Active Galaxies

Active galaxies are galaxies with unusually luminous centers powered by supermassive black holes actively consuming matter. Unlike normal galaxies where most light comes from billions of stars, active galaxies have compact nuclei that outshine their entire stellar populations. These active galactic nuclei, or AGN, are among the most energetic phenomena in the universe.

The power source is straightforward. A supermassive black hole millions to billions of times the sun’s mass sits at the galaxy’s center. Matter falling toward the black hole forms an accretion disk where friction heats the gas to hundreds of thousands of Kelvin in the disk, and to millions of Kelvin in the hot corona above it. The disk emits radiation across the entire electromagnetic spectrum from radio waves to X-rays and gamma rays. Jets of matter and energy can blast outward from the poles at nearly the speed of light, extending for millions of light-years into intergalactic space.

Seyfert galaxies are relatively common active galaxies with moderately bright nuclei. They look like normal spiral galaxies with a brilliant point source at the center. The nucleus often outshines the entire galaxy by factors of 10 or more. Seyfert galaxies come in two types based on their spectra. Type 1 Seyferts show broad emission lines from gas moving at thousands of kilometers per second near the black hole. Type 2 Seyferts show only narrow lines from gas farther out.

The difference between Type 1 and Type 2 may be just viewing angle. The unified model of AGN proposes that a thick torus of dust surrounds the accretion disk. When viewed face-on through the torus opening, we see broad lines from the inner regions - a Type 1. When viewed edge-on through the dusty torus, the inner regions are obscured and we see only narrow lines from outer gas - a Type 2. It’s the same object, just different perspectives.

Radio galaxies are active galaxies with powerful radio emission from jets launched by the central black hole. Centaurus A, visible from the southern hemisphere, shows a peculiar elliptical galaxy bisected by a dark dust lane. Radio images reveal enormous lobes extending over a million light-years on either side, powered by jets from the nucleus. The total energy in these lobes exceeds 10^60 ergs - equivalent to converting the entire mass of a star into energy.

Blazars are active galaxies viewed nearly down the jet axis. Relativistic beaming amplifies their brightness enormously. Light emitted in the direction of the jet appears brighter and more variable than light emitted in other directions. Blazars can change brightness by factors of 10 or more in days or hours. Their spectra are dominated by non-thermal synchrotron radiation from relativistic electrons spiraling in magnetic fields.

BTW active galaxies were more common in the past. Looking at distant galaxies means looking back in time. Surveys show AGN activity peaked when the universe was 2 to 3 billion years old. Today, most supermassive black holes are dormant, having exhausted nearby fuel. The Milky Way’s central black hole, Sagittarius A*, accretes matter at a trickle - it was probably much more active billions of years ago.

Active galaxies affect their host galaxies dramatically. Jets and radiation from the AGN can heat intergalactic gas, preventing it from cooling and forming new stars. This AGN feedback is crucial to galaxy evolution models. Without it, simulations produce galaxies with far too many stars compared to observations. The energy from AGN regulates star formation across cosmic time.

M87 in the Virgo Cluster is a giant elliptical galaxy with an extremely massive black hole - 6.5 billion solar masses. This is the black hole imaged by the Event Horizon Telescope in 2019, showing the shadow of the event horizon silhouetted against hot glowing gas. A jet extends from M87’s core for at least 5,000 light-years, visible in optical images as a blue spike of light.

NGC 4151 is sometimes called the “Eye of Sauron” for its appearance in infrared images. It’s a relatively nearby Seyfert galaxy just 62 million light-years away, making it valuable for detailed AGN studies. X-ray observations show iron emission lines with relativistic broadening from gas orbiting very close to the black hole’s event horizon, within just a few Schwarzschild radii.

Active galaxies serve as cosmological probes. Their extreme brightness makes them visible across vast distances. Quasars - the most luminous AGN - can be detected at redshifts exceeding 7, corresponding to less than a billion years after the Big Bang. Their spectra contain absorption lines from intervening gas clouds, mapping the distribution and evolution of matter over cosmic history.