Astron. Astrophys. 363, 93-107 (2000)

2. Observations

2.1. Interferometric observations at OVRO

We have obtained interferometric maps of the transitions of ¹²CO, ¹³CO, HCN and in the inner 1´ (9 kpc) of NGC 7479 using the Caltech six-element Owens Valley Radio Observatory (OVRO) millimeter array. All observations were centered on the radio continuum peak. The quasar 3C454.5 was observed every  min as a phase and amplitude calibrator. 3C345 was used as an additional passband calibrator, and flux calibration was done relative to the planets Neptune and Uranus. Calibration was done with the standard package developped for OVRO. Continuum emission with a peak flux of 3-4  mJy beam^-1 has been subtracted from all maps.

The observations were carried out in the low resolution and equatorial configurations of the OVRO array between February and June 1996. The naturally weighted synthesized beam sizes are 4 6 4 25 for ¹²CO, 4 9 4 05 for ¹³CO and 7 2 5 2 for HCN. At D=32 Mpc, equals 155 pc. The positional accuracy of structures in the maps is and thus considerably higher than the resolution. Due to the limited uv-coverage, the maps are not sensitive to structures larger than for CO and for HCN. On-source integration times were 7.2 hr for ¹²CO, 23.5 hr for ¹³CO and 11.5 hr for HCN.

The parameters of the observations are summarized in Table 2.

Table 2. Summary of the interferometric observations of NGC 7479.
Notes:
a) The array configurations we used at OVRO are labelled L (low resolution) and E (equatorial)
b) This is the rms noise per channel (20 km s^-1 for CO, 40 km s^-1 for HCN). On a  mJy beam^-1-scale, the values correspond to 18  mJy beam^-1 (¹²CO), and 5  mJy beam^-1 (¹³CO and HCN).

For a beamsize of , i.e. ¹²CO, a brightness temperature () of 1 K corresponds to 0.210 Jy beam^-1 at a wavelength of 2.6 mm. The spectral resolution of our data is 4 MHz or 10.04 km s^-1, and the total velocity range covered by the autocorrelator is 1120 km s^-1, centered on = 2500 km s^-1. For the final presentation, the data were smoothed to a velocity resolution of 20 km s^-1 (¹²CO and ¹³CO) or 40 km s^-1 (HCN).

Imaging was done using the NRAO AIPS package. For image display and analysis of the data cubes, the ATNF Karma package was also used.

2.2. Single dish observations at OSO

Single dish spectra of the transitions of ¹²CO and ¹³CO were taken at the Onsala Space Observatory 20 m telescope in 1997. The central position of NGC 7479 was observed with a beamsize of 33" and system temperatures of 500 K - 600 K (¹²CO) and 200 K - 300 K (¹³CO). The spectra were smoothed to a velocity resolution of 30 km s^-1.

© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
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