AST LABS

1

Name _________________________________ Date ________________ Section _________________________________ Score ________________ Partner _________________________________ I.I’s ________________

Lab 16 – Classification of Galaxies and Galaxies in Color

Materials Needed: Provided in the lab: the Hubble Atlas of Galaxies (http://shelf2.library.cmu.edu/Tech/00537465.pdf), Hubble Key:

Figure 1 Hubble Key (Wiki) Objectives: To become familiar with the basic Hubble system for galaxy classification and their colors. This exercise provides background information for the lectures, identifies characteristic features for telescopic observations, and in addition, allows the student a firsthand look at photographs that are part of every night life of professional astronomers.

INTRODUCTION

2

Galaxies are enormous collections of stars containing billions and even trillions of stars. Our sun is part of a galaxy that we sometimes call the Milky Way Galaxy since the center of our galaxy is towards the middle of the Milky Way. All the stars that we can see with our naked eyes are part of our galaxy. Indeed, there are only three galaxies besides our own that can be seen with the naked eye. The Andromeda Galaxy, the largest nearby galaxy, appears as a faint smudge on a clear night to persons with good eyesight. In the Southern Hemisphere we can also see two faint, misty objects called the Magellanic Clouds. They are two minor satellite galaxies of our galaxy. With large telescopes, however, these and many other faint, nebulous objects reveal themselves as vast systems of star far beyond the Milky Way. In this exercise, we will first examine some photographs from the Hubble Atlas of Galaxies, the standard reference for galaxy classification. As a sequel to this exercise, you may wish to examine some of the color photographs of the Wray Color Atlas of Galaxies (Lab 17), the CD ROM images from the Malin Galaxy Collection (Lab 19), or look at some fainter images of more distant galaxies found on the Palomar Sky Survey Prints. The Hubble Space Telescope (Lab 20) also includes some unusual galaxies in its collection of images. The internet also offers some interesting possibilities.

Basic Distinction: Spirals and Ellipticals All galaxies do not look alike and so astronomers generally classify them according to a scheme first developed in detail by William Hubble. According to this scheme, most galaxies are classified as being either spirals or ellipticals. Spirals are galaxies with swirling arms containing billions of stars apparently whirling about the centers or nuclei of the galaxies. The nearest large galaxy, the Andromeda Galaxy or M31, is a spiral galaxy as is our own galaxy, the Milky Way. Ellipticals are galaxies whose images have an elliptical outline. Ellipticals appear rather featureless at first glance, but some ellipticals are quite unusual. Some giant ellipticals contain perhaps as many as a hundred times as many stars as our own galaxy. One of the galaxies that you will examine in this exercise, M87 is such a giant galaxy. Most astronomers also believe that M87 has a massive black hole at its center. [Not from these photos, however.] Spiral galaxies are associated in the public’s mind with galaxies partially because of their spectacular arms which makes their pictures so stunning. Besides the Andromeda galaxy there are many other picturesque galaxies such as M51, the Whirlpool galaxy, and M104, the Sombrero galaxy. If we look at them more closely, we will find that some of them are more closely wound up than others. In addition to the arms, we also find that spirals also have a central bulge of stars that is not part of the spiral arms. The tightly wound spirals normally have a large central bulge and are called Sa galaxies. The loosely wound or open spirals with small central bulges are called Sc galaxies. Type Sb spirals, of course, are in between. Our galaxy is probably an Sb galaxy. We will use the Hubble Atlas to acquire some familiarity with these principles. Your Hubble Key also illustrates these features with a sort of visual shorthand. Spiral galaxies contain large amounts of dust and gas as well as stars. Indeed, the arms are so spectacular primarily because young, hot blue giants are formed out of the dust and gas. One reason it took astronomers so long to recognize that we are inside a galaxy is precisely because our galaxy is a spiral; the dust of our galaxy makes more than half of

