Investigating the origin of the Milky Way streams. A revised look at their orbital pole distribution in light of precession effects

TL;DR

The paper re-examines whether Milky Way stellar streams share a common origin with the Disc of Satellites/VPOS by analyzing 91 streams from the updated galstreams catalogue and comparing their orbital-pole distribution to a VPOS reference. Using a Bernoulli framework, it finds no strong clustering overall, but reveals a distance-dependent trend where poles of more distant streams align closer to the VPOS, consistent with precession in a non-spherical potential. Hydrodynamical MOND-based MW–M31 fly-by simulations corroborate that initial pole anisotropy can be diluted for nearby material but remains detectable at larger Galactocentric distances, a result quantified with mock catalogues. The work highlights precession as a major factor shaping observed pole distributions and suggests that distant streams (d ≳ 150 kpc) are essential to decisively test a common-origin scenario for MW substructures, with implications for both streams and the globular-cluster population.

Abstract

Stellar streams around the Milky Way (MW) can provide valuable insights into its history and substructure formation. Previous studies have suggested that several MW streams could have an origin related to that of the disc of satellite galaxies (DoS) and the young halo globular clusters of the MW, given that many of these structures present a similar orbital pole orientation. In this work we test the validity of this hypothesis by revising the orbital pole distribution of the MW streams with the latest stream dataset (galstreams). For a sample of 91 streams at Galactocentric distances of $d<100$ kpc we find that the pole distribution has no preferred orbital direction. However, as we subtract the streams closer to the Galactic centre, by imposing several lower distance cuts, we find that the larger the Galactocentric distance of the streams, the higher the fraction of stream poles pointing in a direction similar to the DoS. This trend could be explained if the stream pole distribution were originally anisotropic, but precession effects displaced the orbital poles of the streams closer to the Galactic centre. From the pole distribution and the estimated precession rates of the streams in the sample, we infer that the streams nearer the Galactic centre are indeed quite likely to be affected by precession. Finally, we corroborate with hydrodynamical simulations that, even in a scenario in which the MW substructures had a common origin, an overdensity in their orbital pole direction cannot be appreciated until the selected sample also includes material at $d \gtrsim 150$ kpc.

Figures (7)

• Figure 1: Distribution of the orbital poles of the MW streams included in the galstreams catalogue (represented with a Mollweide projection). Since we only consider one orbital sense, just one half ($120^{\circ} < l < 300^{\circ}$) of the $l$ range is shown. The nominal orbital poles for each stream are represented by the large coloured points. Their $1\sigma$ uncertainty is shown by the point clouds of the same colour. The green cross marks the position of the observed VPOS direction ($l$, $b$) = ($164.0^{\circ}$, $-6.9^{\circ}$) Pawlowski_2015 and the green circle surrounding it represents the threshold of the VPOS membership area considered in our statistical analysis (see Section \ref{['Obs_analysis']}). The red star marks the position of the Sagittarius dwarf galaxy. The colour gradient of the streams represents their Galactocentric distance.
• Figure 2: Tidal debris pole distribution at different Galactocentric distance ($d$) cuts from the present-day Banik_2022 simulation data (represented with a Mollweide projection). All figures have a lower distance cut of $z > 50$ kpc to remove the contamination from the disc material. For a better comparison with the observed sample, we have also assumed that the simulated substructures are all orbiting with the same orbital sense (see Section \ref{['Data']}), even though $-$ unlike the observed data $-$ the simulation does provide information on the orbital sense of its particles and gas cells. The green cross marks the position of the observed VPOS direction ($l$, $b$) = ($164.0^{\circ}$, $-6.9^{\circ}$) Pawlowski_2015 and the green circle surrounding it represents the threshold of the VPOS membership area considered in our statistical analysis (see Section \ref{['Obs_analysis']}).
• Figure 3: Distance distribution of the stream's nominal poles in bins of $15$ kpc.
• Figure 4: Figure similar to Fig. \ref{['poles_galstreams']} but for a 2D projection. This figure also includes contour plots that enclose different stream pole areas as a function of their pole fraction. Solid lines correspond to a $1\sigma$ contour level and dashed lines correspond to a $2\sigma$ level in this plot. For obtaining the fraction of poles per bin, we divided the parameter space in 384 bins of equal area and we distributed the statistical 'weight' of each stream pole equally among its uncertainty pole track.
• Figure 5: Orbital poles of the streams considered in Pawlowski_2012 (hexagons) that are also included in the Riley_2020 (diamonds) and in the galstreams (circles) catalogue. Stream poles corresponding to the same stream in different catalogues are represented with the same color. The $1\sigma$ uncertainty is also included for the galstreams orbital poles.