Astro 3 Lab Exercise

Transcription

1 Astro 3 Lab Exercise Lab #4: Measuring Redshifts of Galaxies Dates: August 5 6 Lab Report due: 5 pm Friday August 15 Summer

2 Introduction This project involves measuring the redshifts of distant galaxies and using this information to determine the speeds at which they are receding away from us. You will be using a software program named Large Scale Structure of the Universe to measure the redshifts of spectral lines. The program simulates what it is like to measure the spectra of galaxies and observe shifts in the spectral lines. It places you at Kitt Peak National Observatory using the same research telescopes that astronomers use, including us at Dartmouth! This project is to be done in groups of two. 2

3 Introduction The velocity of a galaxy is determined by measuring the redshift of spectral lines in the spectrum of the galaxy. With sound, waves become shifted depending on whether the source is moving away or towards an observer. Think of a car racing by: as it comes towards you it has a higher frequency, and as it races away it has a lower frequency. This is known as Doppler shifting. In a similar manner, light of all wavelengths becomes shifted depending on whether the source is moving towards or away from the observer. The light from objects moving towards us shifts towards shorter, more blue wavelengths, while light from objects moving away from us shifts towards longer, more red wavelengths. We call this blueshift and redshift, respectively. The faster the source moves, the more shift that results. Edwin Hubble noted that the galaxies he observed were almost always redshifted; i.e., the spectral lines were shifted towards red wavelengths. Moreover, that more distant galaxies had spectral lines that were increasingly redshifted. This was the beginning of the development of the Big Bang model of the universe. 3

4 Introduction Just like Hubble, you will be measuring the spectral lines of galaxies to observe their redshifts and determine their radial velocities. The absorption lines due to ionized calcium, the Ca II K and H lines at and Angstroms, respectively, will be among the strongest ("deepest") of all the lines. A good first step with any of the spectra is to find these two lines. You ll also want to look for the G Band line, which is a blend of two lines at Angstroms. 4

5 Determining Radial Velocity V V C = λ - λ o λ o λ o = Rest Wavelength λ = Measured wavelength v = Velocity of object being measured c = Speed of light: 3 x 10 5 km/s Example: An absorption line that is found at Å in the lab is found at Å when analyzing the spectrum of a particular galaxy. Therefore this galaxy is moving with a velocity v = (8.5/3968.5) * c = km/sec away from us. 5

6 Using the Software Find the icon for the program labeled Large Scale Structure. Log in using the names of each person in your group. You can ignore the lab table number box. When you start you will be using the 0.9 m telescope at Kitt Peak National Observatory. Using the Telescope menu option, you can switch between three telescopes: the 0.4, 0.9, and the 4.0 m telescopes. (We ve hacked the program so that you can do this!) Feel free to experiment between them. The following controls are available to you: DOME: Opens and closes the dome of the observatory. This will be the first thing you ll want to do. TRACKING: This starts/stops the telescope from tracking the sky. Telescopes must compensate for the Earth s rotation. This feature also allows you access to slew (i.e. move) the telescope across the sky. SLEW RATE: Changes how quickly the telescope moves in response to slew commands. Start with a high slew rate (8 or 16) to move to your object, then center it with smaller slew rates (1 4). N, W, S, E: These buttons shift the telescope across the sky. Right Ascension and Declination : These are the coordinates astronomers use for positions in the sky. Right Ascension increases from west to east, and Declination increases from south to north. CHANGE VIEW: Use this to switch to the Instrument view. You ll need to do this in order to take spectra. The next page explains how to use this mode. 6

7 Scan around the sky looking for galaxies. When you have one centered in your box, you ll want to switch to Instrument view to take a spectrum. (See page 8 for an example.) When you select Instrument view, you ll see two lines representing the slit of the spectrometer. Center the slit on the object of interest. Then click on Take Reading which you ll find to the upper right of the program window. A new window will pop up, representing the readout from the spectrometer. Select Start/Resume count to begin taking data. (See page 4 for an example.) When you are happy with the readout, select stop. Before you will be a spectrum of the object. (Note: the name of the object will be displayed). Click Record Meas. You will be presented with a list of three absorption lines. Use the mouse and click on the peak of the absorptions in your spectrum. At the top you ll find the wavelength and intensity. In the box next to the appropriate line, enter the wavelength you record. Be aware that there may be a significant shift in wavelength from the rest wavelength and the measured wavelength you will be entering. Click Verify/Average to compute the redshifts. It is good practice to try a few of these calculations by hand. See the example on page 5. When you re done recording the measurements, select Return from the menu. Repeat the process for at least 10 objects. NOTE: Use the Field menu option to change to a different field of the sky. Use the File/Save feature to save your data. Ask the TA on how you can save your data. 7

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10 What to include in your lab report Please write the date and time you completed the project, and the name of your TA on the report. Include a print out of your results. In addition to including any observations you had while using the simulation software, please answer these additional questions: How does using the 4.0 m differ from using the 0.4 m telescope? How does telescope size affect ability to study galaxies? Might you do better using the 10 m Keck telescopes? If so, how much better than using the 4.0 m telescope in Arizona? You will have observed few galaxies amongst many stars in your field of view. How can you tell the difference between the stars and the galaxies? What differences do you note in their spectra? Can you always see all of the lines? Why might some lines be missing from your spectra? 10