GHOSTS

The Galaxy Halos, Outer disks, Substructure, Thick disks and Star clusters (GHOSTS) is a survey of resolved stellar populations in the outskirts of nearby spirals using the Hubble Space Telescope, the largest of its kind to date. The survey comprises 18 disk galaxies with distances from 3 to 17 Mpc and rotation velocities of 80-250 km/s. The color-luminosity diagrams of the stars allow us to make estimates of the ages and chemical enrichment of the stars around these galaxies. These faint stars can be traced to very large distances from the galaxies, allowing us to measure the shape and structure of the stellar halos around the host galaxies, thought to be the remains of tidally disrupted dwarf galaxies. GHOSTS has further been used to show that the age distribution of stars across disk breaks is constant with a picture of strong radial migration of stars, to show that disk heating takes place rapidly, and that gas warps on the outskirts of galaxies only contain young stars, no old stars, ruling out torques as the main driver of warp formation.