The abundance/distribution of low-mass satellite galaxies is one of the most effective tools to distinguish different dark matter models. However, current observations of faint low-mass satellites are mainly focused on those in our Milky Way (MW). Previous studies on MW satellites have revealed a series of so-called small-scale crisis to the standard cosmological model, which are, however, still limited by two major sources of uncertainties: 1) It is dangerous to draw strong conclusions based on only one galaxy, our MW, and it still awaits follow-up studies on extra-galactic satellites to reveal whether our MW is special and whether conclusions based on our MW is universal; 2) the total mass of our MW measured in the past has a large scatter, and if the mass of our MW is small enough, the previously raised crisis would have gone. In this talk, I will introduce my previous attempts of trying to understand the two sources of uncertainties.
In the first part, we have developed a statistical method of counting faint photometric satellites around spectroscopic central galaxies, which has enabled us to obtain the intrinsic properties of low-mass satellites without spectroscopic observations. We apply the method to count sources from DECaLS, HSC and SDSS around central primary galaxies sfrom the SDSS/DR7 spectroscopic galaxy sample. Our results reveal that there are on average 1.5 to 2.5 extra-galactic satellites with MV<-16 per MW-mass central primary, consistent with our MW. However, the averaged satellite luminosity functions (LFs) of centrals selected in different ways all show steeper slopes and have more satellites than the MW system at fainter magnitudes, which cannot be fully accounted for by the large scatter. Besides, it seems our MW system is more atypical amongst other MW-mass systems in the Local Volume (LV), perhaps indicating the LV is an under-dense region. We also discovered that the bright end cutoff of satellite LFs and the satellite abundance are both sensitive to the magnitude gap between the primary and its companions, indicating galaxies with larger magnitude gaps are on average hosted by less massive dark matter haloes.
In the second part, I will introduce our efforts of testing tranditional dynamical modelling methods of constraining the total mass of our MW, by applying the methods to numerical simulations to investigate its performance and systematics. We found violations of the steady-state and spherical symmetry assumptions can introduce ~0.1 dex of uncertainty for dark matter particles as tracers and a factor of 2-3 uncertainties for star particles as tracers, even when the full 6-dimensional information of tracers is used. Stars more significantly violate the steady-state assumption because they are stripped later and have less time to reach the steady state. As a result, steady-state tracers out to large radius and having proper motion measurements are important for precise determination of the total mass for our MW.
