The Merging of Galaxies: From Dwarf to Compact

Galaxies come in many shapes and sizes, from massive spirals like the Milky Way to tiny dwarf galaxies. Some of these dwarfs can turn into something unique when they interact with larger galaxies. This article explores how compact elliptical (CE) and ultra-compact dwarf (UCD) galaxies form through the merging of different galaxies, particularly when smaller dwarf galaxies are swallowed up by larger spiral ones.

#What Are cE and UCD Galaxies?

Compact elliptical galaxies are small, spherical collections of stars that have a brightness similar to larger dwarf elliptical galaxies. Ultra-compact dwarf galaxies are even smaller, often resembling massive globular clusters but with unique features. Both types of galaxies are fascinating because they challenge our understanding of how galaxies can develop and change over time.

#The Dwarf Galaxy Merger

In the cosmic dance of galaxies, dwarf galaxies often find themselves in a precarious situation when they get too close to larger galaxies. When a dwarf disc galaxy merges with a larger spiral galaxy, like the Milky Way, dramatic changes occur. The gravitational forces during this merge can strip away the outer layers of the dwarf, leaving behind a compact stellar core. This core shares characteristics with both cE and UCD galaxies, creating what we call transitional cE/UCD galaxies.

#How Do We Study These Interactions?

To figure out what happens during these mergers, scientists create numerical models that simulate how stars, gas, and dark matter behave when galaxies collide. This is done using computer simulations that can track thousands of particles. By doing this, researchers can visualize how the dwarf galaxy falls towards the main galaxy, crosses through its disc, and experiences various gravitational effects over billions of years.

#The Role of Gas in the Dwarf Galaxy

One interesting aspect of these mergers is how the initial gas content of the dwarf galaxy impacts its transformation. If the dwarf galaxy is rich in gas, it can produce a more compact core when it merges with a larger galaxy. In contrast, a gas-free dwarf will result in a larger and less compact galaxy after the merge. This is because gas contributes to the gravitational pull that holds stars together in the core.

#The Process of Stripping

When the dwarf galaxy approaches the larger galaxy, it undergoes a process called "stripping." Tidal forces act like cosmic scissors, cutting away parts of the dwarf’s outer layers. As time passes and the dwarf makes repeated passes through the main galaxy's disc, it continues to lose mass. Over billions of years, this leads to the formation of a compact stellar core that eventually becomes a cE or UCD.

#Time and Trajectories Matter

Timing is essential in this process. How quickly a dwarf galaxy falls into a larger one and the angle at which it approaches can significantly influence its fate. A nearly radial approach (where the dwarf comes in straight) can result in more significant stripping than a more tangent-like trajectory. This is significant when evaluating how long it takes for transitional cE/UCD galaxies to form. Those that fall in with more gas tend to take about 4 to 5 crossings to form into a cE/UCD, while those with less gas or different angles may take longer.

#Characteristics of Transitional cE/UCDs

The transitional cE/UCD galaxies exhibit unique characteristics. They are generally small and dense with a size range of about 100 to 200 parsecs and have low masses. Their effective radii, which determine how far out their light extends, are relatively compact compared to many other galaxy types. These galaxies are often found near larger galaxies, as their formation is linked to larger cosmic structures.

#Loss of Dark Matter

Interestingly, the simulations show that these compact galaxies don't retain much dark matter. After merging, they end up with significantly less dark matter compared to their original dwarf galaxy state. This means the newly formed cE/UCDs consist mainly of stars and very little dark matter, a surprising fact given how prevalent dark matter is in the universe overall.

#What Happens Over Time?

Over billions of years, the stars within these compact galaxies may continue to evolve, but the core remains fairly stable. The processes involved stabilize the remaining stellar mass into a relatively long-lived structure. The new cE/UCD galaxies can persist through cosmic time, indicating that they have robust characteristics that allow them to survive through further interactions.

#The Importance of Angular Momentum

Angular momentum, which is essentially the rotational momentum of a body, plays a vital role in the evolution of these galaxies. As the dwarf galaxy interacts with the larger galaxy, it can lose angular momentum, causing changes in its shape and movement. A gas-rich dwarf may result in a more spherical structure, while a gas-free satellite might lead to a more extended shape as it retains some of its original rotational motion.

#Different Scenarios for Formation

There are various scenarios under which these transitions can occur:

1. Tidal Stripping: This occurs in clusters where tidal forces are strong, effectively removing outer layers of the dwarf galaxy.
2. Mergers with Supergiant Clouds: Some studies suggest transitional cE/UCDs might form from massive molecular clouds, emphasizing the role of gas.
3. Interactions with Bars: If the dwarf galaxy has a stellar bar, it can lead to complex interactions that influence how the galaxy loses stars and gas.

#Observational Challenges

One of the challenges in studying these galaxies is their small size. Because they are often faint and compact, it can be tricky to gather enough data to understand their structure fully. Most observations can only be performed on nearby galaxies, making it harder to analyze many of these interesting smaller structures that are scattered throughout the universe.

#The Connection to Galactic Evolution

Understanding how these compact galaxies form is crucial for comprehending galactic evolution. Dwarf galaxies are often seen as building blocks in the grand structure of the universe. Their interactions and subsequent transformations into cE/UCDs provide insight into the complex history of galaxy formation and evolution.

#The Gaia-Sausage-Enceladus Event

A significant event in galactic history that interests researchers is the Gaia-Sausage-Enceladus merger. This merger, believed to have occurred around 10 billion years ago, contributed a significant portion of stars to our Milky Way's halo. Studying these processes in the context of the GSE helps scientists paint a clearer picture of the history of our galaxy.

#Future Research Directions

There’s still much to uncover about these fascinating transitions. Future studies may involve investigating different initial conditions, such as varying the gas and stellar content of dwarf galaxies. Researchers may also explore the impact of supermassive black holes in the centers of these galaxies, which could drastically change the dynamics and outcomes of mergers.

#Conclusion

In summary, the formation of transitional cE/UCD galaxies through the merging of dwarf and massive disc galaxies is a captivating area of study in astronomy. By understanding the processes involved in these interactions, including the role of gas, angular momentum, and tidal stripping, we can gain deeper insights into how galaxies evolve in the universe. This research not only enhances our knowledge of individual galaxy behaviors but also contributes to the broader understanding of cosmic evolution. So, the next time you look up at the night sky, remember that behind those twinkling stars, there are countless stories of galaxies colliding, merging, and transforming into something new.