Strömgren and near-infrared photometry of metal-rich bulge globular clusters. I. NGC 6528 and its surrounding field

We present Strömgren and near-infrared (NIR) photometry of the bulge cluster NGC 6528 and its surrounding field in Baade’s Window. uvby images were collected with EFOSC2 on the New Technology Telescope (NTT, La Silla, ESO). The NIR catalogs are based on J,K-band VIRCAM at VISTA (Paranal, ESO) and SOFI at NTT photometry. We matched the aforementioned data sets with Hubble Space Telescope photometry to obtain proper-motion-cleaned samples of NGC 6528 and bulge stars. Furthermore, we were able to correct the Strömgren-NIR photometry for differential reddening. The huge color sensitivity of the Strömgren-NIR color–magnitude-diagrams (CMDs) helped us in separating age and metallicity effects. The red giant branch (RGB) of NGC 6528 is well reproduced in all the CMDs by adopting scaled solar isochrones with solar abundance, that is Z = 0.0198, or α-enhanced isochrones with the same iron content, that is Z = 0.04, and an age range of t = 10–12 Gyr. The same isochrones well reproduce most of the color spread of Baade’s Window RGB. These findings support the literature age estimates for NGC 6528. We also performed a new theoretical visual-NIR metallicity calibration based on the Strömgren index m1 and on visual–NIR colors for red giant (RG) stars. Scaled solar and α-enhanced models were adopted and we validated the new metallicity-index-color (MIC) relations by applying them to estimate the photometric metal abundance of a sample of field RGs and of a metal-poor (M 92, [Fe/H] ~−2.3) and a metal-rich (NGC 6624, [Fe/H] ~ −0.7) globular cluster. We applied the calibration to estimate the mean metal abundance of NGC 6528, finding [Fe/H] = [M/H] = −0.04 ± 0.02, with a mean intrinsic dispersion of σ = 0.27 dex, by averaging the metallicities obtained with the scaled solar [m], y − J and [m], y − K MIC relations, and of −0.11 ± 0.01, with σ = 0.27 dex, by using the m1, y − J and m1, y − K relations. These findings support results based on high-resolution spectroscopy, which approximately give a solar metallicity for NGC 6528 with a modest α-enhancement. By applying the scaled solar MIC relations to the sample of Baade’s Window RGs, we find a metallicity distribution that extends approximately from [Fe/H] ~ −1.0 up to [Fe/H] ~ 1 dex, with two metallicity peaks at [Fe/H] ≈ −0.2 and [Fe/H] ≈ 0.55 ([m] , y − J and [m] , y − K relations), and [Fe/H] ≈ −0.25 and [Fe/H] ≈ 0.4 (m1, y − J and m1, y − K relations). These findings fairly agree with previous spectroscopic studies for Baade’s Window and the recent results of the ARGOS surveys for the bulge