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Modeling the 60μm/20cm Infrared-to-Radio Ratio Within Spiral Galaxies

Published online by Cambridge University Press:  19 July 2016

M. D. Bicay
Affiliation:
Infrared Processing and Analysis Center California Institute of Technology Pasadena, CA 91125, USA
G. Helou
Affiliation:
Infrared Processing and Analysis Center California Institute of Technology Pasadena, CA 91125, USA

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The remarkably tight global correlation between integrated far-infrared and radio continuum emission from spiral galaxies has recently stimulated interest in determining whether the relation holds spatially within galaxies (Wainscoat et al. 1987; Beck and Golla 1988; Bicay et al. 1989, hereafter Paper I). We report here on a detailed comparison of the distribution of 60μm infrared and 20cm radio continuum emission within 25 galaxies, mostly disk spirals. Local maxima in the thermal infrared and predominantly nonthermal radio maps are found to be spatially coincident on scales <0.3h−1 kpc in nearby galaxies. Superimposed on this broad correlation, we observe in the disks of most sample galaxies a slow decrease in the 60μm-to-20cm ratio Q60 with increasing radius. Values of Q60 within the central regions are often enhanced by a factor of 3 or more compared to the outer disks, whereas the corresponding enhancement in radio surface brightness is greater by at least an order of magnitude. The radial gradient in Q60 is most easily identified in nearby, face-on galaxies (e.g. NGC 5236, NGC 6946) due to the limited IRAS angular resolution. However, the gradient is also observed along the major axis of highly inclined systems (e.g. NGC 55).

Type
5. The Polarization, Magnetic Field and Velocity Structure of External Spiral Galaxies
Copyright
Copyright © Kluwer 1990 

References

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