Let the students read through this section on their own. It might be a good chance to review the electromagnetic spectrum with your students. Remind them that radio waves are much longer than visible light, meaning that it is much more difficult to pin down the exact location of their source. That is why it took so long to identify the first active galaxies and quasars. In this section, you might also review the concept of redshift and how it is used to help estimate the distances to objects.
You might also wish to mention that some of these radio sources are so strong that they can be picked up be amateur radio astronomers using dishes no larger than an average satellite dish!
VLA FIRST is a companion survey to SDSS. FIRST is mapping the sky in the radio band with a wavelength of 21 cm. Distant galaxies and quasars may appear as normal stars in a visual-light photograph, but they give us clues to their true nature by their strong radio emissions.
In SDSS’s visible images, the quasars are classified as stars or galaxies and look rather unremarkable. The radio images reveal a source. Two of the sources have particularly interesting structures. One of the sources has two faint jets emanating from the source; another actually has two lobes. The two lobes are due to the jets interacting with gas clouds – a well-understood, yet interesting, phenomenon.
Quasars are distinguished from stars and galaxies by their emission lines. Most galaxy and star spectra are dominated by absorption lines. Some active galaxies do have emission lines in their spectra and are sometimes misclassified as quasars. To tell the difference, you need to look at the width of the emission lines. Due to their great distances and to the rapid rotation of gas around their central black holes, quasars have very wide emission lines. Emission lines from active galaxies tend to look like spikes. There are formal mathematical definitions based on the widths of the lines, but these are beyond the scope of this lesson.
What are Quasars?
In this section, students will look at the spectra of a variety of quasars up to a redshift of about 5, the most distant quasar in SDSS’s Early Data Release. The quasars on the list in Exercise 2 are ordered by increasing redshift. As students look at higher-redshift quasars, they will see the emission lines march to the right in the spectra (i.e. longer wavelength, or red end of the spectrum). Some emission lines will disappear off the right side of the graph and new ones will appear on the left side.
Power Source of Quasars
No one has ever actually seen the heart of a quasar. The best model we have for the power source of a quasar is a supermassive black hole. A black hole is a region of space with gravitational fields so large that not even light can escape from it. The black holes in the center of quasars are surrounded by huge disks of gas and dust. As this gas and dust fall into the black hole, they are subjected to enormous pressure and heat up to millions of degrees. The gas and dust are so hot that they give off huge amounts of thermal radiation, including radio waves, visible light, and x-rays.
There is a limit to how bright a quasar can be. As the gas heats up, its pressure increases. The pressure limits the rate at which gas can fall into a black hole, and hence, the luminosity of the quasar. It is similar to a large number of people trying to exit a room. There are only so many people that can get though the door per unit time. If more people try to push through, the “pressure” they exert prevents them from doing leaving.
One of the most important facts about quasars is that they all have a high redshifts. The closest quasar is about 800 million light years away, meaning that the youngest quasars ceased to exist about 800 million years ago. The universe is not static; it changes as time goes on. For most of the history of astronomy, people assumed the universe was constant and unchanging. So the discovery of quasars led to a major shift in our understanding of the universe.
In the next section, students look at quasars from SDSS. Looking for them one by one with the Navigation tool would not work, since the Navigation tool usually misclassifies quasars as stars. To look at many objects, students need to run a query on the SkyServer data. Ask students to name advantages of querying a database to retrieve information on thousands of objects at once.
Most large databases can be searched using Structured Query Language, or SQL. There is a SQL search engine in SkyServer. This engine limits students to searches lasting 30 seconds and returning 1000 objects. If you want more objects, you can always break down your query into multiple smaller queries. For example, if you wanted to find more quasars, you could search for all quasars with z < 0.5, and then another query for z > 0.5. Both queries would return up to 1000 entries.
Students do not need to understand SQL to search the database; they can copy the query directly from the page and past it into the query box. Be sure they erase what is already in the box.
However, students should have a basic understanding of SQL if they want to attempt the final challenge. An SQL query consists of three parts:
In the SELECT block, the user specifies what data to return. In this query, we ask for the plate number, the mjd number, the fiber number, the red shift, the magnitudes in all five filters, the RA, the Dec, and the object ID.
The FROM block specifies which part of the database to search. Instead of searching the entire database of 14 million objects, we tell the computer to focus on the approximately 50,000 objects for which we have spectra. It is a lot faster to search a smaller database.
The WHERE block tells the computer what features to look for. We are looking for objects with spectra and well-measured redshifts that are classified as quasars (class = 3).
Students can have the data returned as a HTML file in their browser or a comma separated value file (csv). If they choose HTML, they can select the data in the table and paste it into an Excel spreadsheet. If they choose a csv file, they will have to save the file to a disk and open it using Excel. Neither method is significantly faster or easier, and you should consider what your students are comfortable doing on the computer.
Research Using Quasar Data
In the next section, students attempt two advanced projects students. Exercise 4 has students create color-redshift diagrams for quasars. These diagrams can be used to determine the redshift of a quasar from its colors rather than from its spectrum. Since we do not take spectra of every object, we can obtain approximate quasar redshifts even when no spectra are available.
In Exercise 5, students make color-color diagrams of quasars. Quasars with similar redshifts tend to have similar colors. Students can examine these diagrams and find what colors quasars with low redshifts have, and what colors quasars with high redshifts have. Students who are good at using spreadsheets can even use different colors to represent different red shifts on their graphs.
Both of these exercises reproduce an SDSS paper that was published in The Astronomical Journal in May 2001. This paper is also available online in PDF format.
Students can try their own queries of the data to see what else they can find. Possible ideas are looking at quasars in a certain ranger of colors or in a certain range of redshifts. They can also look at quasars for which we do not have high confidence redshift measurements.
The final challenge, Exercise 6, will challenge students to do scientific inquiry and find the data necessary to answer their question. They may need to do some research on how to structure an SQL query. This page is an excellent SQL Tutorial.