9.3 Evolution of galaxies over cosmic time

Galaxies change dramatically over cosmic time, evolving from small, irregular structures in the early universe to the diverse morphologies we observe today. Understanding this evolution requires connecting observations across billions of years of lookback time, tracing how star formation, mergers, AGN activity, and environment collectively shape galaxies. This topic ties together the physical processes from earlier in the unit and shows how they play out on cosmological timescales.

Galaxy Evolution

Evidence for galaxy evolution

Deep imaging surveys provide direct evidence that galaxies looked fundamentally different in the past. Because light takes time to reach us, observing distant galaxies means observing them as they were billions of years ago.

• Hubble Deep Fields and Ultra Deep Fields capture galaxies across a wide range of redshifts, sampling multiple cosmic epochs in a single image. The Hubble eXtreme Deep Field, for instance, reveals that higher-redshift galaxies tend to be smaller, bluer, and more irregular than nearby galaxies.
• Morphological evolution shows the early universe was dominated by irregular and spiral galaxies. Elliptical galaxies become increasingly common at lower redshifts, a trend described by the evolving Hubble sequence.
• Star formation rate evolution is tracked by the Madau-Lilly plot, which shows that the cosmic star formation rate density was much higher in the early universe and has been declining ever since.
• Galaxy mass function evolution reveals that the number of massive galaxies increases at lower redshifts, while the population of low-mass galaxies decreases. This pattern is called downsizing and has important implications for how we think about galaxy assembly.

Concept of cosmic downsizing

Cosmic downsizing refers to the observation that more massive galaxies formed their stars earlier and more rapidly than less massive galaxies. Over time, the site of active star formation shifts from high-mass systems to low-mass systems.

This has two major implications:

1. Massive galaxies undergo rapid star formation followed by quenching at early epochs, becoming "red and dead" relatively quickly.
2. Less massive galaxies continue forming stars at later epochs, maintaining blue colors and active star formation for longer.

Downsizing is somewhat counterintuitive because the standard hierarchical (bottom-up) model of structure formation predicts that small structures form first and merge into larger ones. The fact that the most massive galaxies have the oldest stellar populations suggests that additional physics, particularly efficient early star formation and strong quenching mechanisms, must be at work in high-mass halos.

Star formation and AGN influence

Star formation is the primary driver of galaxy growth and evolution. The rate at which a galaxy forms stars depends on gas availability, gas density, and turbulence, relationships captured quantitatively by the Kennicutt-Schmidt law, which relates gas surface density to star formation rate surface density. Feedback from supernovae and stellar winds can regulate this process by driving galactic winds that heat or expel gas from the disk.

Active galactic nuclei (AGN) are powered by accretion onto supermassive black holes and exert their own powerful feedback on their host galaxies:

1. Negative feedback: AGN-driven outflows and relativistic jets heat surrounding gas and can expel it from the galaxy entirely, suppressing or quenching star formation.
2. Positive feedback: In some cases, AGN outflows compress nearby gas clouds, triggering new episodes of star formation (jet-induced star formation).

AGN activity peaks at $z \approx 2-3$, roughly coinciding with the peak of cosmic star formation. This overlap during the so-called quasar epoch points to a deep connection between black hole growth and galaxy-wide star formation.

Environmental impacts on galaxies

The environment a galaxy inhabits strongly shapes its evolutionary path. Galaxies in dense regions evolve very differently from those in sparse regions.

Galaxy clusters (high-density environments):

• Ram pressure stripping removes gas from galaxies as they move through the hot intracluster medium. This is well-documented in the Virgo cluster, where galaxies show truncated gas disks.
• Tidal interactions and mergers occur more frequently due to the high galaxy density. The Antennae galaxies are a famous example of an ongoing merger.
• Morphology-density relation (Dressler's relation): elliptical and S0 galaxies are far more common in cluster cores, while spirals dominate in lower-density regions.

