3 H I data for VV 747

For VV 747, the interpretation of the optical data remains ambiguous regarding its multiplicity (see Sect. 4.3). In order to settle this ambiguity we investigated the dynamical state of the system and performed single-dish H I observations.

3.1 Observations and data reduction

The 21 cm H I-line observations were carried out in July 1999 with the Nançay 300m telescope (NRT). The NRT has a half-power beam width of 3.7$^{\prime}$ (EW) $\times$ 22$^{\prime}$ (NS) at the declination $\delta = 0^\circ$.

A dual polarization receiver has been used, with a system temperature of $\approx$ 40 K in the horizontal and vertical linear polarizations. The gain of the telescope was 1.1 K Jy^-1 at a declination of $\delta = 0^\circ$. The observations were made in position switching mode with 1-minute on-source and 1-minute off-source integrations.

Since VV 747 had a known optical redshift, we split the autocorrelator into two, each bank covering a bandwidth of 6.4 MHz, and centred at the frequency corresponding to the optical redshift. The velocity coverage was $1\,350$ km s^-1. The channel width was 2.6 km s^-1 and after averaging pairs of adjacent channels the effective resolution was 6.3 km s^-1.

The data has been reduced using the software developed by the telescope's staff. Both polarization spectra were calibrated and processed independently, and were averaged to improve the signal-to-noise ratio. Errors were calculated following Schneider et al. ([1986]).

   
3.2 Results. H I-profile and its parameters

For the total integration time of 234 minutes (ON+OFF source) the rms noise is 4 mJy. The galaxy is detected with S/N ratio of 12. The spectrum is presented in Fig. 4. The line shape is close to Gaussian and typical of low-mass galaxies. Its width at 50% of peak W_0.5 = 90 km s^-1is close to the median value of the distribution derived for the statistical sample of BCGs from the zone of the Second Byurakan Survey (SBS) by Thuan et al. ([1999]). The line width at 20% of peak W_0.2 = 117 km s^-1 is very close to that of VV 432. This is important in the following discussion about the applicability of the Tully-Fisher relation ([1977], hereafter TF) for the distance problem of the latter galaxy.

The H I-mass of VV 747 of 0.8 10⁸ $M_{\odot}$, is typical of low luminosity gas-rich galaxies, like the ratio of the hydrogen mass over the blue luminosity: M(H I)/L_B = 1.0.