radio galaxy
/ RAY-dee-oh /
Point an optical telescope at certain galaxies and you see a fairly ordinary elliptical blob. Point a radio telescope at the same spot, and suddenly the galaxy sprouts two enormous glowing clouds, often stretching far beyond the visible galaxy — sometimes for millions of light-years, among the largest single structures in the universe. These radio-bright giants are radio galaxies.
A radio galaxy is an AGN whose relativistic jets are powerful and oriented roughly in the plane of the sky (so we see them from the side, not down the barrel). The jets shoot out from the central black hole, plow into the surrounding intergalactic gas, and at their ends inflate vast 'radio lobes'. The radio light is synchrotron emission from electrons spiraling in magnetic fields at near-light speed. Because the jets here are not pointed at us, we are not blinded by beaming, so we can map the whole jet-and-lobe structure cleanly. Radio galaxies usually live in giant elliptical galaxies with very massive central black holes.
Radio galaxies showed astronomers that AGN reach far beyond their host galaxies. In the unification picture, a radio galaxy and a blazar can be the very same kind of object — a radio-loud AGN — differing only in viewing angle: side-on we call it a radio galaxy, end-on (jet aimed at us) we call it a blazar. The colossal energy stored in radio lobes is also a vivid demonstration of AGN feedback, since these bubbles can heat and displace enough gas to slow or quench star formation across an entire galaxy cluster.
Cygnus A, one of the brightest radio sources in the sky, looks like a modest elliptical galaxy in visible light. In radio it reveals two lobes spanning roughly 500,000 light-years, fed by thin jets from its central black hole — a textbook radio galaxy seen side-on.
Twin radio lobes, fed by jets, dwarfing the visible galaxy at their center.
Only a minority of AGN (the 'radio-loud' ones) launch such powerful jets; most AGN are 'radio-quiet'. A radio galaxy is not a different beast from a blazar — point its jet at us and the same object would look like a blazar instead.