Scientific calculator, Macintosh computer, and the National Institutes of Health computer program ImageJ.

Transcription

1 Name: Lab Partner: Instructor's Signature: Lab Title: Structure of Galaxies Equipment: Scientific calculator, Macintosh computer, and the National Institutes of Health computer program ImageJ. Purpose: Using digital images, we will take a brief tour of galaxies and study some of their structure. They are among the most elegant and beautiful objects in the universe. Requirements: This lab is to be performed individually (or in groups of no more than two if there are not enough computers). If you must work in pairs, take turns manipulating the computer and taking the data. You should switch off from time to time so everyone gets a chance to use the computer. Although you may use the computer and the program with your partner to collect data, all calculations, graphing, and any narratives in your lab report must be your own original work! Introduction: When we look into the dark night sky without the aid of a telescope, we see thousands of stars. However, stars are only a small subset of the fascinating variety of objects and phenomena visible with even a small telescope. Nebulae, open clusters, globular clusters, and galaxies are among the other residents of our sky. Besides the stars, planets, the Moon, and occasional comets, there are only four other "objects" we can see without a telescope. They are all galaxies: the Large and Small Magellanic Clouds (two satellite galaxies to our own, visible from the Southern Hemisphere), the Andromeda Galaxy, and the Milky Way (our own Galaxy). Being embedded within it, the Milky Way looks the brightest. We see our Galaxy edge-on, a wide swath of splotchy, translucent white running from horizon to horizon across the night sky. The description Milky Way is fitting. -1-

2 It is important to realize that all the stars we see in the sky are, like our own Sun, part of the Milky Way Galaxy. With the aid of large telescopes, we see as many galaxies beyond our own as there are stars within the Milky Way itself. Each of the galaxies we see, with or without a telescope, contains billions of stars. They also harbor star clusters, clouds of gas, and perhaps even black holes. For all but the nearest galaxies, viewed through the world's best telescopes, we don't see the stars as individual points. Instead, the galaxy as a whole appears fuzzy, with different degrees of clumpiness. Galaxies also come in a variety of sizes and shapes. The most obvious distinction between galaxies is that some of them are elliptical and some are spiral in shape. Most galaxies fall into one of these two divisions; those that do not are termed irregular. The classification of galaxies is less clear if they are far away, or if their orientation with respect to us makes their structure difficult to see. Figure 1 shows a more detailed classification scheme for galaxies used by astronomers. The ellipticals are divided into eight classes, E0 through E7, with E0 being spherical and E7 being the most elongated. Normal spirals are identified as type S and those with central bars are called type SB. They are further classified by the form of the arms and the size of the central bulge. S0 and SB0 forms show a central bulge and disc, but no apparent development of arms. Sa and SBa galaxies have large central bulges and loosely wound arms; Sc and SBc galaxies have a tiny core and tightly wound arms. Sb and SBb are intermediate forms. The spirals are all viewed face-on in this diagram. They will appear different at other inclinations. Normal Spirals Ellipticals S0 Sa Sb Sc E0 E3 E7 SB0 Barred Spirals SBa SBb SBc Figure 1. The Hubble Galaxy Classification Scheme In this lab, we will begin our study of galaxies by looking at NGC 891 and M95. These are both spiral galaxies, but one is edge-on from our perspective and the other is seen face-on. NGC stands for the New General Catalogue, a catalog of galaxies and nebulae first published in the 1870's, so it's really not all that new. M (as in M95) stands for the Messier catalog, a list of 110 objects compiled in the 1770's by Charles Messier, a French astronomer. His primary interest was searching for new comets. He made a list of the -2-

