$S^5$: Probing the Milky Way and Magellanic Clouds potentials with the 6-D map of the Orphan-Chenab stream

Abstract

We present a 6-D map of the Orphan-Chenab (OC) stream by combining the data from 5 years of Southern Stellar Stream Spectroscopic Survey (𝑆 5 ) observations with Gaia EDR3 data. We reconstruct the proper motion, radial velocity, distance and on-sky track of stream stars with spline models and extract the stellar density along the stream. The stream has a total luminosity of 𝑀 𝑉 = −8.2 and an average metallicity of [Fe/H] = −1.9, similar to classical MW satellites like Draco. The stream shows drastic changes in its physical width varying from 200 pc to 1 kpc, a constant line of sight velocity dispersion of 5 km s −1 , but an increase in the velocity dispersion along the stream near pericenter to ∼ 10 km s −1 . Despite the large apparent variation in the stellar number density along the stream, the flow rate of stars along the stream is remarkably constant. We model the 6-D stream track by a Lagrange-point stripping method with a flexible MW potential in the presence of a moving extended LMC potential. This allows us to constrain the mass profile of the MW within the distance range 15.6 < r < 55.5 kpc, with the best measured enclosed mass of (2.85 ± 0.1) × 10 11 M within 32.4 kpc. With the OC stream's closest approach distance to the LMC of ∼ 21 kpc, our stream measurements are highly sensitive to the LMC mass profile with the most precise measurement of the LMC's enclosed mass being at 32.8 kpc with 𝑀 = (7.02 ± 0.9) × 10 10 M . We confidently detect that the LMC DM halo extends to at least 53 kpc. The fitting of the OC stream allows us to constrain the past LMC trajectory and the degree of dynamical friction it experienced. We demonstrate that the stars on the OC stream show large energy and angular momentum spreads caused by the LMC perturbation and revealing the limitations of orbital invariants for substructure identification in the MW halo.

The effect of the deforming dark matter haloes of the Milky Way and the Large Magellanic Cloud on the Orphan–Chenab stream

“…We find that, compared to the LMC, it has a more distant approach to the OC stream, suggesting it will have a negligible effect. Along similar lines, Koposov et al (2022) explore this in more detail by modelling the gravitational effect of the SMC on the OC stream and find that it has almost no effect on the stream.…”

“…This allowed us to evaluate the time-dependent forces at any position in the system, and thus evolve the OC stream in a deforming Milky Way and LMC system. We compared our simulations to the exquisite 6-D phasespace map of the OC stream from Gaia and the S 5 survey (Koposov et al 2022).…”

“…Koposov et al 2019), we now have a 6D view of the OC stream. The presentation and detailed analysis of these data is published in Koposov et al (2022). The measurements of various observables are done through modelling of unbinned data as a function of angle along the stream track by cubic splines, similar to Erkal et al (2017) and Koposov et al (2019).…”

“…Ibata et al 2020;Ramos et al 2020;Vasiliev et al 2021) and the OC stream (e.g. Koposov et al 2019Koposov et al , 2022. These are the longest streams in the Milky Way that, along the stream, experienced different effects of the deformations at different times.…”

Unravelling the mass spectrum of destroyed dwarf galaxies with the metallicity distribution function

“…Johnston et al 2008). Furthermore, even in the space of conserved quantities the clumps may not stay compact due to perturbations from massive systems such as the LMC (Koposov et al 2022b). This is where chemical information can be crucial, as galaxies of different mass (and star formation history) can have distinct chemical signatures (e.g.…”