6. Large-Scale Aspects of the Distribution of Interstellar Matter

1988 ◽  
Vol 20 (01) ◽  
pp. 385-396
Keyword(s):  
Large Scale ◽  
Galactic Center ◽  
Rotation Speed ◽  
Hii Regions ◽  
Distance Scale ◽  
Interstellar Matter ◽  
B Stars ◽  
Structure And Dynamics ◽  
Galactic System ◽  
The Galaxy

The previous report on structure and dynamics of the galactic system was given by Wielen (41.155.100). The recently recommended values for solar distance to the galactic center (8.5 kpc) and our rotation speed around it (220 km.s-1) were discussed by Trimble (42.155.043).The distance scale of the Galaxy was reviewed by Barkhatovaet al. (40.155.088). A discussion of typical corrugation scales in the Galaxy was given by Spicker and Feitsinger (42.155.003), who concluded that three distinct scales seem to exist: 1-2 kpc, 4-8 kpc, and > 13 kpc. These corrugations are reflected in the distribution of O and B-stars and HII regions, and to a lesser extent in the HI distribution. Feitzinger and Spicker (39.155.026) investigated the corrugation phenomenon for the (heliocentric) longitude range 10° ≤ l ≤ 240° as derived from HI studies.

scholarly journals Low-frequency measurements of synchrotron absorbing HII regions and modeling of observed synchrotron emissivity

2019 ◽  
Vol 621 ◽  
pp. A127 ◽  
Author(s):  
I. M. Polderman ◽  
M. Haverkorn ◽  
T. R. Jaffe ◽  
M. I. R. Alves
Keyword(s):  
Synchrotron Radiation ◽  
Magnetic Fields ◽  
Large Scale ◽  
Milky Way ◽  
Galactic Center ◽  
Three Dimensional ◽  
Low Frequency ◽  
Hii Regions ◽  
The Galaxy ◽  
Absorption Measurements

Context. Cosmic rays (CRs) and magnetic fields are dynamically important components in the Galaxy, and their energy densities are comparable to that of the turbulent interstellar gas. The interaction of CRs and Galactic magnetic fields (GMF) produces synchrotron radiation clearly visible in the radio regime. Detailed measurements of synchrotron radiation averaged over the line-of-sight (LOS), so-called synchrotron emissivities, can be used as a tracer of the CR density and GMF strength. Aims. Our aim is to model the synchrotron emissivity in the Milky Way using a three-dimensional dataset instead of LOS-integrated intensity maps on the sky. Methods. Using absorbed HII regions, we measured the synchrotron emissivity over a part of the LOS through the Galaxy, changing from a two-dimensional to a three-dimensional view. Performing these measurements on a large scale is one of the new applications of the window opened by current low-frequency arrays. Using various simple axisymmetric emissivity models and a number of GMF-based emissivity models, we were able to simulate the synchrotron emissivities and compare them to the observed values in the catalog. Results. We present a catalog of low-frequency absorption measurements of HII regions, their distances and electron temperatures, compiled from literature. These data show that the axisymmetric emissivity models are not complex enough, but the GMF-based emissivity models deliver a reasonable fit. These models suggest that the fit can be improved by either an enhanced synchrotron emissivity in the outer reaches of the Milky Way or an emissivity drop near the Galactic center. Conclusions. Current GMF models plus a constant CR density model cannot explain low-frequency absorption measurements, but the fits improved with slight (ad hoc) adaptations. It is clear that more detailed models are needed, but the current results are very promising.

Evidence for Large Scale Cosmic Ray Electron Gradients in the Galaxy

1976 ◽  
Vol 3 (1) ◽  
pp. 1-6 ◽  
Author(s):  
W. R. Webber
Keyword(s):  
Large Scale ◽  
Spiral Structure ◽  
Cosmic Ray ◽  
Point Sources ◽  
Interstellar Matter ◽  
Interstellar Gas ◽  
The Galaxy ◽  
Ray Density ◽  
Γ Rays ◽  
Γ Ray

In recent years observations of γ-ray emission from the disk of the galaxy have provided a new opportunity for research into the structure of the spiral arms of our own galaxy. In Figure 1 we show a map of the structure of the disk of the galaxy as observed for γ-rays of energy > 100 MeV by the SAS-2 satellite (Fichtel et al. 1975). The angular resolution of these measurements is ~ 3°, and besides two point sources at l = 190° and 265° several features related to the spiral structure of the galaxy are evident in the data. Most of these γ-rays are believed to arise from the decay of π° mesons produced by the nuclear interactions of cosmic rays (mostly protons) with the ambient interstellar gas. As a result, the γ-ray fluxes represent a measure of the line of sight integral of the product of the cosmic ray density NCR and the interstellar matter density N1

scholarly journals Pulsars as a galactic population

1979 ◽  
Vol 84 ◽  
pp. 119-123
Author(s):  
Joseph H. Taylor
Keyword(s):  
Carbon Monoxide ◽  
Statistical Analysis ◽  
Supernova Remnants ◽  
Galactic Center ◽  
Hii Regions ◽  
The Sun ◽  
Disk Thickness ◽  
Galactic Population ◽  
The Galaxy

