According to the hierarchical clustering scenario, galaxies are assembled by merging and accretion of numerous satellites of di †erent sizes and masses. This ongoing process is not 100% efficient in destroying all of the accreted satellites, as evidenced by the satellites of our Galaxy and of M31. Using published data, we have compiled the circular velocity distribution function (VDF) of galaxy satellites in the (V circ ) Local Group. We Ðnd that within the volumes of radius of 570 kpc (400 h~1 kpc assuming the Hubble constant1 h \ 0.7) centered on the Milky Way and Andromeda, the average VDF is roughly approximated as km s~1)~1.4B0.4 h3 Mpc~3 for in the range B10È70 km s~1.The observed VDF is compared with results of high-resolution cosmological simulations. We Ðnd that the VDF in models is very di †erent from the observed one :Mpc~3. Cosmological models thus predict that a halo the size of our Galaxy should have about 50 dark matter satellites with circular velocity greater than 20 km s~1 and mass greater than 3 ] 108 within M _ a 570 kpc radius. This number is signiÐcantly higher than the approximately dozen satellites actually observed around our Galaxy. The di †erence is even larger if we consider the abundance of satellites in simulated galaxy groups similar to the Local Group. The models predict D300 satellites inside a 1.5 Mpc radius, while only D40 satellites are observed in the Local Group. The observed and predicted VDFs cross at B50 km s~1, indicating that the predicted abundance of satellites with km s~1 V circ Z 50 is in reasonably good agreement with observations. We conclude, therefore, that unless a large fraction of the Local Group satellites has been missed in observations, there is a dramatic discrepancy between observations and hierarchical models, regardless of the model parameters. We discuss several possible explanations for this discrepancy including identiÐcation of some satellites with the high-velocity clouds observed in the Local Group and the existence of dark satellites that failed to accrete gas and form stars either because of the expulsion of gas in the supernovae-driven winds or because of gas heating by the intergalactic ionizing background.

We describe the Sloan

We used fully cosmological, high‐resolution N‐body + smooth particle hydrodynamic (SPH) simulations to follow the formation of disc galaxies with rotational velocities between 135 and 270 km s−1 in a Λ cold dark matter (CDM) universe. The simulations include gas cooling, star formation, the effects of a uniform ultraviolet (UV) background and a physically motivated description of feedback from supernovae (SNe). The host dark matter haloes have a spin and last major merger redshift typical of galaxy‐sized haloes as measured in recent large‐scale N‐body simulations. The simulated galaxies form rotationally supported discs with realistic exponential scalelengths and fall on both the I band and baryonic Tully–Fisher relations. An extended stellar disc forms inside the Milky Way (MW)‐sized halo immediately after the last major merger. The combination of UV background and SN feedback drastically reduces the number of visible satellites orbiting inside a MW‐sized halo, bringing it in fair agreement with observations. Our simulations predict that the average age of a primary galaxy's stellar population decreases with mass, because feedback delays star formation in less massive galaxies. Galaxies have stellar masses and current star formation rates as a function of total mass that are in good agreement with observational data. We discuss how both high mass and force resolution and a realistic description of star formation and feedback are important ingredients to match the observed properties of galaxies.

The fourth generation of the Sloan

We performed a series of high-resolution collisionless N-body simulations designed to study the substructure of Milky Way-size galactic halos (host halos) and the density profiles of halos in a warm dark matter (WDM) scenario with a non-vanishing cosmological constant. The virial masses of the host halos range from 3.5 × 10 12 h −1 M ⊙ to 1.7 × 10 12 h −1 M ⊙ and they have more than 10 5 particles each. A key feature of the WDM power spectrum is the free-streaming length R f,W DM which fixes an additional parameter for the model of structure formation. We analyze the substructure of host halos using three R f,W DM values: 0.2, 0.1, and 0.05 Mpc and compare results 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 counterparts in a CDM model. The small number of guest halos that we find in the WDM models with respect to the CDM one is the result of a lower guest halo accretion and a higher satellite destruction rate. These two phenomena operate almost with the same intensity in delivering a reduced number of guest halos at z = 0. For the model with R f,W DM = 0.1 Mpc the number of accreted small halos is a factor 2.5 below that of the CDM model while the fraction of destroyed satellites is almost twice larger than that of the CDM model. The larger the R f,W DM value the greater the size of these two effects and the smaller the abundance of satellites. Under the assumption that each guest halo hosts a luminous galaxy, we find that the observed circular velocity function of satellites around the Milky Way and Andromeda is well described by the R f,W DM = 0.1 Mpc WDM model. In the R f,W DM = 0.1 − 0.2 Mpc models, the surviving guest halos at z = 0 -whose masses are in the range M h ≈ 10 9 − 10 11 h −1 M ⊙ -have an average concentration parameter c 1/5 (= r(M h )/r(M h /5)) which is approximately twice smaller than that of the corresponding CDM guest halos. This difference, very likely, produces the higher satellite destruction rate found in the WDM models. The density profile of host halos is well described by the NFW fit whereas guest halos show a wide variety of density profiles. A tendency to form shallow cores is not evident; the profiles, however, are limited by a poor mass resolution in the innermost regions were shallow cores could be expected.