A sharper view of Pal 5's tails: Discovery of stream perturbations with a novel non-parametric technique (1609.01282v2)

Abstract: Only in the Milky Way is it possible to conduct an experiment which uses stellar streams to detect low-mass dark matter subhaloes. In smooth and static host potentials, tidal tails of disrupting satellites appear highly symmetric. However, perturbations from dark subhaloes, as well as from GMCs and the Milky Way bar, can induce density fluctuations that destroy this symmetry. Motivated by the recent release of unprecedentedly deep and wide imaging data around the Pal~5 stellar stream, we develop a new probabilistic, adaptive and non-parametric technique which allows us to bring the cluster's tidal tails into clear focus. Strikingly, we uncover a stream whose density exhibits visible changes on a variety of angular scales. We detect significant bumps and dips, both narrow and broad: two peaks on either side of the progenitor, each only a fraction of a degree across, and two gaps, $\sim2^{\circ}$ and $\sim9^{\circ}$ wide, the latter accompanied by a gargantuan lump of debris. This largest density feature results in a pronounced inter-tail asymmetry which cannot be made consistent with an unperturbed stream according to a suite of simulations we have produced. We conjecture that the sharp peaks around Pal 5 are epicyclic overdensities, while the two dips are consistent with impacts by subhaloes. Assuming an age of 3.4 Gyr for Pal 5, these two gaps would correspond to the characteristic size of gaps created by subhaloes in the mass range of $10^6-10^7 M_\odot$ and $10^7-10^8 M_\odot$ respectively. In addition to dark substructure, we find that the bar of the Milky Way can plausibly produce the asymmetric density seen in Pal 5 and that GMCs could cause the smaller gap.

• The paper uses a novel non-parametric technique with adaptive cubic splines to precisely characterize density variations and structural properties of the Pal 5 stellar stream.
• Observed density fluctuations in the stream's tails are consistent with interactions from dark matter subhaloes in the $10^6$ to $10^8 M_\odot$ mass range.
• The study also suggests that the Milky Way's rotating bar can significantly influence stream perturbations, potentially causing observed asymmetries.

Overview of "A Sharper View of Pal 5's Tails: Discovery of Stream Perturbations with a Novel Non-Parametric Technique"

This paper presents a detailed analysis of the Palomar 5 (Pal~5) stellar stream using new, high-quality photometric data. The authors focus on identifying density variations in the stream, which may provide insights into the nature of dark matter substructure in the Milky Way. Their approach employs a novel non-parametric technique that allows for flexible modeling of the stream's properties, such as width, centroid track, and density. This methodology leverages adaptive cubic splines, providing remarkable precision in characterizing the stream's structure.

Key Findings

1. Density and Stream Structure: The analysis reveals significant perturbations in the stream density, showcasing both small-scale peaks near the progenitor and large-scale gaps further along. There is a conspicuous asymmetry in the stream density, with the trailing arm displaying a more pronounced density peak and broader variations compared to the leading arm. Notably, the stream's width is not constant, diverging between the leading and trailing tails.
2. Subhalo Interactions: These observed density fluctuations are consistent with predictions from interactions with dark matter subhaloes. Specifically, the paper suggests that the density gap in the trailing arm could be attributed to a subhalo with a mass in the range of $10^7-10^8 M_\odot$. Such interactions are not isolated, with additional evidence pointing to a $10^6-10^7 M_\odot$ subhalo influencing the leading arm.
3. Rotating Bar Effects: In addition to subhalo interactions, the authors explore the potential influence of the Milky Way's rotating bar on stream perturbations. They demonstrate that the bar can induce significant asymmetries in the stream, contingent on the pattern speed and orbital alignment.

Implications and Future Directions

The findings hold substantial implications for understanding dark matter substructures and the dynamics of galactic tidal streams. By providing a method to reliably detect density fluctuations, this paper enhances our capacity to probe the granularity of the dark matter halo. The distinction between perturbations caused by dark subhaloes and those induced by baryonic structures, such as giant molecular clouds, is particularly pivotal.

The paper advocates for further observational efforts, notably through radial velocity measurements and proper motion data from upcoming surveys like WEAVE, 4MOST, and DESI, alongside Gaia's proper motion catalogues. These data will refine the current models and potentially differentiate between the impacts of dark subhaloes and baryonic factors.

Conclusion

The paper successfully illustrates a cutting-edge technique to decipher the complex phenomena governing stellar streams and underlines the intricate interplay between dark matter and observed galactic structures. By drawing connections between stream perturbations and potential interactions with dark matter subhaloes and the Milky Way's rotating bar, this research opens new avenues for constraining the properties of dark matter. Future work in this domain will likely focus on exploiting the synergy between observational advancements and sophisticated modeling to unravel the dark contents of our galaxy.