2. Observations and data reduction
The maps of total power and linearly polarized radio emission of NGC 4449 at 8.46 GHz and 4.86 GHz were obtained using the Very Large Array (VLA) of the National Radio Astronomy Observatory (NRAO) 1. To attain highest sensitivity to smooth extended structures the most compact (D) configuration was used. The observations were carried out on 30 August and 1 - 2 September 1996 using 27 antennas at two independent IFs, each with a bandwidth of 50 MHz, separated by 50 MHz, with 11h integration time on NGC 4449 at 6.2 cm and 16h at 3.5 cm. The data were reduced using the standard AIPS software package. The flux density scale and the position angle of polarization was calibrated by observing the point source 3C 286. Instrumental polarization was corrected by observing 1216+487, which was also used for gain and phase calibration. The calibrated and edited visibility data were cleaned and self-calibrated (in phase only) using the AIPS package, yielding maps of Stokes parameters I, Q and U.
These IUQ data were combined with Effelsberg measurements using the program EFFMERG, a version of the SDE task IMERG (Cornwell et al. 1995) modified by P. Hoernes (see Beck & Hoernes 1996a). This program deconvolves both clean maps with their beams, and Fourier transforms them back into the UV plane. Then a combination of Effelsberg data for small spacings and VLA data for large spacings is performed with a linear interpolation in the overlap domain. The combined data are transformed back into the image plane with the synthesized VLA beam. To avoid ring-like distortions around strong unresolved sources, introduced by the combination technique, the bright unresolved sources were first subtracted from the maps and added again after the combination (see Beck et al. 1997). For our 8.46 GHz map we used the single dish data at 10.55 GHz from Klein et al. (1996), scaled to our frequency. We used mean spectral index of -0.7 (). For 4.86 GHz we performed separate observations with the 100-m Effelsberg radio telescope at 4.85 GHz.
The Q and U maps were combined to get maps of the linearly polarized emission (corrected for the positive zero level offset) and of the position angle of polarization E-vectors. The final maps have the synthesized beam of at 8.46 GHz and at 4.86 GHz. To obtain the map of Faraday rotation the data at both frequencies were convolved to a common beam of .
© European Southern Observatory (ESO) 2000
Online publication: March 17, 2000