RR Lyrae stars in the Milky Way stellar halo

Abstract

A current important research topic in modern Astrophysics is to understand the formation process of galaxies like the Milky Way. The Milky Way Stellar Halo preserves the most useful information about the Galaxy evolution. Despite a number of detailed studies over many years, the shape and extent of the Galactic Halo is still debated and knowledge about it is incomplete. The existence of the halo stellar streams provides important clues to the hierarchical formation of our galaxy. We study these substructures through the characteristics of various stellar populations. RR Lyraes (RRLs) are very good Galactic halo tracers because of their relatively high intrinsic and well established absolute magnitudes. RRLs are metal poor and very old and can hold information of galaxy evolution. Their characteristic colors and light curves made them easy to be identified. Keller et al. (2008) found that the power-law slope of the RR Lyrae space density distribution is steepened beyond the Galactocentric radius 45 kpc. They identified 2016 RRL candidates derived from the analysis of archival observations of the Southern Edgeworth-Kuiper Belt Object (SEKBO) survey. We have investigated this result by following up on a subset of 137 candidates with a range of magnitudes (V{u0303}14-20) using the Faulkes Telescope (FT) database and confirmed 57 candidates as real RRLs. A cross-match between SEKBO RRL survey and Sloan Digital Sky Survey Data Release-7 (SDSS DR-7) revealed 272 RRL candidates in common. Applying the color selection criteria proposed by Ivezic' et al. (2005) resulted in 193 likely RRLs. The completeness as a function of magnitude was calculated empirically from the combined set of SEKBO RRL candidates from current FT data, SDSS cross-matched data, and the Prior et al. (2009) catalog. This resulted in a spatial density distribution characterized by two power laws with a break radius R within a range between 45 kpc and 50 kpc, similar to the results of Keller et al. We find the power-law slopes for the inner and the outer halo as -2.78+/-0.02 and -5.0+/- 0.2. The SkyMapper telescope survey promises to deliver better quality data for the entire southern hemisphere sky. We quantify the likely efficiency and completeness of the survey as regards the detection of RRL stars via observations of the RRL-rich globular cluster NGC3201. For single-epoch 'uvgri' observations followed by two further epochs of 'gr' imaging, as per the intended 3-epoch survey strategy, we recover known RRLs with >90% efficiency. We also investigate boundaries in the gravity-sensitive single-epoch 2-colour diagram that yield high completeness and high efficiency (i.e. minimal contamination by non-RRLs). The availability of multi-epoch data is one of the big advantages of the SkyMapper survey over SDSS. The primary selection for RRLs in SkyMapper through variability and then location in the gravity sensitive diagram will help to find thousands of RRLs in the southern sky over the entire halo and will likely identify many new substructures. These will make a major contribution to the shape and the extent of the halo and will constrain Galaxy formation models.