The formation of the Galactic bulge and halo: Observational signatures

Abstract

The evolution of tidal debris within the Galactic halo has been simulated to determine its detectability within the constraints imposed by currently available telescopes and instrumentation. Observations of the simulations show that there is a very high probability of detecting and quantifying the presence of tidal debris with a pencil beam survey of 100 square degrees. The debris is readily detectable via the presence of kinematic substructure in the radial velocities. The detection probabilities show surprisingly little change with the age of the debris. Accretion events that occurred up to ≳ 10 Gyr ago can be detected. In the limiting case of a single 10⁷ M(⊙) satellite contributing 1% of the luminous halo mass the detection probability is a few percent using just the velocities of 100 halo stars in a single 1 deg² field. The detection probabilities scale with the accreted fraction of the halo and the number of fields surveyed. Accurate CMDs in the Washington photometric system have been derived for four fields spanning the range of Galactocentric distances from 1.5 to 5.5 kpc. The differential reddening variations within each field were corrected by a new technique optimized for the highly variable reddening variations found in bulge fields. Abundance distributions in the four fields were derived from color-color diagrams in the Washington system. The quality of the photometry which yields photometric abundances with σ[Fe/H] ≲ 0.25 dex (including reddening errors) supplemented by the luminosity information from observations in the 51 filter allows contamination by foreground and background stars to be eliminated from the bulge sample. A clear abundance gradient is seen which is consistent with the change in morphology of the CMDs. The abundance gradient is predominantly due to a decrease in the fraction of stars in the metal-rich shoulder of the abundance distributions. The modal abundance changes little. Relative to Baade's window the magnitude distribution of clump stars in the L354 B-06 field implies a bar angle of ≃ 40 deg.