3 positions of galaxies, nebulae, and other "fuzzy" objects he ran across, so he wouldn't confuse them with comets! Procedure: Your instructor will inform you about which computers on campus currently have the program installed on their hard drives. Position the mouse cursor arrow over the icon (picture) of the Mac's hard disk and double-click on the mouse button. A list of the contents of the hard disk will appear. Move the mouse cursor over the Astronomy 30 folder's icon and again double-click the mouse button to reveal the contents. Clicking the mouse button twice when the cursor arrow is on the picture of the Image Processing folder will open the folder and reveal a picture (icon) of the program itself called ImageJ in the file list. You can now start the program by positioning the mouse cursor over the ImageJ icon and double-clicking to run the program. ImageJ is a public-domain Java imageprocessing program inspired by the National Institutes of Health s NIH Image. It is used widely throughout the scientific research community to study and analyze digital images obtained from a wide variety of sources. Downloadable versions of ImageJ are available for Windows, Mac OS, Mac OS X, and Linux. After the program has finished loading, a menu bar will appear along the top portion of the screen. Each word in this bar (File, Edit, Image, Process, Analyze, Plugins, Window, Help) is a menu consisting of a list of commands to ImageJ. You can open a menu by positioning the cursor over a word in the menu bar. While pressing the mouse button, move the mouse downward. As you drag the mouse, an item list appears and each item is highlighted with a black bar as the cursor moves over it. Some items under names in the menu bar have a series of three dots (... ) next to them. Releasing the button on these dotted items causes a dialog box to appear on the screen. If an item is a command, releasing the mouse button with the item highlighted executes the command. The Quit command in the File menu lets you exit from the ImageJ program. An item that is lighter than the other items in a menu is inactive and can not be used until the program decides it is appropriate to do so. An Edge-On Spiral Galaxy The first image is of NGC 891, an edge-on spiral in the constellation of Andromeda. This image was taken by George Jacoby of the National Optical Astronomy Observatory. Let's take a look. Select File Open... and open the NGC 891 image file. Since this spiral galaxy is viewed edge-on, the diameter of the spiral corresponds to the length of that image on the screen, from upper left to lower right. Note the dark band running the length of the galaxy. Select the tool and place the crosshairs in a dark corner of the image and check the brightness value. Then do the same for a white star. For grayscale images, a pixel brightness value of 255 represents pure white and a pixel brightness value of 0 represents pure black. Pixel values like 25, 71, and 193 represent various shades of gray between white and black. If this is not the case, choose Image Type 8 bit, even if the 8 bit -3-

4 choice is already selected (by a check-mark). Re-check the brightness values to be sure they are now correct. Figure 2. Image of NGC 891 This galaxy is approximately 30,000 parsecs in diameter, roughly the same size as our own Milky Way Galaxy. The parsec is a unit of length often used in astronomy, and is equal to approximately 3.26 light-years. Thus, light travels 1 parsec in 3.26 years. The parsec is defined using the principle of parallax, the apparent motion of a star against a background of much more distant stars, caused by the motion of the Earth around the Sun. A thousand parsecs or one kiloparsec (abbreviated kpc) is a useful unit of distance when discussing galactic sizes. Use the tool to help you enlarge and examine the image. The bright region around the nucleus is the central bulge. Notice the shape of the bulge. A spheroidal bulge would look perfectly round; an ellipsoidal bulge would look like an oval (i.e., like a squashed sphere). -4-

5 1. Is the bulge spheroidal or ellipsoidal in shape? 2. Notice again the dark band running across the midplane of this archetypical edge-on galaxy. What could be causing this? (Hint: something is blocking the light, a process called extinction.) When dust is found in the halo of a galaxy, astronomers often point to supernova (exploding stars) as the culprits. Massive stars end their lives in these violent explosions, which blow dust-forming heavy elements into the interstellar medium and reshape the gas and dust that are already there. Supernovae can blow holes all the way through a galaxy's disk of interstellar matter, sending streamers of gas and dust into the halo. Frothy and nebulous, NGC 891 has one of the dustiest inner halos of any known galaxy and provides a fascinating look at the outflow of matter from a galaxy's disk. Where Would the Sun Be? NGC 891 is very similar in size and shape to our own Galaxy. We will use this image to help us visualize the position of the Sun in the Milky Way. If our Sun is 8 kiloparsecs from the center of our Galaxy, you can locate the approximate position of the Sun. This point would be in the plane of the galaxy, about halfway out from the center. Select Image Adjust Brightness/Contrast, and click Auto to find a good brightness and contrast setting. You can adjust it further using the Brightness and Contrast slider bars until you think you can detect the furthest extent of the galactic disk. If you can't see all of the galaxy on the screen, resize the window by clicking-and-dragging on a corner of the image window. Use the tool to drag out a line selection along the entire length of the galaxy. Select Analyze Measure, and the length in pixels (short for picture element) will be reported in the Results window. The smallest unit of the digitized image is a called a pixel. 3. What is the diameter of the galaxy in pixels? Given that Pluto is about kilometers from the Sun, let's find out how big the solar system is on this galactic size scale. Remember that a parsec is equal to 3.26 lightyears, or roughly kilometers. -5-

