasing energy in the process; hydrogen is the “fuel” that causes stars to shine. This process is called nuclear fusion. In brown dwarfs, fusion also occurs, but it lithium and deuterium that act as the fuel, not hydrogen.

1 Visit the following website for a lesson on the nature of light: http://mwmw.gsfc.nasa.gov/

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Figure 2: This equation shows how lithium and deuterium (hydrogen with one extra neutron) can fuse to make beryllium.

For most brown dwarfs, fusion does not last for long. After a brown dwarf stops fusion and cools down, it generates energy only through gravitational contraction. The masses of brown dwarfs range from about 13 times the mass of Jupiter to about 80 times the mass of Jupiter. Objects less massive than 13 times the mass of Jupiter would be classified as giant planets; objects more massive than 80 times the mass of Jupiter probably fuse hydrogen and would be called stars. Brown dwarfs are one kind of object that the new WISE mission is searching for. ULIRGS An Ultra-Luminous Infrared Galaxy (ULIRG) is a type of galaxy that is a 100 times more luminous in infrared light than it is in visible light. The key to this profusion of infrared light is dust, which mostly emits infrared light and which astronomers observe wherever stars are forming. Thus, ULIRGs may be found in merging galaxies whose collisions lead to dust-enshrouded bursts of star formation. WISE will discover ULIRGs all over the sky.

Figure 3: A near-infrared image of the peculiar galaxy Arp 220 from the Hubble Space Telescope. More luminous than 100 Milky Ways and radiating most of its energy in the

infrared, Arp 220 is a ULIRG.

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Grade Levels: Primary activity: Grades 5-8 With advanced steps: Grades 9-12 Standards Correlation: See Appendix A for California, Texas, NSES, and AAAS alignment. Objectives:

• Students will learn how to access images taken by the WISE mission. • Students will measure and analyze the infrared light, using computer

software such as Salsa J, from objects in three images taken by the WISE mission.

• By examining the brightness of stars at different wavelengths of infrared light, students will be able to identify brown dwarfs and ULIRGs.

• Students will learn how observing an object in different wavelengths of light reveals different information about the properties of the object.

Time: Between 50 to 100 minutes (1 to 2 50-minute class sessions): Acknowledgements: This activity was produced by the Center for Science Education at the University of California at Berkeley. It was written by Nia Imara, with input from Bryan Mendez and Kyle Fricke. Materials (per group of students):

• Pencil • Data table and graph paper (Appendix B) • Access to a computer with internet access and on which SalsaJ (or some

other image analysis program) is installed2 Grading: Please refer to the ‘Evaluate’ section at the end of this Teacher Guide. Getting Ready: Engage: Astronomers as light detectives

1. Find the brown dwarf: Raw data – or the images astronomers receive before they are processed – have lots of information that may be difficult to interpret by eye. Show students the WISE image below and ask them if they can find the brown dwarf WISE 1828+2650, the coldest brown dwarf known.

2 SalsaJ for all computers can be downloaded here: http://www.euhou.net/index.php/salsaj-software-mainmenu-9/download-mainmenu-10

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Figure 4: WISE image of the sky containing brown dwarf WISE 1828+2650.

2. How astronomers know what they know. (I): One of the ways astronomers identify and learn about celestial objects is to measure their color. The characteristic wavelengths of light associated with a star, for instance, depend on the specific physical and chemical processes that take place within and around that star. It is the ratio of two different wavelengths of light associated with a celestial object that astronomers call color. The WISE mission takes images of the sky in four wavelengths of infrared light: 3.4 µm (microns), 4.6 µm, 12 µm, and 22 µm. We will refer to these as the W1, W2, W3, and W4 bands. The image below is a combined false-color image of the same part of the sky as the above Figure 4. Light in the W1 band is designated blue, the W2 band is designated green, and the W3 band is designated red. Since brown dwarfs are deficient in 3.4 µm light and relatively bright in 4.6 µm light, they appear very green in color images. Based on this information, ask your students if they can now identify the brown dwarf in the image below (Figure 5).

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Figure 5: WISE color image of the sky containing brown dwarf WISE 1828+2650, the small green dot in the center.

3. How astronomers know what they know. (II): In order to find out detailed

information about lots of stars and brown dwarfs, astronomers have had to develop techniques for precisely measuring their light. Photometry is the technique of measuring a celestial object’s brightness or intensity – the amount of light in a given wavelength. In the following section, your students will use the computer program Salsa J to systematically measure the intensity of stars and brown dwarfs.

Procedure: Engage: Collecting the data To save time, the teacher may choose to do the following steps prior to class and have students begin the activity in the next section.

