Astronomy Lab
Properties of Quasars
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 begin to develop an appreciation of
the immensity of the universe revealed by our current knowledge.
The spectra are ones that were used in the FIRST Survey, a large project done at the Very Large
Array to identify faint, unusual radio sources. The vertical or y-axis of the spectral graph
measures flux, the amount of energy emitted. The x-axis represents wavelengths in Angstroms.
The continuum is represented by the general trace or trend of the graph. Emission lines are
spikes above the continuum, while absorption lines are dips or spikes below the continuum.
Background: 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. Emission lines are “spikes” above the continuum of the blackbody curve. You
have been given a sheet of reference composites of quasar spectra averaged together from
various surveys and labeled with common quasar emission lines. They are set at zero redshift
(also known as rest wavelengths) - that is, as if the quasar was not moving at all relative to our
line of sight. Use this as a guide to help in identifying the shapes, patterns and strengths of
emission lines, but not their locations!
Remember 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 they are not
where they are expected! Not only will the position of the emission lines be redshifted, but the
relative spacing between two features also expands. Keep this in mind as you attempt to
identify your lines. The zero redshift emission lines are known as rest wavelengths. They are
measured in Angstroms (Å). 1 Angstrom = 1 x 1010 meters.
Procedure: Use 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. Put a title on the spectrum, using the quasar’s name for
the title. Maximize the spectrum and the size of the graph so that it takes up most of the paper
in landscape orientation and print the spectrum. Turn this in with the accompanying data sheet.
Use the Examine and Zoom tools to measure the exact position (to the nearest Angstrom) of
three of the major emission lines. Beside each line, write its possible identity and the measured
(observed) wavelength. Next, work through the calculations for the data table. If you have
correctly identified the emission lines, all three of the wavelength ratios will be very close to
the same values. If they are not, then one or more of the lines is misidentified.
Properties of Quasars
Data Sheet
Name ________________________
Identify 3 lines using the composite quasar diagrams. Write their identities on the printed
graph next to each line. Once you have identified 3 lines, fill out the data table below and
calculate the 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. Round the ratio to three decimal places.
If 2 of the 3 show close agreement, throw out the outlier when averaging or re-identify the
mismatched line.
Properties of Quasars Data Table
Quasar name
Line
Identity
Observed
Wavelength (Å)
Rest Wavelength
(Å)
Line
Ratio
Z
Average ratio:
Properties of Quasars Analysis Table
Velocity (km/s) Distance (Mpc)
Distance (LY) Luminosity Distance (Mpc)
Calculations
To calculate redshift (Z), use this formula: Z = (Avg. line ratio) -1
(Z has no unit.)
Next, use the below equation to calculate the velocity (v) of the quasar. Show your calculation
below. Circle your final answer and include a unit on it. Enter this value in the analysis table.
c (the speed of light) = 3.0 x 105 km/sec
Sample calculation:
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 Space Telescope.
Present estimates put it at 71.2 km/sec/Mpc. Show your work, circle your final answer and
include a unit on it. Enter this value in the analysis table.
Distance (in Mpc) = (cz/H) * ((1 + 0.5 z)/(1 + z))
Sample calculation:
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. Enter this value in the analysis
table.
Sample calculation:
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
previously by your distance in LY). 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 by using this calculator:
http://www.astro.ucla.edu/~wright/CosmoCalc.html
Just enter your redshift in the Z box at left and click the General button (not RETURN!).
Copy the luminosity distance on the right in Mpc. Enter this value in the analysis table.
Remember to attach your labeled quasar spectrum to this data sheet.