How do astronomers know that quasars are located in galaxies? In what sorts of galaxies are they found?

Active Galactic Nuclei (AGN) represent some of the most energetic and enigmatic objects in the universe. They are brightly shining regions at the very heart of certain galaxies. The source of this incredible energy is a supermassive black hole which accrues material through its immense gravitational pull. This process is highly chaotic and releases an enormous amount of radiation, observable across various wavelengths, from radio waves to gamma rays.

AGNs vary widely in size, luminosity, and the types of radiation they emit, but they are united by some common characteristics. Firstly, they produce jets of high-speed particles that can extend far beyond the confines of their host galaxy, creating brilliant lobes of emission. Secondly, the area surrounding the black hole, called the accretion disk, is exceptionally bright due to the heat generated from the friction and collisions among particles as they spiral inward.

It’s within these regions that quasars, the most luminous type of AGN, are found. They are so bright that they can outshine their entire galaxy. This luminosity allows them to be visible even from great distances, making them valuable tools for studying the early universe.

Redshift is a key concept in understanding the movement and distance of celestial objects like quasars. It refers to the phenomenon where light is stretched to longer, or redder, wavelengths as an object moves away from us. This is similar to the change in the pitch of a siren as an ambulance drives past, known as the Doppler effect.

In the context of astronomy, we observe the redshift of spectral lines—specific wavelengths of light that are emitted or absorbed by elements, like hydrogen or helium, in the disciplined dance of quantum mechanics. These lines are fingerprints for the elements and can tell astronomers much about the object's composition, temperature, and velocity.

By measuring the extent to which these spectral lines are redshifted, astronomers can calculate how fast an object, such as a quasar, is receding from Earth. This motion is largely due to the expansion of the universe. The farther away an object is, the more its light is redshifted, allowing us to estimate its distance. Therefore, high redshift values indicate great distances, confirming that quasars are indeed far away from us, situated in the centers of distant galaxies.

Elliptical and spiral galaxies are two of the most common types of galaxies where quasars are observed. Elliptical galaxies are typically more massive than spiral galaxies and have a smooth, oval-shaped appearance. They contain older, redder stars and have very little gas or dust, which means they have a limited potential for new star formation.

Spiral galaxies, on the other hand, have a flat, disk-like structure with a central bulge, surrounded by spiral arms. These arms are sites of ongoing star formation and are characterized by their blue color, indicating the presence of younger, hotter stars. They also contain significant amounts of gas and dust.

Both types of galaxies can host quasars, which are indicative of active galactic nuclei. The presence of a quasar signifies that there is a supermassive black hole at the center of the galaxy that is actively feeding on material. This infalling matter is responsible for the AGN's brightness. Spiral galaxies with quasars often have an active nucleus in their central bulge, whereas elliptical galaxies hosting quasars shine brightly across their entire structure due to the presence of an active galactic nucleus at their core.