We describe the public ESO near-IR variability survey (VVV) scanning the Milky Way bulge and an adjacent section of the mid-plane where star formation activity is high. The survey will take 1929 h of observations with the 4-m VISTA telescope during 5 years (2010-2014), covering ˜109 point sources across an area of 520 deg2, including 33 known globular clusters and ˜350 open clusters. The final product will be a deep near-IR atlas in five passbands (0.9-2.5 μm) and a catalogue of more than 106 variable point sources. Unlike single-epoch surveys that, in most cases, only produce 2-D maps, the VVV variable star survey will enable the construction of a 3-D map of the surveyed region using well-understood distance indicators such as RR Lyrae stars, and Cepheids. It will yield important information on the ages of the populations. The observations will be combined with data from MACHO, OGLE, EROS, VST, Spitzer, HST, Chandra, INTEGRAL, WISE, Fermi LAT, XMM-Newton, GAIA and ALMA for a complete understanding of the variable sources in the inner Milky Way. This public survey will provide data available to the whole community and therefore will enable further studies of the history of the Milky Way, its globular cluster evolution, and the population census of the Galactic Bulge and center, as well as the investigations of the star forming regions in the disk. The combined variable star catalogues will have important implications for theoretical investigations of pulsation properties of stars

We combine V I photometry from OGLE-III with V V V and 2MASS measurements of E(J − K s ) to resolve the longstanding problem of the non-standard optical extinction toward the Galactic bulge. We show that the extinction is wellfit by the relation. The optical and near-IR reddening law toward the inner Galaxy approximately follows an R V ≈ 2.5 extinction curve with a dispersion σ R V ≈ 0.2, consistent with extragalactic investigations of the hosts of type Ia SNe. Differential reddening is shown to be significant on scales as small as as our mean field size of 6 . The intrinsic luminosity parameters of the Galactic bulge red clump (RC) are derived to be (M I,RC , σ I,RC,0 , (V − I) RC,0 , σ (V −I) RC , (J − K s ) RC,0 ) = (−0.12, 0.09, 1.06, 0.121, 0.66). Our measurements of the RC brightness, brightness dispersion and number counts allow us to estimate several Galactic bulge structural parameters. We estimate a distance to the Galactic center of 8.20 kpc. We measure an upper bound on the tilt α ≈ 40 • between the bulge's major axis and the Sun-Galactic center line of sight, though our brightness peaks are consistent with predictions of -2an N-body model oriented at α ≈ 25 • . The number of RC stars suggests a total stellar mass for the Galactic bulge of ∼ 2.3×10 10 M if one assumes a canonical Salpeter IMF, or ∼ 1.6×10 10 M if one assumes a bottom-light Zoccali IMF.We adopt (M I,RC , σ I,RC,0 , (V −I) RC,0 , σ (V −I) RC , (J −K s ) RC,0 ) = (−0.12, 0.09, 1.06, 0.121, 0.66) for the mean absolute magnitude and magnitude dispersion in I, intrinsic (V −I) color,

