27.3 Quasars as Probes of Evolution in the Universe

Quasar Activity and Galaxy Evolution

Quasars are among the brightest objects in the universe, and their activity has changed dramatically over cosmic time. By studying when and where quasars "turned on" and "turned off," astronomers can trace how galaxies and their central black holes grew together. This section covers the timeline of quasar activity, the feedback loop between galaxies and black holes, how the first black holes may have formed, and why most supermassive black holes today sit quietly at galactic centers.

Timeline of Quasar Activity

Quasar activity peaked roughly 10 billion years ago, at a redshift of about $z \sim 2\text{–}3$. This era is called cosmic noon because it also corresponds to the peak of star formation and galaxy growth across the universe. Quasars and galaxies were building up together.

After that peak, quasar activity dropped steeply. The quasar population fell by a factor of about 100 between $z \sim 2$ and the present day ($z = 0$). Gas supplies ran low, mergers became less frequent, and there was simply less fuel to power these engines.

• Early universe ($z > 6$, less than 1 billion years after the Big Bang): Quasars are rare, but the ones that exist are extremely luminous, powered by black holes already exceeding 1 billion $M_{\odot}$. How they grew so massive so quickly is still an active area of research.
• Cosmic noon ($z \sim 2\text{–}3$): Peak quasar activity. Galaxy mergers are common, funneling gas into galactic centers and lighting up accretion disks.
• Local universe ($z < 0.5$): Quasars are uncommon. Most massive galaxies, including the Milky Way and Andromeda, host supermassive black holes that are essentially dormant, with little or no accretion activity.

Galaxies and Black Holes: Co-Evolution

Galaxies and their central black holes don't evolve independently. They influence each other through a feedback loop that regulates both black hole growth and star formation.

How galaxies feed black holes:

• Black holes grow by accreting gas and dust from their host galaxy.
• This accretion is most efficient during galaxy mergers and interactions, which funnel large amounts of gas toward the galactic center.
• Stellar winds and supernovae also provide a steady (though smaller) supply of gas for the black hole to consume.

How black holes regulate galaxies (quasar feedback):

• When a black hole accretes rapidly, it can launch powerful outflows and jets that heat and expel gas from the host galaxy.
• This expelled gas is no longer available for star formation, so the galaxy's growth slows down.
• Over time, this feedback process also starves the black hole itself of fuel, causing quasar activity to fade.

This back-and-forth produces a tight observed relationship called the M-sigma relation: the mass of a galaxy's central black hole correlates with the properties of the galaxy's bulge (specifically, the velocity dispersion of stars in the bulge). The feedback loop is the leading explanation for why this correlation exists.

Theories of Early Black Hole Formation

One of the biggest puzzles in this field is how billion-solar-mass black holes existed when the universe was less than a billion years old. They needed massive "seeds" and rapid growth. Two main formation pathways have been proposed:

1. Collapse of Population III stars

• The first generation of stars (Population III) were extremely massive, often exceeding 100 $M_{\odot}$, and burned through their fuel quickly.
• When they died, they left behind black hole remnants of roughly 10–100 $M_{\odot}$.
• These are relatively small seeds, so they'd need to accrete gas very efficiently and merge with other black holes to reach billion-solar-mass scales in under a billion years.

2. Direct collapse of primordial gas clouds

• Under specific conditions, such as the suppression of molecular hydrogen cooling, a massive gas cloud can collapse directly into a black hole without first forming stars.
• This produces much larger seeds, on the order of $10^{4}$–$10^{5}$ $M_{\odot}$.
• Starting with a bigger seed makes it easier to reach the enormous masses seen in early quasars.

In either scenario, the seed black holes must grow rapidly through a combination of gas accretion and mergers with other black holes. Whether one pathway dominates, or both contribute, is still an open question.

Active Quasars vs. Quiescent Black Holes

The difference between a blazing quasar and a quiet black hole comes down to one thing: how much gas is falling in.

Active quasars:

• Powered by rapid accretion onto a supermassive black hole, making them a type of active galactic nucleus (AGN).
• Accretion rates approach the Eddington limit, which is the maximum rate where radiation pressure from the infalling material balances the gravitational pull of the black hole. Exceed this, and radiation would blow the gas away.
• High accretion rates produce a luminous accretion disk and often powerful relativistic jets.
• These quasars tend to live in gas-rich galaxies, frequently ones undergoing mergers that push gas toward the center.

Quiescent black holes:

• Have low or negligible accretion rates, producing little or no observable emission.
• Most supermassive black holes in the local universe fall into this category. Sagittarius A*, the 4-million-solar-mass black hole at the center of the Milky Way, is a well-known example.
• Found in gas-poor galaxies where most gas has already been consumed by star formation or expelled by earlier episodes of quasar feedback.

The transition from active quasar to quiescent black hole is a natural part of galaxy evolution. As a galaxy uses up or ejects its gas reserves, the fuel supply for the black hole dwindles, and the quasar gradually fades.

Quasars as Cosmic Probes

Because quasars are so luminous, they can be observed at enormous distances, making them useful tools for studying the universe itself.

• Lyman-alpha forest: As light from a distant quasar travels toward us, it passes through clouds of neutral hydrogen in the intergalactic medium. Each cloud absorbs light at a slightly different redshift, creating a series of absorption lines in the quasar's spectrum. This "forest" of lines maps out the distribution of gas between galaxies across billions of light-years.
• Cosmic reionization: Quasars contributed to ionizing the neutral hydrogen that filled the early universe. Studying the spectra of the most distant quasars helps astronomers understand when and how reionization occurred.
• Gravitational lensing: When a massive object (like a galaxy cluster) sits between us and a distant quasar, it bends the quasar's light. Analyzing these lensed images reveals information about the intervening mass distribution and the structure of the cosmic web.
• Large-scale surveys: Projects like the Sloan Digital Sky Survey (SDSS) have cataloged hundreds of thousands of quasars, giving astronomers a statistical picture of how quasar populations change with redshift and environment.