The Sagittarius Dwarf Galaxy (Sgr), a small satellite of the Milky Way, holds valuable clues to the early history of galaxies. Xiaowei Ou and collaborators sought to unravel its secrets by analyzing the chemical makeup of 37 stars within its core and tidal streams. Their findings provide new insights into how galaxies form and evolve, focusing on the processes that enriched early stars with heavy elements.

Introduction to the Sgr Galaxy and Its Stars

Sgr is a nearby dwarf galaxy currently being torn apart by the Milky Way's gravitational forces. Its stars are scattered between a dense core and long streams that wind around our galaxy. By studying the chemical elements in these stars, especially the ancient, metal-poor ones, researchers can reconstruct the galaxy’s history and understand the processes that created heavy elements like gold and uranium.

Metal-poor stars are stars with very little heavy elements, as they formed when the universe was young and lacked the building blocks produced by later generations of stars. These stars act as cosmic fossils, preserving details about the early universe.

Selecting Stars for Study

To find suitable stars, the team combined data from SkyMapper and Gaia, two observatories that measure star positions, movements, and colors. Using special techniques to estimate the metallicity (amount of heavy elements) of stars, they identified 37 promising targets: 10 extremely metal-poor (EMP) stars, 25 very metal-poor (VMP) stars, and 2 moderately metal-poor stars. This study expanded the number of known EMP stars in Sgr by fivefold.

High-resolution spectroscopy was used to study the stars’ chemical fingerprints, allowing the team to identify the abundances of 20 different elements, from light ones like carbon to heavy ones formed in neutron star mergers.

A Galaxy Shaped by the r-process

One of the most intriguing findings was the high abundance of r-process elements in many of the stars. The r-process (short for "rapid neutron-capture process") is a mechanism that produces heavy elements during rare events like neutron star mergers or certain types of supernovae. Over half of the stars studied showed an element pattern matching the r-process, suggesting that early in its history, Sgr experienced one or more major enrichment events that spread these elements across the galaxy.

Comparing the Core and Streams

Interestingly, the stars in Sgr’s core and tidal streams shared similar chemical patterns, indicating that both regions likely experienced similar enrichment histories. This suggests that the tidal stripping of Sgr didn’t introduce major chemical differences between its stars.

Unique Stars and Their Stories

One star stood out with unusually low amounts of certain elements, hinting that it might have formed in a smaller galaxy that was later absorbed into Sgr. Such findings highlight how galaxies grow by merging with smaller systems, adding to the complexity of their chemical evolution.

The Bigger Picture

The study adds Sgr to a small but growing list of galaxies known to have experienced r-process enrichment. It also demonstrates the importance of studying dwarf galaxies to understand how the universe built up heavy elements over time.

Source: Ou