The Radio Spectrum Across Three Tailed Radio Galaxies
Extragalactic Radio Sources
Multifrequency observations with large single dishes are the ideal tool to examine the variation of the shape of the radio spectrum across extended extragalactic radio sources. Three complex low luminosity sources with angular extent 20 T <9<30 1 have been mapped with the 100-m telescope of the MPIfR at frequencies 2.7, 4.9 and 10.7 GHz (HPBW = 4!4, 216 and 112 resp.). To extend the frequency range we used published low frequency maps for the spectral comparison, too. Thus at least four maps
... h angular resolution ^ 4!4 were available for each source. All maps were (if necessary) corrected for sidelobes, then cleaned from obvious background sources and finally smoothed to the same beam of 4 J 4 HPBW. To look for spectral curvature a spectrum of the form In S = a-j In v +an (In v) 2 was fitted to the brightness data for each sam pling point of the map. We chose two parameters to characterize the spectral shape. The first is the mean spectral index, a, defined as the slope of the fit curve at the geometric mean of the lowest and highest observing frequency. As a measure of the spectral curvature we derived the change of spectral index, Aa, along the fit curve between the lowest and highest observing frequency, i.e. Aa positive for a concave (= flat tening) spectrum and vice versa. In the following the results are briefly summarized (see Andernach, 1981, for details). Figure 1a shows our 10.7 GHz map of 3C40 in Abell 194. In this map the main part of the source appears as an asymmetric wide-angle-tailed radio galaxy with N547 as the parent galaxy. A separate component is due to emission from N541 and "Minkowski's object". The 2.7 and 4.9 GHz data suggest a highly polarized, steep spectrum radio bridge connecting N541 and the N545/7-system. The far northern tail is the only place in our sample of sources, where we could detect a spectral break in the radio spectrum as expected from synchrotron losses. The age derived from the break frequency and equipartition arguments corresponds to a particle transport speed of 2000 km s*" 1 , not too far from bulk veloci ties in other radio jets inferred from different arguments. The need for in situ particle acceleration is evident from Figure 1b , since the spectrum flattens with frequency (i.e. Aa>0) over a large extent of the source. This feature is also present in the two other sources, the 41 D. S. Heeschen and C M. Wade (eds.), Extragalactic Radio Sources, 41 -42. Copyright © / 982 by the I A U. available at https://www.cambridge.org/core/terms. https://doi.