MORE ACTIVE GALAXIESAND QUASARS

BRYAN NOZALEDA

How many AGNs and QSOs are there in the universe?

About 5 to 10 percent of all large galaxies in the nearby universe contain AGNs or QSOs. The brighter the QSO, the rarer it is. Only a small fraction of QSOs, for example, are as luminous as 3C 273.

If they are so uncommon, why are AGNs and QSOs important in the universe?

QSOs and AGNs are such bright yet compact objects

that they shine like cosmic searchlights. Thus, they are relatively easy to detect even if they are very far away. When we observe a distant QSO, then, all the material between it and us is lit up. We can search the spectra of QSOs to see if there is evidence of matter that we cannot see directly, except with the illumination of the QSO light.

How bright can such a QSO searchlight appear in the sky?

The brightest QSO as seen from Earth is 3C 273. It is about two billion light-years from Earth, which makes it a challenge for most small amateur telescopes to find. Compared to other distant objects, however, QSOs are brilliantly bright and relatively easy to detect with large astronomical telescopes.

What is a quasar absorption line?

If the spectrum of a quasar (or, more generally, an AGN or QSO) contains an absorption feature that was not produced by the quasar itself, that means that the quasar’s light has shined through some material or object that absorbed some of that light.

What causes a quasar absorption line?

A quasar absorption line is usually caused by the interstellar medium within or surrounding a galaxy. The quasar’s light goes through the medium, and the atoms in the medium absorb the quasar’s light at specific wavelengths.

An artist’s concept of two active galaxies with active nuclei containing black holes.The idea that galaxies without a central bulge like the one on the right could not contain black holes has been proven to be erroneous. (NASA/JPL-Caltech)

What is the Lyman-alpha forest?

When the spectrum of a QSO contains a very large number of absorption lines, the majority of those absorption lines are usually caused by Lyman-alpha clouds. These sub-galaxy-sized clumps of gas populate the distant universe at different redshifts. Each cloud produces a single absorption line caused by hydrogen atoms called the Lyman-alpha line (hence the name “Lyman-alpha cloud”).

What can astronomers learn from the Lyman-alpha forest?

These clouds are not luminous enough to be observed directly, but they are important constituents of matter in the universe. Understanding the population of Lyman-

alpha clouds, therefore, helps astronomers understand how gaseous matter is distributed throughout the cosmos.