Examining Thick Disks in Disk Galaxies

Thick Disks are an important part of many disk Galaxies, including our Milky Way. They can account for a big portion of a galaxy's overall Mass, with estimates ranging from a small fraction to as much as 100%. This variation depends on the galaxy in question and how scientists measure thick disks.

In this study, we use a sophisticated simulation called NewHorizon to look closely at thick disks. This simulation offers high precision in both space and the mass of stars, allowing us to investigate how much of a galaxy's disk is made up of the thick disk. We also use a second simulation called NewHorizon2, which is based on the same initial setup but tracks nine different Chemical Elements, giving us even more information about the stars.

From our analysis, we found that in 27 massive disk galaxies, the thick disk contributes roughly a significant part of the total light and mass of the galaxy. The details show a range of behaviors depending on the galaxy type. For instance, some galaxies are best described with only one type of disk component, while others need two or three different components to accurately depict their structure.

One of the exciting findings from the NewHorizon2 data is that using a simple method to identify thick disk stars can lead to errors, as it often includes stars that belong to other categories. To get a clearer view of thick disks, we need to measure their properties carefully. Observations show that most stars form near the galaxy's center and gradually spread out over time due to various forces at play.

Different galaxies create thick disks in unique ways. Some have a significant proportion of their stars tightly packed, while others develop looser structures over time, depending on their history, including how they formed and whether they merged with other galaxies.

While most big galaxies have a spiral shape, their thin disks are often more noteworthy than their spiral arms. The Milky Way, for instance, has a thin disk that's only about 300 parsecs tall, but its radius is much greater. This thin structure is thought to result from how galaxies gather angular momentum during their formation.

In our exploration, we found that the Milky Way and other galaxies show a thicker component within their disks. Early studies suggested that the mass of the thick disk was very low, around a few percent, and that its scale height was several times that of the thin disk. However, later research indicated that thick disks might be a more prominent feature than previously thought.

More recent work suggests that regions within the galaxy have a more complex structure. Some stars appear to belong to a thick disk based on their movements and chemical compositions. Observations indicate that within certain distances from the center of the galaxy, a significant part of the stars could be classified as part of a thick disk.

To address this topic, we aim to answer key questions about thick disks: How significant are they? Do different measurement methods yield different results regarding their importance? What processes help form these structures? And how do thick disks in other galaxies compare?

To provide clarity, we rely heavily on the NewHorizon simulation, while also considering the additional details from NewHorizon2 when looking at chemical properties. The combination of high resolution and chemical tracking gives us the chance to better understand thick disks.

The primary data we examine comes from NewHorizon, a simulation that models how galaxies form. It can resolve dense regions very finely, down to 34 parsecs. This capability allows us to see the structure of thick disks clearly, as the simulation represents galaxies with millions of star particles. Additionally, the simulation captures changes over time with great detail, allowing us to monitor how star movements evolve.

In our analysis, we focus on massive galaxies to ensure that we are studying structures that are well-formed and widely recognized in current research. We end up with a sample of 27 disk galaxies for deeper investigation.

#How We Measure Thick Disks

To analyze the structural properties of these galaxies, we begin by taking measurements at specific locations. We identify the effective radius where most stellar mass is located and measure how thick the disks are vertically at varying distances from the galaxy mid-plane.

We also compare these measurements to available observational data to confirm accuracy. By using statistical methods to fit profiles of the stars, we can separate them into thin and thick disk categories. We have found that the properties of the thick disks align reasonably well with what we see in actual galaxies.

Interestingly, our results indicate that thick disks can play a significant role in how the total mass of galaxies is distributed. In general terms, thick disks often make up about a third of a galaxy's total light and can contribute even more to overall mass.

We also look at how spatial distribution plays a role in understanding the properties of thick and thin disk stars. We observe that most stars form close to the center of the galaxy and tend to be grouped together within the plane of the disk.

#Investigating Chemical Elements

The chemical properties of stars are another important factor in analyzing thick disks. Chemical information can provide insights into a star's formation history and help us understand their origins. For instance, a range of chemical compositions might indicate different star formation events happening at various times in the galaxy's history.

As we analyze the chemical elements present in our sample galaxies, we start to see clear patterns. For example, stars classified as chemically thick often occupy a thicker region of the disk than those identified as chemically thin. The chemical abundances can be related to their ages and star formation histories, giving hints at how and when different populations of stars formed.

#The Role of Kinematics

Kinematic data, which details the movements of stars, is very useful in this research. Stars in thick disks often show a more varied motion, reflecting their complex formation histories. In other words, stars that are part of a thick disk generally move differently compared to those in a thin disk.

By applying advanced data analysis methods, such as Gaussian Mixture Models, we identify various star groups within the galaxies. This approach allows us to sort stars based on their energy and angular momentum, revealing different kinematic components. This helps us understand not just how stars are distributed, but also how they interact with one another and evolve over time.

#Comparison to Observational Data

Overall, our findings suggest the thick disks in our sample of galaxies are prominent features. The roles they play vary depending on the galaxy's history and structure. Measurements show that the mass contributions of thick disks are significant and align well with observational data from edge-on galaxies and our Milky Way.

The existence of thick disks is not unexpected. It appears that star formation in galaxies predominantly occurs in thin disks filled with gas, and as time goes by, these stars are influenced by surrounding forces, causing them to move in ways that eventually lead to a thicker distribution in the disk.

#Conclusion

In summary, thick disks are a key feature of many disk galaxies, and through careful observation and analysis, we can better understand their significance. The use of high-resolution simulations like NewHorizon, combined with detailed chemical and kinematic data, gives us valuable insights into the formation and evolution of these structures.

Our research not only highlights the complexity of thick disks but also sheds light on how galaxies as a whole evolve over time. Different galaxies might have unique histories and formation patterns, which ultimately influence the characteristics of their thick disks.

As we continue to gather more data and refine our methods, we hope to uncover even more about these intriguing components of galaxies and what they reveal about the universe's history overall.