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Application – Hubble’s Galaxies

Background and Theory

A galaxy is an assembly of between a billion (109) and a hundred billion (1011) stars. In addition to stars, there is often a large amount of dust and gas, all held together by gravity. The Sun and the Earth are in the Milky Way Galaxy (sometimes referred to as “the Galaxy”). Galaxies have many different characteristics, but the easiest way to classify them is by their shape (or “morphology”), and Edwin Hubble devised a basic method for classifying them in this way. In his classification scheme, there are three types of galaxies: spirals, ellipticals, and irregulars.

Spiral galaxies were the first to be discovered, because the most luminous galaxies close to the Milky Way are spirals. These galaxies get their name from the spiral distribution of light seen in photographs. A subclass of spirals contains the barred spirals. Ordinary spirals have a nucleus which is approximately spherical, while barred spirals have an elongated nucleus which looks like a bar. Spirals are labeled as Sa, Sb, or Sc; barred spirals are designated SBa, SBb, or SBc. The subclassification (a, b, or c) refers both to the size of the nucleus and the tightness of the spiral arms.

Elliptical galaxies are classified according to the relative sizes of their apparent major and minor axes. All elliptical galaxies have n between 0 and 7.

Irregular galaxies have no obvious spiral or elliptical structure. It is thought that many irregulars were once spiral or elliptical, but that a close encounter with a larger galaxy disrupted the organization of the material by gravitational forces. Irregular galaxies come in two flavors: Irr I’s are resolvable into individual stars, and Irr II’s are not.

Not all galaxies are easily classified. Quasars are the bright, superluminal cores of very distant active galaxies. These galaxies are so distant in fact that the quasars look like stars in most images. However, their redshifts are so high that we know that they can not be stars. These quasars are moving away from us at extremely high velocities. Quasar 3C273, for example, is moving away from us at 43,700 km/sec!

The relationship between galaxy types is not clear. Because there is little evidence of star formation in elliptical galaxies, and because they seem to have extremely small angular momentum, it was thought that perhaps elliptical galaxies are much older than spirals. If this is true, then we would expect to see more spiral galaxies as we look farther out into the universe (that is, back in time). Recent observations made by Hubble Space Telescope do show more spirals in distant clusters of galaxies, however, there are also many more distorted galaxies and blue irregulars with enormous star formation rates.

We do know that there is a correlation between the environment and the type of galaxy that formed there. Dense clusters have much higher percentages of elliptical galaxies, indicating that dense galaxy formation regions are more likely to form ellipticals. The entire problem is not yet well understood, and many explanations rely heavily on the postulated existence of dark matter.

In the late 1920’s, Edwin Hubble discovered one of the most fundamental properties of the universe, namely that it is expanding in all directions with a speed proportional to the distance. He used the redshift of spectral lines from distant galaxies (calculated by Slipher) whose distances could be determined by other means (for example, by Cepheid variable observations or measuring the angular sizes of HII regions). He interpreted the observed spectral shift as a Doppler shift, and determined that all galaxies (except a few very close ones that are in the same group of galaxies as the Milky Way) are receding from the Milky Way Galaxy with speeds proportional to their distances:

v=H·d,

where d is the galaxy’s distance (in Mpc), H is Hubble’s constant (with a modern value of about 65 km/s/Mpc), and the speed v is found from the Doppler shift of the galaxy.

Procedure

Galaxy images, click each.

1. NGC 1381 2. NGC 1398 3. NGC 224 4. NGC 3031 5. NGC 3384 6. NGC 5055 7. NGC 5184 8. NGC 5236 9. NGC 7331 10. 3C273

1. Examine the images of the galaxies listed above. When there is more than one galaxy in the

image, use the finder chart to identify the galaxy in question. 2. Fill in the table (this is worth 10 pts)

Galaxy Name

x y Type Velocity (km/s)

Distance (Mpc)

Light Travel Time (yrs)

Earth-history Reference

NGC 1381 1630

NGC 1398 1299

NGC 224 -59

NGC 3031 95

NGC 3384 642

NGC 5055 587

NGC 5236 337

NGC 7331 1105

3C273 43,700

3. Identify each galaxy’s type. 4. Estimate the subgroup of the spirals using this image. 5. For ellipticals, measure the major and minor axes of the ellipticals

a. You can calculate the subclass. Use any scale (inches or millimeters) you like to measure the major and minor axes, but be sure to measure both axes on the same scale.

b. For example, the longest dimension becomes the A value. Measure it in centimeters or inches. Measure the dimension perpendicular to that one as the B value.

