421 is listed as both a quasar and a galaxy. So which is it? Like
all quasars, it is neither; it is both. Technically, Markarian 421 is a BL
Lacertae object, which is a class of Blazar.
In a six-inch scope it is a challenge to
simply see this 12th magnitude blazar. Like the astronomers who discovered
the first far away quasars, all you will see is a star-like point of light.
It will take larger apertures to see the fuzz around it. "No problem
at 12th magnitude," you say? It isn't quite that simple. The
magnitude of the blazar varies from around 12.5 to nearly 13.5 over the course
of about a month.
In ten-inch or larger instruments you should
begin to be able to make out the fuzz around the star-like blazar. This
fuzz is the galaxy of which the blazar is the bright, star-like core.
Users of very large scopes should look for the illusive companion galaxy, a mere
13" to the northeast. This galaxy is apparently interacting with Markarian
While you look at this bizarre and interesting
member of the cosmic zoo take a few minutes to stop and think about what you are
seeing. Here is some food for thought:
The redshift of this object is 0.03, which
doesn't exactly set any records. On the other hand, that translates to a
recession velocity of 9000 km/sec, which in turn indicates a distance of around
360 million light years. So the light you see from this galaxy started on
its journey some 360 million years before your were born--the Devonian period
here on earth. At this time the sea levels were high worldwide.
Trilobites and fish were plentiful in the oceans. The first trees could be
found on land along with the first seed bearing plants. About this time
massive extinctions occurred.
The field in a 6-inch at 150x. North is
down and east is right.
The light from this galaxy traveled all those
millions of years across the emptiness of intergalactic space. In the last
few thousand years it passed through our own galaxy, where some of the light was
absorbed, and in the last few hours it entered our own solar system.
Finally it passed through the air above your head, down your telescope tube, and
ultimately this long journey ended at your eye. Don't let the journey go
to waste--stop and think!
So what is Markarian 421? Unlike a
quasar, BL Lacertae objects have no absorption lines. Their light output
is highly variable over a few weeks to months and they produce high intensity
emission from radio to gamma ray wavelengths. The lack of absorption lines
indicates that the source of the light is not stars; the emission indicates the
presence of very hot gas. The variability indicates that the region of
bright gas is small--very much smaller than the galaxy as a whole, yet it emits
as much light as the billions of stars that surround it. In addition, the
gas is moving at very high speeds toward us in an apparent jet of material.
So what's causing all this? Like the
distant quasars, it appears that the root cause of all the "trouble"
at the center of this galaxy is a supermassive black hole. This black hole
contains the mass of perhaps millions of stars, yet remains a very compact
object. Since the force of gravity depends on both the mass of the object
and how close you are too it, a supermassive black hole holds the potential for
an enormous gravitational force. Unlike most people assume, this force
alone isn't enough to draw material into it from far away--this material will
simply orbit the black hole just as the earth orbits the sun (black holes do not
"suck"). But if any material, such as an interstellar gas cloud,
happens to pass close enough to the black hole it will form a flat disk as it
orbits. Due to the way orbits work, the inside of the disk will rotate
more rapidly than the outside. The friction of adjacent particles moving
at different speeds will cause the gas to slowly spiral ever closer.
Eventually this friction heats the gas to very high temperatures, which makes it
emit light brightly at wavelengths from radio to gamma rays. Untimely as
the gas spirals in and jams together, rather than falling into the black hole,
much of it gets ejected. But because of the thick disk of material
falling-in the gas can only be easily ejected at right angles to the disk, where
there is nothing blocking the way out. The result is two high speed jets
of very hot gas going in opposite directions. In the case of
Markarian 421, one of the jets appears to be pointed right at us. When you
look at it, imagine that gas coming right at you!
When we look far back in time we see many
quasars--far more than we find nearby (near our own time). The reason for
this has to do with the material falling into the black hole at the center of
the quasar. There is evidence that supermassive black holes inhabit the
central region of all or most galaxies, even our own. Despite their
enormous gravity, eventually the gas falling into a black hole will simply run
out. Without the brightly glowing gas, both falling in and being ejected
in jets, the presence of the black hole is hidden from us. In other words,
in time a quasar will simply "turn off" as the gas supply dries up.
In fact, it generally requires some sort of interaction, either between
colliding gas clouds or between two colliding galaxies, to send enough gas
falling into the central region to power the quasar (or blazar).
There is evidence that in the early days of
the universe galaxy collisions were much more frequent; therefore there was more
gas to fuel the quasars. That is why our own galaxy has no quasar (or
blazar) today. But galaxy collisions still occur, and Markarian 421 has a
close interacting companion. The interaction with this smaller galaxy
seems to have sent fresh gas into the heart of Markarian 421, fueling the black
hole that lurks there. And even from 360 million light years away you can
see the result!