Observational Constraints on the Diffusion of Cosmic-Ray Electrons in Spiral Galaxies

January 1998 • 1998ApJ...493..121M

Authors • Marsh, K. A. • Helou, G.

Abstract • We present quantitative tests of the ``smearing'' model which relates radio and infrared images of spiral galaxies. We test whether the radio image appears as a smeared version of the infrared image in 24 spirals and one irregular, using 20 cm radio images and 60 μm infrared images processed to the same resolution of <~1'. Previous work by Bicay & Helou, assuming circularly symmetric, exponentially profiled disks, suggested that the smearing function is best approximated by an exponential radial profile. Our detailed assessment of the shape and size of the smearing function is free of those assumptions. We used two different approaches: (1) parameterized fits to exponential and Gaussian functions, each either circularly symmetric or elongated along the disk's tilt axis, and (2) direct estimation of the form of the kernel by deconvolution of the radio image against the infrared image. We conclude that:

1. The radio images are indeed reproduced to reasonable accuracy by smearing the infrared images with positive-valued kernels with model-error residuals <=10% of the local radio intensity over an intensity range of almost 2 orders of magnitude.

2. Most estimated smearing scales are smaller than 2 kpc and are typically 50% smaller than those reported by Bicay & Helou. This discrepancy is most likely due to the specific infrared intensity distribution assumed by Bicay & Helou.

3. The discrimination between exponential and Gaussian smearing kernels is marginal in most cases: above the 90% confidence level, only four galaxies are better fit by exponential kernels, and none is better fit by a Gaussian. There is also no improvement in the goodness of fit if we use direct deconvolution rather than parametric Gaussian or exponential fitting.

4. There is no evidence for a change of shape of the smearing kernel from one quadrant of the galaxy disk to another. The smearing is therefore unlikely to be elongated in the radial direction.

5. For some but not all galaxies inclined to the line of sight by 45° or more, the smearing function is significantly elongated parallel to the apparent major axis. The elongation tends to be associated with exponentially shaped smearing kernels and with smearing scale lengths exceeding 1 kpc. This suggests that confinement heights in the disks are on the order of 1 kpc, and smaller scale smearing corresponds to negligible escape of cosmic-ray electrons.

6. The radio-to-infrared ratio is statistically indistinguishable between galaxies with exponential smearing kernels and galaxies with Gaussian kernels. This argues in favor of the Helou & Bicay smearing model.

Links

View on ADS

IPAC Authors
(alphabetical)

George Helou

IPAC Executive Director