Context. The Milky Way bulge is the nearest galactic bulge and the most readily accessible laboratory for studies of stellar populations in spheroids based on individual stellar abundances and kinematics. These studies are challenged by the strongly variable and often large extinction on a small spatial scale. Aims. We use the Vista Variables in the Via Lactea (VVV) ESO public survey data to measure extinction values in the complete area of the Galactic bulge covered by the survey at high resolution. Methods. We derive reddening values using the method described in Paper I. This is based on measuring the mean (J − K s ) color of red clump giants in small subfields of 2 × 2 to 6 × 6 in the following bulge area:• . To determine the reddening values E(J − K s ) for each region, we measure the RC color and compare it to the (J − K s ) color of RC stars measured in Baade's Window, for which we adopt E(B − V) = 0.55. This allows us to construct a reddening map sensitive to small-scale variations minimizing the problems arising from differential extinction. Results. The significant reddening variations are clearly observed on spatial scales as small as 2 . We find good agreement between our extinction measurements and Schlegel maps in the outer bulge, but, as already stated in the literature the Schlegel maps are unreliable for regions within |b| < ∼ 6• . In the inner regions, we compare our results with maps derived from DENIS and Spitzer surveys. While we find good agreement with other studies in the corresponding overlapping regions, our extinction map is of higher quality owing to both its higher resolution and a more complete spatial coverage of the bulge. We investigate the importance of differential reddening and demonstrate the need for high spatial resolution extinction maps for detailed studies of bulge stellar populations and structure. Conclusions. We present the first extinction map covering uniformly ∼315 sq. deg. of the Milky Way bulge at high spatial resolution. We consider a 30 arcmin window at a latitude of b = −4• , which corresponds to a frequently studied low extinction window, the so-called Baade's Window, and find that its A Ks values can vary by up to 0.1 mag. Larger extinction variations are observed at lower Galactic latitudes. The extinction variations on scales of up to 2 −6 must be taken into account when analyzing the stellar populations of the Galactic bulge.

Context. The Milky Way (MW) bulge is a fundamental Galactic component for understanding the formation and evolution of galaxies, in particular our own. The ESO Public Survey VISTA Variables in the Vía Láctea is a deep near-IR survey mapping the Galactic bulge and southern plane. Particularly for the bulge area, VVV is covering ∼315 deg 2 . Data taken during 2010 and 2011 covered the entire bulge area in the JHK s bands. Aims. We used VVV data for the whole bulge area as a single and homogeneous data set to build for the first time a single colour−magnitude diagram (CMD) for the entire Galactic bulge. Methods. Photometric data in the JHK s bands were combined to produce a single and huge data set containing 173 150 467 sources in the three bands, for the ∼315 deg 2 covered by VVV in the bulge. Selecting only the data points flagged as stellar, the total number of sources is 84 095 284. Results. We built the largest colour-magnitude diagrams published up to date, containing 173.1+ million sources for all data points, and more than 84.0 million sources accounting for the stellar sources only. The CMD has a complex shape, mostly owing to the complexity of the stellar population and the effects of extinction and reddening towards the Galactic centre. The red clump (RC) giants are seen double in magnitude at b ∼ −8• −10 • , while in the inner part (b ∼ −3 • ) they appear to be spreading in colour, or even splitting into a secondary peak. Stellar population models show the predominance of main-sequence and giant stars. The analysis of the outermost bulge area reveals a well-defined sequence of late K and M dwarfs, seen at (J − K s ) ∼ 0.7−0.9 mag and K s 14 mag. Conclusions. The interpretation of the CMD yields important information about the MW bulge, showing the fingerprint of its structure and content. We report a well-defined red dwarf sequence in the outermost bulge, which is important for the planetary transit searches of VVV. The double RC in magnitude seen in the outer bulge is the signature of the X-shaped MW bulge, while the spreading of the RC in colour, and even its splitting into a secondary peak, are caused by reddening effects. The region around the Galactic centre is harder to interpret because it is strongly affected by reddening and extinction.

Aims. We present the analysis of the [α/Fe] abundance ratios for a large number of stars at several locations in the Milky Way bulge with the aim of constraining its formation scenario. Methods. We obtained FLAMES-GIRAFFE spectra (R = 22 500) at the ESO Very Large Telescope for 650 bulge red giant branch (RGB) stars and performed spectral synthesis to measure Mg, Ca, Ti, and Si abundances. This sample is composed of 474 giant stars observed in 3 fields along the minor axis of the Galactic bulge and at latitudes b = −4•• . Another 176 stars belong to a field containing the globular cluster NGC 6553, located at b = −3• and 5• away from the other three fields along the major axis. Stellar parameters and metallicities for these stars were presented in Zoccali et al. (2008, A&A, 486, 177). We have also re-derived stellar parameters and abundances for the sample of thick and thin disk red giants analyzed in Alves-Brito et al. (2010, A&A, 513, A35). Therefore using a homogeneous abundance database for the bulge, thick and thin disk, we have performed a differential analysis minimizing systematic errors, to compare the formation scenarios of these Galactic components. Results. Our results confirm, with large number statistics, the chemical similarity between the Galactic bulge and thick disk, which are both enhanced in alpha elements when compared to the thin disk.

