Building Systems: Galaxy Formation Models Quiz

Monolithic collapse, often associated with the ELS model, proposes that galaxies formed early in the history of the universe through the rapid contraction of huge gas clouds. This "top-down" approach suggests that all stars in a galaxy should have roughly the same age. Modern data suggests this model primarily explains the formation of some ancient elliptical galaxies.

The hierarchical merging model is frequently called the "bottom-up" approach because it describes the universe starting with small structures that grow over time. In this scenario, gravity pulls together small gas clouds and dwarf galaxies to assemble the massive galaxies we see today. This process is consistent with observations of the distant, early universe and dark matter simulations.

Dark matter is a critical component in modern galaxy formation theories because it provides the necessary gravitational pull to cluster matter together. While invisible, its presence is inferred by the way it influences the movement of visible stars and gas. This invisible scaffold allowed the first small structures to form and eventually merge into larger, more complex galactic systems.

The monolithic collapse model is characterized by a "top-down" process where a large cloud collapses all at once. This leads to a burst of star formation where most stars are created in a short cosmic timeframe, resulting in a population of stars with very similar ages. It does not account for the slow assembly of galaxies from smaller fragments or mergers.

Observations from deep-space telescopes like Hubble show that galaxies in the very early universe were much smaller, clumpier, and more irregular than modern galaxies. This evidence directly supports the hierarchical merging theory, as it shows that large, modern galaxies were likely built over time from the collisions and mergers of these smaller, primitive galactic building blocks.

When two large spiral galaxies collide and merge, the gravitational interactions strip away their organized disk structure and randomize the orbits of their stars. This violent process often results in the formation of a single, large elliptical galaxy. This explains why elliptical galaxies are often found in dense clusters where galactic collisions and mergers are much more frequent.

Galactic cannibalism is a specific type of hierarchical merging where a massive galaxy consumes a smaller satellite galaxy. As the smaller galaxy passes through the larger one, tidal forces tear it apart, and its stars and gas become integrated into the larger system. This process is currently happening in our own Milky Way as it interacts with smaller dwarf galaxies.

Evidence for merging includes long "tidal tails" of stars and gas pulled out by gravitational forces during a collision. Furthermore, galaxies that have undergone mergers often show distinct populations of stars with different ages and chemical compositions. Distorted or asymmetrical shapes are also common indicators that a galaxy has recently interacted with another massive object in space.

The monolithic collapse model suggests that most stars form in a single, early burst. However, observations of the Milky Way’s disk reveal stars with a wide range of ages, indicating that star formation has been ongoing for billions of years. This diversity is better explained by the hierarchical model, where gas is added and star formation is triggered by successive mergers.

In the hierarchical "bottom-up" model, the first stars actually formed in much smaller, lower-mass systems called protogalactic fragments. These small systems were the first to collapse within dark matter halos in the early universe. Over billions of years, these small star-forming regions merged together through gravitational attraction to eventually build the massive, complex galaxies that we observe in the modern cosmos.

The ELS model, proposed in the 1960s, was one of the first major attempts to explain how our galaxy formed through the rapid collapse of a giant gas cloud. While it successfully explained some aspects of the Milky Way's halo stars, it was eventually supplemented by the Searle-Zinn model, which introduced the idea of smaller fragments merging over a longer period.

When galaxies merge, the compression of gas clouds often triggers a massive surge in star formation known as a starburst. The process also reshapes the galaxies, often turning spirals into ellipticals. Additionally, mergers funnel gas toward the center, providing fuel that allows the central supermassive black holes to grow significantly in mass through the process of accretion and eventual merging.

For a galaxy to form, gas must lose its internal thermal energy and cool down. As it cools, the pressure that opposes gravity decreases, allowing the gas to collapse into dense regions where stars can ignite. Without efficient cooling mechanisms, the gas would remain too hot and dispersed to ever form the stars and structures that define a galaxy.

The monolithic model describes a smooth, large-scale collapse, which does not match the clumpy, fragmented appearance of the early universe seen in deep-field observations. The hierarchical model is much more successful at predicting this "clumpiness," as it assumes that the early universe was filled with many small, independent structures that only later combined to form the larger galaxies we see today.

Because galaxy formation takes billions of years, astronomers cannot watch it happen in real-time. Instead, they use complex computer simulations that incorporate the laws of physics, dark matter, and gas dynamics. By comparing the results of these simulations with actual observations from telescopes, scientists can determine that the hierarchical merging model is the most accurate description of cosmic evolution.

Early protogalactic clouds were composed almost entirely of hydrogen and helium, which were the primary elements created during the Big Bang. They also contained a significant amount of dark matter, which provided the gravity for the collapse. Heavier elements, or metals, were only created much later through nuclear fusion inside the first stars that formed within these collapsing gas clouds.

The hierarchical merging model accounts for varied chemical compositions because it suggests that a galaxy is built from many different smaller systems, each with its own history of star formation. As these different systems merge, they bring in stars and gas with different levels of heavy elements. This creates the complex chemical patterns observed in the stars of large, modern galaxies.

Galaxy formation and evolution are ongoing processes in the universe. While the most intense period of formation occurred in the distant past, galaxies continue to interact, collide, and merge today. Our own Milky Way is currently on a collision course with the Andromeda Galaxy, and they are expected to merge into a single elliptical galaxy in about four billion years.

The hierarchical model predicts a clear evolution in the galactic population over time. In the early universe, there were many small, chaotic fragments of gas and stars. As time passed, gravity caused these fragments to collide and merge, resulting in a modern universe that is dominated by a smaller number of much larger, more structured spiral and elliptical galaxies.

During the hierarchical assembly of a galaxy, gas that is not consumed in initial starbursts can eventually settle into a stable, rotating disk. This conservation of angular momentum leads to the formation of the disk structures seen in spiral galaxies. Subsequent star formation in these disks produces the younger, metal-rich stars that we often observe in the arms of modern spiral galaxies.