A1101, Lab 11: Galaxies and Rotation Lab Worksheet

Transcription

1 Student Name: Lab Partner Name: Lab TA Name: Part 1: Classifying Galaxies A1101, Lab 11: Galaxies and Rotation Lab Worksheet In the 1930s, Edwin Hubble defined what is still the most influential system used to classify galaxies, but astronomers have long debated the physical significance of Hubble s classification and considered alternative schemes. In this part of the assignment, you and your lab partner will devise and describe your own galaxy classification system. Your envelope contains images of 32 well known galaxies, an image collection put together by astronomer Tyler Nordgren of Redlands University for a similar laboratory exercise. These are all black- and- white images even though some are printed with a more sepia tone; ignore any color differences. Working with your partner, group these galaxies into classes based on similarities. You should have at least two but no more than five classes. You may also have a category of peculiar galaxies that don t fit into any of your classes, but if you have more than four or five of these peculiar galaxies then your classification system probably needs some improvement. Take your time, and think carefully about what kinds of similarities and differences you want to draw on to define your classes. Give particular thought to the distinction between intrinsic properties of the galaxies, which would be the same regardless of how they are oriented in space and how far away they are, and apparent properties, which depend on the distance and orientation relative to Earth. Even if you know what Hubble s classification system is, there is no reason you need to use a similar system yourself. Once you have decided on your categories, come up with a descriptive name for each class and, more importantly, a description of the characteristics that define the category. Your descriptions should be complete enough that someone else could read them and decide how to classify a new galaxy. In fact, after you have written down your classification criteria, you will exchange your description with another group s and they will use your description to classify two more galaxies, and you will do the same with theirs. List the category names and descriptions on the opposite side of this page. Also list which of the 32 galaxies you assigned to each class. Space for five classes is provided, but you do not need to use them all.

2 Class 1 Class 2 Class 3 Class 4

3 Class 5 What galaxies, if any, did you classify as peculiar not fitting in any of your classes? What is it that makes these galaxies peculiar? Which two of these 32 galaxies do you think are closest (in distance) to the Milky Way? Explain why you think these two are the closest. What do you think is going on in the galaxy Arp 252? Exchange your written description with that of a neighboring group. Read their classification system and compare it to your own. The TA will give you two additional galaxies to classify using the other group s criteria. Were the other goup s descriptions clear enough that you knew how to classify these galaxies?

4 Did the other group s classification of the two new galaxies under your system agree with the way you would have classified them? Why did you get an artist s conception for the Milky Way instead of an actual telescope image? Whose classification system do you think was better, yours or the other group s? Explain your answer. Before answering the last question of this Part, read this passage from the introduction to the book The Order of Things, by the French philosopher Michel Foucault. It s a complicated passage, so read it twice. This book first arose out of a passage in Borges, out of the laughter that shattered, as I first read the passage, all the familiar landmarks of my thought --- our thought, the thought that bears the stamp of our age and our geography, breaking up all the ordered surfaces and all the planes with which we are accustomed to tame the wild profusion of existing things, and continuing long afterwards to disturb and threaten with collapse our age-old distinction between the Same and the Other. This passage quotes 'a certain Chinese encyclopaedia' in which it is written that animals are divided into: (a) belonging to the Emperor, (b) embalmed, (c) tame, (d) sucking pigs, (e) sirens, (f) fabulous, (g) stray dogs, (h) included in the present classification, (i) frenzied, (j) innumerable, (k) drawn with a very fine camelhair brush, (l) et cetera, (m) having just broken the water pitcher, (n) that from a long way off look like flies. In the wonderment of this taxonomy, the thing we apprehend in one great leap, the thing that, by means of the fable, is demonstrated as the exotic charm of another system of thought, is the limitation of our own, the stark impossibility of thinking that. As you can see from these and other galaxy images, in detail every galaxy looks different from every other galaxy. What is the value of classifying galaxies into categories based on similar characteristics?

