Astron. Astrophys. 346, 45-57 (1999)

2. Observations

2.1. Interferometric ¹²CO observations

Interferometric ¹²CO observations were obtained at the Owens Valley Radio Observatory (OVRO) millimeter array. The six 10.4 m telescopes are equipped with SIS receivers providing system temperatures of typically K (single sideband). Details of the observations are given in Table 2. With natural weighting, we obtained a roughly circular beam of size (PA 46^o and sensitivity to extended structure up to 17". The quasar 2345-167 was observed as a phase and amplitude calibrator every minutes. Passband calibration was done using stronger sources, usually 0528+134 or 3C 454.4. These sources, calibrated against the planets Neptune and Uranus, were also taken for flux calibration. The tracks were calibrated using software developed for the OVRO array. After calibration, the data were inspected for quality, and low-coherence data points were edited out on a baseline-to-baseline basis.

Table 2. Summary of the interferometric observations.
Notes:
a) The array configurations at OVRO are labelled C (compact), L (low resolution) and H (high resolution)

For a beam size of , a brightness temperature () of 1 K corresponds to 0.304 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 = 1200 km s ^-1.

Imaging was done using the NRAO AIPS package. To cover the long bar of UGC 2855 fully, we obtained a 3-field mosaic with a spacing of between the pointing centers. The fields were CLEANed individually and combined linearly, correcting for the attenuation of the primary beam, using the AIPS task LTESS.

To analyse the innermost structure (see Sect. 4.1), the beam size could be reduced to (PA 78^o) applying robust weighting (Briggs 1995), while a beam size of (PA -19^o) was reached when only the longer baseline data from the high resolution track were used. This (naturally weighted, since the signal-to-noise in the robustly weighted maps is too low) map includes baselines from 230 m to 35 m length, which means that structures larger than cannot be imaged. The increased resolution also goes along with a loss in sensitivity: The rms noise per channel increases from 0.11 K (34 mJy beam^-1) in the naturally weighted map to 0.43 K (52 mJy beam^-1) in the robustly weighted and 1.34 K (62 mJy beam^-1) in the high-resolution-only maps.

The interferometric ¹²CO observations of UGC 2866 are summed up in Table 2. The naturally-weighted map of UGC 2866 has a beam size of (PA -68^o) and an rms noise of 0.20 K (38 mJy beam^-1). The velocity resolution, velocity range and central velocity of these observations were the same as those of UGC 2855.

2.2. Interferometric ¹³CO observations

Data in the ¹³CO transition of the same three fields in UGC 2855 (see Table 2) that were mapped in ¹²CO were taken at OVRO in June 1998 under excellent weather conditions, using the same spectrometer configuration as for ¹²CO. The calibration and mapping procedure was identical to the ¹²CO measurements. The (naturally weighted) synthesised beam has a size of (PA ), and the sensitivity reached is 0.09 K (23 mJy beam^-1).

2.3. Single-dish observations of UGC 2855

Single dish ¹²CO and ¹³CO data of UGC 2855 were obtained with the 20 m telescope at Onsala Space Observatory in December 1997 and January 1998. We observed 7 spectra along the bar in ¹²CO, spaced by 15" along the bar, and 5 positions in ¹³CO (see Fig. 3). The central position ( almost exactly coincided with the interferometric CO emission peak. The typical system temperature was 500 K - 600 K for ¹²CO and 200 K - 300 K for ¹³CO. The beam size is , and the spectra have been smoothed to a velocity resolution of km s ^-1. The Onsala spectra do not fully sample the entire area of the OVRO interferometer map and have pointing uncertainties of , preventing us from combining the single-dish data with the interferometry data into a single map.

2.4. H

An H image of UGC 2855 and UGC 2866 was taken using HoLiCam (Reif et al. 1995) at the 1.06 m telescope at Hoher List Observatory in October 1997. The conditions were not photometric, and the image provides relative information on fluxes only. However, no other deep H imaging of these galaxies is available at this time. The image was processed using the IRAF package.

© European Southern Observatory (ESO) 1999

Online publication: May 6, 1999
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