6 4. If the diameter of the Solar System is defined as twice the distance of the Sun to Pluto, what would the diameter of our Solar System be in kilometers? 5. What fraction of a parsec is this Solar-System diameter? Remember that the diameter of the galaxy is about 30,000 parsecs. So 30,000 pc divided by your answer in Question 3 should give the number of parsecs per pixel on the scale of this image. 6. So each pixel represents how many parsecs? 1 pixel = parsecs Now find how many pixels our Solar System would span on this galactic scale, by dividing your answer for Question 5 by your answer for Question 6: Solar-System diameter = pixels 7. Repeatedly use the tool to zoom in on a single pixel at about the position in the galaxy where the Sun would be. What do you think are the chances of seeing a Solar-System-sized object in this nearby galaxy? Explain your answer. Our Sun is about parsecs above the plane of the Galaxy. Locate the plane of the galaxy in this image and notice some small knots in the disk. These knots are star-forming regions that are larger than our own Galaxy's Orion complex! 8. The very thin region is the disk of the galaxy. The central region is the bulge. Measure the diameter (in pixels) of the bulge in the plane of the galaxy using the tool. 9. What would the diameter of the bulge be in parsecs? (See Question 6 for your conversion factor from pixels into parsecs.) -6-

7 10. From the Sun's position in our Galaxy, a line-of-sight towards the galactic center intersects a great many stars. Which line-of-sight would intersect the least number of stars? This latter line of sight provides the clearest direction through a galaxy and allows one to view the most extragalactic objects. In other words, looking through a telescope pointed in this direction, you would see a minimum number of foreground stars in the way. The brightness of the bulge can be measured by taking a density profile. A density profile is a graph of the brightness of the image along a selected line. If necessary, re-draw the straight line through the galactic bulge, perpendicular to the galactic plane. Choose Analyze Plot Profile to measure the brightness gradient along the line. 11. Sketch and label the resulting brightness density profile in the space below. Identify directly on your sketch the region most affected by the obscuring dust band. Brightness Position (pixels) Using your answer to the first part of Question 6, also indicate how far 1 kpc would extend on the horizontal scale of this graph. What is the thickness (both in pixels and in pc) of this dust band? For a different view of this image, try one or more of the other color tables. Lookup table choice Image Lookup Tables Fire can be especially revealing when used along with enhancing the image with the Brightness and Contrast slider-bar controls in the B&C window. When you are finished exploring this image, select File Close. A Face-On Spiral Galaxy Next we will look at an image of a face-on spiral called M95. It is also known by its New General Catalogue name, NGC Select File Open... the image file M95. DO NOT ADJUST the brightness or contrast of this image, and answer the following questions. -7-

8 12. From the galaxy classification diagram shown in Figure 1, is it easy to classify M95's type? If you had to choose a type, what would it be and why? 13. How many bright spots are visible in the central region of the galaxy? Now you may adjust the brightness and contrast of this image as before until a large, fuzzy bar appears. Continue adjusting the controls until the entire galaxy is visible. Again, you should experiment with different color tables such as Image Lookup Tables Spectrum (Inverted) or Image Lookup Tables Fire. Use the controls in the B&C window to create a satisfactory image when using these other color tables that shows the outermost parts of the disk but does not wash out the bright, central ring. Enhancing this image illustrates the danger of classifying galaxies on the basis of limited information! 14. After the adjustments that you made, to the image would you make any changes to your classification of M95 s type? Dynamic range is a measure of the ability to detect or record information that varies across a range of values, such as the surface brightness of a galaxy. A good astronomical photographic emulsion has a limited dynamic range, say 100. This means that the brightest objects visible (in full detail) in a photograph are at most 100 times brighter than the dimmest recorded objects. If the objects being observed have a brightness difference greater than this factor of 100, considerable information will be lost. For example, a photograph of a galaxy exposed correctly for the bright center will not allow one to see very far from the center, since the edges will be severely underexposed. Similarly, a photograph taken so that the outer regions are properly exposed will end up overexposing and washing out the detail in the center. New astronomical cameras using electronic imaging chips called CCDs (Charge Coupled Devices) have dynamic ranges that reach as high as 32,000:1. These cameras have little difficulty recording good detail in all parts of a galaxy in a single exposure. Though our eyes have a much lower dynamic range, computers allow us to view all of the information contained in a CCD image, though not all at once. By adjusting the brightness, we can extract information about the brightest and faintest portions of the image. Bars and ring structures often occur in galaxies that have survived a near collision with another galaxy! The bar in the M95 galaxy is composed of stars, each of which moves independently in its own orbit around the center of the galaxy. The bar somehow retains its identity for many revolutions (orbital periods) of its constituent stars -8-

