27.1 Quasars

Quasars

Quasars are the most luminous objects in the known universe, powered by supermassive black holes devouring matter at the centers of distant galaxies. Despite being compact enough to fit within our solar system, a single quasar can outshine an entire galaxy by a factor of 100 or more. Because they're so bright and so far away, quasars act as cosmic flashlights that let astronomers study conditions in the early universe.

Discovery and Significance of Quasars

Quasars were first discovered in the 1960s during radio astronomy surveys. Objects like 3C 273 and 3C 48 appeared as point-like sources of radio emission that didn't match any known type of star or nebula. When astronomers found their optical counterparts, the spectra looked strange: emission lines like Lyman-alpha and C IV were present, but shifted to much longer wavelengths than expected.

That shift turned out to be a massive redshift, meaning these objects were billions of light-years away. Nothing that distant had ever been seen so clearly before.

Why quasars matter for astronomy:

• They're the most energetic objects known, reaching luminosities up to $10^{47}$ erg/s
• They let us observe conditions as far back as about 1 billion years after the Big Bang
• They reveal how supermassive black holes grew over cosmic time
• They help map the large-scale structure of the universe, including the cosmic web and galaxy clusters

Quasar Redshifts and Cosmic Distances

A quasar's distance is determined from its redshift, the shift of spectral lines toward longer (redder) wavelengths. This isn't caused by the quasar moving through space like a car driving away from you. Instead, it's a cosmological redshift: the expansion of the universe itself stretches the light while it travels to us.

Hubble's law connects redshift to distance:

$v = H_0 \times d$

where $v$ is the recession velocity, $H_0$ is the Hubble constant, and $d$ is the distance.

Quasar redshifts range from about 0.056 (relatively nearby) all the way up to 7.642 for the most distant known quasar, J0313-1806. A redshift of 7.6 means we're seeing that quasar as it existed when the universe was less than 700 million years old, during the era of reionization when the first light sources were transforming the universe.

By observing quasars at different redshifts, astronomers can study how galaxies, star formation rates, and chemical enrichment changed across billions of years of cosmic history.

Energy Output vs. Size of Quasars

The puzzle of quasars is that they produce staggering amounts of energy from a tiny region. For comparison, the entire Milky Way has a luminosity of roughly $10^{44}$ erg/s. A bright quasar can reach $10^{47}$ erg/s, about 1,000 times greater.

How we know they're small: Quasars vary in brightness on timescales of hours to weeks. An object can't change its brightness coherently faster than light can cross it. So if a quasar brightens noticeably in a few days, the emitting region must be smaller than a few light-days across. That's comparable to the size of our solar system.

How they produce so much energy from so little space:

1. A supermassive black hole ($10^6$ to $10^{10}$ solar masses) sits at the center of the host galaxy.
2. Gas spirals inward and forms a hot accretion disk around the black hole.
3. As matter falls deeper into the gravitational well, friction and compression heat the disk to extreme temperatures.
4. The gravitational energy of the infalling matter converts to radiation. Accretion is remarkably efficient, converting up to about 40% of the matter's rest-mass energy into light and other radiation. (For comparison, nuclear fusion in stars converts less than 1%.)

The Eddington limit sets a ceiling on how bright an object of a given mass can be. Beyond this limit, radiation pressure would blow away the infalling gas and shut off the fuel supply:

$L_{Edd} = 3.2 \times 10^4 \left(\frac{M}{M_{\odot}}\right) L_{\odot}$

Most quasars emit near the Eddington limit, which is how astronomers estimate the black hole masses required to power them. Reaching quasar-level luminosities demands black holes of millions to billions of solar masses.

Active Galactic Nuclei (AGN) and Related Phenomena

Quasars are the most extreme members of a broader family called active galactic nuclei (AGN). All AGN are powered by accretion onto supermassive black holes, but they vary widely in brightness and behavior.

Some key AGN phenomena:

• Relativistic jets: Some AGN launch narrow, high-speed outflows of plasma from near the black hole. These jets can extend thousands of light-years and emit synchrotron radiation as charged particles spiral through magnetic fields.
• Blazars: These are AGN whose jets happen to point almost directly at Earth. Relativistic beaming makes them appear especially bright and rapidly variable, with high polarization in their light.
• Seyfert galaxies and radio galaxies: Lower-luminosity AGN that share the same basic engine as quasars but at reduced power.

The unified model of AGN ties all these types together. The core idea is that quasars, Seyferts, blazars, and radio galaxies are fundamentally the same kind of object. They look different to us mainly because of viewing angle and the amount of dust and gas obscuring our view. A quasar seen from the side, through a thick dusty torus, might be classified as a different type of AGN entirely. This single framework explains a huge range of observational diversity with one underlying physical picture.