Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe SKA view of the Neutral Interstellar Medium in Galaxies
148
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
Two major questions in galaxy evolution are how star-formation on small scales leads to global scaling laws and how galaxies acquire sufficient gas to sustain their star formation rates. HI observations with high angular resolution and with sensitivity to very low column densities are some of the important observational ingredients that are currently still missing. Answers to these questions are necessary for a correct interpretation of observations of galaxy evolution in the high-redshift universe and will provide crucial input for the sub-grid physics in hydrodynamical simulations of galaxy evolutions. In this chapter we discuss the progress that will be made with the SKA using targeted observations of nearby individual disk and dwarf galaxies.
rate research
Read More
Magnetohydrodynamic (MHD) turbulence is a crucial component of the current paradigms of star formation, dynamo theory, particle transport, magnetic reconnection and evolution of structure in the interstellar medium (ISM) of galaxies. Despite the importance of turbulence to astrophysical fluids, a full theoretical framework based on solutions to the Navier-Stokes equations remains intractable. Observations provide only limited line-of-sight information on densities, temperatures, velocities and magnetic field strengths and therefore directly measuring turbulence in the ISM is challenging. A statistical approach has been of great utility in allowing comparisons of observations, simulations and analytic predictions. In this review article we address the growing importance of MHD turbulence in many fields of astrophysics and review statistical diagnostics for studying interstellar and interplanetary turbulence. In particular, we will review statistical diagnostics and machine learning algorithms that have been developed for observational data sets in order to obtain information about the turbulence cascade, fluid compressibility (sonic Mach number), and magnetization of fluid (Alfvenic Mach number). These techniques have often been tested on numerical simulations of MHD turbulence, which may include the creation of synthetic observations, and are often formulated on theoretical expectations for compressible magnetized turbulence. We stress the use of multiple techniques, as this can provide a more accurate indication of the turbulence parameters of interest. We conclude by describing several open-source tools for the astrophysical community to use when dealing with turbulence.
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.
Thanks to their proximity, local starbursts are perfectly suited for high-resolution and sensitivity multiwavelength observations aimed to test our ideas about star formation, evolution of massive stars, physics and chemical evolution of the interstellar medium (ISM). High-resolution UV spectroscopy with FUSE and STIS has recently given the possibility to characterize in great detail the neutral ISM in local starbursts thanks to the presence in this spectral range of many absorption lines from ions of the most common heavy elements. Here we present the results for two nearby starburst galaxies, I Zw 18 and NGC 1705, and show how these results relate to the star-formation history and evolutionary state of these stellar systems.
We present an analysis of the properties of HI holes detected in 20 galaxies that are part of The HI Nearby Galaxy Survey (THINGS). We detected more than 1000 holes in total in the sampled galaxies. Where they can be measured, their sizes range from about 100 pc (our resolution limit) to about 2 kpc, their expansion velocities range from 4 to 36 km/s, and their ages are estimated to range between 3 and 150 Myr. The holes are found throughout the disks of the galaxies, out to the edge of the HI; 23% of the holes fall outside R25. We find that shear limits the age of holes in spirals (shear is less important in dwarf galaxies) which explains why HI holes in dwarfs are rounder, on average than in spirals. Shear, which is particularly strong in the inner part of spiral galaxies, also explains why we find that holes outside R25 are larger and older. We derive the scale height of the HI disk as a function of galactocentric radius and find that the disk flares up in all galaxies. We proceed to derive the surface and volume porosity (Q2D and Q3D) and find that this correlates with the type of the host galaxy: later Hubble types tend to be more porous. The size distribution of the holes in our sample follows a power law with a slope of a ~ -2.9. Assuming that the holes are the result of massive star formation, we derive values for the supernova rate (SNR) and star formation rate (SFR) which scales with the SFR derived based on other tracers. If we extrapolate the observed number of holes to include those that fall below our resolution limit, down to holes created by a single supernova, we find that our results are compatible with the hypothesis that HI holes result from star formation.
We present 21-cm absorption measurements towards 12 radio continuum sources with previously identified thermally-unstable warm neutral medium (WNM). These observations were obtained with the Expanded Very Large Array (EVLA) and were complemented with the HI emission spectra obtained with the Arecibo Observatory. Out of 12 sources, HI absorption was detected along 5 lines of sight (seven new absorption features in total), resulting in a detection rate of ~42%. While our observations are sensitive to the WNM with a spin temperature T_s<3000 K, we detected only two wide absorption lines with T_s=400-900 K. These temperatures lie above the range allowed for the cold neutral medium (CNM) by the thermal equilbrium models and signify the thermally unstable WNM. Several absorption features have an optical depth of only a few x10^{-3}. While this is close or lower than what is theoretically expected for the CNM, we show that these weak lines are important for constraining the fraction of the thermally unstable WNM. Our observations demonstrate that, for the first time, high bandpass stability can be achieved with the VLA, allowing detection of absorption lines with a peak optical depth of ~10^{-3}.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
