▪ Abstract Turbulence affects the structure and motions of nearly all temperature and density regimes in the interstellar gas. This two-part review summarizes the observations, theory, and simulations of interstellar turbulence and their implications for many fields of astrophysics. The first part begins with diagnostics for turbulence that have been applied to the cool interstellar medium and highlights their main results. The energy sources for interstellar turbulence are then summarized along with numerical estimates for their power input. Supernovae and superbubbles dominate the total power, but many other sources spanning a large range of scales, from swing-amplified gravitational instabilities to cosmic ray streaming, all contribute in some way. Turbulence theory is considered in detail, including the basic fluid equations, solenoidal and compressible modes, global inviscid quadratic invariants, scaling arguments for the power spectrum, phenomenological models for the scaling of higher-order structu...

http://www.ifa.hawaii.edu/%7Ereipurth/reviews/elmegreen1_araa.pdf

Interstellar Turbulence I: Observations and Processes

This article reviews the current knowledge and understanding of the interstellar medium of our galaxy. The author first presents each of the three basic constituents---ordinary matter, cosmic rays, and magnetic fields---of the interstellar medium, with emphasis on their physical and chemical properties as inferred from a broad range of observations. The interaction of these interstellar constituents, both with each other and with stars, is then discussed in the framework of the general galactic ecosystem.

The interstellar environment of our galaxy

We present an analysis of star formation and feedback recipes appropriate for galactic smoothed particle hydrodynamics simulations. Using an isolated Milky Way-like galaxy, we constrain these recipes based on well-established observational results. Our star formation recipe is based on that of Katz (1992) with the additional inclusion of physically motivated supernova feedback recipes. We propose a new feedback recipe in which type II supernovae are modelled using an analytical treatment of blastwaves. With this feedback mechanism and a tuning of other star formation parameters, the star formation in our isolated Milky Way-like galaxy follows the slope and normalisation of the observed Schmidt law. In addition, we reproduce the low density cutoff and filamentary structure of star formation observed in disk galaxies. Our final recipe will enable better comparison of N-body simulations with observations.

/pdf/star-formation-and-feedback-in-smoothed-particle-1pzg6upasl.pdf

Star formation and feedback in smoothed particle hydrodynamic simulations – I. Isolated galaxies

This paper presents new maps of the soft X-ray background from the ROSAT all-sky survey. These maps represent a significant improvement over the previous version in that (1) the position resolution of the PSPC has been used to improve the angular resolution from ~2? to 12', (2) there are six energy bands that divide each of the previous three into two parts, and (3) the contributions of point sources have been removed to a uniform source flux level over most of the sky. These new maps will be available in electronic format later in 1997. In this paper we also consider the bright emission in the general direction of the Galactic center in the 0.5-2.0 keV band, and the apparent absorption trough that runs through it along the Galactic plane. We find that while the northern hemisphere data are confused by emission from Loop I, the emission seen south of the plane is consistent with a bulge of hot gas surrounding the Galactic center (in our simple model, a cylinder with an exponential fall-off of density with height above the plane). The cylinder has a radial extent of ~5.6 kpc. The X-ray emitting gas has a scale height of 1.9 kpc, an in-plane electron density of ~0.0035 cm-3, a temperature of ~106.6 K, a thermal pressure of ~28,000 cm-3 K, and a total luminosity of ~2 ? 1039 ergs s-1 using a collisional ionization equilibrium (CIE) plasma emission model.

https://iopscience.iop.org/article/10.1086/304399/pdf

ROSAT Survey Diffuse X-Ray Background Maps. II.

