The VVV Survey RR Lyrae Population in the Galactic Center Region

Deep near-IR images from the VISTA Variables in the Via Lactea (VVV) Survey were used to search for RR Lyrae stars within 100 arcmin from the Galactic Center. A large sample of 960 RR Lyrae of type ab (RRab) stars were discovered. A catalog is presented featuring the positions, magnitudes, colors, periods, and amplitudes for the sample, in addition to estimated reddenings, distances, and metallicities, and measured individual relative proper motions. We use the reddening-corrected Wesenheit magnitudes, defined as W-Ks = K-S - 0.428 x (J - K-s), in order to isolate bona fide RRL belonging to the Galaxy Center, finding that 30 RRab are foreground/background objects. We measure a range of extinctions from A(Ks) = 0.19 to 1.75 mag for the RRab in this region, finding that large extinction is the main cause of the sample incompleteness. The mean period is P = 0.5446 +/- 0.0025 days, yielding a mean metallicity of [Fe/H] = 1.30 +/- 0.01 (sigma = 0.33) dex for the RRab sample in the Galactic Center region. The median distance for the sample is D = 8.05 +/- 0.02 kpc. We measure the RRab surface density using the less reddened region sampled here, finding a density of 1000 RRab/sq deg at a projected Galactocentric distance R-G = 1.6 deg. Under simple assumptions, this implies a large total mass (M > 10(9) M-circle dot) for the old and metal-poor population contained inside R-G. We also measure accurate relative proper motions, from which we derive tangential velocity dispersions of sigma V-i = 125.0 and sigma V-b = 124.1 km s(-1) 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. In summary, 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 with respect to 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, has high velocity dispersions and zero rotation, suggesting a formation via dissipational collapse.