9 around the center of the galaxy. This is long after any galactic collision that may have formed it. Why is this? Part of the answer is that the gravitational field is not purely central, as is assumed for Kepler's Law to be valid. Still, this is only part of the answer, and resolving this question is currently a hot research topic. Bars may have the ability to preserve themselves, despite forming rather suddenly! The bar in M95 is an "interesting phenomenon" (which is what scientists call things that they don't understand very well...). Of course, most scientists are excited by the challenges of trying to explain phenomena that are not well understood, like bars in galaxies. This is more fun than confirming knowledge that most people in the field are already "sure" of. However, it often turns out that what we are sure of has plenty of surprises as well. When you are finished exploring the image of M95, select File Close. -9-

10 Star Formation in Galaxies Next, choose File Open... the image M51 continuum. This is the famous Whirlpool Galaxy in the constellation of Canes Venatici ("The Hunting Dogs"). These images were provided by Richard Elston of the National Optical Astronomy Observatories. This particular image of M51 was taken through a continuum red filter broadly centered around a wavelength that is used to measure star formation in galaxies. Such spectral filters allow only light within a certain wavelength band to pass through the filter and actually reach the detector. In contrast to this broad continuum emission, regions where new stars are being formed emit significant amounts of light from the surrounding clouds of hydrogen gas that are excited by the radiation emitted by the embedded young stars. This light is a very special color of red that originates from the n=3 to the n=2 transition in excited neutral hydrogen atoms. This particular shade of red is called Hα (pronounced H-alpha ) and is hydrogen line emission at a precise wavelength of 6563 Ångstroms. Figure 3. Image of the Whirlpool Galaxy (M51) -10-

11 By comparing images taken in the special, very narrow-band Hα filter with the broad-band images taken with the continuum red filter, we can determine where star formation is taking place in the galaxy, and how vigorous it is. We'll be doing just this with the image you've already loaded (M51 continuum) and an additional Hα image (named M51 H-alpha). The diagrams below show the light curves produced by regions containing older stars and regions of new star formation. A normal region of older stars is on the left, and a region with new star formation is on the right. Continuum Red filter Light curves Continuum Red filter Intensity Intensity Narrow Hα filter Wavelength Wavelength 15. Note the Hα spike in the light curve on the right. What causes this? Look first at the red image M51 continuum. Images such as this, recording broadband red light, emphasize the light from older stars. However, we are really looking for regions of new star formation. Stars tend to be born in clumps called Star-Formation Regions, such as the Orion Nebula in our own Galaxy. You will try to determine if any of the clumps seen in the continuum image could be large star-formation regions. Locate three clumpy regions in the spiral arms and record their pixel coordinates below (use the tool to help you examine the image). DO NOT include the small, bright areas those are actually foreground stars in our own galaxy that happen to lie along the line of sight to M Star-forming Region l X: Y: Star-forming Region 2 X: Y: Star-forming Region 3 X: Y: -11-

12 Let's see if your guesses are correct that these clumps are indeed star-forming complexes. Choose File Open... the M51 H-alpha image. Select Window Tile to place the two images side-by-side. The Hα image shows regions that emit strongly in this 6563-Ångstrom hydrogen emission line. However, only by subtracting the continuum red emission near Hα can the true Hα line emission be seen. 17. The older stars that are already formed, such as our Sun, are emitting most of their energy in what wavelength interval? 18. Does any light from an older star pass through an Hα filter? Why or why not? You will now subtract the red continuum image pixel-by-pixel from the Hα image using the following steps. First, click on each image one at a time to activate it and choose Image Type 8 bit, even if the 8 bit choice is already selected (by a check-mark). Now choose Process Image Calculator and set: Image 1: M-51 H-alpha Operation: Subtract Image 2: M-51 continuum Check Create New Window and click OK. A third image called Result of M51 will appear and be quite dark on the screen until you select Image Adjust Brightness/Contrast, and click Auto to find a good brightness and contrast setting. For an enhanced view of this differenced image, choose Image Lookup Tables Fire and then adjust the image by adjusting the brightness and contrast controls in the B&C window. Compare the locations of the three star-formation clumps you guessed from the continuum red image to see if they coincide with any of the actual star-forming regions revealed by the Hα subtraction process. Any young star-forming regions should appear as bright clumps in this differenced image. 19. How accurate were your previous guesses of star-forming regions in Question 16? Which of your initial three choices in the spiral arms actually turned out to be starformation regions? -12-

