Beacons in the Gloom

Quasars as Probes of the Distant Universe

Absorption of their light by intervening objects

Quasars are ideal objects to use to learn about the distant — and for us practically invisible — Universe. In one situation, we can use light from quasars to probe the material between us and them because that light may pass through one or more galaxies or gas clouds on its way to us. The gas inside the galaxies or the intergalactic clouds acts then as a filter, absorbing selected wavelengths from the quasar light. When the light that ultimately reaches us is analyzed by astronomers, we find distinctive "finger prints" of the gas since each kind of gas absorbs in a different way. These finger prints of the gas appear in quasar spectra as dark lines, places where the light has been taken by the absorbing gas. It’s as easy as that!

Light from a quasar must pass through clouds of gas and even faint galaxies -- each leaving its imprint on the light -- before we receive it here on Earth. Illustration by James White.

Consider this analogy: When you look at a lamp through a red filter, you see a red lamp because the filter has absorbed all other colors. When analyzing light from quasars, astronomers do exactly the same exercise. The only difference is that a red filter absorbs almost all of the light, letting only the red rays through. Thus, you see red, but you also see the light source quite a lot fainter. The gas in the galaxies, however, absorbs only a small part of the light, letting most of it pass through, and we see quasars shining bright.

But there is more to gain from study of quasar spectra. All of the clouds between the quasar and us have different recessional speeds because of the expansion of the Universe. Now, remember the Doppler effect, and you will understand why the spectral lines caused by absorption of one gas cloud will be shifted from those caused by absorption from a second cloud, and so on. It becomes our job as astronomers to sort out all the clouds between us and the quasar using the clouds' redshifted spectral lines. And knowing the clouds' redshifts enables us to estimate the distances to the clouds.

Gravitational lensing

Much as an optical lens, a galaxy can bend, distort, and magnify light from objects behind it. If the mass of the intervening galaxy (or cluster of galaxies) is large enough and the geometry of the situation just right, we can detect multiple images of the same object. Illustration by James White.

When the light coming from quasars passes near great concentrations of matter, such as in clusters of galaxies or even individual galaxies, it may be deflected by the gravitational effect of all that matter on the fabric of space. Remember that light can be considered as a beam of particles called photons. Being particles, photons are also subject to the gravitational force. The figure shows how, in such a case, those light rays reaching us from a distant quasar seem to come from different sources which seem to be far apart on the sky. Due to the analogy of this effect to what happens with glass lenses, these instances are called gravitational lensing, and the structures responsible for the lensing, gravitational lenses. In addition, just as glass lenses do, images of gravitationally lensed objects may be amplified. This is why in most cases the direct image of the source is too faint to be seen. In some cases of gravitational lensing, measurement of the strength of the lensing effect permits us to infer how much matter is producing it — matter that is quite often undetectable by other means. Thus, studying cases of gravitational lensing enables us to search for seemingly invisible matter.

Classroom Activities

Quasars and the unified model (for middle school ages and up)

Have your students build a cardboard model of the unified model for AGNs. Have them paint the light source yellow, and then place the model — light source, torus, and accretion disk — in the center of the classroom. Now ask them to draw what they see from different angles: from one corner of the classroom, from the top of a desk, etc. After they are finished, you can construct the analogy between their drawn "AGN observations" and the real astronomical observations of different AGNs.

Absorption of quasar light by foreground gas (for high school ages and up)

Place a slide or overhead projector on one side of the classroom and a simple spectrometer on the other. Select five to six students to play the "intervening gas," and give each a monochromatic filter. The other students note which of the gas-cloud students has each filter. While a selected "observer" waits outside the classroom, two or three of the filters are held in the path of the light. Now, the "observer" has to guess who has placed a filter in the line of sight to the light source by checking the spectrum of the light. This procedure is repeated several times with different combinations of filters (and with no filters). The "observers" must draw the observed spectrum and represent it in a simplified intensity-wavelength diagram (colors in place of wavelength and intensity only as a histogram with values of zero or one for no detection and detection, respectively).

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