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Markarian 421

BL Lac Object

aka UGC 6132, PGC 33452

R.A.: 11h04m27.4s Dec.: +3812'34" (2000)

Magnitude: 12.5-13.5

Size: 51"x 45"

Light Time: 360 myr

Minimum requirements to detect: 6-inch and larger telescope

Markarian 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 421.

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!

Millennium Star Atlas Vol II Chart 636
Sky Atlas 2000 Chart 6
Uranometria 2000 Vol I Chart 106
Herald-Bobroff Astroatlas B-05 C-21