Chapter 1: Introduction to Nyx

Recent discoveries have unveiled Nyx, a cluster of approximately 250 stars that are believed to have originated outside our galaxy. Astrophysicist Lina Necib discusses the implications of this finding.

The Milky Way is home to billions of stars, yet not all are native to this galaxy. Astronomers have identified a unique stellar stream within our galaxy that formed externally. The Nyx cluster exhibits velocities indicating its origins lie beyond the Milky Way. It is thought to be remnants of a dwarf galaxy that merged with our own in the distant past. As Nyx approached the Milky Way, gravitational forces stretched the star cluster, akin to pulling taffy.

The GAIA spacecraft has been instrumental in charting a billion stars both within and outside the Milky Way. Image credit: ESA.

On July 28, Dr. Lina Necib, the astrophysicist behind this groundbreaking discovery, will discuss her findings on Astronomy News with The Cosmic Companion.

Chapter 2: The Role of GAIA

Using FIRE (Feedback In Realistic Environments) simulations, astronomers can model stellar groups, shedding light on their origins. These simulations, among the largest of their kind, illustrate how galaxies formed post-Big Bang. Even with advanced supercomputers, these nine simulations required months to finalize.

The GAIA spacecraft, launched in 2013, aims to create 3D maps of a billion stars in and beyond our galaxy.

“It represents the most extensive kinematic study to date. GAIA provides motion data for one billion stars, with seven million having 3D velocities. This allows us to pinpoint a star’s location and trajectory, marking a significant advancement in our understanding of the Milky Way’s structure,” explains Lina Necib, a postdoctoral researcher in theoretical physics at Caltech.

FIRE simulations have produced virtual galaxies for analysis. Image credit: Northwestern University.

By merging the insights from GAIA and FIRE, Necib and her team utilized deep learning techniques to model these stars. They discovered that the Milky Way may have absorbed more small galaxies than previously thought, contrary to the belief that it had a relatively quiet merger history.

“Galaxies grow by assimilating others. We had assumed the Milky Way’s merger history was uneventful, but the volume of smaller structures we now observe indicates a more tumultuous past,” Necib states.

The video titled NGC 4945: The Milky Way's Not-So-Distant Cousin explores similar themes, detailing how galaxies interact and merge over time.

Chapter 3: The Gaia Mock Catalogs

Analyzing data from a billion stars is a formidable task without advanced computational assistance.

“We can’t manually track the activities of seven million stars. Our approach involved utilizing the Gaia mock catalogs,” Necib explains.

Developed by Robyn Sanderson at the University of Pennsylvania, the Gaia mock catalogs simulate what astronomers would observe if the FIRE simulations were accurate and seen through GAIA’s lens.

Dr. Lina Necib, the leading astrophysicist on this discovery, will appear on The Cosmic Companion on July 28.

To ensure accurate interpretation of the simulation data, the team adjusted their algorithms to focus on key elements. They tested their findings against known features of the Milky Way, including the Gaia Sausage—a distinctive star group from a dwarf galaxy that merged with the Milky Way billions of years ago.

By examining baryons, a class of subatomic particles, the Ananke framework from Northwestern University models stellar behavior effectively.

“The Ananke framework generates realistic synthetic stellar surveys based on cosmological baryonic simulations, preserving vital observational relationships among gas, dust, stellar populations, and dark matter,” the Northwestern researchers report.

The model identified a group of 250 stars moving toward the Milky Way's center while orbiting the galaxy.

“The Nyx stars were discovered in two phases. Initially, a machine learning algorithm developed by my collaborator Bryan Ostdiek distinguished between accreted stars and native disk stars. Following this, I applied a clustering algorithm to identify these specific stars based on their unique motion, which indicates a merger occurring parallel to the disk,” Necib shares with The Cosmic Companion.

When Dr. Necib first encountered the data, she suspected an error and withheld her findings for three weeks. As the reality of her discovery settled in, she shared the information with her peers.

Upon reviewing earlier studies, she realized no one had previously documented this stellar group, granting her the opportunity to name it. She chose the name Nyx, after the Greek goddess of night.

Chapter 4: The Future of Research

Although FIRE simulations provide valuable insights, they are not tailored specifically for the Milky Way. The model was enhanced using lessons learned from particle physics experiments at the Large Hadron Collider (LHC) in Switzerland.

“At the LHC, we have top-notch simulations, yet we are cautious about machines learning from them, potentially mistaking simulation for real physics. Similarly, while FIRE simulations offer excellent data, they aren’t the Milky Way. We had to discern how to extract useful information to apply to our galaxy,” explains physicist Bryan Ostdiek from Harvard University.

This synthetic survey image was created by Professor Robyn Sanderson, showcasing how GAIA would observe starlight from simulated galaxies.

The team developed techniques to track stars within the simulations, categorizing them as being born within or outside the galaxy under study. This approach facilitated training of the deep learning model, which was then applied to other FIRE galaxies.

“Accreted stars near the sun constitute merely 1–2% of the total, making them elusive without specialized techniques, hence the usefulness of machine learning in this project,” Necib explains.

Looking ahead, Necib intends to continue her investigation into extragalactic stars using GAIA's ongoing data collection and by utilizing some of the world’s most advanced telescopes.

“I eagerly await the third data release from GAIA, expected next year. Additionally, my collaborator Alexander Ji and I have two approved observing proposals at Keck in Hawaii and Magellan in Chile to analyze the chemical compositions of Nyx stars to validate their origins,” Necib elaborates.

This research was published in the journal Nature Astronomy. Findings like these deepen our understanding of the galaxy we inhabit.

James Maynard, founder and publisher of The Cosmic Companion, shares his passion for astronomy from his home in Tucson, Arizona, alongside his wife Nicole and their cat, Max.

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