3

the stars of our galaxy invisible even in the larges telescopes. Even when we have a very faint image of a spiral galaxy, we can still recognize the spiral by its clumpy blobs of dust and gas. This will be very important when you look at some of the faint images on the Sky Survey Print. Ellipticals are much “smoother” in appearance than spirals. They seem to be composed primarily of very old stars and have very little gas. The only prominent difference between them is in the degree of flattening of the image. A nearly circular image is characteristic of an E0 galaxy; the most elongated image is characteristic of an E7 galaxy. Again, the intermediate types are labeled E1 through E6. You will see numerous examples throughout the exercise. The basic scheme is also shown on the Hubble Key (provided in the lab). Oddballs: Barred Spirals, Intermediate Types, and Irregulars While these general principles apply to most galaxies, there are a number of interesting complications. Some spiral galaxies have a long linear or barlike feature in their central regions. These galaxies are called barred spirals or SB galaxies. SB galaxies are classified exactly like other spirals. SBa galaxies are barred spirals with very tightly bound arms and larger central bulges. SBc galaxies are barred spirals with open arms and small central bulges. Again, the SBb galaxies are an intermediate type of barred spiral. Some galaxies are, of course, halfway between elliptical and spirals. Such galaxies are too smooth to be spirals and too dusty to be ellipticals. These intermediate types of galaxies are called S0 galaxies. They sometimes show a disk, but they do not possess “arms”. Galaxies that are smooth, but not rounded or elliptical in shape or outline are frequently S0 galaxies. Finally, it should be noted that even this modified scheme works only for about 95% of the galaxies. Galaxies, which do not fit any classification scheme, are called irregular or Irr galaxies. Two nearby irregular galaxies are the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). Naturally enough, the irregulars include some of the most interesting galaxies. In this introductory exercise we will not emphasize the SB’s, the SO’s and the Irr’s, but we will examine a few of these oddballs.

4

ADDITIONAL READING

[Any astronomy textbook has additional information about galaxies and their classification. An especially good treatment can be found in Robbins & Jefferys Discovering Astronomy. Frequent articles of interest

are found in Astronomy, Sky, and Telescope and Scientific America.] Ira Sprague Brown (Aug 1955). “Completing the Atlas of the Universe”, National

Geographic Magazine vol 108, #2, pp 185-190. Timothy Ferris (1982). Galaxies. Stewart, Tabor & Chung: New York, 1982. 192 pages.

[a stunning collection of beautiful color photographs and non-technical excellent prose] Alan Hirshfeld (April 1980). “Inside Dwarf Galaxies”, Sky and Telescope vol 59, #4, pp

287-291. [small is also interesting] Edwin P. Hubble (1958). The Realm of the Nebulae. Dover: New York.

[a reprint of Hubble’s 1936 classic] Allan Sandage (1961). The Hubble Atlas of Galaxies. Carnegie Institute of Washington:

Washington, D.C. [the lab’s primary source]

Ronald A. Schorn (Jan 1988). “The Extragalactic Zoo – I”, Sky and Telescope vol 75, #1,

pp 23-27. Raymond Talbot, Eric Jensen & Reginald Dufour (July 1980). “Anatomy of a Spiral

Galaxy”, Sky and Telescope vol 60, #1, pp 23-28. Sidney Van Den Bergh (Dec 1976), “Golden Anniversary of Hubble’s Classification

System”, Sky and Telescope vol 52, #6, pp 410-414.

5

PART I: Galaxy Classification

I. Getting Acquainted with the Hubble Atlas [Tutorial Worksheet] Open the Hubble Atlas from the link above. The name of the galaxy is either the page before of after each image page. The list of galaxies by NGC (New Galactic Catalogue) number is after the introductory pages.

II. Classifying Large Images (from the Atlas) [Individual Worksheet] You will apply your newly learned information to the task of classifying the additional galaxies listed at the beginning of your individual worksheet. This will provide you with a little more practice. A suggested procedure:

(1) First classify each galaxy as either elliptical (E) or spiral (S). (2) Using the Hubble Key, determine whether the elliptical is an E0, E1, E2, E3,

E4, E5, E6, or E7. (3) For the spirals, determine whether they are Sa, Sb, or Sc. Be sure to account

for the tilt of the spiral. (4) Decide what to do with the oddballs. Some spirals have a bar (SB). One or

two galaxies may be intermediate between spiral and elliptical (SO). Real oddballs are called irregulars (Irr).