We present a novel interpretation of the previously puzzling different behaviours of stellar populations of the Milky Way's bulge. We first show, by means of pure N -body simulations, that initially co-spatial stellar populations with different in-plane random motions separate when a bar forms. The radially cooler populations form a strong bar, and are vertically thin and peanut-shaped, while the hotter populations form a weaker bar and become a vertically thicker box. We demonstrate that it is the radial, not the vertical, velocity dispersion that dominates this evolution. Assuming that early stellar discs heat rapidly as they form, then both the in-plane and vertical random motions correlate with stellar age and chemistry, leading to different density distributions for metal-rich and metal-poor stars. We then use a high-resolution simulation, in which all stars form out of gas, to demonstrate that this is what happens. When we apply these results to the Milky Way we show that a very broad range of observed trends for ages, densities, kinematics and chemistries, that have been presented as evidence for contradictory paths to the formation of the bulge, are in fact consistent with a bulge which formed from a continuum of disc stellar populations which were kinematically separated by the bar. For the first time we are able to account for the bulge's main trends via a model in which the bulge formed largely in situ. Since the model is generic, we also predict the general appearance of stellar population maps of external edge-on galaxies.

Context. Several recent studies have demonstrated that the Galactic bulge hosts two components with different mean metallicities, and possibly different spatial distribution and kinematics. As a consequence, both the metallicity distribution and the radial velocity of bulge stars vary across different lines of sight. Aims. We present here the metallicity distribution function of red clump stars in 26 fields spread across a wide area of the bulge, with special emphasis on fields close to Galactic plane, at latitudes b = −2 • and b = −1 • , that have not been explored before. Methods. This paper includes new metallicities from a sample of approximately 5000 K giant stars, observed at spectral resolution R ∼ 6500, in the Calcium II Triplet region. These represent the main dataset from the GIRAFFE Inner Bulge Survey. As part of the same survey we have previously published results for a sample of approximately 600 K giant stars, at latitude b ∼ −4 • , derived from higher resolution spectra (R = 22 500). Results. The combined sample allows us to trace and characterize the metal poor and metal rich bulge populations down to the inner bulge. We present a density map for each of the two components. Contrary to expectations from previous works, we found the metal poor population to be more centrally concentrated than the metal rich one, and with a more axisymmetric spatial distribution. The metal rich population, on the other hand, is arranged in a boxy distribution, consistent with an edge-on bar. By coupling metallicities and radial velocities we show that the metal poor population has a velocity dispersion that varies rather mildly with latitude. On the contrary, the metal rich population has a low velocity dispersion far from the plane (b = −8.5 •), yet has a steeper gradient with latitude, becoming higher than the metal poor one in the innermost field (b = −1 •). Conclusions. This work provides new observational constraints on the actual chemodynamical properties of the Galactic bulge, that will help discrimination between different formation models.

We have combined optical and near-infrared data of known RR Lyrae (RRL) stars in the bulge in order to study the spatial distribution of its metal-poor component by measuring precise reddening values and distances of 7663 fundamentalmode RRL stars with high-quality photometry. We obtain a distance to the Galactic center of R 0 = 8.33 ± 0.05 ± 0.14 kpc. We find that the spatial distribution of the RRL stars differs from the structures traced by the predominantly metal-rich red clump (RC) stars. Unlike the RC stars, the RRL stars do not trace a strong bar, but have a more spheroidal, centrally concentrated distribution, showing only a slight elongation in its very center. We find a hint of bimodality in the density distribution at high southern latitudes (b < −5 • ), which needs to be confirmed by extending the areal coverage of the current census. The different spatial distributions of the metal-rich and metal-poor stellar populations suggest that the Milky Way has a composite bulge.