GHOSTS logo title

About GHOSTS

We have made remarkable progress in recent years in our understanding of galaxy formation and evolution. Deep, high redshift observations have constrained the star formation history of the Universe and have unveiled the evolution of galaxy morphologies. These observations have helped to build and solidify our current model of hierarchical structure and galaxy formation - the Lambda CDM paradigm. Yet, the nature of look-back observations does not permit study of the evolution of individual galaxies: our understanding of the galaxy formation process is incomplete. Which high redshift galaxy building blocks end up in what kind of local galaxies? How much of the stellar content of the different galaxy components (bulge, thin and thick disk, halo) is created in situ and how much is accreted? How does the current accretion rate compare to Lambda CDM predictions? One of the best ways to address these questions is to look at the fossil record preserved in the resolved stellar populations of local galaxies.

The discovery of the Sagittarius Dwarf galaxy (Ibata et al. 1994) decisively demonstrated that satellite accretion is ongoing in the Milky Way. Observations of the stellar halo population of the Andromeda galaxy (M31) have revealed that it too contains a highly structured halo with low surface brightness loops and spurs (Ibata et al. 2001; Ferguson et al. 2002; Zucker et al. 2004a,b; Fig. 1). In contrast, M33 shows virtually no halo nor signs of accretion: it seems to consist of a pure quiescent disk formed in situ (McConnachie et al. 2004). Yet, despite the similarly violent histories of the Milky Way and M31 halos, they are remarkably different. The Milky Way halo seems to consist mainly of old, metal poor stars; the halo of M31 contains a significant population of intermediate age stars (~50%) and has overall a much higher metallicity (Brown et al. 2003). Clearly the formation histories of the halos of the Milky Way, M31, and M33 have been very different. A larger sample is therefore needed before general conclusions can be drawn.

Figure 1: The spatial distribution of RGB stars around M31 with an inset image of M31 to scale (Ferguson et al. 2002). There is substantial substructure and the halo is quite flattened, quite unlike the textbook picture of a galaxy halo. The ellipse semi-major axis is 55 kpc. For reference we show our proposed aperture positions for NGC5907 when put on the M31 physical scale.

The GHOSTS survey will provide the definitive HST analysis of extra-planar stellar populations of nearby disk galaxies. We propose to obtain deep ACS and parallel NICMOS and WFPC2 images of several edge-on galaxies with a range in masses (Vrot=80-260 km/s) sampled along one major,one minor, and one intermediate axis. These unique observations will provide:

  • The first systematic study of the shapes of the diffuse stellar halos of spiral galaxies, using the bright Red Giant Branch (RGB) stars as tracer for the faint underlying population.
  • The most detailed comparative study to date of thick disk morphologies and stellar populations.
  • A comprehensive analysis of the stellar halo and thick disk metallicity distributions as a function of galaxy type and position within the galaxy.
  • A sensitive search for tidal streams and dwarf galaxies, using color-magnitude diagram (CMD)- based population contrast enhancement techniques.
  • An unparalleled direct comparison of globular cluster systems and the field stellar populations.

Seven galaxies are the minimum needed to study halo and thick disk characteristics as functions of galaxy properties (mass, morphological type, scale size, etc.) and observing edge-on galaxies is absolutely essential to disentangle the true halo, thick disk, and thin disk components. When put on the same physical scale, our observations will reach at least the same depth and spatial resolution as the Sloan observations of M31 that clearly showed its tidal streams and allowed discovery of the MV = -8 dwarf galaxy Andromeda IX (Zucker et al. 2004a,b)