c. Calculate the n value by dividing a by b. So, an E2 is an elliptical galaxy that is 2x as long as it is wide.

d. Note: you only need to measure the axes for the elliptical galaxies! 6. Use the Hubble constant and the formula given in the “Background and Theory” section

above to find the distance to each galaxy. a. Remember the assumptions behind Hubble’s Law. b. Convert the distance from Mpc to light years. (1 Mpc = 3.26·106 l.y.) c. Converting to light years gives the amount of time the light traveled between leaving

the galaxy and arriving at the telescope. d. Check to make sure that all of your answers make sense. e. For example, check that none of the galaxies’ light has been traveling for more than the

age of the Universe. 7. It is often difficult to make astronomical numbers meaningful.

a. For each of the galaxies, indicate what was happening in the Earth’s history when the light left that galaxy.

b. For example, the dinosaurs became extinct about 65 million years ago, Pangaea split into multiple continents about 200 million years ago, the Earth is about 4.5 billion years old, and the Universe is about 15 billion years old. Find these dates using the cosmic calendar in Chapter 01 or elsewhere on the Net.

Part 1 Questions (3 pts each)

1. What color do these galaxies tend to be (some areas may be saturated and look white, so look closer to the edge)? If different regions are different colors note that.

2. The velocity of NGC224 is negative. What does this mean? What are the implications for applying the Hubble Law to this galaxy?

3. 3C273 is one of the brightest radio sources in the sky. But the type of galaxy 3C273 is impossible to find from these images. Does this make sense? Hint: Look at the distance.

4. Look again at the color image of NGC5194. What color are the arms? What color is the bulge? Explain the colors that you see in terms of what is going on to the stars in that galaxy.https://www.americaspace.com/wp-content/uploads/2016/03/HubbleTuningFork2w-500×272.jpg

Part 2: Hubble Ultra-Deep Field

Galaxies — those vast collections of stars that populate our universe — are all over the place. But how many galaxies are there in the universe? Counting them seems like an impossible task. Sheer numbers is one problem — once the count gets into the billions, it takes a while to do the addition. Another problem is the limitation of our instruments. To get the best view, a telescope needs to have a large aperture (the diameter of the main mirror or lens) and be located above the atmosphere to avoid distortion from Earth’s air. Perhaps the most resonant example of this fact is the Hubble eXtreme Deep Field (XDF), an image made by combining 10 years of photographs from the Hubble Space Telescope. The telescope watched a small patch of sky in repeat visits for a total of 50 days, according to NASA. If you held your thumb at arm’s length to cover the moon, the XDF area would be about the size of the head of a pin. By collecting faint light over many hours of observation, the XDF revealed thousands of galaxies, both nearby and very distant, making it the deepest image of the universe ever taken at that time. So if that single small spot contains thousands, imagine how many more galaxies could be found in other spots.

Procedure

1. Look at this Hubble Ultra Deep Field (UDF) image segment. Note there are no stars, only galaxies in this image. Light from most of the galaxies in this image left when the universe was about 1 billion years old, so they provide the earliest snapshot yet of what galaxies were like when the universe was young.

2. Take the portion of sky imaged by the telescope and count the number of galaxies a. In this case, you can either print out the image and take a ruler to one corner and make

a square and count the number of galaxies in that square. b. OR you could not print it out and instead do the same thing on your monitor – just use

two pieces of paper, taped carefully to the edge of the monitor to make a paper square outline. Count the number of galaxies inside that square.

3. Then — using the ratio of the square to the entire image — you can estimate the number of galaxies in the whole image.

a. Measure the whole image using a ruler, both sides. b. Multiply the two sides to get the area of the whole image. c. Now measure the two sides of your measured square. d. Make the scaling factor – area of whole image divided by area of measured square. e. Estimate the total number of galaxies by multiply your value from 3d above by the

number of galaxies you actually counted (answer 2a or 2b above).

Questions (4 pts each)

1. Estimate the number of galaxies in the image. Describe how you estimated the number. 2. Estimate the number of galaxies you can classify as spiral, elliptical or lenticular. What fraction

of the total number of galaxies is this? 3. Estimate the number of irregular galaxies. What fraction of the total number of galaxies is this? 4. How is the Hubble classification system useful for the galaxies in the UDF? What does this

tell us about the galaxies we find in the earliest universe?https://upload.wikimedia.org/wikipedia/commons/6/69/NASA-HS201427a-HubbleUltraDeepField2014-20140603.jpg