1. Go to the WISE Image Service: http://irsa.ipac.caltech.edu/applications/wise Here is what the home screen looks like:

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2. In the “Single Objects” tab, type the following coordinates into the space following “Name or Position”:

04h23m35.13s -04d14m02.5s Equ J2000

This is the location of the field in which you will search for Brown Dwarfs and ULIRGs.

3. Where it says “Return Image Size (leave blank for full images)” enter 23 and select the “Arc Minutes” option. This is the size of the field you will search.

4. Leave the other default items selected and click the “Search” button.

5. On the next page you will see four images taken in each of the WISE wavelengths and one composite image. To download the image data, select the first three images in the results box and click the “Prepare Download” button. This brings up a dialog box (see below). Under “Zip File Structure” select “Flattened (no folders)”. Leave the other default items as they are and click “Prepare Download.”

6. Once the download is complete, save the data file “WISE_Files.zip” to a location on the computer that you will be able to access, such as the desktop.

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Explore: Finding Brown Dwarfs The diagram below is called a Finder Chart. It contains a list of the targets you will measure. The numbers are arbitrary. The same targets appear in each of the three images you will analyze.

1. Open Salsa J on your computer.

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2. Go to the File menu (or click on the picture of the folder in the upper left-hand corner) and open the images you just downloaded.3 The first image will have w1 in the file name. This is the WISE image taken in the W1 3.4 µm band. Compare the image to the finder chart and make sure you can identify all the objects that are labeled on the chart.

3. Click the red double arrow on the right and select “Photometry”. Then click

on the Analyse menu and choose “Photometry Settings”. Choose the following settings:

o Star’s Center: Auto o Star Radius: Forced Star Radius – 10 o Sky: Forced Sky Radius – 20

4. Choose the bulls-eye button with the red circle from the button bar. (You can

also pick Analyse>Photometry.) When you put your mouse over the image, a cross will appear. Use the finder chart to locate star #1 and click on it. A data table will pop up that shows many numbers, but you only need a few.

5. From the table, copy the following numbers onto your data table:

X, Y, Star’s intensity

The first two numbers, X and Y, are the location of the object. The third number, the intensity in units of digital numbers (DN), tells you how much light is coming from the object.

6. Measure all of the finder chart objects in this way.

3 Say something about the default memory on SalsaJ and how to get more of it so as to open multiple images.

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7. Open the next image (the W2 image in 4.6 µm). It is important you identify the same stars in the image. The finder chart and the star’s location (x, y) can help you with this. For each of the stars you measured before, measure the same star again in the new image and add that data to the table. This new data goes in the next column labeled W2 [DN].

8. Repeat for the third image (W3 in 12 µm).

Plotting the Color-Color Diagram

9. Astronomers can distinguish brown dwarfs from other objects by comparing their intensities. To do this, compute the ratio of the fluxes. This means that for each star divide the W2 intensity by the by the W1 intensity. Also take W3 and divide by W2Record these values in the last two columns of your table. And keep in mind that your ratios may be less than 1.

10. When you plot the values from Table 1, this is called a color-color diagram, and you can identify brown dwarfs and ULIRGs by the location these points occupy on the plot. For each object, plot W3/W2 on the x-axis and plot the corresponding value of W2/W1 on the y-axis.

Explain: Where are the Brown Dwarfs?

11. Identify object(s) on your finder chart that might be brown dwarfs. Brown dwarfs are noted for having an absorption line near the 3.4 µm wavelength (W1). That means that due to the chemical composition of the brown dwarf, it absorbs light with wavelengths near 3.4 µm, and the intensity in that wavelength tends to be unusually low. This means that the ratio of the 4.6 µm intensity to the 3.4 µm intensity (W2/W1) will tend to be high in comparison to other objects, which will have smaller values. However, brown dwarfs are not the only objects that might have 4.6 µm to 3.4 µm ratios in that high range. Certain giant stars or dusty galaxies may also have similar ratios (or “color” as astronomers use the term). To separate out those objects we can look at the intensity ratio between 12 µm and 4.6 µm. Brown dwarfs and stars will tend to have smaller flux ratios compared to the dusty galaxies and red giant stars.

Thus, brown dwarfs will tend to be higher on the color-color diagram than normal stars. Since dusty galaxies such as ULIRGs emit lots of light in the W3 band due to warm dust, their W3/W2 ratios are high, and they tend to be farther to the right of brown dwarfs on the color-color diagram.

Helpful Hint Finder chart locations. To ensure that your

students are measuring the correct objects, you can refer to the Example Data

table in the appendix. It provides the locations (x, y

coordinates) of the finder chart objects.

