Epicyclic density variations in the Indus stellar stream

Longitudinal density fluctuations observed in stellar streams can result from gravitational interactions with massive perturbers in the Milky Way, such as dark matter subhaloes. Analysing these density variations provides a powerful probe of properties (motion, mass, size, etc.) of the perturbing objects. However, caution is needed because density variations may arise naturally from internal dynamics of streams, namely epicycles. In this work, we focus on the Indus stellar stream, a remnant of an ancient dwarf satellite of the Galaxy. An Indus stream spanning ∼ 90^◦ is revealed in the southern Galactic sky using a comprehensive matched-filter analysis utilizing data from the Gaia mission. A spatial density model is fitted to the filtered map to quantitatively characterize the morphology, which demonstrates episodic density peaks and gaps in the stream. Through N-body simulations, we show that there are strong epicyclic motions of stars happening during tidal disruptions. The present-day longitudinal densities from simulations are comparable to the measurement from data, with similar numbers and locations of peaks and gaps, suggesting that the observed density should mainly be caused by epicycles. We also find that a cuspy dark matter halo for the Indus dwarf is likely to produce milder stellar epicyclic peaks compared to a cored halo, which results in steeper peaks. This arises from different instantaneous mass-loss due to distinct central mass distributions of haloes, where a cored halo usually leads to severer tidal stripping. The observed density exhibits moderate peak sharpness, implying that Indus may have originally possessed a cuspy halo.