After you complete the worksheet, you will turn in your results. If you have done well you will receive full credit. If not, you will normally still receive partial credit. If you are careful, your results should be quite satisfactory. TUTORIAL: USE THE HUBBLE ATLAS Elliptical Galaxies M87: The Nearest Giant Elliptical or NGC 205: The Nearest Elliptical First, look carefully at either NGC 4486 (M 87) or at NGC 205. Notice how smooth the general distribution of light is. Does the galaxy in your Atlas have a sharp edge? ___________________________________ (#1) M87 is an unusual elliptical galaxy containing approximately a trillion stars; NGC 205 is another smaller but much closer elliptical galaxy. No individual stars may be seen in the photograph of M87 and only a very few are seen in the photograph of NGC 205. The galaxies are too far away for that. Both photographs are “cluttered” by a number of foreground stars from our own galaxy that just happen to be in the same general direction. Many of the larger stars have a diffraction pattern with four spikes. Most galaxies are elliptical galaxies and most elliptical galaxies are smaller than NGC 205, which itself has a stellar population perhaps a hundred times smaller than that of our own galaxy. A typical dwarf elliptical (Leo II) is also found on page B1. The very largest galaxies are generally giant ellipticals such as M87. The hundred or so small soft dots that seem to be part of the M87 galaxy are not individual stars but globular clusters – groups that may have as many as one hundred thousand stars. Do you see the 7 or 8 satellite galaxies? __________________________ Several additional elliptical galaxies listed below. These are fairly typical ellipticals. Except for a few overexposed central regions, their images are very regular and smooth. Their primary distinguishing characteristic is their shape – from circular (E0) to highly

6

elongated ellipses (E7). Use your Hubble Key and attempt to classify the following elliptical galaxies of your Atlas as either E0, E1, …. or E7: NGC 4406 (M86) ________ NGC 751 ________

NGC 205 ________ NGC 4636 ________

NGC 4486 (M87) ________ NGC 4697 ________ Actually, M87 is a little peculiar, probably due to a massive black hole near the center of the galaxy. The peculiar feature may be seen on B6 and in recent HST photos. Large Spiral Galaxies M31: The Andromeda Galaxy, Our Nearest Spiral Neighbor or M81: A Large Nearby Spiral Galaxy Now let us look at either the Andromeda Galaxy (M31, NGC 224) in the constellation of Andromeda or at NGC 3031 (M81), another large spiral galaxy found in the constellation of Ursa Major. M31 is the nearest large spiral, about 2.5 million light years from the sun; M81 is slightly further away, about 10 million light years from the sun. The spiral arms are part of a somewhat flattened collection of stars, gas and dust that form a disk. Do the arms reach all the way to the galaxy core? Can you see some of the very faint extensions of the arms into the darker regions? The pictures are actually somewhat misleading; radio observations reveal that the galaxy actually reaches about two or three times as far into space as the apparent visible edge suggests. Notice too that M31 has two fairly impressive satellite galaxies. The small galaxy to the lower right of M31 is NGC 205. Ellipticals versus Spirals Comparing M31 & M87 or M81 & NGC 205 What are the most obvious differences between the above spirals and ellipticals in your Atlas? Which type of galaxy is marked by prominent dark streaks or lanes due to dust, the spiral or elliptical? ________________________________________________________ (#2a) Does the core or central bulge of your spiral galaxy most resemble the outer regions of your spiral or the central regions of the elliptical? Hint – is it smooth or does it have large clumpy regions of dust and gas? ________________________________________ (#2b) Spirals: Sa, Sb, Sc; Open Spirals and Tightly Wound Spirals

