Astronomy Lab Distance to a Quasar

Astronomy Lab Distance to a Quasar Introduction: In this lab you will measure the redshift of a quasar through spectral analysis, and use it to calculate its distance, speed and age. In so doing, you will employ one of the many techniques by which distances in space are measured, and develop an appreciation of the immensity of the universe revealed by our current knowledge. These spectra are from the FIRST Survey, a large project done at the VLA to identify faint, unusual radio sources. The vertical or y-axis of the graph measures flux, the amount of energy emitted. The x-axis represents wavelengths in Angstroms. Emission lines are spikes above the continuum, while absorption lines are dips or spikes below the continuum. If you have not already done so, sign up for your quasar and write its name on the blank below:_________________________ Quasars: Classic AGN's Before beginning your analysis it is helpful to get acquainted with the spectrum of a typical quasar. Among types of active galaxies, quasars are distinguished by their strong emission lines. Below is a composite of over 100 quasar spectra averaged together labeled with common quasar emission lines. The wavelengths of the lines shown are at rest wavelengths- that is, their normal positions. Use this as a guide to help in identifying the shapes, patterns and strengths of emission lines, but not their locations! Recall that as objects move away, the light (and all emission or absorption lines with it) shifts towards the longer (redder) wavelengths, and as they approach, the light shifts to shorter wavelengths (blue shifts). The challenge of identifying lines in quasars is that the lines are not at rest wavelengths! Not only will the position of the emission lines be redshifted, but the relative spacing between two lines also expands. Keep this in mind as you attempt to identify your lines. Procedure: You will be using Graphical Analysis to analyze the spectra. This is located under the Astronomy programs menu. Open the program and click OK. Select File/Import from text. Navigate through the Directories: S drive/North/Herrold/QSO. A list of the quasar names will now appear. Open your spectrum. The spectrum will initially look like a mess. Double-click on the actual graph points. A window will appear. Check OFF Point Protectors, then close the window. The graph should appear normal now. You may need to select and expand an area, to optimize it. When you have it optimized, print a copy and write the name of the quasar at the top. Turn this in with the accompanying data sheet. Use the Examine and Zoom tools to measure the exact position of 3 of the major emission lines. Beside each line, write its possible identity and measured wavelength. Common Emission Lines found in Quasars Ne VIII 774 Angstroms C IV 1549 Angstroms H4102 Angstroms O II 834 He II 1640 H4340 C III 977 O III 1664 He II 4686 N III 991 N III 1750 H 4861 Lyman 1026 Al III 1857 O III 4959 O VI 1035 Ar I 1067 Si III 1892 C III 1909 O III 5007 He I 5876 Lyman 1216 C II 2324 Fe VII 6087 N V 1240 O I 1304 C II 1335 Si IV, O IV 1400 N IV 1486 Mg II 2799 O III 3133 Ne V 3426 O II 3727 Ne III 3869 O I 6300 Fe X 6374 H 6563 N II 6583 S II 6716, 6731 Distance to a Quasar Name ________________________ Data Sheet Period ___ Quasar name: ______________ 1. Identify 3 lines using the Composite Quasar and the table of Common Quasar Emission lines. Write their identities on the printed graph next to each line. 2. Use Graphical Analysis to determine the observed wavelength of these lines. 3. Once you have identified 3 lines, fill out the data table below and calculate their line ratios. The line ratio is the observed wavelength divided by the rest wavelength. The ratios should all show close agreement to 2 decimal places. If 2 of the 3 show close agreement, throw out the outlier when averaging or identify other lines until they all show agreement on their calculated line ratios. Then calculate the average ratio. Line Identity Observed Wavelength / Rest Wavelength = Ratio AVERAGE Ratio: 4. To calculate redshift (Z), use this formula: Z= (Line ratio) – 1. Z= _________ 5. Next, use the below equation to calculate the velocity (v) of the quasar. Show your work. Circle your final answer and include a unit on it. c (the speed of light) = 3.0 x 105 km/sec v = c (1 + Z)2 -1 (1 + Z)2 +1 6. Now calculate the distance to the quasar using the equation below. H is the Hubble Constant, the experimental value for the expansion rate of the universe. Determining a precise value for the Hubble Constant was one of the primary mission directives for the Hubble SpaceTelescope. Present estimates put it at 71 km/sec/Mpc. Show your work, circle your final answer and include a unit on it. Distance in Mpc = cZ 1 + 0.5Z H 1+Z 7. Convert your answer to a distance in light-years. 1 Megaparsec (Mpc) = 3 260 000 LY. Show your work, circle your final answer and include a unit on it. BONUS: Where a quasar is today and where it was when it emitted the light that we now see are two very different places! In the time that the light has traveled through space, the quasar has been moving father away due to the expansion of the universe. The look-back time of an object is the time calculated to where it was when it emitted the light (this is the number of years calculated in question 7). Where the quasar is today is known as its luminosity distance. This is a very complicated calculation, since it involves factors such as the shape of the universe, and constants added for relativity and the cosmological constant. Calculate the luminosity distance to your quasar Copy the luminosity distances on the right in Mpc and Gly: Mpc distance: ____________________ Gly distance: ______________________