Merging Histories Shape Galaxy Structures

This study looks at how the stars in the outer parts of Milky Way-like galaxies relate to their recent merging history. Mergers are when two galaxies collide and combine. The focus is on understanding how the past merging events of these galaxies have affected their structure today, particularly the stars found outside their main bodies, known as stellar haloes.

#Key Findings

1. The ratio of stars involved in the most recent major merger is closely linked to how many stars came from outside the galaxy. However, it shows a weak connection to the total amount of stars in the halo, especially in disk-shaped galaxies.
2. Surprisingly, Disk Galaxies with few stars in their haloes can actually have busy merging histories. About 25% of these galaxies have experienced a major merger recently.
3. Disk galaxies with low halo mass and active merging histories tend to merge with smaller galaxies that have more circular paths and are roughly flat with the disk itself. This leads to a larger fraction of outside stars being incorporated into the disk instead of just increasing the halo.
4. Such mergers can also provide new material that can trigger significant star formation in the disk.

#The Importance of Mergers

Mergers are essential to how galaxies grow. They bring new stars and gas, which can lead to bursts of star creation. Big galaxies typically gain most of their stars through these merging events, with disk galaxies often shifting into elliptical shapes as a result.

While it is known that merging can happen, it is also evident that disks can recover or even reform after significant mergers, as long as there is enough gas left. This means that a galaxy's current shape does not necessarily indicate its merging history.

#Stellar Halos and Merging Histories

Observations of stellar haloes can provide insights into how galaxies have assembled over time. The Milky Way and its neighbor, M31, show different merging histories based on their haloes. The Milky Way appears to have had a quiet merging past, while M31 has a richer and more varied halo likely due to a major merger happening not too long ago.

However, studying other galaxies is challenging due to the difficulty in observing faint structures. Existing studies show that there is a significant variation in the properties of stellar haloes between different galaxies, a result that may be caused by unique formation paths.

To understand these variations, simulations are important. They can help us connect what we see in the present-day haloes to past merging events.

#Merging Histories in Cosmic Models

In cosmic models, galaxies are expected to gain mass through merging. Yet, there are two challenges when studying stellar haloes:

1. Typical cosmic models show that the mass fractions in the haloes of Milky Way-like galaxies are often lower than what is observed. This calls for higher-resolution simulations to accurately capture the formation process without biases.
2. The definitions used to classify and study halo properties often vary, which can create confusion. Different methods for identifying the disk and halo structures can lead to different interpretations of the data.

#Study Design

This study examines the relationship between the stellar halo and merging history using data from a detailed simulation. By breaking down galaxies into their main parts-like the disk, bulge, and halo-we can investigate how their recent merging histories influenced their current structures.

#Sample of Galaxies

The focus is on a sample of 1254 central galaxies, which represent a range of Milky Way masses. The sample aims to exclude galaxies that have not undergone any merging events recently.

To categorize these galaxies based on their structure, a dynamic analysis technique is applied. This technique looks at the different components of each galaxy to see how they relate to their merging histories.

#Analyzing Galaxy Structures

Each galaxy is examined using a coordinate system that centers on it, aligning the axes to its overall shape and rotation. The galaxies are divided into three components: disk, bulge, and halo. The analysis employs a statistical approach to classify these components based on how the stars move and their energy levels.

This classification allows us to understand the shapes and compositions of the different galaxies in the sample, providing insights into their overall structure.

#Recent Mergers

The key goal is to see how recent merging events affect the galaxy's structure. By identifying the most significant merger each galaxy has gone through, we can look for patterns and correlations between merging history and current properties.

#Observing Merging Events

The process taken involves tracking secondary progenitors-smaller galaxies that merged with the main galaxy. The most substantial of these mergers is labeled as the "peak merger." The study specifically considers the Mass Ratios of these mergers and how they relate to the disk's mass fraction.

#Correlations Between Structures and Merging History

The analysis shows that there are strong correlations between the amount of merging activity and the structure of the galaxy. Disks that have undergone significant merging typically have a higher mass fraction than those with quieter histories, indicating that merging can significantly alter a galaxy's profile.

#The Role of Merging on Stellar Halos

The research emphasizes that stellar haloes provide valuable information about a galaxy's past. It was revealed that, contrary to common beliefs, a galaxy's stellar halo does not necessarily tell the complete story about its merging activity. Many galaxies that look quiet in terms of their halo can have complex histories.

#Merging Histories and Stellar Halo Properties

Observations show variations in stellar halo characteristics, and the study aims to determine if certain halo properties can be traced back to the merging events. Different galaxies show significant diversity in their stellar halo properties.

#Key Observations and Results

1. Disk galaxies are generally less affected by major mergers than spheroids. The observed mass ratios of the most significant recent mergers suggest that disks often engage in more minor merging activities.
2. Despite this, many disk galaxies exhibit high mass fractions, indicating that their recent histories involve substantial merging.
3. The amount of external stars from merging is less correlated to the halo mass than previously thought.

#Active Merging Histories

The distinctive nature of merging histories for disk versus spheroid galaxies suggests that even those with low halo mass can have a vivid history of mergers. For instance, a significant portion of disk galaxies with low-mass stellar haloes experienced noteworthy mergers in their past.

#Merging Outcomes

Mergers often lead to the distribution of stars in the disk but may not always contribute significantly to the stellar halo. The study suggests that many mergers do not result in an increase in halo mass but rather affect the disk and bulge components.

#Connection Between Merging Paths and Disk Growth

Disk galaxies that underwent significant mergers tend to have thicker and more extended structures. The galaxies that experienced co-planar mergers particularly show a tendency for the material to integrate into the disk rather than becoming part of the halo.

#Summary of Findings

The findings suggest that the merging paths of galaxies play a crucial role in shaping their present-day structures. The study notably highlights that high stellar mass ratios indicate an active merging history, but low ratios do not necessarily signify a quiet past, indicating a richer complexity in galaxy formation.

#Implications for Future Research

These results open avenues for future studies to better understand how the structure of galaxies can be interpreted through their stellar haloes. While stellar haloes can provide some insights, growing evidence suggests that analyzing the halo's features or density profiles may yield better information about the merging history of galaxies.

#Conclusion

This research showcases the intricate relationship between the stellar halos of Milky Way-like galaxies and their merging histories. The results point toward a more nuanced view of galaxy evolution, suggesting that even galaxies perceived as quiescent in their merging activity may have complex and varied histories. This challenges the traditional understanding of how we interpret the assembly histories of these galaxies based solely on their stellar halo masses. The study underscores the importance of merging events in shaping the structure and composition of galaxies, paving the way for improved insights into galaxy formation in the universe.