▪ Abstract The interstellar medium in the vicinity of the Sun is arranged in large-scale structures of bubble walls, sheets, and filaments of warm gas, within which close to the midplane there are subsheets and filaments of cold dense material; the whole occupies roughly half the available volume and extends with decreasing mean density to at least a kiloparsec off the plane. The remainder of the volume is in bubble interiors, cavities, and tunnels of much lower density, with some but not all of those lower density regions hot enough to be observable via their X-ray emission. This entire system is pervaded by a rather strong and irregular magnetic field and cosmic rays, the pressures of which are confined by the weight of the interstellar gas, particularly that far from the plane where gravity is strong. Observations suggest that the cosmic rays and magnetic field have an even more extended vertical distribution than the warm gas, requiring either the weight of additional coronal material or magnetic tens...

The three-phase interstellar medium revisited

A map is presented of the soft X-ray diffuse background flux in the C band (approximately 0.13-0.28 keV) covering almost all of the southern galactic hemisphere. A comparison at constant galactic latitude of both C band and B band (about 0.1-0.18 keV) soft X-ray data with neutral-hydrogen maps shows that the intensity does decrease with increasing neutral-hydrogen column density but in a manner that is inconsistent with photoelectric absorption. It is suggested that the inverse correlation is a displacement effect. X-ray emission regions appear to be where the cool gas is not. Further, the evidence against photoelectric absorption implies that the bulk of the cool gas is beyond the X-ray emitting regions. Fewer than 10 to the 20th power H atoms per sq cm can be between the sun and the X-ray emitting regions. The sun appears to be surrounded by a soft X-ray emission region consisting of gas at a temperature of about 1 million K.

Soft diffuse X-rays in the southern galactic hemisphere

Two rocket observations of very soft X-ray diffuse intensity show three emission regions of large angular extent. A large portion of the sky as seen with wide angular resolution is mapped. These three regions coincide with broad regions of small column densities of neutral hydrogen. It is suggested that a component of the interstellar medium is very hot and is responsible for the soft X-ray emission and the O VI.

Observation of features in the soft X-ray background flux

The Diffuse X-ray Spectrometer (DXS) flew as an attached payload on the STS-54 mission of the space shuttle Endeavour in 1993 January and obtained spectra of the soft X-ray diffuse background in the 148-284 eV (84-44 ?) band using a Bragg-crystal spectrometer The spectra show strong emission lines, indicating that the emission is primarily thermal Since the observations were made at low Galactic latitude, this thermal emission must arise from a nearby hot component of the interstellar medium, most likely the Local Hot Bubble, a region within ~100 pc of the Sun characterized by an absence of dense neutral gas The DXS spectrum of the hot interstellar medium is not consistent with either collisional equilibrium models or with nonequilibrium ionization models of the X-ray emission from astrophysical plasmas Models of X-ray emission processes appear not yet adequate for detailed interpretation of these data The DXS data are most nearly consistent with models of thermal emission from a plasma with a temperature of 1061 K and depletions of refractory elements magnesium, silicon, and iron to levels ~30% of solar

Spectra of the ¼ keV X-Ray Diffuse Background from the Diffuse X-Ray Spectrometer Experiment

Soft cosmic X ray background flux from rocket observations on 21 September 1968 from White Sands, discussing flux origins

Soft X-ray background flux.

Soft X-ray sky survey data for high galactic latitudes are used to constrain simple geometric models for the source of the diffuse X-ray background at 1 keV. The intensity maps show two extended and enhanced features, in Eridanus and in the direction of the galactic center, with a relatively uniform sky away from these features and an observed degree of isotropy consistent with a model in which the 0.5-1.2 keV background consists of an isotropic extragalactic component and a thick disk galactic component. A temperature of 2-3 million K and an emission pressure of 0.004 per cm to the 6th pc are derived for the galactic component from an assumed spectrum of 11E to the -1.4 power photons/sq cm s sr keV. While consistent with the data, a local, isotropically distributed source model is shown to pose physical difficulties.

W. L. Kraushaar

Papers

Soft diffuse X-rays in the southern galactic hemisphere

Observation of features in the soft X-ray background flux

Spectra of the ¼ keV X-Ray Diffuse Background from the Diffuse X-Ray Spectrometer Experiment

Soft X-ray background flux.

On the origin of the 1 keV diffuse X-ray background