In the standard cosmic structure formation paradigm, galaxies form in dark matter halos. Smaller galaxies merge with larger halos, becoming satellite galaxies. Satellites lose their mass due to tidal stripping, which form stellar streams and halo stars around the central dominating galaxy. The observation of low-mass satellites and faint diffuse stellar halos can thus help to verify the standard theory. We developed a statistical method to study these low-mass satellites in extra-galactic systems, when their spectroscopic redshifts are not available. It has been applied to three surveys (HSC, DECaLS and SDSS), and the signals are measured for central galaxies spanning a wide range in stellar mass. I will introduce the properties of these faint satellites and the connection to their central galaxies and host halos. A comparison is made against MW satellites, indicating our Milky Way (MW) system is statistically atypical. By stacking galaxy images, we also measured the signals of extended stellar halos for these central galaxies. The PSF-deconvolved surface brightness profiles are close to universal once scaled by the halo virial radius. Red galaxies have more extended stellar halos, redder and shallower color profiles and more satellites than blue counterparts, consistent with the galaxy formation theory that red galaxies stopped forming stars due to feedback, while their host halos, population of satellites and stellar halos keep growing. In the end, I will go back to our MW, whose population of satellites, halo stars and globular clusters can be resolved, and their 6-dimensional phase-space distributions are powerful tools to help constraining the underlying dark matter distribution and substructures. I will briefly advertising our efforts of measuring the total mass of our MW and of measuring the proper motions for blue main sequence halo stars.
Wenting Wang mainly works on galaxy formation and evolution, galaxy-halo connection and dynamical modelling of the Milky Way Galaxy. Wenting got her PhD. at Shanghai Astronomical Observatory in 2012, and meanwhile she visited the Max-Planck Institute for Astrophysics as an exchange student. Afterwards, she worked as a postdoc at Durham University, and at Kavli-IPMU (Tokyo University), before joining the Astronomy department of Shanghai Jiaotong University in 2020. She has got the support from the Youqing fundation of NSFC.