Voids (low-density environments):

• Void galaxies tend to be gas-rich, blue, and actively forming stars at higher specific rates. Their isolation means fewer interactions and slower overall evolution. The Boötes void is one well-known example of such an underdense region.

Cosmic web (filaments, walls, and nodes):

• Galaxies in filaments and walls experience intermediate environmental effects, between the extremes of clusters and voids. The Sloan Great Wall is a prominent large-scale filamentary structure. Galaxies at the nodes of the cosmic web, where clusters reside, experience the strongest environmental processing.

Factors Influencing Galaxy Evolution

Evidence for galaxy evolution

Beyond morphology and star formation rates, several other observable properties trace galaxy evolution across cosmic time.

• Color evolution: Early-universe galaxies tend to be bluer, reflecting vigorous star formation. As star formation declines and stellar populations age, galaxies migrate to the red sequence, a tight correlation between color and magnitude populated by quiescent galaxies.
• Metallicity evolution: Galaxies at high redshift have lower metallicities because fewer generations of stars have enriched the interstellar medium. Over time, successive stellar generations increase metallicity, producing the observed mass-metallicity relation, where more massive galaxies are more metal-rich at a given epoch.
• Size evolution: High-redshift galaxies are typically more compact than their present-day counterparts of similar mass. Galaxies grow in physical size over time through mergers and continued gas accretion, a trend captured by the size-mass relation.

Concept of cosmic downsizing

Observational evidence for downsizing comes from multiple independent lines:

1. Archaeological downsizing: Stellar population analysis shows that massive galaxies have older average stellar ages than less massive galaxies, even though they all exist at the same redshift today.
2. Chemical downsizing: The mass-metallicity relation itself evolves with redshift, indicating that massive galaxies enriched their gas earlier.

Two leading explanations work together to produce this pattern:

• Efficient early star formation in massive halos: Gas cooling is more efficient in the deep potential wells of massive halos, allowing rapid early conversion of gas into stars.
• Stronger quenching in massive galaxies: AGN feedback is more powerful in massive systems, shutting down star formation once the central black hole grows large enough.

Star formation and AGN influence

The cosmic star formation rate density peaks at $z \approx 2$, sometimes called "cosmic noon." After this peak, the rate declines steadily toward the present day, as shown in the Madau-Lilly plot.

The AGN luminosity function also evolves with redshift, peaking at $z \approx 2-3$ during the quasar epoch. This temporal coincidence is not accidental. AGN and star formation appear to co-evolve through interlinked processes:

• AGN feedback regulates galaxy growth: The tight correlation between black hole mass and bulge properties (the black hole-bulge relation, also called the $M_{\text{BH}}-\sigma$ relation) implies that black hole growth and galaxy assembly are coupled.
• Star formation fuels black hole growth: Gas funneled toward the galactic center during mergers or disk instabilities feeds both star formation in the nuclear region and accretion onto the supermassive black hole.

Environmental impacts on galaxies

Environmental quenching describes the observation that galaxies in denser environments are quenched earlier and more completely than galaxies in less dense environments. Several mechanisms contribute:

• Ram pressure stripping removes cold gas directly.
• Strangulation (also called starvation) cuts off the supply of fresh gas from the halo, causing star formation to decline gradually as existing gas is consumed.
• Tidal interactions disturb gas distributions and can trigger bursts of star formation followed by rapid gas exhaustion.

The Butcher-Oemler effect provides direct evidence for environmental quenching: galaxy clusters at higher redshifts contain a larger fraction of blue, star-forming galaxies than nearby clusters, showing that quenching has progressed over time.

Assembly bias adds another layer. Halos in higher-density regions tend to assemble earlier, giving their galaxies more time to evolve, merge, and quench. Halos in lower-density regions assemble later, and their galaxies are correspondingly less evolved at any given epoch.

Void galaxies represent the opposite extreme. Galaxies in voids like the Boötes void retain more of their gas reservoir and sustain star formation over longer timescales, providing a useful comparison case for understanding how environment accelerates or delays the evolutionary processes that all galaxies eventually undergo.