The Sun's distance from the Galactic Centre and mid-plane, and the Galactic old bulge's morphology: 715 VVV Type II Cepheids

A statistical method is employed in tandem with new VISTA Variables in the Via Lactea (VVV) near-infrared observations to determine the Sun's distance from the Galactic Centre (r<SUB>0</SUB>, GC), the Sun's height from the local mid-plane (z<SUB>0</SUB>), and to likewise infer the shape of the Galactic ∼10 Gyr old bulge. Specifically, the conclusions stem from an investigation of 715 high-latitude (|b| > 1°) and centrally symmetric concentrated Type II Cepheids (T2Cs) recently identified in the VVV survey by Braga et al. The analysis yields r<SUB>0</SUB> = 8.35 ± 0.10 kpc and z<SUB>0</SUB> = 10 ± 2 pc. The T2Cs distribution within the effective bulge radius r<SUB>bulge</SUB> = 2-3 kpc is an ellipsoid exhibiting axial ratios of ≍1:0.7:0.6, with the major axis inclined at an angle θ ≍ -3° to the Sun-GC sightline. T2Cs do not trace a prominent barred structure at distances >1 kpc from the GC. A key conclusion is that analyses of independent optical and infrared Optical Gravitational Lensing Experiment (OGLE) and VVV observations yield consistent results (e.g. r<SUB>0</SUB> > 8.0 kpc and both observations display a comparable shape of an ellipsoid), thus providing a constrained and reduced systematic uncertainty.