Auriga Streams III: the mass-metallicity relation does not rule out tidal mass loss in Local Group satellites

TL;DR

The paper addresses whether the small scatter in the Local Group mass-metallicity relation (MZR) excludes tidal mass loss in satellites. Using Auriga cosmological zoom-in simulations, it distinguishes intrinsic (total) and tidally evolved (bound progenitor) MZRs for satellites with $M_\star \gtrsim 5\times10^5\,M_\odot$, and analyzes how disruption, via $f_\text{bound}$, reshapes metallicity while preserving the overall relation. It finds $\sigma_{\rm intr} \approx 0.14$ dex for the intrinsic MZR and $\sigma_{\rm evol} \approx 0.27$ dex for the tidally evolved one, with the evolved scatter and offset arising from mass loss and negative metallicity gradients; many satellites on the intrinsic relation can nonetheless lose substantial mass, and some on the evolved relation still reflect their total stellar content. Applying this framework to Local Group satellites, the authors derive an evolved MZR fit with $[Fe/H] = -1.80 + 0.35\log_{10}(M_\star/10^6\,M_\odot)$ and show that offsets correlate with disruption level, enabling predictions of tidal tails and a disruption classification (Classes 1–5). The work demonstrates that a small observed scatter does not rule out tidal disruption and provides a practical method to identify disrupting satellites and forecast faint streams for upcoming Rubin, Euclid, and Roman surveys, significantly impacting our interpretation of dwarf galaxy evolution and the baryon cycle in the Local Group.

Abstract

The mass-metallicity relation is a fundamental galaxy scaling law that has been extended to the faintest systems in the Local Group. We show that the small scatter in this relation, which has been used to argue against tidal mass loss in Local Group satellites, is consistent with the level of disruption in the Auriga simulations. For every accreted system in Auriga, we compute stellar masses and metallicities two ways: considering the total system (bound + lost material) and only considering the progenitor. Accreted systems in Auriga have a tight relation between total stellar mass and metallicity, with scatter at a fixed stellar mass driven by age. When only considering the progenitor, the tidally evolved mass-metallicity relation has similar scatter ($\sim$0.27 dex) as observed for the Local Group satellites ($\sim$0.23 dex). Satellites that lie above the relation have experienced substantial mass loss and typically have low metallicity for their total stellar mass. Even satellites that fall exactly on the evolved relation can lose over half of their stellar mass. Only satellites substantially below the evolved relation are reliably intact. Based on their offset from the observed relation, we predict which Milky Way and M31 satellites have tidal tails waiting to be discovered.

Figures (5)

• Figure 1: The intrinsic mass-metallicity relation for accreted systems in Auriga. Squares indicate systems that have been fully disrupted ($f_\text{bound} = 0$) while crosses indicate systems that still have a bound progenitor at the present day ($f_\text{bound} > 0$). The scatter at fixed total stellar mass is driven by age (colour of points), parametrised here as $t_{90}$.
• Figure 2: The intrinsic and tidally evolved mass-metallicity relations for Auriga satellites that have $f_\text{bound} > 0$. Individual systems are shown both considering bound progenitors (circles) and the total system (crosses). We provide linear fits to both bound (solid red) and total (dashed black) cases, along with the vertical scatter about these relations. We only include systems that have 10 or more bound star particles at the present day.
• Figure 3: The mass-metallicity relation for observed satellites of the Milky Way (blue points) and M31 (orange points). We provide a linear fit to these satellites (solid) and the relation from Kirby:2013 (dashed). We also highlight individual satellites known or believed to be disrupting and their reconstructed 'total' values (star symbols, see text for details).
• Figure 4: Top panel: $f_\text{bound}$ vs. scaled offset from the evolved mass-metallicity relation for Auriga satellites. The dashed lines are the boundaries between the five broad categories of mass loss detailed in Section \ref{['sec:local-group-sats']} and Appendix \ref{['app:individual-sats']}. Bottom panel: histogram of scaled offsets for both Auriga and Milky Way and M31 satellites from their respective mass-metallicity relations.
• Figure 5: Similar to Figure \ref{['fig:mass-feh-fbound']} but excluding the effect of radial metallicity gradients.