5 Part 2: Galaxy Rotation To complete this part of the Lab, you will need the figure handout, which the TA will provide once there has been enough time for everyone to complete and discuss the galaxy classification section. On this handout, Figure 1 shows a visible light image of the galaxy NGC 3198, a galaxy that is roughly half the total luminosity and half the size of the Milky Way. Figure 2 shows the luminosity profile of NGC 3198: the total luminosity of all stars inside radius R, where R is the distance from the center of the galaxy. The vertical axis is marked in billions of solar luminosities. The distance on the horizontal axis is marked in kiloparsecs (kpc, thousands of parsecs); for purposes of this Lab, you just need to know that 1 kpc is about 3000 light years. As you can see, the total luminosity of the stars in NCG 3198 is about 9 billion Lsun. About half of that light is inside the radius R = 5 kpc and about half of it is between 5 and 15 kpc. The curve stops at 15 kpc because there are almost no stars beyond that radius in NGC 3198, as you can see from the image. Figure 3 shows the rotation curve of NGC 3198, a plot of the disk s rotation speed (measured from Doppler shifts) vs. distance from the center of the galaxy. The rotation curve is measured from radio observations of hydrogen gas, and it extends to 30 kpc, roughly twice the size of the visible image. If all of the stars in NGC 3198 were exactly like the Sun, we could just multiply the total luminosity by 1 Msun / Lsun (1 solar mass per solar luminosity) to infer that the total mass of all the stars in the galaxy is about 9 billion Msun. However, the light actually comes from a mix of main sequence stars and red giant stars, with a wide range of luminosities, and by the time you do the right averaging over all the stars it turns out that you need to multiply by a mass- to- light ratio of about 4 Msun / Lsun to infer the stellar mass. Thus, the total stellar mass of NGC 3198 is about (9 billion Lsun) (4 Msun / Lsun) = 36 billion Msun. We can also use Newton s laws of gravity to infer the mass of the galaxy from the velocities of the stars, just as we have used the orbital speeds of moons to infer the mass of Jupiter or the orbital speeds of stars to infer the mass of the Galactic Center black hole. Here we use our familiar formula M = v 2 r / G, but to make your lives easier I have done all the unit conversions for you to express it in a scaled form: Mass = ( solar masses) (velocity in km/s) 2 (radius in kpc). When we apply this formula to the measured rotation speed at radius R, it tells us the mass of the galaxy interior to radius R. For example, if you measured a velocity of 50 km/s at a radius of 5 kpc, you would infer that the mass inside 5 kpc is = Msun (2.9 billion solar masses).

6 Based on the image in Figure 1, what would the classification of the galaxy NGC 3198 be in your classification scheme from Part 1? Based just on this image, do you think the galaxy is rotating clockwise (as seen from our vantage point) or counter- clockwise? Explain the basis of your answer. Using the mass formula on the previous page and the observed rotation curve shown in Figure 3, compute the total mass of NGC 3198 interior to radii R = 2 kpc, 5 kpc, 15 kpc, 20 kpc, and 30 kpc. Using the luminosity profile in Figure 2, compute the stellar mass interior to these radii. Remember to multiply the luminosity by 4 Msun / Lsun to get stellar mass. R = 2 kpc R = 5 kpc R =15 kpc R = 20 kpc R = 30 kpc Is the stellar mass interior to 2 kpc sufficient to explain the rotation speed measured at 2 kpc? In other words, do the stars themselves produce enough gravity to keep the galaxy rotating at this speed? Is the stellar mass interior to 30 kpc sufficient to explain the rotation speed measured at 30 kpc?

7 Between 2 kpc and 5 kpc, how much does the stellar mass increase? Between 2 kpc and 5 kpc, how much does the total mass increase? Between 15 kpc and 30 kpc, how much does the stellar mass increase? Between 15 kpc and 30 kpc, how much does the total mass increase? What are objects you know about that have mass but emit little or no light? Propose a hypothesis to explain the observed rotation curve of NGC 3198 specifically, to explain why the change of rotation with radius is different from what is predicted based on the star light alone.