Author(s): , , , , ,
Institution(s): 1. Leibniz-Institut für Astrophysik Potsdam (AIP), 2. Max Planck Institut für Astrophysik, 3. University of Alabama, 4. University of Leiden, 5. University of Michigan
The stars in the halos around galaxies are thought to be for a significant fraction the result of tidally disrupted infalling smaller galaxies, an important process in the mass build up of galaxies. However, in recent year the realization has grown that a fraction of halo stars may have formed in situ or in the disk of the host galaxy.
To understand the nature of stellar halos, the GHOSTS project has used the Hubble Space Telescope to observe the resolved stellar populations in the outskirts of 18 nearby massive galaxies, with a range of masses and seen from face-on to edge-on. From two filter observations (F814W and F606W) with both WFC3 and ACS, we map the different giant populations in these haloes: typically Asymptotic, Red, and He-burning Giants (AGB, RGB and HeB). By studying the distribution of these stellar populations, the GHOSTS project aims to characterize on a statistical basis the size, shape, age, amount of sub-structure, and chemical composition of stellar halos, thereby allowing us to constrain models of the hierarchical galaxy formation process.
In this presentation we will in particular show the structural parameters of the observed stellar halos out to 70 kpc and present the rather small range in color gradients present in the RGB population indicative in small metallicity gradients in these halos. We also present the discovery of a surprisingly large population of AGB stars with ages <2 Gyr seen to ~20 kpc above the disks of many edge-on galaxies and speculations on the origin of these young populations at a location where no current star formation is seen.