3 The environment

Among the warped objects, 10 galaxies are members of interacting systems. The relative fraction of interacting galaxies - 17% - is higher than the analogous fraction - 6% - for our complete sample of 540 galaxies (Reshetnikov & Combes 1998). The fraction of isolated galaxies (9 objects $-15\%$) is smaller in the warped sample than in the control sample (25%) while the relative number of galaxies with companions (68%) is the same in both samples. This supports our conclusion that S-shaped warps are connected with galaxy environment (Reshetnikov & Combes 1998). But this connection is not perfectly tight since there are warped galaxies among relatively isolated objects (an interpretation could be in terms of recent accretion).

To get more insight on the large-scale environment of warped galaxies, we have tried to compute the average density of galaxies around the S-shape warped population, and compare it with a control sample. The control has been selected from the un-warped FGCE galaxies, with the condition that the asymmetry index along the minor axis is lower than 1.05 (cf. Reshetnikov & Combes 1998). We have used the Southern Sky Redshift Survey (SSRS2, da Costa et al. 1998), where redshifts and magnitudes are reported for 5369 galaxies. Unfortunately, the redshifts are not known for all of the FGCE catalog, and we have only extracted from NED 15 redshifts for the warped sample, and 17 for the control (non-warped) galaxies. Some of them are plotted in Fig. 5 superposed on the SSRS2 points.

  

Figure 5: Location of some of the FGCE galaxies (filled triangles) among the SSRS2 survey objects (dots). The left sector corresponds to warped galaxies and right to un-warped ones, between declinations -24 and $-37^\circ$. Right ascensions run from 0 to 5 h, and the maximum distance is 250 Mpc (H0=75 km s-1/Mpc)

Around each object of our sample, we compute the average distance of the SSRS2 galaxies, given a search radius $R_{\rm s}$.This average distance $d_{\rm m}$ is computed taking the luminosity of galaxies as weight. The mean density is then estimated as

$$\rho_{\rm m}=\frac{3 N_{\rm gal}}{4 \pi d_{\rm m}^3}$$

where $N_{\rm gal}$ is the total number of objects within $R_{\rm s}$. Taking a common weight for all galaxies only changes $d_{\rm m}$ by 10% at most. The results obtained for the warped and control samples are displayed in Table 2. The average density appears 3-4 times higher for the warped objects. This result has to be confirmed by more statistics, when the redshifts for the whole FGCE catalog are known.

  

Table 2: Average density around warped and non-warped galaxies

$R_{\rm s}$ is the search radius, and the average densities are in gal Mpc^-3