7

Spirals display a little more variety than ellipticals. Part of the variety comes from differences both in the arms and in the central regions. Also, because of the obscuring material in the dark dust lanes, spirals may look quite different when observed from different viewing angles. The most tightly wound arms belong to the Sa spirals; the most loosely wound or open arms belong to the Sc spirals. Sb spirals have characteristics intermediate between Sa and Sc galaxies. First, let us compare a typical Sa with a typical Sc. NGC 4274 Sa NGC 3898 Sa NGC 5457 (M 101) Sc NGC 5194 (M 51) Sc Does the Sa or Sc galaxy have the tightest arms? ___________________________ (#3a) Does the Sa or Sc have the largest central bulge (allow for the photo size)? ___________________________________________________________________ (#3b) You will find that this is a general rule – large central bulges are almost always found together with tight arms in Sa galaxies; while very open arms and small bulges are found together in Sc galaxies. If NGC 4274 is an Sa & M101 is an Sc, what is M31? ____________________________ If NGC 3898 is an Sa & M51 is an Sc, what is M81? _____________________________ Edge-on Spirals Most of the photographs of galaxies that we have been examining so far have been viewed “face on”, that is, we view from a perspective that allows us an immediate inspection of their arms and central regions. Sometimes, though, we see galaxies “edge on”. In this case we generally cannot see the arms. Instead, what we see are primarily dust lanes. However, we can determine the size of the central bulge – and that is generally enough to classify the galaxy. Can you classify the galaxy (or galaxies) in your set by comparing the size(s) of the bulge and the arms? NGC 253 _________________________________________________ Which galaxy is an Sb and which an Sc? NGC 4565 ____________ NGC 5907 ________ Notice that the outline of NGC 253 looks something like that of an elliptical galaxy. However, an elliptical galaxy would have no dark dust lanes. Subtleties I: Intermediate Type of Galaxy or SO Galaxies The more closely we look at anything the more complicated it seems. Some galaxies are truly borderline cases. Galaxies that do not quite fit the simple scheme developed so far are NGC 5866 and NGC 1201 & NGC 7457 . While they are too irregular and/or too flattened in shape to be elliptical and may even have a disk structure, they lack the arms- and-dust of true spirals. We classify such borderline galaxies as SO galaxies. Subtleties II: Barred Spirals or SB Galaxies

8

Some spiral galaxies have large concentrations of stars that form a more or less rectangular outline. Such concentrations are called bars – and spiral galaxies with such bars are called barred spirals or SB galaxies. NGC 1398 & NGC 1300 are examples of such SB galaxies. Since both of these galaxies have medium size bulges both are classified as SBb’s. The big B is for the bar; the little b is for the medium bulge as with normal spirals. Classify your other SB galaxy as either SBa, SBb, or SBc. NGC 4394 __________________ NGC 1073 ______________________ Subtleties III: Irregular Galaxies or Irr Galaxies Find the three pictures of NGC 3034 (M 82), a galaxy apparently disturbed by violent starburst activity – intense movements of gas and dust due to the infall of huge gas clouds into the denser central regions of the galaxy. It is truly an irregular galaxy or Irr galaxy. Find the picture of the Large Magellanic Cloud (LMC), a small satellite galaxy of our own galaxy. The two Magellanic Clouds are the closest external galaxies but cannot be seen in most of the Northern Hemisphere since they are close to the South Celestial Pole. Galaxies without obvious structures and/or violently disrupted galaxies are called irregular or Irr galaxies. Practice: Since you will be graded on this exercise, it will probably help to have a dry run before doing your individual worksheets. See if you can classify the following galaxies in the appropriate set. Galaxy Type Galaxy Type

Sextans ________ NGC 628 (M 74) ________

NGC 598 (M 33) ________ NGC 185 ________

NGC 4736 (M 94) ________ NGC 891 ________

NGC 3368 (M 96) ________ NGC 2525 ________

NGC 3377 ________ NGC 2681 ________

NGC 4762 ________ NGC 3329 ________

NGC 7331 ________

NGC 7332 ________

9

PART II: Galaxies in Color

1) Open http://skyserver.sdss.org/dr13/en/proj/advanced/galaxies/spectra.aspx and click on each of the galaxies in the table about mid page. You will receive this image:

2) Note the galaxy name, redshift, and type in the table below:

10

Name Galaxy Type Redshift Spectra Shape (Flat or Curved?)

Distance of K and H line (How many tick marks from 4,000 Angstroms?)

3) Click on the spectrum and note if the spectrum is flat or curved and how many ticks the K and H lines are from the 4,000 Angstrom lines in the table above. The spectrum looks like this:

11

4) Answer the following questions:

• Is there relationship between spectra flatness and galaxy type?

• Is there a relationship between number of tick marks and redshift?