Introduction: Students learn the basics of galaxy classification and grouping, then use Hubble Space Telescope images to discover the 'morphology-density effect' and make hypotheses about its causes.McDonald Observator

McDonald Observatory

University of Texas at Austin

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                         	 	 	 	 	 		 	 	 	 		 	 	 			 	 	 	 	 		 	 	 	 		 	                                            Coma Cluster of Galaxies national science education standards •	 Content Standard in 9-12 Science as Inquiry (Abilities necessary to do sci-entific inquiry, Understanding about scientific inquiry) •	 Content Standard in 9-12 Earth and Space Science (Origin and evolution of the universe) InvIsIble Cluster If you aim a big telescope at the ComaCluster, you’ll see galaxies galore— thousands of galaxies of all sizesand shapes, from little puffballs tobig, fuzzy footballs. Even so, you won’tsee most of the cluster because it’s invis­ible to human eyes. Some of the cluster’s “dark side” is inthe form of superhot gas that glows in X-rays. All together, the gas is several timesas massive as the galaxies themselves. There’s a dynamic interplay betweenthe hot gas and the galaxies. As galaxies “fall” toward the center ofthe cluster, they fly through the hot gas, which strips away the cold gas insidethe galaxies. Without their cold gas, thegalaxies can’t give birth to new stars. Thathelps transform the appearance of someof the galaxies. Spiral galaxies lose theirspiral arms, so they look like featurelessdisks. But the galaxies may have an effect onthe hot gas, too. Over the eons, it shouldhave cooled, but it hasn’t. Hot “jets” ofparticles from the centers of some galax­ies may act like big blowtorches, keepingthe gas hot. Yet even the gas and the galaxies com­bined make up only a small fraction of theComa Cluster. As much as 80 percent ofits mass may consist of dark matter — aform of matter that produces no detectableenergy, but that exerts a gravitational pullon the visible matter around it. The darkmatter ensures that most of this impressivecluster remains invisible. This is the transcript of a StarDate radio episode that aired May 6, 2008. Script by Damond Benningfield,©2008. In 2006, Hubble Space Telescope aimed at a nearby collection of galaxies called the Coma Cluster. Using the HST images, astronomers gained fascinating insights into the evolution of galaxies in dense galactic neighborhoods. In this activity, students will first learn the basics of galaxy classification and grouping, then use HST images to discover the “morphology-density effect” and make hypotheses about its causes. Materials & PreParation •	 Each student needs a copy of the next 7 pages (not this page). You may copy the pages out of this guide, but it is recommended that you go to mcdonaldobservatory.org/teachers/classroom and download the student worksheets. The galaxy images in the online worksheets are “negatives” of the real images, which provides better detail when printing. Supple­mental materials for this activity are also available on the website. •	 Each student or student team will need a calculator and a magnifying glass (a linen tester works well). •	 Knowledge of percentages is needed before doing this activity. suggested grading •	 Page 31 (5 pts): Student provides clear explanations of the scheme. •	 Page 32 (2 pts total, 2 pts each): Answers: (E/S0/SB0 – 2,6,9), (S – 1,8,12), (SB – 3,4,10), (IR – 5,7,11) •	 Pages 34 and 35: Not graded; based on student’s subjective interpreta­tion. •	 Page 36 (30 pts): Graded for completion, not accuracy. Students will get different numbers, but math should be correct. Answers for percentages are typically in the following range: (Cluster: E 50 percent, L 30 percent, S 20 percent) (Field: E 20 percent, L 10 percent, S 70 percent). Students usually find a higher percentage of spirals in the field. •	 Page 37 (bottom, 30 pts): Student hypothesis should mention the effects of interactions and ram-pressure stripping in changing past gas-rich spirals into current gas-poor ellipticals and lenticulars in clusters. S ta r D at e / U n i v e r S o t e a c h e r  G U i D e 1         GEMS COLLABORATION engage The diagram above shows a mosaic of 40 galaxies. These images were taken with Hubble Space Telescope and show the variety of shapes that galaxies can assume. When astronomer Edwin Hubble first started studying these vari­ous types of galaxies in the 1920s, he realized he needed to develop a way to organize and categorize them. He cre­ated a classification scheme in which he grouped similar galaxies together. Your job is to do the same thing. In the chart, invent your own four galaxy types and provide a description and three examples for