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Register a new userGalactic dust evolution with rapid dust formation in the interstellar medium due to hypersonic turbulence
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Authors
Lars Mattsson
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Turbulence can significantly accelerate the growth of dust grains by accretion of molecules. For dust dynamically coupled to the gas, the growth rate scales with the square of the Mach number, which means that the growth timescale can easily be reduced by more than an order of magnitude. The limiting timescale is therefore rather the rate of molecular cloud formation, which means that dust production in the ISM can rapidly reach the levels needed to explain the dust masses observed at high redshifts. Thus, turbulence may be the solution to the replenishment problem in models of dust evolution in high-redshift galaxies and explain the dust masses seen at $z = 7 - 8$. A simple analytic galactic dust-evolution model is presented, where grain growth nicely compensates for the expected higher rate of dust destruction by supernova shocks. This model is simpler, relies on fewer assumptions and seems to yields a better fit to data derived from observations, compared to previous models of the same type.
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Typical galaxies emit about one third of their energy in the infrared. The origin of this emission reprocessed starlight absorbed by interstellar dust grains and reradiated as thermal emission in the infrared. In particularly dusty galaxies, such as starburst galaxies, the fraction of energy emitted in the infrared can be as high as 90%. Dust emission is found to be an excellent tracer of the beginning and end stages of a stars life, where dust is being produced by post-main-sequence stars, subsequently added to the interstellar dust reservoir, and eventually being consumed by star and planet formation. This work reviews the current understanding of the size and properties of this interstellar dust reservoir, by using the Large Magellanic Cloud as an example, and what can be learned about the dust properties and star formation in galaxies from this dust reservoir, using SPICA, building on previous work performed with the Herschel and Spitzer Space Telescopes, as well as the Infrared Space Observatory.
The properties of interstellar grains, such as grain size distribution and grain porosity, are affected by interstellar processing, in particular, coagulation and shattering, which take place in the dense and diffuse interstellar medium (ISM), respectively. In this paper, we formulate and calculate the evolution of grain size distribution and grain porosity through shattering and coagulation. For coagulation, we treat the grain evolution depending on the collision energy. Shattering is treated as a mechanism of forming small compact fragments. The balance between these processes are determined by the dense-gas mass fraction $eta_mathrm{dense}$, which determines the time fraction of coagulation relative to shattering. We find that the interplay between shattering supplying small grains and coagulation forming porous grains from shattered grains is fundamentally important in creating and maintaining porosity. The porosity rises to 0.7--0.9 (or the filling factor 0.3--0.1) around grain radii $asim 0.1~mu$m. We also find that, in the case of $eta_mathrm{dense}=0.1$ (very efficient shattering with weak coagulation) porosity significantly enhances coagulation, creating fluffy submicron grains with filling factors lower than 0.1. The porosity enhances the extinction by 10--20 per cent at all wavelengths for amorphous carbon and at ultraviolet wavelengths for silicate. The extinction curve shape of silicate becomes steeper if we take porosity into account. We conclude that the interplay between shattering and coagulation is essential in creating porous grains in the interstellar medium and that the resulting porosity can impact the grain size distributions and extinction curves.
This paper presents the first results of comparison of Planck along with IRAS data with Green Bank Telescope 21-cm observations in 14 fields covering more than 800 square degrees at high Galactic latitude. Galactic dust emission for fields with average HI column density lower than 2 x 10^20 cm^-2 is well correlated with 21-cm emission. The residual emission in these fields, once the HI-correlated emission is removed, is consistent with the expected statistical properties of the cosmic infrared background fluctuations. Fields with larger column densities show significant excess dust emission compared to the HI column density. Regions of excess lie in organized structures that suggest the presence of hydrogen in molecular form, though they are not always correlated with CO emission. Dust emission from intermediate-velocity clouds is detected with high significance. Its spectral properties are consistent with, compared to the local ISM values, significantly hotter dust (T~20 K), lower sub-millimeter dust opacity, and a relative abundance of very small grains to large grains about four times higher. These results are compatible with expectations for clouds that are part of the Galactic fountain in which there is dust shattering and fragmentation. Correlated dust emission in HVCs is not detected; the average of the 99.9% confidence upper limits to the emissivity is 0.15 times the local ISM value at 857 and 3000 GHz, in accordance with gas phase evidence for lower metallicity and depletion in these clouds. Unexpected anti-correlated variations of the dust temperature and emission cross-section per H atom are identified in the local ISM and IVCs, a trend that continues into molecular environments. This suggests that dust growth through aggregation, seen in molecular clouds, is active much earlier in the cloud condensation and star formation processes.
We study infrared emission of 17 isolated, diffuse clouds with masses of order solar masses, to test the hypothesis that grain property variations cause the apparently low gas-to-dust ratios that have been measured in those clouds. Maps of the clouds were constructed from WISE data and directly compared to the maps of dust optical depth from Planck. The mid-infrared emission per unit dust optical depth has a significant trend toward lower values at higher optical depths. The trend can be quantitatively explained by extinction of starlight within the clouds. The relative amounts of PAH and very small grains traced by WISE, compared to large grains tracked by Planck, are consistent with being constant. The temperature of the large grains significantly decreases for clouds with larger dust optical depth; this trend is partially due to dust property variations but is primarily due to extinction of starlight. We updated the prediction for molecular hydrogen column density, taking into account variations in dust properties, and find it can explain the observed dust optical depth per unit gas column density. Thus the low gas-to-dust ratios in the clouds are most likely due to `dark gas that is molecular hydrogen.
Turbulence is ubiquitous in the insterstellar medium and plays a major role in several processes such as the formation of dense structures and stars, the stability of molecular clouds, the amplification of magnetic fields, and the re-acceleration and diffusion of cosmic rays. Despite its importance, interstellar turbulence, alike turbulence in general, is far from being fully understood. In this review we present the basics of turbulence physics, focusing on the statistics of its structure and energy cascade. We explore the physics of compressible and incompressible turbulent flows, as well as magnetized cases. The most relevant observational techniques that provide quantitative insights of interstellar turbulence are also presented. We also discuss the main difficulties in developing a three-dimensional view of interstellar turbulence from these observations. Finally, we briefly present what could be the the main sources of turbulence in the interstellar medium.
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