The VVV Survey RR Lyrae Population in the Galactic Centre Region

Abstract

Deep near-IR images from the VVV Survey were used to search for RR Lyrae type ab (RRab) stars within 100' from the Galactic Centre (GC). A sample of 960 RRab stars were discovered. We use the reddening-corrected magnitudes in order to isolate RRab belonging to the GC. The mean period for our RRab sample is $P=0.5446$ days, yielding a mean metallicity of $[Fe/H] = -1.30$ dex and a median distance from the Sun of $D=8.05$. We measure the RRab surface density using the less reddened region sampled here, finding $1000$ RRab/sq deg at a projected Galactocentric distance $R_G=1.6$ deg. This implies a large total mass ($M>10^9 M_\odot$) for the old and metal-poor population contained inside $R_G$. We measure accurate relative proper motions, from which we derive tangential velocity dispersions of $\sigma V_l = 125.0$ and $\sigma V_b = 124.1$ km/s along the Galactic longitude and latitude coordinates, respectively. The fact that these quantities are similar indicate that the bulk rotation of the RRab population is negligible, and implies that this population is supported by velocity dispersion. There are two main conclusions of this study. First, the population as a whole is no different from the outer bulge RRab, predominantly a metal-poor component that is shifted respect the Oosterhoff type I population defined by the globular clusters in the halo. Second, the RRab sample, as representative of the old and metal-poor stellar population in the region, have high velocity dispersions and zero rotation, suggesting a formation via dissipational collapse.