5 Environment effects

Many publications describe a segregation of galaxies by morphological types and HI content as dependent on the number density of surrounding galaxies. The most distinct deficiency of HI-rich galaxies is seen in the central parts of clusters [7, (Dressler 1984;] [5, Giovanelli & Haynes 1991).] But the data on segregation outside galaxy clusters look rather controversal.

To describe the local mass density around a galaxy "i'' we introduced the so-called "tidal index" or isolation index [16, (Karachentsev & Makarov 1998):] where M_k is the total mass of any neighbouring galaxy separated from the considered galaxy by a distance of D_ik. For every galaxy "i" we found its "main disturber", producing the maximum density enhancement, $\Delta\rho\sim M_k/D^3_{ik}$. The value of constant C is choosen so that $\Theta=0$when the Keplerian cyclic period of the galaxy with respect to its main disturber equals the cosmic Hubble time, 1/H. Therefore, galaxies with $\Theta < 0$ may be considered as well isolated objects.

 

Figure 8: The plot of HI mass-to-luminosity ratio versus the "tidal index" defined above and proportional to the local mass density. The solid line shows the least-squares regression. Some galaxies with extreme parameters, like NGC 205, are indicated with their name in the figure. The quantities "r" and "k" in a corner correspond to the regression parameters in the Cols. (4) and (7) of Table 1

The distribution of the LV galaxies in $M_{\rm HI}/L$ versus index $\Theta$ is plotted in Fig. 8. As it can be seen from this diagram, in the whole range of local densities, $\Delta{\rm log}\rho_k = 6$, the mean hydrogen mass-to-luminosity ratio remains approximately constant, showing an insignificant segregation effect from environment. It should be, however, noted that many spheroidal dwarf galaxies, whose HI-fluxes lie below a threshold detection, are not represented in Fig. 8, which produces an effect of observational selection. Another global parameter of galaxies, M₂₅/L, seems also to have the same mean value as for very isolated galaxies as well as for tight companions of massive galaxies (see line 24 in Table 1).