From astrobites: “The “Shocking” Mystery about Filaments”

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From astrobites

Dec 24, 2019
Michael Foley

Title: The isothermal evolution of a shock-filament interaction
Authors: K. J. A. Goldsmith and J. M. Pittard
First Author’s Institution: School of Physics and Astronomy, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK

Status: Open access on arXiv

Galaxies are built from black holes, stars, gas, and dust. While the stars and black holes may get most of the attention, the gas and dust play crucial roles in the evolution of a galaxy. Gas and dust are responsible for forming stars, feeding the central supermassive black hole, and regulating the chemical composition of the galaxy. Consequently, understanding the dynamics of the gas and dust is very important if we want to learn how galaxies work.

One critical mechanism in the workings of galaxies is shocks.

This X-ray image was produced by combining a dozen Chandra observations made of the central region of the Milky Way. The colors represent low (red), medium (green) and high (blue) energy X-rays. Chandra’s unique resolving power has allowed astronomers to identify thousands of point-like X-ray sources due to neutron stars, black holes, white dwarfs, foreground stars, and background galaxies. What remains is a diffuse X-ray glow extending from the upper left to the lower right, along the direction of the disk of the Galaxy. The Chandra data indicate that the diffuse glow is a mixture of 10-million-degree Celsius gas and 100-million-degree gas. Shock waves from supernova explosions are the most likely explanation for heating the 10-million degree gas, but how the 100-million-degree gas is heated is a mystery.

Gas in galaxies can frequently become supersonic, such as when a supernova explodes, meaning that it travels faster than the local sound speed.

Bullet Cluster NASA Chandra NASA ESA Hubble, evidence of shock

This supersonic gas will generate shocks, just like an airplane traveling supersonically will produce a shockwave in the air that creates a sonic boom. The interactions of these shocks with other shocks or gas features can create turbulence or interesting substructures in the interstellar medium, potentially laying the fertile ground for stars to form.

Today’s paper looks at the interaction of these shocks with filaments of gas. Filaments are long, coherent structures that are found throughout the interstellar medium and that serve as the birthplaces of stars (Figure 1). We know that filaments can be destroyed by shocks, but the exact conditions necessary to destroy a filament remain a mystery. The authors of ran a number of simulations to try to recreate these crime scenes.

Figure 1: A 50-light-year long filament of star-forming gas found in the Orion Nebula. Image: R. Friesen, Dunlap Institute; J. Pineda, MPE; GBO/AUI/NSF. Taken from the Dunlap Institute.

Interstellar Medium
While the space between the stars of the Milky Way Galaxy appears empty, it is actually filled with gas and dust which, together with stars, form a galactic ecosystem called the Interstellar Medium (ISM)—a dynamic landscape of vast, turbulent structures, radiation, nucleosynthesis, shockwaves, and stellar birth and death.

The ISM is comprised mostly of gas, existing as ions, atoms or molecules. The gas is predominantly hydrogen, but there is also helium, both the products of primordial nucleosynthesis. The ISM also contains trace amounts of carbon, oxygen and nitrogen. Ionized gas is heated to millions of degrees K, while cold molecular, star-forming gas sits at temperatures measured in tens of degrees K.

From this rich and dynamic medium come stars which form from dense concentrations of molecular clouds. Then, in a galactic circle of life, they replenish the ISM through the stellar wind they generate, and through supernovae and neutron star mergers that synthesize and disseminate heavier elements into the galaxy.

The ISM is shaped by many forces: stellar radiation, stellar winds, as well as shockwaves from supernovae that can create “bubbles” in the ISM. Turbulence and magnetic fields also play roles in shaping this environment.

As such, questions about the birth and death of stars are intricately linked to questions about the ISM. What’s more, a better understanding of the medium is critical in understanding extra-galactic phenomena, like the Cosmic Microwave Background, because we cannot observe them without peering through the filter of the ISM.

At the Dunlap and U of T:

Prof. Bryan Gaensler
Dr. Cameron van Eck
Dr. Jennifer West
Jessica Campbell

How did the authors gather clues? It’s filamentary, my dear Watson.

To investigate this interaction, the authors conducted simulations of a single shock that crashes into a single filament. In order to better understand the physics, they varied multiple properties in the different simulation runs: the speed of the shock, the density of the filament, the orientation of the filament relative to the shock, and the length of the filament. By looking at how the remnants of the filament changed after varying certain parameters, they can gain clues about which parameters really matter for destroying a filament. Plots of one of these simulations are shown in Figure 2, where a filament is oriented sideways, or parallel to, the shock front. This filament is essentially blown up over time, and it develops interesting turbulence in its wake.

Figure 2: Filament (100 times more dense than surrounding material and oriented sideways to the shock) being hit by a shock (traveling at 3 times the speed of sound). The gas is color-coded by density, with red corresponding to high-density material and blue representing diffuse material. The development of turbulence and a ‘three-rolled’ structure in the filament material is clear in the last two snapshots. Taken from Figure 2 of the paper.

The authors found that the filament gas behaved differently when they changed the orientation of the filament relative to the shock, the speed of the shock, and the length and density of the filament. For example, much less turbulence developed when the filament was closer to perpendicular to the shock (Fig 3). Additionally, the filament in Figure 3 was slowly stripped of its material rather than blown up somewhat rapidly like the filament in Figure 2. This means that we could potentially determine the orientation of a filament relative to a shock with only the properties of the remnant gas in the wake.

Figure 3: Plot of shock (traveling at 3 times the speed of sound) moving through a filament (100 times denser than the surrounding material and oriented at a 60 degree angle to the shock). The shock front is marked by the transition between light grey and white, and it is moving to the right. The filament is shown in dark grey, and each of the 7 panels depicts a single time snapshot. The authors model the shock as a continuous inflow of material, so the simulation remains full of shock material (light grey) even after the shock front reaches the other end. Taken from Figure 5 of the paper.


The authors notice many of these trends among the different properties that they tested. They found:

Filaments oriented closer to perpendicular to the shock had longer and less turbulent wakes.
Only sideways-oriented filaments developed a three-rolled structure (see Fig. 2), and filaments with densities different than that of the filament in Figure 2 formed more of a ‘C’ shape.
Faster shocks stripped material from the filament much more quickly than slower shocks did.
Longer filaments ended up moving faster than shorter filaments since they were exposed to more of the shock.
These isothermal simulations were able to push the filaments much faster than in adiabatic simulations.

These results are important for understanding the impact of shocks moving through the interstellar medium. Throughout their journey, they may encounter filaments with various properties. This work will help us to begin to understand what types of shocks are responsible for destroying various types of filaments, bringing us one step closer to solving this ‘shocking’ mystery.

See the full article here .


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