No Stream Left Unscathed: The Imprint of a Host Galaxy

Stellar streams from disrupted globular clusters are excellent probes of dark matter (DM) subhalos. Observed Milky Way streams display a remarkable diversity of features: spurs, gaps, kinks, cocoons, and density variations, many attributed to subhalo encounters. But how much of this diversity arises from the host itself? We simulate ∼15,000 globular cluster streams across four Milky Way–mass halos from the FIRE-2 cosmological simulations, evolved in basis-function expansion potentials capturing the evolving disk, halo, and large-scale structure while excluding small-scale perturbers such as DM subhalos and giant molecular clouds. We find that roughly three-quarters of streams develop complex features from the host potential, such as spurs, kinks, and cocoon-like envelopes. Even the smoothest streams exhibit 10%–25% width variation along their track and host overdensities and gaps at scales of ∼2°, squarely in the 1°–5° range predicted for subhalo-induced gaps. Pericentric distance is the primary predictor of stream morphology, with ∼15 kpc separating smooth from disturbed streams and circular orbits beyond ∼20 kpc producing the smoothest streams. Only ∼70 out of ∼15,000 streams are free of detectable wiggles in the track at any scale. Analogs to observed features, such as the GD-1 spur and the ATLAS–Aliqa Uma kink, emerge even without the presence of subhalos. As next-generation surveys (LSST, Euclid, and the Nancy Grace Roman Space Telescope) resolve stream structure across hundreds of streams, the baseline established here, i.e., streams evolved without small-scale perturbers, becomes essential for extracting DM substructure constraints.