In their study titled "The Panchromatic Hubble Andromeda Treasury XX: The Disk of M31 is Thick," Julianne J. Dalcanton and colleagues present a novel method for determining the thickness of a partially face-on stellar disk by utilizing dust geometry. They investigate the fraction of reddened stars in a moderately-inclined disk galaxy and observe a systematic variation in this fraction across M31. This finding suggests that M31's red giant branch (RGB) stars possess an exponential scale height (h_z) that is approximately 0.14 times its exponential scale length (h_r~5.5kpc), indicating a substantial thickness compared to the Milky Way's thin disk but similar to its thick disk. The presence of dust in a thin midplane typically results in an expected 50% reddening across all regions in moderately-inclined disks. However, as the disk thickness increases, the projection of a wide range of radii onto the line of sight leads to varying numbers of stars on the near and far sides of the dust layer. This deviation from the predicted 50% reddening becomes more pronounced in thicker disks. This research sheds light on the structural properties of galactic disks and highlights the importance of considering dust geometry when assessing stellar populations in galaxies. The absence of a significant thin disk in M31 aligns with its interaction history and elevated disk velocity dispersion. The authors propose using asymmetric reddening as a criterion for identifying systems dominated by a "thick disk" and discuss future possibilities for conducting three-dimensional tomographic mapping of gas and stars within M31.
- Novel method for determining disk thickness using dust geometry
- Systematic variation in fraction of reddened stars across M31
- M31's red giant branch stars have substantial thickness compared to Milky Way
- Deviation from expected 50% reddening in thicker disks due to varying radii projection
- Importance of considering dust geometry when assessing stellar populations in galaxies
- Absence of significant thin disk in M31 due to interaction history and elevated disk velocity dispersion
- Proposal of using asymmetric reddening as a criterion for identifying systems dominated by a "thick disk"
- Future possibilities for conducting three-dimensional tomographic mapping of gas and stars within M31
Summary- Scientists found a new way to measure how thick disks are by looking at dust shapes.
- They noticed that the number of reddish stars changes in M31 galaxy.
- Stars in M31 are thicker compared to our Milky Way galaxy.
- Thicker disks in M31 have more red stars than expected due to different sizes.
- It's important to think about dust shapes when studying stars in galaxies.
Definitions- Novel: Something new or original
- Disk: A flat, circular object like a plate
- Geometry: The shape and size of objects
- Reddened: Becoming reddish in color
- Deviation: Going away from what is expected
Introduction
The structure of galactic disks has been a topic of interest for astronomers for many years. Understanding the properties and dynamics of these disks can provide valuable insights into the formation and evolution of galaxies. In their recent study, Julianne J. Dalcanton and colleagues present a novel method for determining the thickness of a partially face-on stellar disk by utilizing dust geometry. This research sheds light on the structural properties of galactic disks and highlights the importance of considering dust geometry when assessing stellar populations in galaxies.
Background
Galaxies are made up of stars, gas, and dust that are held together by gravity. The majority of stars in galaxies reside in thin, rotating disks that have a flattened shape. These disks play a crucial role in shaping galaxy morphology and driving star formation processes. However, not all galactic disks are created equal - some are thicker than others.
The Milky Way is an example of a galaxy with both thin and thick disk components. The thin disk is characterized by young stars that follow nearly circular orbits around the center, while the thick disk consists mainly of older stars with more eccentric orbits. Previous studies have shown that these two components have different scale heights (the vertical distance from the midplane to where 90% of stars are found) - with the thin disk having a scale height approximately 0.1 times its scale length (the radial distance at which surface brightness decreases by e^-1) while the thick disk has a scale height approximately 0.3 times its scale length.
In contrast to our own galaxy, M31 (also known as Andromeda) does not seem to possess a significant thin disk component despite being similar in size to the Milky Way. This discrepancy raises questions about how M31's structure differs from our own galaxy.
Methodology
To investigate this further, Dalcanton et al utilized data from Hubble Space Telescope observations as part of The Panchromatic Hubble Andromeda Treasury (PHAT) survey. They focused on the red giant branch (RGB) stars in M31, which are bright and easily observable even at large distances.
The team used a novel method to determine the thickness of M31's disk by studying the fraction of reddened stars in a moderately-inclined disk galaxy. This method takes into account the effects of dust geometry on observed colors and can provide an accurate measure of scale height.
Results
Dalcanton et al found that there is a systematic variation in the fraction of reddened stars across M31, with regions closer to the center having a higher fraction compared to those farther away. This finding suggests that M31's RGB stars possess an exponential scale height (h_z) that is approximately 0.14 times its exponential scale length (h_r~5.5kpc), indicating a substantial thickness compared to the Milky Way's thin disk but similar to its thick disk.
Furthermore, they observed that as the disk thickness increases, there is a deviation from the expected 50% reddening due to dust in thin midplanes. This deviation becomes more pronounced in thicker disks, highlighting the importance of considering dust geometry when assessing stellar populations in galaxies.
Implications
The absence of a significant thin disk component in M31 aligns with its interaction history and elevated disk velocity dispersion - both factors that can contribute to thickening of galactic disks. These findings suggest that interactions with other galaxies may play a crucial role in shaping galactic structure.
Moreover, Dalcanton et al propose using asymmetric reddening as a criterion for identifying systems dominated by a "thick disk." This could be applied not only to other nearby galaxies like M31 but also distant ones where detailed observations are not possible.
Future Possibilities
This research opens up possibilities for conducting three-dimensional tomographic mapping of gas and stars within M31 using this new method. By combining data from different wavelengths, it may be possible to create a detailed picture of the distribution and properties of dust and stars in M31's disk.
Conclusion
In their study, Dalcanton et al present a novel method for determining the thickness of galactic disks by considering dust geometry. Their findings shed light on the structural properties of M31's disk and highlight the importance of interactions in shaping galaxy structure. This research has implications not only for understanding our neighboring galaxy but also for identifying thick disk-dominated systems in distant galaxies. With future advancements in technology and observations, we may gain even more insights into the complex structures of galactic disks.
