Disentangling the inner galaxy

Abstract

One of the principal targets in the observational study of galactic evolution is the Milky Way,
which represents the optimal subject for our capacity of resolving individual stellar proper-
ties and thus inferring its history. Despite this focus, the central regions of our Galaxy,
characterized by a bulge and a bar, remain a mystery. The confluence of diverse stellar
populations, such as metal-rich stars from the thin disc and metal-poor stars from the thick
disc and halo, provides a complex environment that makes the deciphering of the formation
mechanisms challenging. One key aspect is whether the bulge of spiral galaxies represents an
independent feature, or if is merely the result of the addition of the discs and halo populations.
This thesis seeks to bridge the gap in our current understanding of the Milky Way bulge and bar by performing a metallicity distribution analysis of several stellar samples. In or- der to do so, we take a multi-faceted approach, incorporating observational data from the APOGEE catalog and machine learning models. We then identify two hidden structures in the central regions (1 kpc < RGAL < 3.5 kpc) of the Galaxy. One of them is a population that might be associated with the bar, located at [Fe/H] = 0.095 ± 0.005, while the other, at [Fe/H] = -0.70 ± 0.05, is only present in the bulge regions and could represent a chemical distinction from the thick and thin discs and the halo. The distinct chemical composition of these two components not only differentiates them from the rest of the structures, but also prompts speculation about the processes that may have led to their formation over the course of the Milky Way’s evolutionary history. These chemical signatures suggests that the components may have arisen from specific conditions prevalent at different epochs.

Popular Abstract

Since the beginning of time, people have looked at the stars and wondered about the mysteries of space. Over time, we have learned how to build better tools to observe the sky, ranging from small telescopes to the largest ones we now have on Earth and even in space. Stars, like us, undergo a life cycle, marked by numerous parameters. Gas, a fundamental constituent of galaxies, also undergoes cycles intimately tied to the stars, affecting the whole galaxy. On this note we have come to relate the stars chemical composition to past events and thus, how the galaxy was when they were born. This framework is called galactic archaeology, and it allows us to trace the history of the Milky Way. We can differentiate multiple structures within our Galaxy. At the very centre lies a densely packed bulge accompanied by a central bar. Beyond this, we encounter a dispersed structure called the thick disk, populated with mainly old stars and longer and thinner one called the thin disc, hosting generally younger stars, including our Solar System. Surrounding the entire Galaxy is the halo, mostly composed of dark matter and few older stars. In the past, some studies have detected a change between the chemical composition of the thick and thin disc stars, but we don’t know if this difference can also be detected in the bulge region. In this thesis we will try to tackle this by searching for some signature that distinguishes these central regions from the rest of the Galaxy. The way to do this is to perform a thorough analysis of stellar data from the APOGEE catalogue, which comes from two telescopes operating in Las Campanas (Chile) and New Mexico (USA). Our data will be analysed by using different fitting techniques for the iron distribution of four samples at different distances from the center of our Galaxy. One is directly comparing their profiles considering that in the same regions of the galaxy we expect the same contribution from the thick and thin discs. The other is by employing a machine learning algorithm, where we try to get the different populations in these central regions (i.e. stars that belong to the two discs and possibly the bar and the bulge). The halo stars are not considered because their number is very small. This algorithm, based in two steps, is very useful in situations where you have incomplete data or hidden parameters. In the first step it estimates the missing parameters, and in the second, using the values from the first one, it calculates the probability of observing our data. The algorithm learns by repeating these two steps until the probability do not change significantly. Once we have carried out our search, we find four different populations. The first two, present in the four samples, are related to the thick and thin disc, while the other two are exclusive to the central regions. One of them is associated with the bar and the other to the bulge. The bar seems to change the thick disc metallicity when compared to the samples that do not show this bar population, meaning that its formation and presence has, most likely, an effect in this disc. On another note, the bulge population might be from an old bulge, formed in the early stages of the Galaxy, indicating a complex forming scenario. The report of these two components seems to be in line with other studies and complements the current knowledge of how the Milky Way became what it is today.

• author: Ruiz Del Pozo, Andrea
• supervisor: Thomas Bensby
• organization: Astrophysics; Department of Physics
• course: ASTM32 20241
• year: 2024
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Astronomy, Galactic Archaeology, Milky Way, APOGEE
• report number: 2024-EXA228
• language: English
• id: 9165321
• date added to LUP: 2024-11-06 13:35:36
• date last changed: 2024-11-06 13:35:36