Observations of galactic gamma-rays and their implications for galactic structure studies

Journal Article · Tue Feb 15 04:00:00 EST 1977 · Astrophys. J.; (United States)

DOI:https://doi.org/10.1086/155019· OSTI ID:7322668

Stecker, F W

The relation of the recent SAS-2 observations of galactic ..gamma..-rays and other surveys to the large-scale distribution of cosmic rays and interstellar gas in the Galaxy is reviewed and reexamined. Starting with a discussion of the production rates, the case for pion-decay being the predominant production mechanism in the galactic disk above 100 MeV is reestablished, and it is also pointed out that Compton ..gamma..-rays can be a significant source near l=0degree. To facilitate discussion the concepts of four distinct galactic regions of various thicknesses are defined, viz., the nebulodisk, ectodisk, radiodisk, and exodisk. Bremsstrahlung and pion-decay ..gamma..-rays are associated with the first two regions (primarily the first region), and Compton ..gamma..-rays and synchrotron radiation are associated with the latter two regions. On a large scale in the radial dimension, the cosmic rays, interstellar gas (primarily H/sub 2/ clouds in the inner Galaxy), and ..gamma..-ray emissivity all peak in a region between 5 and 6 kpc from the galactic center. The nonuniform galactic cosmic-ray distribution argues against the trapping of cosmic rays in a large quasi-spherical halo.Correlations with arm features do not appear to be evident in the range 0degree< or =l< or =180degree. Some evidence for correlation with arm features may exist in the range 180degree< or =l< or =360degree, but arguments against confinement of cosmic rays in spiral arms (with I/sub c r/proportionaln/sub gas/) are given on the basis of ..gamma..-ray evidence, lifetime of cosmic rays, isotropy, etc.The galactic contribution to the observed high-latitude ..gamma..-ray background is discussed, and this contribution is shown to account reasonably for the observed spectrum at high latitudes between 35 and 200 MeV when added onto the extragalactic background.

• Research Organization: Theoretical Studies Group, NASA Goddard Space Flight Center
• OSTI ID: 7322668
• Journal Information: Astrophys. J.; (United States), Journal Name: Astrophys. J.; (United States) Vol. 212:1; ISSN ASJOA
• Country of Publication: United States
• Language: English

Related Subjects

640105* -- Astrophysics &amp; Cosmology-- Galaxies
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
BASIC INTERACTIONS
BOSONS
BREMSSTRAHLUNG
COMPTON EFFECT
COSMIC GASES
COSMIC PIONS
COSMIC RADIATION
COSMIC RADIO SOURCES
ELASTIC SCATTERING
ELECTROMAGNETIC INTERACTIONS
ELECTROMAGNETIC RADIATION
ELEMENTARY PARTICLES
FLUIDS
GALAXIES
GAMMA RADIATION
GASES
H2 REGIONS
HADRONS
INTERACTIONS
IONIZING RADIATIONS
MESONS
MILKY WAY
PIONS
PSEUDOSCALAR MESONS
RADIATIONS
SCATTERING
SYNCHROTRON RADIATION