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The Electromagnetic Spectrum

The electromagnetic spectrum refers to radiation composed of waves of oscillating
electromagnetic fields. One example of such radiation is visible light, but other examples
are radio waves, x-rays, gamma-rays, etc. All objects emit electromagnetic radiation with a
range of wavelengths (or, equivalently, energies). This spread of wavelengths is called the
objectʼs spectrum. By studying the spectrum it is possible to learn quite a lot about the
object emitting the radiation. Spectra are recorded by an instrument called a spectrograph.
When a spectrograph is mounted onto a telescope it can be used to measure the spectra
of stars or galaxies.
In this set of exercises we compare emission in the optical, infrared and gamma-ray parts
of the spectrum from several active galaxies. The optical and infrared emission has long
wavelengths (this type of light has relatively low energy), whereas the gamma-rays have
much shorter wavelengths (and higher energies). We present one possible model for how
the radiation is produced, and we compare the emission at different wavelengths with our
model in order to try to learn something about the modelʼs validity.
To learn more about galaxies and their spectra see the GTN Resources pages using the
link at the end of the next section.

Different Kinds of Galaxies

Galaxies come in three basic types. These are spiral, elliptical and irregular. The names
are descriptive. Spirals have prominent spiral arms, usually two of them, winding outward
from a central bright bulge. Ellipticals look like smooth ellipses on the sky. They have a
bright core and fade gradually toward their edges, at which point they become too faint to
be seen. Irregulars are galaxies that do not fit into either of the other two categories.
The different kinds of galaxies are thought to arise from their different formation histories.
Our understanding of the details of galaxy formation is still imperfect, but it seems clear
that large galaxies are built up by the merger of smaller ones. In many irregulars we catch
this merger process mid-stream. That is why irregulars often have a disturbed or chaotic
appearance. Ellipticals can also be the result of mergers, especially the largest ellipticals.
Some of these even have multiple galactic nuclei, each of which is the nucleus from one of
the former galaxies that have been consumed in making the large galaxy. In spirals we
also see evidence of the merging of smaller systems, but the formation process had to
differ quite a bit from that of ellipticals in order to leave an orderly disk; disks are fragile
and are easily disrupted.
To learn more about the properties of the different types of galaxies, go to the GTN
Resource pages:


Active Galaxies and Normal Galaxies!

For decades it has been known that some galaxies, a percent or so, have unusual things
going on in their nuclei. In some, the nucleus is unusually bright. Others have jets of
material being ejected from the nucleus at speeds approaching the speed of light. Others
show very broad emission lines of hydrogen and other elements in spectra taken of their
nucleus. Some of these galaxies have only one of these characteristics, while others have
all three… and this is not a complete list of their distinguishing characteristics.
These galaxies have come to be known as active galaxies, but what they really have in
common is an active nucleus, one that pours out much more energy in the form of jets and
radiation than the entire rest of the galaxy does. It has taken us a long time to understand
what these objects are, but over the past three decades or so, the evidence has mounted
that they contain supermassive black holes (black holes with masses measured in millions
or even billions of solar masses), and that these black holes are feeding: material is
funneling down into the gravitational bottomless pit at the center of the black hole. And in a
somewhat counterintuitive twist, a feeding black hole is an extremely bright object. Much of
the material that falls into the central parts of these galaxies ends up getting ejected, either
in the jets or as electromagnetic radiation, before it crosses the event horizon of the black
Our Milky Way is not an active galaxy at the moment, but it almost certainly was billions of
years ago. Like all large galaxies, the Milky Way has a supermassive black hole at its
center. Ours is a modest 4 million solar masses, but our neighbor Andromeda has a 70
million solar mass monster. Neither of those sounds very impressive compared to the 3
billion solar mass behemoth in the center of the giant elliptical galaxy Messier 87 that lies
at the center of our local supercluster of galaxies (M87 is itself a radio galaxy, a type of
active galaxy). Whatʼs more, as we look farther out into the universe, we see the number
of quasars, the most energetic kind of active galaxy, increases. Since looking farther away
also means looking farther back in time, this means that many more quasars existed in the
past than do in the present (there are no quasars at all in the nearby, present-day
universe). Taken together, the ubiquity of supermassive black holes in the centers of large
galaxies, and the increasing numbers of quasars as one looks earlier in the universe,
suggest that all galaxies have undergone a stage as an active galaxy early in their
existence. And while they tend to quiet down in later life, they can flare up again if they
merge with a neighbor (as we will with Andromeda in several billion years). All that is
required for a galaxy to be active is that material falls into the central black hole, feeding it
and lighting it up. Mergers can bring quiescent central black holes back to life by providing
food for the monster down there in the middle.
One type of active galaxy does not necessarily have a supermassive central black hole.
Starburst galaxies have greatly increased star formation rates as compared to normal starforming
galaxies. They are always associated with galaxy mergers or interactions of some
sort, and while they are termed active, they do not usually have an active nucleus.
To learn more about active galaxies, go to the GTN page on active galaxies at the
following link:

Imagery Used

The images used in this game are all on the Web and are publicly available. They were
taken with telescopes like the NASA/ESA Hubble Space Telescope, NASAʼs Chandra Xray
Observatory and Spitzer Space Telescope, and the Very Large Array radio telescope,
operated by the National Radio Astronomy Observatory.

