Faint Streams Hidden in Plain Sight: What the Mass–Metallicity Relation Tells Us About Tidal Disruption

Astronomers have long used the mass–metallicity relation to understand galaxies. This relation links the stellar mass of a galaxy (how many stars it has) with its metallicity (the amount of heavy elements like iron in those stars). Observations show that this relation is tight, with very little scatter, even among the tiny satellite galaxies of the Milky Way and Andromeda. Traditionally, researchers argued that such small scatter means these satellites must be largely untouched, since mass loss from tidal forces would cause them to shift away from the relation. But in this new paper, Alexander Riley and collaborators test this assumption using state-of-the-art simulations.

The Auriga Simulations

The authors use the Auriga project, a collection of high-resolution computer simulations of galaxies similar in mass to the Milky Way. These simulations carefully follow the formation and evolution of stars, gas, and dark matter, making it possible to study the satellites orbiting around Milky Way–like galaxies. Riley and colleagues focus on six of these simulated systems, examining both the stars that remain bound to satellite galaxies today and those stripped away by tidal forces.

Intrinsic vs. Tidally Evolved Relations

The team compares two versions of the mass–metallicity relation. The first, the intrinsic relation, uses the total stellar population a satellite ever formed. The second, the tidally evolved relation, only counts the stars still gravitationally bound to the satellite today. The difference is striking, as satellites lose stars, their remaining bound population becomes both less massive and slightly more metal-rich, because the stars stripped away tend to be the less enriched ones on the outskirts. Surprisingly, though, the scatter in the evolved relation remains small, like what is observed in the Milky Way and Andromeda.

Connecting to Real Satellites

With this framework, the authors turn to actual observations. By comparing the positions of real Local Group satellites in the mass–metallicity plane with their simulated counterparts, they classify galaxies by how disrupted they likely are. For example, familiar systems like Sagittarius and Crater II are predicted to have lost most of their original mass, while others such as Fornax and several Andromeda satellites show signs of significant but less extreme disruption. This method even suggests some satellites that appear intact today may in fact be missing large fractions of their stars.

Hidden Streams Awaiting Discovery

One implication of this work is that tidal disruption is more common than previously thought. Many of the predicted tidal streams are likely too faint to have been detected yet with current telescopes. However, upcoming surveys with instruments such as the Rubin Observatory’s LSST, Euclid, and the Roman Space Telescope will have the sensitivity to uncover these stellar remnants, dramatically increasing the number of known streams in the Local Group.

Conclusion

Riley and collaborators show that the small scatter in the mass–metallicity relation does not rule out heavy tidal disruption. Instead, the relation itself carries a record of how much mass satellites have lost. The Local Group may be littered with faint tidal features awaiting discovery, and this work provides a roadmap for identifying which galaxies to watch most closely when the next generation of sky surveys begins.

Source: Riley