Annu. Rev. Astron. Astrophys. 1979. 17: 135-87 Copyright © 1979 by Annual Reviews. All rights reserved

6.5 Conclusion

It is still impossible to give a definitive value for the mass-to-light ratios of small groups of galaxies because the remaining uncertainties are large. As a temporary expedient, one can use the numerical simulation of Aarseth et al. to estimate the effect of contamination on the median M / L_B for unculled TG groups. This is the only bias-free method available at present. We employ the model with = 0.1, including only those simulated groups with three or more members. The median M / L_B for this subsample is 200, compared to the true value of 70 for the model as a whole. The median M / L_B for observed unculled TG groups having three or more members is 80. Assuming the simulated universe is a fair match to the real one, we apply the correction factor 70/200 to this value, obtaining 30 as the best unbiased estimate for the median M / L_B of small groups. This value is to be compared with values of 30 and 40 for the culled TG groups found by Gott & Turner and Rood & Dickel, and 90 for both the TG and STV groups having the best membership assignments (Rood & Dickel 1978a). These values probably encompass the allowable range. Though still very uncertain, these estimates seem distinctly higher than M / L_B within R_HO for individual spirals (~ 5) yet very compatible with the range of 35-50 estimated for binaries in the previous section. The mass-to-light ratios in groups therefore do not imply the existence of additional dark matter beyond that sampled by the orbits of wide binary galaxies.