The distance and properties of hydrogen clouds in the Leading Arm of the Magellanic System

Abstract

We present a high-resolution study of five high-velocity clouds in the Magellanic Leading Arm region. This is a follow-up study of our widefield Parkes survey of the region in order to probe the multiphase structures of the clouds and to give an insight to their origin, evolution and distance. High-resolution data were obtained from the Australia Telescope Compact Array. By combining with single-dish data from the Galactic All-Sky Survey (GASS), we are able to probe compact and diffuse emission simultaneously. We identify resolved and unresolved clumps. Physical parameters were derived for both diffuse structure and compact clumps. The latter are cold with typical velocity linewidths of 5 km s −1 . We find a gradient in thermal halo pressure, hydrogen density and H i column density of HVC as a function of Galactic latitude. This is possibly the first observational evidence of varying distance in the Leading Arm region, with the leading part of the Leading Arm (LA II and III) probably being closer to the Galactic disc than the trailing end (LA I).

Radiative Turbulent Mixing Layers and the Survival of Magellanic Debris

“…If in the strong cooling regime, which seems applicable, the present-day Leading Arm is likely a series of clumps in the long cooling tail behind the progenitor cloud. This is consistent with observations, which find a clear distance variation along the Leading Arm (Venzmer et al 2012;For et al 2016;Antwi-Danso et al 2020). The clumpy, fragmented nature of the Leading Arm, rather than a coherent tail, could be due to cutoffs in observational sensitivity, i.e., that we observe only the highest column density clumps formed through thermal instability (see the distinctly clumpy morphology in Figure 4) or disruptive turbulence in the Milky Way halo.…”

“…The number of head-tail clouds decreases along the length of the Stream, consistent with a decrease in ram pressure and hence a distance gradient along the Stream. There is similarly strong evidence for a distance variation in the Leading Arm (see, e.g., Figure 5 in Antwi-Danso et al 2020 for a compilation of data points), with some clumps believed to be within ∼20 kpc of the Milky Way disk (McClure-Griffiths et al 2008;Venzmer et al 2012;For et al 2016;Richter et al 2018) in spite of ram pressure from the hot Milky Way halo.…”

Radiative Turbulent Mixing Layers and the Survival of Magellanic Debris

“…If in the strong cooling regime, which seems applicable, the present-day Leading Arm is likely a series of clumps in the long cooling tail behind the progenitor cloud. This is consistent with observations, which find a clear distance variation along the Leading Arm (Venzmer et al 2012;For et al 2016;Antwi-Danso et al 2020). The clumpy, fragmented nature of the Leading Arm, rather than a coherent tail, could be due to cut-offs in observational sensitivity, i.e.…”

“…There is similarly strong evidence for a distance variation in the Leading Arm (see e.g. Figure 5 of Antwi-Danso et al 2020 for a compilation of data points), with some clumps believed to be within ∼ 20 kpc of the Milky Way disk (McClure-Griffiths et al 2008;Venzmer et al 2012;For et al 2016;Richter et al 2018) in spite of ram pressure from the hot Milky Way halo.…”

Cold HI ejected into the Magellanic Stream

“…They find that the cool core, represented by the narrow components, can survive while the outer layer of warmer H i is being stripped away by ram pressure stripping by the hot MW halo. For et al (2016) examined five HVCs and found that the resolved clouds all had cold cores and thermal pressures consistent with a two-phase equilibrium in SMC metallicities. Looking back to the MS, Putman et al (2003) found that similar head-tail clouds were prevalent across the Stream, with many elongated along the MS.…”