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Figure 6 shows the regions on the color-color diagram where different classes of objects detected by WISE would fall. Notice that although both brown dwarfs and a class of stars known as asymptotic branch (AGB) stars have high W2/W1 ratios, the two classes of objects can be distinguished because we can also compare their W3/W2 ratios. Though the regions have some overlap, AGB stars tend to have much higher W3/W2 ratios than brown dwarfs.

Figure 6: Color-color diagram showing where brown dwarfs, ULIRGs, normal galaxies, stars, and asymptotic giant branch (AGB) stars fall. The dashed-line box indicates the

region where students will plot their data.

Going Further (optional): Observing an object in different wavelengths of light helps astronomers to more fully understand that object’s properties. The various physical and chemical interactions taking place in and around a star, for instance, produce different wavelengths of light; so the star looks different depending on the wavelength in which it is observed.

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Compare Figures 3 and 7, near-infrared and optical images of the ULIRG Arp 220. Much of the infrared light of ULIRGs comes from warm dust associated with star formation (Figure 3). However, dust blocks light at optical wavelengths, but stars emit a large part of their energy in the optical, and so in an optical image of Arp 220, the structure of the galaxy’s stars and dust lanes are prominent (Figure 7).

Figure 7: An optical image of Arp 220, a ULIRG, from the Hubble Space Telescope.

In this extension activity, students will search for ULIRGs using data from the Hubble archive.

1. Go to the WISE Image Service and enter the following coordinates:

6h20m54.00s -63d18m09.0s Equ J2000

Where it says “Return Image Size (leave blank for full images)” enter 23 and select the “Arc Minutes” option. This is the size of the field you will search.

2. Leave the other default items selected and click the “Search” button. Then follow Steps 4 and 5 on page 4 of this activity.

Finding ULIRGs The diagram below is called a Finder Chart. It contains a list of the targets you will measure. The numbers are arbitrary. The same targets appear in each of the three images you will analyze.

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3. Open Salsa J on your computer.

4. Go to the File menu (or click on the picture of the folder in the upper left-hand corner) and open the images you just downloaded.4 The first image will have w1 in the file name. This is the WISE image taken in the W1 3.4 µm band. Compare the image to the finder chart and make sure you can identify all the objects that are labeled on the chart.

5. ULIRGs are very faint since they are very distant from the Milky Way Galaxy.

You may need to adjust your image in order to compare them with the finder 4 Say something about the default memory on SalsaJ and how to get more of it so as to open multiple images.

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chart. Click on the Image menu and choose “Adjust > Brightness/Contrast”. Adjust the maximum brightness until you can identify all the objects shown in the finder chart.

6. Click the red double arrow on the right and select “Photometry”. Then click

on the Analyse menu and choose “Photometry Settings”. Choose the following settings:

a. Star’s Center: Auto b. Star Radius: Auto c. Sky: Auto

7. Choose the bulls-eye button with the red circle from the button bar. (You can

also pick Analyse>Photometry.) When you put your mouse over the image, a cross will appear. Use the finder chart to locate star #1 and click on it. A data table will pop up that shows many numbers, but you only need a few.

8. From the table, copy the following numbers onto your data table:

X, Y, Star’s intensity

The first two numbers, X and Y, are the location of the object. The third number, the intensity in units of digital numbers (DN), tells you how much light is coming from the object.

9. Measure all of the finder chart objects in this way.

10. Open the next image (the W2 image in 4.6 µm). It is important you identify the same stars in the image. The finder chart and the star’s location (x, y) can help you with this. For each of the stars you measured before, measure the same star again in the new image and add that data to the table. This new data goes in the next column labeled W2 [DN].

11. Repeat for the third image (W3 in 12 µm).

Plotting the Color-Color Diagram

12. Astronomers can distinguish ULIRGs from other objects by comparing their intensities. To do this, compute the ratio of the fluxes. This means that for each star divide the W2 intensity by the by the W1 intensity. Also take W3 and divide by W2. Record these values in the last two columns of your table. And keep in mind that your ratios may be less than 1.

13. When you plot the values from Table 1, this is called a color-color diagram, and you can identify brown dwarfs and ULIRGs by the location these points occupy on the plot. For each object, plot W3/W2 on the x-axis and plot the corresponding value of W2/W1 on the y-axis.

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Explain: Where are the ULIRGs?