each one. Galaxy Type (name and draw) Defining Characteristics (write a short description, provide enough detail so that anyone could use your scheme) Three Examples (give 3 grid coordinates) S ta r D at e / U n i v e r S o t e a c h e r  G U i D e 2                            1 2 4 5 6 7 8 9 10 11 12 •	 ORDINARYSPIRALS BARRED SPIRALS ELLIPTICAL GALAXIES Boxy Disky SBO SO SBa SBb SBc IBm ImSa Sb Sc IRREGULARS exPlore The image on the left is the classification scheme that Hubble himself came up with. He thought that the “tuning fork” sequence represented the evolutionary progression of galaxies. This concept turned out to be wrong, although astronomers still use these general categories and labels to describe galaxies. the Main galaxy tyPes •	 Elliptical (E): Spherical or elliptical shape (like a football), has no flat disc or spiral arms •	 Lenticular (S0): Smooth, flat disk shape without spiral structure, often hard to distin­guish from ellipticals Barred Lenticular (SB0): Same as above, but with an elongated (barred) nucleus •	Spiral (S): Flat disk shape with notable spiral patterns in the outer disk, also contains a large bright central bulge 3 •	 Barred Spiral (SB): A special type of spiral characterized by an elongated nucleus with the spiral arms springing from the ends of the bar There are two other categories for classifying galaxies: • Irregular (IR):	 An oddly shaped galaxy that doesn’t fit into any other category • Interacting (INT):	 Two or more galax­ies that are so close together that they are affecting each other’s shape Using the definitions above, place the 12 gal­axies on the left into their proper morphology categories: Morphology Picture Numbers (3 each) E/S0/SB0 S SB IR JOhN KORMENdy/TIM JONES ESO (TOp RIGhT); ALL OThERS NASA The smallest galaxies are often called dwarf galaxies (No. 5 and No. 7 are dwarf galaxies). These contain only a few billion stars — a small number compared to the Milky Way’s 200 billion. The largest ellipticals contain sev­eral trillion stars. S ta r D at e / U n i v e r S o t e a c h e r  G U i D e 3                     the coMa galaxy cluster The Coma Cluster, which is centered about 320 million light-years away, contains several thousand individual galaxies. The cluster has a roughly spherical shape and is about 20 million light-years across. (For compari­son, the Milky Way is 100,000 light-years across). That many galaxies in a relatively small space makes the Coma Cluster one of the richest and densest galaxy clusters in our region of the universe. NW W Arcturus COMA BERENICES BIG DIPPER On the following pages you will be asked to count different types of galaxies. Use the labels on this picture as an example of how to count the various objects. I) Ellipticals or Lenticulars It can be hard to tell these apart. If you know it’s either an E or S0/SB0, it is okay to guess between these two. I II?II) Spirals and Barred Spi­rals It can be hard to tell these ? I apart. If you know it’s either an S or SB, it is okay to guess between these two. ? II III) Irregular galaxy I IV) Uncertain I An edge-on view of a gal- ? axy that could possibly be ?Ian S0, SB0, S, SB, or IR. Star There are too many pos­sibilities, so do not count these. II Star)Any object that has “crosshairs” sticking out of I it is a foreground star in the Milky Way galaxy, so do not IV IIVcount these. Star?) Star Don’t count small, faint IIIIV objects like these that are II too hard to classify. NASA/STScI/COMA hST ACS TREASuRy TEAM (3) TIM JONES S ta r D at e / U n i v e r S o t e a c h e r  G U i D e 4 II      A Count the number of galaxies of each morphological type and write down the number in the correct spot in the table. Use the guidelines on page 4 to help you decide which objects to count. E S0 /SB0 S SB IR / INT Top Image (A) Bottom Image (B) B S ta r D at e / U n i v e r S o t e a c h e r  G U i D e 5        C E S0 /SB0 S SB IR / INT Top Image (C) Bottom Image (D) Count the number of galaxies of each morphological type and write down the number in the correct spot in the table. Use the guidelines on page 4 to help you decide which objects to count. D NASA/STScI/COMA hST ACS TREASuRy TEAM (4) S ta r D at e / U n i v e r S o t e a c h e r  G U i D e 6          =               exPlainGalaxies in Clusters, Groups, and the Field Galaxies are found throughout the universe, from our next door neighbors — the Magellanic Clouds and Andromeda — all the way out to the edge of the visible universe 13 billion light years away. Nobody knows for sure, but it is estimated that there are 100 billion galaxies or more in the visible universe, and many more beyond that. Galaxies live in a variety of envi­ronments. Sometimes large numbers of them are packed close together in clusters, such