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Astron. Astrophys. 331, 428-438 (1998)

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The radio continuum halo of NGC 5775

N. Duric 1, J. Irwin 2 and H. Bloemen 3, 4

1 Institute for Astrophysics, University of New Mexico, Albuquerque, NM 87131, USA
2 Department of Physics, Stirling Hall, Queen's University, Kingston, Ont. K7L 3N6, Canada
3 SRON-Utrecht, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
4 Leiden Observatory, P.O. Box 9513, 2300 RA Leiden, The Netherlands

Received 18 November 1996 / Accepted 15 September 1997


Sensitive, scaled-array VLA radio-continuum observations of the edge-on galaxy NGC 5775, at 6 and 20 cm, show evidence of a radio-continuum halo that is detected up to 10-15 kpc above and below the plane. In addition, the presence of a radio continuum bridge connecting NGC 5775 to neighbouring NGC 5774 is confirmed, lending further support to reports that these galaxies form an interacting pair.

Detailed analysis of the radio continuum halo of NGC 5775 indicates that it is characterized by a relatively steep spectrum with a spectral index of [FORMULA]. The spectral index of the disk is significantly flatter, averaging [FORMULA]. To first order, there is no evidence for large-scale spectral changes as a function of distance from the mid-plane in either component. The superposition of the disk and halo components, with distinctly different spectral indices, accounts for the apparent spectral steepening of the observed (total) emission with distance from the mid-plane.

The spectral results presented here are consistent with the findings for NGC 3556 (Bloemen et al. 1993), which could be studied with a 2-3 times better linear resolution. As with NGC 3556 the relatively flat spectrum of the disk of NGC 5775 appears to be the result of an ensemble of discrete sources, embedded in diffuse steep-spectrum emission as found in the halo. It appears that the gradual spectral steepening away from the mid-plane found in previous studies of edge-on galaxies may be attributed to insufficient angular resolution, supplemented by the missing-flux problem.

In contrast to the absence of global spectral changes in the disk and halo there are localized features of flatter spectral index that appear to connect the disk to the halo. These "tentacles" of flatter spectral index are roughly vertical and may represent chimneys of localized convection.

Key words: cosmic rays – halos – radio continuum – galaxies

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© European Southern Observatory (ESO) 1998

Online publication: February 16, 1998