- About 5-10% of large galaxies contain active galactic nuclei (AGNs) or quasars, with brighter quasars being rarer. - AGNs and quasars are important because they are extremely bright and allow astronomers to illuminate distant material between Earth and the quasar that cannot be seen directly. - Quasar absorption lines are usually caused by interstellar gas and dust within or surrounding a galaxy, which absorbs specific wavelengths of light from the quasar. - The Lyman-alpha forest refers to the many absorption lines seen in quasar spectra caused by hydrogen clouds along the line of sight between Earth and the quasar. Studying these clouds helps astronomers understand the distribution of gas

1. MORE ACTIVE GALAXIES
AND QUASARS
BRYAN NOZALEDA

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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)

8. 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”).

9. 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.