2 HI observations

2.1 HI velocity measurements

The technical procedure has been described in Fouqué et al. (1990). The main characteristics of the Nançay radiotelescope receiver are given in Table 1.

The Nançay radiotelescope has a non circular Half-Power-Beam-Width (HPBW) of roughly $\leq 22'\,{\rm NS}\times 4'\,{\rm EW}$ which actually depends on the elevation along the North-South direction. The variation is presented in Fig. 6 (E. Gérard, private communication). The change of the HPWB intervenes for estimating the confusion of a measurement and for the beam filling correction. In this paper, only raw fluxes are given (i.e. uncorrected for beam-filling effect). It is worth noting that, the beam-filling correction also depends on the position angle of the galaxy as described in Bottinelli et al. (1990).

  

Table 1: Characteristics of the Nançay radiotelescope receiver

  

Figure 6: Variation of the North-South Half-Power-Beam-Width of the Nançay radiotelescope according to E. Gérard (private communication)

Annexe: 21-cm line profiles for sosie candidates of NGC 224 (Fig. 7), NGC 3031 (Fig. 8), NGC 253 (Fig. 10) and NGC 5457 (Fig. 9). The x-axis gives the heliocentric radial velocity $c \Delta \lambda /\lambda$ in km s^-1. The y-axis gives the flux density in mJy

  

Figure 7: 21-cm line profiles for sosie candidates of NGC 224

  

Figure 8: 21-cm line profiles for sosie candidates of NGC 3031

  

Figure 9: 21-cm line profiles for sosie candidates of NGC 5457

The 21-cm line widths are measured using an interactive method. The smoothed 21-cm line is displayed on a screen; the user estimates visually the maximum of the profile; the program calculates the width at 20% and 50% of this maximum by considering the points where both levels intercept the profile. This procedure is impossible when the S/N ratio is poor (less than $\approx$3 like for NGC 4675, NGC 5267 and ESO 404-15). Nevertheless, the user has full control to obtain the most secure result, for instance by smoothing the profile in noisy regions. The most critical part is the proper estimation of the maximum.

  

Figure 10: 21-cm line profiles for sosie candidates of NGC 253

When the profile has two maxima at different levels, the adopted maximum is the mean of both if it is compatible with the noise (otherwise, the higher level is chosen, but it is reduced by the standard deviation of the noise).

2.2 Results

All new measurements are presented in Table 2. Mean errors and S/N ratio are calculated according to Fouqué et al. (1990). Columns are arranged as it follows:

• Column 1: Identification with the PGC name (According to Paturel et al. 1989a,b).
• Column 2: Identification with an alternative name according to the following hierarchy: NGC/IC, UGC, ESO, MCG (Paturel 1989a).
• Column 3: Measured heliocentric velocity measured at the middle of the 20% level in km s^-1.
• Column 4: Measured heliocentric velocity measured at the middle of the 50% level in km s^-1.
• Column 5: Internal mean error on both V 20 and V 50.
• Column 6: 21-cm line width W 20 at 20% of the peak value in km s^-1.
• Column 7: Internal mean error on W 20.
• Column 8: 21-cm line width W 50 at 50% of the peak value in km s^-1.
• Column 9: Internal mean error on W 50.
• Column 10: 21-cm flux FH (area defined by the 21-cm line profile, in Jy km s^-1).
• Column 11: Internal mean error on FH.
• Column 12: S/N ratio.

The HI-profiles are given in Figs. 7 to 9 of the Annexe for sosies candidates of NGC 224, NGC 3031, NGC 253 and NGC 5457, respectively. For each spectrum the galaxy name is given above the frame. The PGC name is given in the upper-right corner. The x-axis gives the heliocentric velocity expressed in km s^-1 with the optical convention ($\Delta \lambda / \lambda$). The y-axis gives the flux in mJy.

  

Table 2: New HI data

 

Table 2: continued

2.3 Discussion

The confusion of HI measurements is an old problem in astronomy. For each galaxy a visual inspection of the field has been made using the charts produced with the LEDA database. The galaxies listed below are confused or possibly confused. For each of them we calculate two coefficients in order to quantify the degree of confusion. The first coefficient gives the degree of confusion in position. It is defined as:

$$C_{\rm p}= 1 - \sqrt{\frac{\Delta {x^2}}{a^2} + \frac{\Delta {y^2}}{b^2}}$$ (3)

where $\Delta x$ and $\Delta y$ are the angular distances (in right ascension and declination) between the observed galaxy and the galaxies of the field and where a and b are the HPBW in the corresponding directions. The HPBW in the $\rm N-S$ direction is calculated according to Fig. 6. The second coefficient gives the degree of confusion in velocity. It is defined as:

$$C_{\rm v}=1-\frac{\vert\Delta V\vert}{W_{20}}$$ (4)

where $\Delta V$ is the velocity difference between the observed galaxy and the galaxies of the field and where W₂₀ is the 21-cm line width for the observed galaxy. A confusion exists when $C_{\rm p}$ and $C_{\rm v}$ are both larger than zero. For each confused galaxy we give the maximum value of $C_{\rm p}$ and $C_{\rm v}$.

• PGC 02791: Confused by PGC 2794 = ESO 540-24 ($V=6259~{\rm km~s}^{-1}$)and/or PGC 2796 = ESO 540-25 ($V=6362~{\rm km~s}^{-1}$); $C_{\rm p}=0.83$ and $C_{\rm v}=0.96$.

  
  

• PGC 10595: Confused by PGC 10597 = NGC 1103 ($V=4156~{\rm km~s}^{-1}$).

  
  

• PGC 42708: Confused by PGC 42725 = MCG 9-21-32 ($V=4913~{\rm km~s}^{-1}$); $C_{\rm p}=0.79$ and $C_{\rm v}=0.97$.

  
  

• PGC 08351: Confused by PGC 08352 = NGC 834 ($V=4594~{\rm km~s}^{-1}$); $C_{\rm p}=0.59$ and $C_{\rm v}=0.58$.

  
  

• PGC 02305: The HI spectrum belongs to PGC 2308 = IC 1562 (face on spiral at $V=3671~{\rm km~s}^{-1}$).

  
  

• PGC 31500: The HI spectrum belongs to PGC 31510 ($V=3799~{\rm km~s}^{-1}$).

  
  

• PGC 51456: Possibly confused by PGC 51445 = NGC 5595 ($V=2697~{\rm km~s}^{-1}$); $C_{\rm p}=0.15$ and $C_{\rm v}=0.88$.

  
  

• PGC 70130: Confused by PGC 70127 = MCG-1-58-9 and PGC 70133 = MCG-1-58-11 (V=3666 and 3898 km s^-1, respectively); $C_{\rm p}=0.77$ and $C_{\rm v}=0.99$.

Further, galaxies PGC 04353, PGC 42998, PGC 44589, PGC 05841, PGC 38712, PGC 44294, PGC 53965, PGC 28388, PGC 37719 may be confused by galaxies with unknown radial velocity and/or morphological type.

Acknowledgements

We express our gratitude to the observers of the Nançay radioastronomical observatory and also to N. Hallet and M. Loulergue for their helpful contribution. We would like to thank also Dr. W.K. Huchtmeier for his helpfull comments on the manuscript.