Subhalo Spreading of Thin Tidal Star Streams

Abstract

Dark matter subhalos that pass a thin tidal star stream change the velocities of the stars near the point of closest encounter. Subsequent orbital evolution increases the stream width and spreads the changes along the stream. We measure the average widths and velocity dispersions of stream populations in three Milky Way–like cold dark matter cosmology simulations containing star particle globular clusters and galactic disks of 0, 1, and 2 times a baseline model. Power-law fits to the velocity dispersion with stream longitude, ϕ, for the overlaid streams in the 10–60 kpc range find σ ≃ 5–15 ϕ 0.2–0.5 km s−1, with the coefficients depending on the subhalo numbers, as well as the stream measurement details. The velocity distributions summed along the entire length of the streams do not require the progenitor location. They also rise with subhalo number and are significantly non-Gaussian, with the ratios of the 6σ to the 3σ clipped velocity dispersions being ∼1.5 ± 0.3 and ∼2.5 ± 1 for measurements within 1° and 5° of the streams, respectively. Streams longer than 50° have an average radial velocity dispersion of 2.1 km s−1 with a disk and 4.2 km s−1 without a disk. However, a few similar thin, low-velocity dispersion streams are present in all three simulations. Statistically reliable conclusions require velocity data extending several degrees from the stream centerline for a set of streams.

Stellar Escape from Globular Clusters. II. Clusters May Eat Their Own Tails

“…Notably, we found in Section 5.6 that the opposing flow of the outgoing and returning tails boosts the stream's velocity dispersion by ≈3 km s −1 -substantial since the stream's dispersion without accounting for the returning tails is only ≈1 km s −1 . This boost is similar in magnitude to that from perturbations by MW substructure, such as DM subhalos or the MW disk (e.g., Carlberg 2009;Carlberg & Agler 2023). It is also much greater than the likely inflation of σ by unresolved binaries in streams.…”

“…Yet, capitalizing on the prospect of DM inference with stellar streams has proven challenging since many phenomena simultaneously and degenerately affect stream morphology and kinematics. Stream gaps, overdensities, and fanning naturally arise via the epicyclic trajectories of slow GC escapers even in static MW potentials without any clumpy substructure (e.g., Capuzzo Dolcetta et al 2005;Küpper et al 2008), and also from perturbations by many baryonic substructures, including giant molecular clouds (Amorisco et al 2016), the MW spiral arms or disk (Banik & Bovy 2019;Carlberg & Agler 2023), infalling MW satellites (Garavito-Camargo et al 2019), and other GCs (Doke & Hattori 2022). Further complicating matters, MW substructures, such as a rotating bar (Hattori et al 2016;Pearson et al 2017) or even modified gravity (Thomas et al 2018;Kroupa et al 2022), can induce stream asymmetry and observational artifacts can cause false gaps/overdensities (Ibata et al 2020).…”

Stream Fanning and Bifurcations: Observable Signatures of Resonances in Stellar Stream Morphology

“…In the large-scale regime, this calculation is greatly dependent on the uncertain nature of the dark matter substructure in the halo of the MW. Carlberg & Agler (2023) predicted that the dominant contribution to stream heating is caused by the largest CDM subhalos; they found that a few encounters with the largest CDM subhalos led to heating of ∼1 km s −1 over 10 Gyr. Given this, we find it safe to assume that Δ < 1 and quite possible that Δ = 1 for streams on halo orbits near the resonances studied in this work.…”

“…Finally, repeated interactions with smaller-scale dark matter substructure can also cause streams to become more diffuse. In CDM, repeated encounters with low-mass subhalos are not expected to greatly affect stream morphology (Yoon et al 2011;Carlberg & Agler 2023), but in other models of dark matter, such as fuzzy dark matter, these kinds of interactions can impact stream morphology to a greater degree (Hui et al 2017;Dalal et al 2021). One may therefore wonder whether the stream fanning discussed above may be confused with such an effect.…”