Tracing magellanic Stellar Stream Gas
By Elijah Bernstein-Cooper, January 21, 2016, 0 comments.

Table of Contents

Stellar Streams

Recently Belokurov et al. (2015) used blue horizontal giant branch stars (BHBs) identified in the Dark Energy Survey (DES) as distance tracers. They found 4 stellar filaments whose distances were consistent with being stellar streams from the LMC, with one stream unambiguously connected to the LMC. Only one stream overlaps with the Magellanic Stream (MS). See Figure 1 for a map of the four streams at different distances.

The stellar streams could potentially be associated with the gaseous stream in a scenario where the Milky Way halo RAM-pressure strips the gas from the two galaxies.

Figure 1

Density of the BHB stars for different distances. Positions of the DES Year 1 satellites (red triangles) are shown as well as the locations and the designations of the newly detected structures. The gray outline represents the DES footprint. Arrows show the proper motion vectors of the LMC and the SMC, as measured by Kallivayalil et al. (2013) and corrected for the Solar reflex. Blue contours are the HI density of the Magellanic Stream as reported by Nidever et al. (2008).

A nearly vertical spray of BHB stars originating in the LMC is clearly visible. As confirmed by the density distribution, the two most prominent overdensities are i) a hook-like structures at X ∼ 0◦, Y ∼ 20◦ and ii) the S4 cloud of stars at X ∼ 35◦ on the sky, the S4 cloud appears to overlap several newly discovered DES satellites (red triangles). Only S3 overlaps with the Magellanic Stream.

HI Streams

Hammer et al. (2015) examine the structure of the HI in the Magellanic system to argue the Milkway halo is stripping the Magellanic Steam gas via ram-pressure.

H+15 decomposed the Galactic All-Sky Survey (GASS) HI data in a similar way to Nidever et al. (2008), fitting Gaussian components to each line of sight. The GASS data have three times the resolution as the LAB data, used by Nidever et al. (2008). With these high resolution data, they are able to identify several features in the Magellanic Stream (MS) such as a bow shock which allow for a more complete understanding of the MCs’ origin.

Bow Shock

H+15 disentangles the components of the MS to find many high-velocity clouds (HVCs) along a line perpendicular to the MS. This is consistent with a bow shock from colliding and heated gas.

Turbulence in the Stream

H+15 assess the turbulent shape of the stream to estimate the density of the surrounding medium. For fluids with high enough turbulent potential, estimated by the Reynolds number, the flow of the fluid will create vortices. They conclude the size of the vortices along the stream are consistent with increasing hot halo gas density along the stream.

The Leading Arm

The Leading Arm (LA) may be gas deposited by a MW dwarf Spheroidal (dSph) whose gas was rapidly stripped from the stars, leaving no associated stellar component.

Connecting the Scenario

H+15 derive a scenario where the MCs were on nearly parallel orbits. The MW halo begins to strip the gas away, giving rise to gaseous tails. These tails become turbulent within the hot halo, leading to vortices. The MCs collide, leading to a huge increase in gas density, which then produces a bow shock, consistent with the large number of HVCs present in the middle of the stream. The leading arm is a relic of another stripped system. See Figure 2 for a N(HI) map of the Magellanic system outlining this scenario.

H+15 run hydrodynamical simulations of this scenario to confirm the scenario is consistent with observations. They find extremely good agreement between the N(HI) profile of the MS.

Figure 2

Top: Figure 7 from H+15. Shows proposed scenario of the Magellanic system. Bottom: Same except for with approximate region of Figure 1 shown in black outline.