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Around the Spindle galaxy: the dark halo mass of NGC3115

We report observations of five dwarf galaxies in the vicinity of the luminous S0 galaxy NGC 3115 performed with the Advanced Camera for Surveys on the Hubble Space Telescope. Their distances determined via the Tip of the Red Giant Branch are: 10.05 Mpc (UGCA 193), 9.95 Mpc (KKSG 17), 10.13 Mpc (2MASX-J0957-0915), 10.42 Mpc (2dFGRS-TGN218Z179) and 11.01 Mpc (KKSG 19). With their typical distance error of about 0.75 Mpc all the five dwarfs are consistent to be true satellites of the host galaxy NGC 3115 (10.2$\pm$0.2 Mpc). Using the DESI Legacy Imaging Surveys we also found 5 new probable dwarf satellites of NGC3115, as well as 4 new probable members of the neighboring group around NGC 3521 situated 3 Mpc away from the NGC 3115 group. Based on the radial velocities and projected separations of 10 dwarf companions, we derived the total (orbital) mass of NGC 3115 to be (4.89$\pm$1.48) $10^{12}$ $M_{\odot}$. The ratio of the total mass-to-K-luminosity of NGC 3115 is (50$\pm$15) $M_\odot/L_\odot$, which is typical for the early-type luminous galaxies.