as the Coma Cluster; sometimes they gather in smaller num­bers called groups, like the Local Group that contains our Milky Way; and sometimes they are isolated far from one another in the field. Number of Galaxies Minimum Number of Non-dwarf Galaxies Diameter (1 Mpc 3.26 million light years) Total Mass Galaxy Cluster Large and dense 50 to thousands 6 2 to 10 Mpc 1014 to 1015 solar masses Galaxy Group Small and dense less than 50 3 1 to 2 Mpc 1013 solar masses The Field Large and deserted very few 0 Voids, can be larger than 100 Mpc < 1010 Clusters, groups, and some isolated galaxies can all be part of even larger structures called superclusters. At the largest scales in the visible universe, superclusters are gathered into filaments and walls surrounding vast voids, often described as resembling large soap bubbles. This structure often is referred to as the “cosmic web.” On the previous two pages, the images on the top (A&C) show the dense central core of the Coma Cluster, and the images on the bottom (B&D) show galaxies out in the field. Fill in the table below using the numbers you wrote down on the previous two pages: Morphology→ E Ellipticals S0 & SB0 Lenticulars S & SB (sum both together) Regular and Barred Spirals Total (E+S0+SB0+S+SB) Image A Image C Sum Total FromA + C (e) (f) (g) (h) Image B Image D Sum Total From B + D (i) (j) (k) (m) Using a calculator, find the percentages of each galaxy type in the cluster versus the field (ignore IRs and INTs). Fill in each of the boxes on the right: i— m j— mk— m Where did you find a higher percentage of spirals — in the Cluster or in the Field? Answer: _________________________ S ta r D at e / U n i v e r S o t e a c h e r  G U i D e 7                   The percentages that you just found tell us which types of galaxies are common in the Coma Cluster versus which types are common in the field. Astronomers have done this same exercise on hundreds of thousands of galaxies in the nearby universe, and discovered that the following percent­ages are pretty typical: •	 In dense clusters, 40 percent of the galaxies are ellipticals, 50 percent are lenticulars, and 10 percent are spirals. •	 In the field, 10 percent of the galaxies are ellipticals, 10 percent are len­ticulars, and 80 percent are spirals. When galaxies are found very close together there are more ellip­ticals and lenticulars. When galaxies are far apart there are more spirals. Astronomers call this the “morphology-density effect” (the word morphology means “type” or “class,” not “transformation,” as in biology). The term basically means that in crowded galaxy neighborhoods, like clus­ters, there are different types of galaxies than are found in open areas, like the field. extend The clues needed to answer the last question are in the following para­graphs. Please read the paragraphs carefully and then answer the question at the right. As a general rule, spiral galaxies (S and SB) have a lot of gas and are still forming lots of new stars. Elliptical and lenticular galaxies (E, S0, and SB0) are gas poor and are not making many new stars. Spirals are Gas-richBoth Ellipticals and Lenticulars are Gas-poor Galaxies that are very close to each other, such as those in clusters, often undergo many violent interactions with each other. When a gas-rich spiral galaxy interacts with another galaxy, it tends to quickly use up most of its gas to make new stars, leaving little gas behind. Galaxy-galaxy interactions often change gas-rich galaxies into gas-poor galaxies. Many lenticular galaxies are the remains of old spirals that have lost their gas, and many elliptical galaxies are the remains of several spiral galaxies that have collided. Galaxy clusters are usually filled with a lot of extremely hot gas that is spread between galaxies throughout the cluster. However, there is no hot gas like this out in the field. When the radiation from this hot gas hits a spiral galaxy, it strips the spiral galaxy of its much cooler gas in a process called ram-pressure stripping. This process quickly converts a gas-rich spiral galaxy into a gas-poor lenticular galaxy. Spiral galaxies have a hard time surviving in the superheated gas environment. Using what you’ve learned, write a hypothesis that might explain why we see the morphology-density effect. In other words, why do we see more elliptical and lenticular galaxies in clusters and more spiral galaxies in the field? Remem­ber that galaxies change and evolve over time, and these galaxies have had a very long time to get to this point. S ta r D at e / U n i v e r S o t e a c h e r  G U i D e 8 

Coma Clusters of Galaxies

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Coma Clusters of Galaxies

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