SATELLITES IN MILKY-WAY-LIKE HOSTS: ENVIRONMENT DEPENDENCE AND CLOSE PAIRS

Previous studies showed that an estimate of the likelihood distribution of the Milky Way (MW) halo mass can be derived using the properties of the satellites similar to the Large and Small Magellanic Clouds (LMC and SMC). However, it would be straightforward to interpret such an estimate only if the properties of the Magellanic Clouds (MCs) are fairly typical and are not biased by the environment. In this study, we explore whether the environment of the MW affects the properties of the SMC and LMC such as their velocities. To test for the effect of the environment, we compare velocity distributions for MC-sized subhalos around MW hosts in a sample selected simply by mass and in the second sample of such halos selected with additional restrictions on the distance to the nearest cluster and the local galaxy density, designed to mimic the environment of the Local Group (LG). We find that satellites in halos in the LG-like environments do have somewhat larger velocities, as compared to the halos of similar mass in the sample without environmental constraints. For example, the fraction of subhalos matching the velocity of the LMC is 23% +/- 2% larger in the LG-like environments. We derive the host halo likelihood distribution for the samples in the LG-like environment and in the control sample and find that the environment does not significantly affect the derived likelihood. We use the updated properties of the SMC and LMC to derive the constraint on the MW halo mass of log (M-200/M-circle dot) = 12.06(-0.19)(+0.31) (90% confidence interval). We also explore the incidence of close pairs with relative velocities and separations similar to those of the LMC and SMC and find that such pairs are quite rare among Delta CDM halos. Only 2% of halos in the MW mass range have a relatively close pair (Delta r < 40 kpc and Delta s < 160 km s(-1)) of subhalos with circular velocities v(circ) > 50 km s(-1). Pairs with masses and separations similar to those of the LMC and SMC (Delta r(MC) = 23.4 +/- 10 kpc and Delta s(MC) = 128 +/- 32 km s(-1)) are found only in one out of approximate to 30,000 MW-sized halos. Interestingly, the halo mass likelihood distribution for host halos constrained to have MC-like close pairs of subhalos is quite different from the global likelihood from which the MW halo mass constraint discussed above was derived. Taking into account the close separation of the MCs in the Busha et al. method results in the shift of the MW halo mass estimate to smaller masses, with the peak shifting approximately by a factor of two.