[Forgiven_Sinner]

Centaurus A, at 11 million light-years' distance, is the closest active galaxy to our own.

What does "active galaxy" mean? Both the Magelanic Clouds (<1 million LY) and the Andromeda galaxy (2 million LY) are closer than Centaurus A.

[petuniasevan]

Its core is wildly active, emitting huge amounts of electromagnetic energy. This seems to be fueled by a black hole. It sucks gas and dust down its infinite gravity well (not even light can escape, thus the name "black hole"). As the material is spun "down the drain", it is heated by friction and tidal forces until it emits x rays. Those are expelled as a jet (see the Chandra images above).

Most galaxies, including our own, have black holes at the core which aren't getting enough material inflow to act like Cen A.

[petuniasevan]

Also known as Seyfert galaxies.

Here is a Hubble image of the inner workings of galaxy NGC 4261:

See the accretion disk? The hard x-rays and gamma rays it emits are astounding.

Even more energetic galaxies exist: we call them quasars.