13 20. Are there any young star-formation regions in the center of M51? How do you know from looking at your differenced image? You might now compare your results with the image file M51 difference, which was created using a similar subtraction process. The use of Hα light is a very powerful tool for finding star formation regions, because the young stars stand out prominently at this wavelength. The use of this and other spectral lines is useful for hunting for objects with a high rate of new star formation, such as in protogalaxies (newly-forming galaxies) or in colliding galaxies! Elliptical and colliding galaxies We haven t spent any time looking at elliptical galaxies. To see why, click File Open and open ngc4647and4649.gif. These are two galaxies in the Virgo cluster of galaxies (named for the constellation in which the cluster can be seen). The two are not actually colliding one is in front of the other. These images were taken by Rebecca Koopman, Jeffrey Kenney, and Judith Young and published in The Astrophysical Journal Supplement (2001, Vol. 135, p. 125). NGC 4649 is an elliptical galaxy, while NGC 4647 is a spiral. The image on the left was taken through the R filter, which allows a wide range of red light. The image on the left was taken through an H-alpha filter. The elliptical NGC 4649 is much more massive, and yet almost disappears in the H-alpha image. A color image of the pair can be seen if you open the file m60.jpg (M60 is the designation of NGC 4649 in the Messier catalog of astronomical objects). Using what you learned for M51, answer the following: 21. Compare the R band and H-alpha images of NGC 4649 and Although we don t have the raw images that you could subtract, think of your results for M51 which of the two galaxies would likely show regions of star formation if you were to subtract the red and H-alpha images? Why? Although elliptical galaxies may not seem exciting, the large ones started their lives with something of a bang. Open up the file Antennae.jpg. This shows two images of galaxies that are colliding. The black-and-white picture on the left was taken with a groundbased telescope. The green irregular outline shows the shape of the zoomed image on the right, which is a Space Telescope image of the central region of the two galaxies. -13-

14 Notice the large number of hot, young, blue stars. As gas clouds collide during the merger, star formation occurs rapidly, using up much of the gas clouds in the two galaxies. The two galaxies were originally spirals, not too different from our own Galaxy. The two streams that you see in the left image, which give the pair their informal name, are tidal streams left behind as the two galaxies fell towards each other. After a few hundred million years, the chaos that we see now will have settled down, and the two galaxies will merge into a single galaxy, which will more resemble a medium-sized elliptical galaxy. It may still be rotating, and will have some gas and dust, but after a few more mergers, it will much more closely resemble NGC If you were awarded observing time on a telescope to search for colliding galaxies, what filter/s would you use, and how would you make use of them? -14-

15 Conclusions and Comments In this section, focus on discussing the different types of galaxies. Go into detail on their different characteristics. Also discuss how we are able to use images taken using different filters to study what goes on in galaxies. -15-

16 Name: Lab Partner: Pre-lab Exercises: Structure of Galaxies The bar in the M95 galaxy is formed of stars, each of which moves independently in its own orbit around the center of the galaxy. One characteristic of orbits in a central gravitational field is that they obey Kepler's Third Law, which says that the (Period) 2 /(Distance) 3 is a constant for each orbit. Outline of "Bar" Orbital Direction + Galactic Center Point 1 Orbital Direction Point 2 Central Portion of the Image of M95 In the initial image above, Point 2 on the bar is twice as far from the center of the galaxy as is Point 1 on the bar. 1. For Kepler's Law, Point 1 has a distance of one unit of distance (let's choose for simplicity an arbitrary unit of distance) from the center and a period of one unit of time. Apply Kepler's Law to Point 1: ( ) 2 /( ) 3 = -16-

17 2. Now, Point 2 is twice as far (2 distance units) from the center. Since the constant you evaluated above in Kepler's Law must be the same for both points, what is the orbital period (in our arbitrary units of time) for Point 2? Show your work below. 3. Given that these two stars (Point 1 and Point 2) have different speeds, draw Point 2 in the box below as it will be after the inner star Point 1 has finished two complete revolutions and is back at its original spot. Initial Outline of "Bar" + Galactic Center Point 1 (after 2 revolutions) Orbital Direction of Bar Point 2 (initially) 4. The results of Exercise 3 illustrate why bar structures in galaxies are not well understood! From the analysis you just did on the orbits of two of the individual stars comprising the bar, discuss why it is apparent that the bar should disrupt itself within just a few orbital periods? -17-

18 Yet the bar apparently retains its identity for many revolutions (orbital periods) of the stars around the center of the galaxy! This is long after any galactic collision that may have formed it. 5. (a) Would the parsec be a longer or a shorter unit of distance if the Earth were farther away in its orbit around the Sun? Why? Illustrate your reasoning with a sketch showing a top view of the motion of the Earth around the Sun relative to the distance to a fixed nearby star lying in the plane of the ecliptic. (b) If the Earth were four times farther away from the Sun in its orbit than it is now, how many years would astronomers have to wait (instead of only 6 months) until the Earth would be on the opposite side of the Sun to make parallax measurements of the distances to nearby stars? -18-