Math Concepts in This Exercise

A spectrum can convey one of the primary ideas behind graphing and reading graphs. It
shows the run of brightness with wavelength or energy. At each point on the horizontal axis
(typically wavelength, frequency or energy… they are equivalent for this purpose), the plot
of a spectrum shows how bright the object is at that particular wavelength. The brightness
is usually given in specific intensity (energy per unit time per unit area per wavelength
interval, e.g., watts per square meter per angstrom).



Accretion Disk - A disk of material that is spiraling in toward the center. The Accretion disk
allows material to transfer angular momentum outward, while the material itself flows
inward. In this way, small objects like stars can be formed out of large extended gas

Active Galaxy - A galaxy that has energetic processes occurring, unlike those found in
normal galaxies. Some galaxies, called starburst galaxies, have elevated levels of star
formation, perhaps hundreds or even thousands of times greater than the star formation
rate found in normal galaxies. Other galaxies have extremely active nuclei powered by
mass accretion onto a supermassive black hole located there.

AGN - Active Galactic Nucleus. The nucleus of a galaxy in which a central supermassive
black hole resides. The black hole must be actively accreting large amounts of mass for
the galaxy to be active. The AGN is typically much brighter than the nucleus in a normal
galaxy. In the most energetic AGN the energy produced in the nucleus outshines the entire
rest of the galaxy. Examples of AGN are quasars, QSOs, Seyfert galaxies, BL Lac objects,
LINERs and radio galaxies.

Black Hole - A black hole is an object that has completely collapsed under the influence
of gravity. Essentially, it is a region of high space-time curvature, i.e., gravity, but nothing
else. They can be formed as the end result of stellar evolution when the core of a massive
star collapses under its own weight. The mass of these stellar black holes is typically a few
times the mass of the sun, perhaps going as high as ten solar masses. Through processes
not yet fully understood, some black holes grow to immense sizes, having masses millions
or even billions of times the sunʼs mass. These supermassive black holes are found in the
centers of all large galaxies. There are also black holes with masses intermediate between
the supermassive black holes and stellar mass black holes. Their sizes are a few hundred
to a few thousand solar masses. The means by which they form is still not understood.

Galaxy - A galaxy is a collection of stars, gas and sometimes dust, all held together by
gravity. However, the dominant component of all galaxies is the so-called dark matter,
material that does not interact strongly by any means other than gravity, thus rendering it
invisible. The dark matter makes up over 80% of the mass of galaxies, with the visible
material making up the rest. Galaxies come in various sizes, but our own galaxy, the Milky
Way, is typical of large galaxies: it is about 100,000 light years in diameter, contains about
100 billion stars, and has several components ranging in age from about 8 billion to nearly
14 billion years. A small fraction of galaxies is larger than the Milky Way (some more than
ten times larger), but most are smaller (down to only a few percent as big as our galaxy).

Light - For our purposes, light refers to any type of electromagnetic radiation. This
includes visible light, but also radio, infrared, ultraviolet, x-rays and gamma-rays. These
different kinds of light have different energies, and are often referred to by their wavelength
or frequency. Radio has the longest wavelength, up to kilometers, the lowest energies and
the lowest frequencies. Gamma-rays have the highest frequencies and energies, with the
highest energies ever measured being more than 1020 electron volts (visible light has
energy of about 1 electron volt, or about 10-19 joules). On the other hand, radio photons
have energies more than 10 million times lower than that of visible light.

Milky Way Galaxy - This is our galaxy, the one in which the Sun is located. The Milky Way
is a spiral galaxy 100,000 light years across. It contains about 100 billion stars. The Milky
Way has three main parts. The disk, where the Sun is located, is about 8 billion years old.
The other two, the halo and the bulge, are very old, more than 13 billion years. Like all
spirals, the disk of the Milky Way is where most of the stars are being formed (a small
amount of star formation goes on in the bulge). The halo has no star formation at present;
all itʼs stars formed more than 13 billion years ago.

Nucleus - The central, bright part of a galaxy. Sometimes also called the core of the

Spectrum (pl - spectra) - The spectrum of a star or galaxy refers to the run of its
brightness with wavelength. The spectrum is often shown as a graph with brightness on
the vertical axis and wavelength on the horizontal axis. The graph shows how much
energy is being emitted by the object at each wavelength.

Torus - The torus surrounding the black hole in an AGN is a large ring-shaped cloud. It is
where material piles up before it is fed into the accretion disk and eventually falls into the
black hole.

Wavelength - The distance between success peaks (or successive troughs) in a wave.
Electromagnetic waves are characterized by either their wavelength (λ) or their frequency
(ν). Either one will suffice since the product of the two is the constant speed of light, c:
λν = c .