Recent pulsar surveys have increased the number of known pulsars to well over 300, and many of them lie at distances of several kpc or more from the sun. The distribution of pulsars with respect to distance from the galactic center is similar to other population I material such as HII regions, supernova remnants, and carbon monoxide gas, but the disk thickness of the pulsar distribution is rather greater, with <|z|>≈350 pc. Statistical analysis suggests that the total number of active pulsars in the Galaxy is a half million or more, and because kinematic arguments require the active lifetimes of pulsars to be ≲5×106 years, it follows that the birthrate required to maintain the observed population is one pulsar every ∼10 years (or less) in the Galaxy.

scholarly journals Filamentary Structures Near the Galactic Center

1989 ◽  
Vol 136 ◽  
pp. 243-263 ◽  
Author(s):  
F. Yusef-Zadeh
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Galactic Center ◽  
Galactic Plane ◽  
Galactic Rotation ◽  
Physical Interaction ◽  
Magnetic Structures ◽  
The Galaxy ◽  
Field Lines ◽  
The Magnetic Field

Recent studies of the Galactic center environment have revealed a wealth of new thermal and nonthermal features with unusual characteristics. A system of nonthermal filamentary structures tracing magnetic field lines are found to extend over 200pc in the direction perpendicular to the Galactic plane. Ionized structures, like nonthermal features, appear filamentary and show forbidden velocity fields in the sense of Galactic rotation and large line widths. Faraday rotation characteristics and the flat spectral index distributions of the nonthermal filaments suggest a mixture of thermal and nonthermal gas. Furthermore, the relative spatial distributions of the magnetic structures with respect to those of the ionized and molecular gas suggest a physical interaction between these two systems. In spite of numerous questions concerning the origin of the large-scale organized magnetic structures, the mechanism by which particles are accelerated to relativistic energies, and the source or sources of heating the dust and gas, recent studies have been able to distinguish the inner 200pc of the nucleus from the disk of the Galaxy in at least two more respects: (1) the recognition that the magnetic field has a large-scale structure and is strong, uniform and dynamically important; and (2) the physics of interstellar matter may be dominated by the poloidal component of the magnetic field.

scholarly journals Abundances and Chemical Evolution of the Galactic Center

1977 ◽  
Vol 45 ◽  
pp. 79-101
Author(s):  
Jean Audouze
Keyword(s):  
Chemical Evolution ◽  
Gamma Rays ◽  
Molecular Clouds ◽  
Galactic Center ◽  
Solar Neighbourhood ◽  
Interstellar Matter ◽  
Low Mass Stars ◽  
Element Abundances ◽  
The Galaxy ◽  
Low Mass

AbstractFrom observations of the galactic center using various techniques radioastronomy, millimeter waves (molecules) – infrared and gamma rays, the interstellar matter of this region* appears to have been strongly processed into stars : the gas density is much lower than in the solar neighbourhood. From CO measurements one knows that there are many molecular clouds such as SgrB2 where stars are forming now. From IR measurements, there are some indication that low mass stars are relatively more numerous in such regions than in the external regions of the galaxy. Finally the heavy element abundances show three important features (i) the possibility of strong enhancements in elements such as N and in a less extent 0 and Ne (the so called abundance gradients), (ii) Some specific enhancements of isotopes such43C,44N and also47O relative to42C,45N and43O (iii) Deuterium seems to have a lower abundance than in other parts of the galaxy such as the solar neighbourhood. Simple models of chemical evolution have been designed to account for such features and are rewiewed here.

scholarly journals 33. Structure and Dynamics of the Galactic System

1985 ◽  
Vol 19 (1) ◽  
pp. 397-406
Author(s):  
R. Wielen ◽  
W. B. Burton ◽  
L. Blitz ◽  
W. Iwanowska ◽  
E. K. Kharadze ◽  
...  
Keyword(s):  
Galactic Center ◽  
Structure And Dynamics ◽  
Subject Category ◽  
Serial Number ◽  
Galactic System ◽  
The Subject ◽  
Astronomy And Astrophysics

Several authors have contributed to this report: L. Blitz (Section V), W.B. Burton (Sections IIIB and IVB), J. Einasto (Section VII), B. Fuchs (Sections VIC and VID), W. Hermsen (Section VIF), G. Lynga (Sections IIIA and IVA), M. Mayor (Section II), M. Miyamoto (Sections VIB and VIE) and R. Wielen (Sections I, VIA, and editing). The layout of this report follows previous practice. The galactic center is included in Sections IV and V. The references are, as far as possible, coded by their numbers (VV.CCC.NNN) in the bibliography “Astronomy and Astrophysics Abstracts” (AAA). VV identifies the volume of AAA, while CCC.NNN gives the subject category and the serial number within that volume.

