Radiative Turbulent Mixing Layers and the Survival of Magellanic Debris

Abstract

The Magellanic Stream is sculpted by its infall through the Milky Way’s circumgalactic medium, but the rates and directions of mass, momentum, and energy exchange through the stream-halo interface are relative unknowns critical for determining the origin and fate of the Stream. Complementary to large-scale simulations of LMC-SMC interactions, we apply new insights derived from idealized, high-resolution cloud-crushing and radiative turbulent mixing layer simulations to the Leading Arm and Trailing Stream. Contrary to classical expectations of fast cloud breakup, we predict that the Leading Arm and much of the Trailing Stream should be surviving infall and even gaining mass due to strong radiative cooling. Provided a sufficiently supersonic tidal swing-out from the Clouds, the present-day Leading Arm could be a series of high-density clumps in the cooling tail behind the progenitor cloud. We back up our analytic framework with a suite of converged wind-tunnel simulations, finding that previous results on cloud survival and mass growth can be extended to high Mach number (  ) flows with a modified drag time t drag ∝ 1 +  and longer growth time. We also simulate the Trailing Stream; we find that the growth time is long (approximately gigayears) compared to the infall time, and approximate Hα emission is low on average (∼ a few milliRayleigh) but can be up to tens of milliRayleigh in bright spots. Our findings also have broader extragalactic implications, e.g., galactic winds, which we discuss.

The Magellanic Corona as the key to the formation of the Magellanic Stream

“…In order for the neutral Stream to survive to the present day, the gas stripped out of the SMC must be dense enough to remain neutral (e.g. Bustard & Gronke 2022).…”

Key Physical Processes in the Circumgalactic Medium

“…For highly supersonic winds, as expected in starburst galaxies, the cloud develops a strong bow shock; it therefore interacts with higher-pressure postshock gas. Simulations of high-Mach number winds show tails and entrainment times that are longer by a factor of ∼(1 + M) (Scannapieco & Brüggen 2015, Bustard & Gronke 2022. This can be attributed to the compression by an oblique shock, so that χ → χ (1 + M), which also produces a streamwise pressure gradient (Scannapieco & Brüggen 2015).…”

The saturation mechanism of thermal instability

“…Despite our pointing out in WP19 that McCourt et al's (2018) simulations are a demonstration of isobaric takeover, the term 'shattering' is still regularly invoked when describing the appearance of small-scale cloud fragments in wind-cloud or shock-cloud interaction simulations that include radiative cooling (e.g., Sparre et al, 2020;Banda-Barragán et al, 2021;Bustard and Gronke, 2022;Jennings et al, 2023). The use of the term here is likely because the size of these fragments appears to be the cooling length evaluated in the cold phase gas, denoted by min(λ cool ), which is the isobaric length scale identified by McCourt et al (2018).…”