14. Identify the ULIRG from your color-color diagram.

Both brown dwarfs and ULIRGs have similar W2/W1 flux ratios. So to distinguish between the two types of objects, you can compare their W3/W2 ratios, as well. Astronomers have evidence showing that ULIRGs are the likely result of two or more colliding galaxies. A galaxy collision typically triggers huge amounts of star formation, and the resulting energy heats the dust, causing it to “glow” very brightly in 12 µm light (W3). Thus, the W3/W2 flux ratios of ULIRGs are very high, and they tend to be on the far right of the color-color diagram.

Exploring ULIRGs in Different Wavelengths The ULIRG that students identified above is called IRAS 06206-6315. Below left is a combined color image of the ULIRG in the same region of sky as the finder chart. To the right is an optical image of the ULIRG taken by the Hubble Space Telescope (corresponding to the region inside the yellow box). Hubble takes images of the sky in optical wavelengths of light – like the light that can be viewed by the naked eye.

Figure 8: A WISE color image (left) and a Hubble image (right) of IRAS 06206-6315. The arrows in the Hubble image point north and east.

The Hubble image shows the starlight from two colliding galaxies, as well as the large tidal tail of the galaxy to the north. The energy associated with star formation is not observable in the optical wavelengths seen by Hubble, however, but is clearly noticed as the bright red region in the WISE color image. (Remember, red designates 12 µm light.) Ask your students to examine these two images and to discuss the different information that they might provide about IRAS 06206-6315.

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Discussion Notes: Elaborate: After finishing the activity, discuss the following questions and talking points with your students.

• What is a brown dwarf? • What is the difference between a brown dwarf and a star? • What does the computer software measure when you click on an object? • What sources of error might there be in this experiment? • What might the objects be that occupy the lower left-hand corner of the

color-color diagram?

• What is a ULIRG? • Why do ULIRGs look different in mid-infrared light (as seen in WISE images)

and optical light (as seen in Hubble Space Telescope images)? • Why are ULIRGs to the right of brown dwarfs in the color-color diagram? • Look at the color-color diagram in Figure 6, and notice how some of the

regions overlap. Suppose you wanted to identify an object whose W1/W2 and W2/W3 ratios made it fall in the overlapping brown dwarf-ULIRG region. How could you use the Hubble Space Telescope to tell whether it was a brown dwarf or ULIRG?

Evaluate: There are ____ questions on the Student Worksheet that students can answer after they have finished the activity. Grading Rubric: Question 1:

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Table:

TABLE 1

Finder chart #

X Y W1 [DN] W2 [DN]

W3 [DN] W2 / W1 W3 / W2

1

2

3

4

5

6

7

8

9

10

11

12

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Example Data and Graphs:

TABLE 1 Brown Dwarfs

Finder chart #

X Y W1 [DN]

W2 [DN]

W3 [DN]

W2 / W1 W3 / W2

1

266 151 11047 4277 827 0.39 0.19

2

504 206 122299 70872 13150 0.58 0.19

3

142 205 24914 9778 1718 0.39 0.18

4

364 299 7278 2822 857 0.39 0.30

5

561 399 7364 2759 552 0.37 0.20

6

389 502 1919 1308 656 0.68 0.50

7

120 449 68034 25340 4741 0.37 0.19

8

465 615 3351 1256 268 0.37 0.21

9

640 713 2538 1182 310 0.47 0.26

10

486 858 8129 2936 576 0.36 0.20

11

373 858 7333 2625 583 0.36 0.22

12

509 927 4294 1611 292 0.38 0.18

Color-color diagram for brown dwarfs.

0.1  

0.2  

0.3  

0.4  

0.5  

0.6  

0.7  

0.8  

0.1   0.3   0.5   0.7  

W2/W1  

W3/W2  

W2/W1  vs.  W3/W2  

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TABLE 1 ULIRGs

Finder chart #

X Y W1 [DN]

W2 [DN]

W3 [DN]

W2 / W1 W3 / W2

1

283

688 11047 4277 827 0.39 0.19

2

342

725 122299 70872 13150 0.58 0.19

3

528

736 24914 9778 1718 0.39 0.18

4

712

627 7278 2822 857 0.39 0.30

5

563

604 7364 2759 552 0.37 0.20

6

548

472 1919 1308 656 0.68 0.50

7

467

468 68034 25340 4741 0.37 0.19

8

358

473 3351 1256 268 0.37 0.21

9

477

366 2538 1182 310 0.47 0.26

10

797

309 8129 2936 576 0.36 0.20

Color-color diagram for ULIRGs.

0  

0.2  

0.4  

0.6  

0.8  

1  

1.2  

0   2   4   6   8  

W2/W1  

W3/W2  

W2/W1  vs.  W3/W2  

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Appendix A: Standards Correlation Appendix B: Correction Factor Appendix C: