28.2 Galaxy Mergers and Active Galactic Nuclei

Galaxy Interactions and Evolution

Galaxy mergers and structural changes

When two galaxies drift close enough together, their mutual gravity creates tidal forces that stretch and distort both galaxies. These interactions can completely disrupt a galaxy's original structure, tearing apart spiral arms and warping central bulges.

During a merger, stars, gas, and dust get redistributed throughout the system. The gravitational disturbances compress gas clouds, which triggers intense bursts of star formation called starbursts. These starbursts tend to occur either in the central regions of the merging galaxies or along tidal tails, the long streams of material pulled outward by gravitational forces.

What the final merged galaxy looks like depends on several factors:

• The mass, gas content, and shape (morphology) of each galaxy going in
• Their relative orientations and trajectories during the collision
• Whether supermassive black holes are present at their centers

One common type of merger is galactic cannibalism, where a larger galaxy gradually absorbs a smaller one. The smaller galaxy loses material over repeated close passes until it's fully consumed.

Influence of supermassive black holes

Supermassive black holes (SMBHs), with masses ranging from millions to billions of solar masses, sit at the centers of most massive galaxies. Their growth is tightly connected to the evolution of the galaxies around them.

When matter falls onto an SMBH, the process (called accretion) releases enormous amounts of energy. This powers active galactic nuclei (AGN), extremely luminous central regions that can actually outshine the entire host galaxy. AGN come in several varieties, including quasars, Seyfert galaxies, and radio galaxies.

AGN influence their host galaxies through two main feedback mechanisms:

• Radiative feedback: Intense radiation from the AGN heats and ionizes surrounding gas, which suppresses star formation in the region.
• Mechanical feedback: AGN drive powerful outflows and jets that physically expel gas from the galaxy, limiting its ability to form new stars.

These feedback processes create a kind of self-regulation. Galaxy mergers can trigger AGN activity by funneling fresh gas toward the central SMBH. The resulting burst of AGN feedback then shapes the galaxy's further evolution.

One of the strongest pieces of evidence for this co-evolution is the $M_{BH}-\sigma$ relation. This is a tight correlation between the mass of the central SMBH ($M_{BH}$) and the velocity dispersion of stars in the galaxy's bulge ($\sigma$). The fact that these two quantities track each other so closely, even though they operate on very different scales, strongly suggests that SMBHs and their host galaxies evolve together.

AGN Physics and Radiation

The intense radiation from AGN is powered by accretion of matter onto the central SMBH. As material spirals inward through an accretion disk, gravitational energy is converted into light across the electromagnetic spectrum.

In AGN jets, a key emission mechanism is synchrotron radiation. This is produced when relativistic electrons (electrons moving near the speed of light) spiral through magnetic fields, emitting radiation as they accelerate.

There's a theoretical ceiling on how bright an AGN can get, called the Eddington limit. This is the luminosity at which outward radiation pressure exactly balances the inward pull of gravity. Beyond this limit, radiation would blow away the infalling material, so it sets a maximum rate at which a black hole can grow through accretion.

Early vs. present galaxy interactions

Galaxy mergers were far more common in the early universe. Galaxies were packed closer together in a denser, smaller cosmos, so collisions happened frequently. The peak of merger activity occurred roughly 8 to 10 billion years ago (at redshifts of $z \approx 1-2$), which lines up with the peak of cosmic star formation. These frequent early mergers played a major role in building the galaxies we see today.

In the present-day universe, galaxies are more widely separated, so mergers are less frequent overall. That said, galaxy clusters and groups still provide dense enough environments for mergers to occur at higher rates than average.

The role of mergers in galaxy evolution has shifted over cosmic time:

1. Early universe: Mergers were a primary driver of galaxy growth and morphological transformation. They were key to forming massive elliptical galaxies and building up galaxy mass.
2. Present day: While mergers still happen, other processes have become more dominant. Secular evolution, which includes internal processes like bar formation and spiral arm development, along with minor mergers (where a large galaxy absorbs a much smaller companion), now plays a bigger role in shaping galaxies.

Tracking how the frequency and impact of galaxy interactions change across cosmic time gives astronomers a window into how galaxies formed and evolved throughout the history of the universe.