Caught in the Act: Dissecting the Sagittarius Dwarf Galaxy’s Heart with Gaia
The Sagittarius dwarf spheroidal galaxy (Sgr dSph) is one of the best local examples of galactic cannibalism, a process where larger galaxies like the Milky Way tear apart smaller companions over billions of years. In this study, Toguchi-Tani and collaborators use new data from the Gaia DR3 and APOGEE DR17 sky surveys to investigate the central core of Sgr dSph and its neighboring globular cluster, Messier 54 (M54), shedding light on whether these two celestial structures share a common origin or were merely brought together by chance.
Identifying Members: Separating Signal from Noise
The team begins by selecting stars likely to belong to the Sgr core and M54 based on their positions, motions across the sky, and distance from Earth. Using Gaia DR3 data, they identify over 4 million stars near the expected location of Sgr, then narrow this down using proper motions (how stars move across the sky) and parallax (a measure of distance). To clean out stars from the Milky Way that lie in front of or behind Sgr, they apply several quality filters and isolate stars showing similar movement patterns to those expected from the Sgr system. This careful filtering leaves them with a high-confidence sample of about 515,000 stars in the Sgr core and over 4,500 in M54.
Stellar Evolution: Mapping Star Life Cycles
To understand the kinds of stars they found, the researchers group them by evolutionary phase, how far along they are in their lifetimes, using color-magnitude diagrams (CMDs), which plot a star’s brightness against its color. Different regions of these diagrams correspond to young stars, red giants, and other phases of stellar evolution. By comparing these CMDs with known patterns, they can map out the detailed structure of both Sgr and M54 and spot any differences in how their stars are distributed.
Measuring Distance: Red Clump Stars as Standard Candles
Next, they use a special kind of star called the red clump, a reliable “standard candle” because its brightness is well understood, to measure the distances to the Sgr core and M54. By comparing the stars' apparent brightness in near-infrared light (less affected by dust) to their known intrinsic brightness, they estimate both structures to be about 25,000 light-years away. This small difference in distance, just about 100 light-years, is well within their margin of error, suggesting that Sgr and M54 likely lie in the same region of space.
Chemical Clues: Are Sgr and M54 Truly Connected?
Despite this spatial closeness, the chemical makeup of stars in the two groups hints at separate origins. Metallicity, how many elements heavier than helium are present in a star, is often used to trace a star’s birthplace. The Sgr core hosts stars with a wide range of metallicities, implying a long history of star formation possibly triggered by three close passages around the Milky Way. In contrast, M54’s stars show a narrower metallicity range, suggesting fewer star-forming events and pointing to a different history.
Motion and Structure: Tracking Galactic Flow
Finally, the team analyzes the motion of stars across the sky to study how the two systems move. Using a visualization technique called line-integral convolution, they show that both groups of stars are being pulled in a similar direction, toward the Sgr tidal stream that stretches across the sky as the galaxy is torn apart. However, subtle differences in how the stars are distributed suggest that M54 might be a later addition to Sgr, possibly captured during one of its galactic flybys.
Conclusion: Coexisting, But Not Coevolved
In summary, this paper presents the largest and cleanest set of stars associated with the Sgr core and M54 to date. While their distances and motions suggest they currently occupy the same space and are being pulled apart together, their differing stellar populations and chemical signatures point to a more complex past, one where M54 may have been a separate globular cluster that was drawn into Sgr during its dramatic collision with the Milky Way.
Source: Toguchi-Tani