scholarly journals Atomic and Molecular Gas in the Inner Regions of the Milky Way and Other Galaxies

1980 ◽  
Vol 5 ◽  
pp. 149-161
Author(s):  
H. S. Liszt
Keyword(s):  
Galactic Center ◽  
Galactic Disk ◽  
Rotation Axis ◽  
Molecular Form ◽  
Marked Change ◽  
Great Majority ◽  
Neutral Gas ◽  
Galactic System ◽  
The Galaxy ◽  
Rotation Motion

It seems likely that we inhabit a galactic system which in its gross characteristics exemplifies the present meaning of the word “normal”. The general framework for interpreting the distribution and kinematics of the observed gas consists of a well-defined rotation curve, applicable over the great majority of a thin, nearly flat disk; the stars and gas within this disk are generally quite cold, having little energy in random as opposed to well-ordered motions. In fact, neutral gas in the galactic disk is sufficiently regular in its structure that no grand pattern of spiral arms stands out in sharp relief.As one moves inward in the Galaxy from the location of the Sun, this regularity persists to within about 4 kpc of the center, at which distance the abundance of gas in either atomic or molecular form drops fairly abruptly. Inside this region, the character of the gas distribution undergoes a marked change and the behaviour observed there, while it may be common to even the best-ordered systems, represents a rather spectacular departure from the organization at larger galacto-centric radii. As the gas abundance increases once more toward the galactic center it is seen to reside in a multitude of individual features having disparate spatial and kinematic properties. Many of these have large and/or obvious components of non-circular motion, usually taken as indicating expulsion of matter from the galactic center region at both large and small angles with respect to the rotation axis of the galaxy at large. Others, while occurring at velocities whose sign is consistent with rotation motion alone, have contradictory changes in velocity with position. Line profiles taken in the inner regions of the Galaxy are generally decomposed according to catalogs of classifiable features but no generally accepted interpretative framework exists as a means of relating the wide variety of observable phenomena.

Structure and Kinematics of the Milky Way

2017 ◽  
Vol 13 (S336) ◽  
pp. 148-153 ◽  
Author(s):  
Mark J. Reid
Keyword(s):  
Rotation Curve ◽  
Milky Way ◽  
Spiral Structure ◽  
Galactic Center ◽  
Rotation Speed ◽  
Information Modeling ◽  
High Mass ◽  
Proper Motions ◽  
Dimensional Phase Space ◽  
The Galaxy

AbstractMaser astrometry is now providing parallaxes with accuracies of ±10 micro-arcseconds, which corresponds to 10% accuracy at a distance of 10 kpc! The VLBA BeSSeL Survey and the Japanese VERA project have measured ≈200 parallaxes for masers associated with young, high-mass stars. Since these stars are found in spiral arms, we now are directly mapping the spiral structure of the Milky Way. Combining parallaxes, proper motions, and Doppler velocities, we have complete 6-dimensional phase-space information. Modeling these data yields the distance to the Galactic Center, the rotation speed of the Galaxy at the Sun, and the nature of the rotation curve.

scholarly journals The galactic distance scale

1979 ◽  
Vol 84 ◽  
pp. 195-200 ◽  
Author(s):  
J. A. Graham
Keyword(s):  
Recent Work ◽  
Galactic Center ◽  
Variable Stars ◽  
Distance Scale ◽  
Rr Lyrae ◽  
A Value ◽  
The Galaxy ◽  
Distance Indicators

Recent work on the distance scale of the Galaxy has largely been in the direction of refining previously established methods. The RR Lyrae variable stars appear to be better distance indicators than was once thought and they have been used in determining Ro, the distance to the Galactic center. Ro is probably somewhat less than 10 kpc but greater than 7 kpc. Most methods point to a value near 8.5 kpc.

scholarly journals Abundance gradients: tracing the chemical properties of the disk

2009 ◽  
Vol 5 (H15) ◽  
pp. 790-790
Author(s):  
Roberto D.D. Costa ◽  
Walter J. Maciel
Keyword(s):  
Time Evolution ◽  
Chemical Evolution ◽  
Galactic Disk ◽  
Chemical Properties ◽  
Planetary Nebulae ◽  
Hii Regions ◽  
B Stars ◽  
Radial Gradient ◽  
The Galaxy ◽  
Vertical Gradients

AbstractAbundance gradients are key parameters to constrain the chemical evolution of the galactic disk. In this review recent determinations for the radial gradient are described, including its slope as derived from different objects such as planetary nebulae, HII regions, cepheids, or B stars, and for different elements. Inner and outer limits for the radial gradient, as well as its time evolution, both related to the chemical evolution of the Galaxy, are also described. The possible existence of azimuthal and vertical gradients is also discussed.

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