Substructure and halo density profiles in a warm dark matter cosmology

We performed a series of high-resolution collisionless N-body simulations d esigned to study the substructure of Milky Way-size galactic halos (host ha los) and the density profiles of halos in a warm dark matter (WDM) scenario with a nonvanishing cosmological. constant. The virial masses of the host halos range from 3.5 x 10(12) to 1.7 x 10(12) h(-1) M-circle dot, and they have more than 10(5) particles each. A key feature of the WDM power spectru m is the free-streaming length R-f,(WDM), which fixes an additional paramet er for the model of structure formation. We analyze the substructure of hos t halos using three R-f,R-WDM values: 0.2, 0.1, and 0.05 Mpc, and compare r esults to the predictions of the cold dark matter (CDM) model. We find that guest halos (satellites) do form in the WDM scenario, but are more easily destroyed by dynamical friction and tidal disruption than their counterpart s in a CDM model. The small number of guest halos that we find in the WDM m odels with respect to the CDM one is the result of a lower guest halo accre tion and a higher satellite destruction rate. These two phenomena operate a lmost with the same intensity in delivering a reduced number of guest halos at z = 0. For the model with R-f,R-WDM = 0.1 Mpc, the number of accreted s mall halos is a factor of 2.5 below that of the CDM model, while the fracti on of destroyed satellites is almost twice as large as that of the CDM mode l. The larger the R-f,R-WDM value, the greater the size of these two effect s and the smaller the abundance of satellites. Under the assumption that ea ch guest halo hosts a luminous galaxy, we find that the observed circular v elocity function of satellites around the Milky Way and Andromeda is well d escribed by the R-f,R-WDM = 0-1 MPC WDM model. In the R-f,R-WDM - 0.1-0.2 M pc models, the surviving guest halos at z = 0, whose masses are in the rang e M-h approximate to 10(9)-10(11) h(-1) M-circle dot, have an average conce ntration parameter c(1/5) = r(M-h)/r(M-h/5), which is approximately twice a s small as that of the corresponding CDM guest halos. This difference very likely produces the higher satellite destruction rate found in the WDM mode ls. The density profile of host halos is well described by the Navarro, Fre nk, & White (NFW) fit, whereas guest halos show a wide variety of density p rofiles. A tendency to form shallow cores is not evident; the profiles, how ever, are limited by a poor mass resolution in the innermost regions where shallow cores could be expected.