Stroemgren - near-infrared photometry of the Baade's Window. I. The
  bulge globular cluster NGC6528 and the surrounding field
  We present Stroemgren-NIR photometry of NGC6528 and its surroundings in the
Baade's Window. uvby images were collected with EFOSC2@NTT, while NIR catalogs
are based on VIRCAM@VISTA and SOFI@NTT data. The matching with HST photometry
allowed us to obtain proper-motion-cleaned samples of cluster and bulge stars.
The huge color sensitivity of Stroemgren-NIR CMDs helped us in disentangling
age and metallicity effects. The RGB of NGC6528 is reproduced by scaled-solar
isochrones with solar abundance or alpha-enhanced isochrones with the same iron
content, and an age of t = 11+/-1 Gyr. These findings support literature age
estimates for NGC6528. We also performed a theoretical metallicity calibration
based on the Stroemgren index m1 and on visual-NIR colors for RGs, by adopting
scaled-solar and alpha-enhanced models. We applied the calibration to estimate
the metallicity of NGC6528, finding [Fe/H] = -0.04+/-0.02, with an intrinsic
dispersion of 0.27 dex (by averaging abundances based on the scaled-solar [m],
y - J and [m], y - K Metallicity-Index-Color relations), and of -0.11+/-0.01
(sig = 0.27 dex), by using the m1, y - J and m1, y - K relations. These
findings support the results of Zoccali et al. (2004) which give [Fe/H] =
-0.10+/-0.2, and a low alpha-enhancement, [alpha/Fe] = 0.1, and of Carretta et
al. (2001), that find [Fe/H] = 0.07+/-0.01, with [alpha/Fe] = 0.2. By applying
the scaled-solar MIC relations to Baade's window RGs, we find a metallicity
distribution extending from [Fe/H] ~ -1.0 to ~ 1 dex, with peaks at [Fe/H] ~
-0.2 and +0.55 ([m], y - J and [m], y - K relations), and [Fe/H] ~ -0.25 and
+0.4 (m1, y - J and m1, y - K relations). These findings are in good agreement
with the spectroscopic studies of Hill et al. (2011) for the Baade's window, of
Uttenthaler et al. (2012) for a region centered at (l,b) = (0, -10), and with
the results of the ARGOS survey (Ness et al. 2013a).
