E. Valenti

Reddening and metallicity maps of the Milky Way bulge from VVV and 2MASS - II. The complete high resolution extinction map and implications for Galactic bulge studies

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 − Ks) color of red clump giants in small subfields of 2 � × 2 � to 6 � × 6 � in the following bulge area: −10.3 ◦ ≤ b ≤ +5.1 ◦ and −10.0 ◦ ≤ l ≤ +10.4 ◦ . To determine the reddening values E(J − Ks) for each region, we measure the RC color and compare it to the (J − Ks) 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 AKs 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.

Reddening and metallicity maps of the Milky Way bulge from VVV and 2MASS - I. The method and minor axis maps

We present a method to obtain reddening maps and to trace structure and metallicity gradients of the bulge using data from the recently started ESO public survey Vista Variables in the Via Lactea (VVV). We derive the mean J-Ks color of the red clump (RC) giants in 1835 subfields in the Bulge region with -8<b<-0.4 and 0.2<l<1.7, and compare it to the color of RC stars in Baades Window for which we adopt E(B-V)=0.55. This allows us to derive the reddening map on a small enough scale to minimize the problems arising from differential extinction. The dereddened magnitudes are then used to build the bulge luminosity function in regions of 0.4 x 0.4 deg to obtain the mean RC magnitudes. These are used as distance indicator in order to trace the bulge structure. Finally, for each subfield we derive photometric metallicities through interpolation of red giant branch colors on a set of empirical ridge lines. The photometric metallicity distributions are compared to metallicity distributions obtained from high resolution spectroscopy in the same regions. The reddening determination is sensitive to small scale variations which are clearly visible in our maps. The luminosity function clearly shows the double RC recently discovered in 2MASS and OGLE III datasets, hence allowing to trace the X-shape morphology of the bulge. Finally, the mean of the derived photometric metallicity distributions are in remarkable agreement with those obtained from spectroscopy. The VVV survey presents a unique tool to map the bulge properties by means of the consistent method presented here. The remarkable agreement between our results and those presented in literature for the minor axis allows us to safely extend our method to the whole region covered by the survey.

Near-Infrared Properties of 24 Globular Clusters in the Galactic Bulge*

We present near-IR color-magnitude diagrams and physical parameters for a sample of 24 Galactic globular clusters toward the bulge. In this paper we discuss the properties of 12 new clusters (out of the 24) in addition to those previously studied and published by our group. The compilation includes measurements of the cluster reddening, distance, photometric metallicity, horizontal branch red clump, and red giant branch morphological (e.g., mean ridgelines) and evolutionary (e.g., bump and tip) features. The compilation is available in electronic form on the World Wide Web, and it will be updated regularly.

The GIRAFFE Inner Bulge Survey (GIBS) - I. Survey description and a kinematical map of the Milky Way bulge

Context. The Galactic bulge is a massive, old component of the Milky Way. It is known to host a bar, and it has recently been demonstrated to have a pronounced boxy/peanut structure in its outer region. Several independent studies suggest the presence of more than one stellar populations in the bulge, with different origins and a relative fraction changing across the bulge area. Aims. This is the first of a series of papers presenting the results of the Giraffe Inner Bulge Survey, carried out at the ESO-VLT with the multifibre spectrograph FLAMES. Spectra of ∼5000 red clump giants in 24 bulge fields have been obtained at resolution R=6500, in the infrared Calcium triplet wavelength region at ∼8500 A. They are used to derive radial velocities and metallicities, based on new calibration specifically devised for this project. Radial velocities for another ∼1200 bulge red clump giants, obtained from similar archive data, have been added to the sample. Higher resolution spectra have been obtained for ∼450 additional stars at latitude b = −3.5, with the aim of investigating chemical abundance patterns variations with longitude, across the inner bulge. In total we present here radial velocities for 6392 red clump stars. Methods. We present here the target selection criteria, observing strategy and the catalogue with radial velocity measurements for all the target stars. Results. We derive a radial velocity, and velocity dispersion map of the Milky Way bulge, useful to be compared with similar maps of external bulges, and to infer the expected velocities and dispersion at any line of sight. The K-type giants kinematics is consistent with the cylindrical rotation pattern of M-giants from the BRAVA survey. Our sample enables to extend this result to latitude b = −2, closer to the Galactic plane than probed by previous surveys. Finally, we find strong evidence for a velocity dispersion peak at (0,−1) and (0,−2), possibly indicative of a high density peak in the central ∼250 pc of the bulge.

The first detailed abundances for M giants in the inner bulge from infrared spectroscopy

We report the first abundance analysis of 17 M giant stars in the inner Galactic bulge at (l, b) = (0°, -1°), based on R = 25,000 infrared spectroscopy (1.5-1.8 μm) using NIRSPEC at the Keck telescope. Based on their luminosities and radial velocities, we identify these stars with a stellar population older than 1 Gyr. We find the iron abundance [Fe/H] = -0.22 ± 0.03, with a 1 σ dispersion of 0.14 ± 0.024. We also find the bulge stars have an enhanced [α/Fe] abundance ratio at the level of +0.3 dex relative to solar stars, and low 12C/13C ≈ 6.5 ± 0.3. The derived iron abundance and composition for this inner bulge sample is indistinguishable from that of a sample of M giants our team has previously studied in Baades window (l, b) = (0.9, -4). We find no evidence of any major iron abundance or abundance ratio gradient between this inner field and Baades window.

SPECTROSCOPY UNVEILS THE COMPLEX NATURE OF TERZAN 5

We present the chemical abundance analysis of 33 red giant stars belonging to the complex stellar system Terzan 5. We confirm the discovery of two stellar populations with distinct iron abundances: a relatively metal-poor component with [Fe/H] = –0.25 ± 0.07 rms and another component with [Fe/H] = +0.27 ± 0.04 rms, exceeding in metallicity any known Galactic globular cluster (GC). The two populations also show different [α/Fe] abundance ratios. The metal-poor component has an average [α/Fe] =+0.34 ± 0.06 rms, consistent with the canonical scenario for rapid enrichment by core collapse supernovae (SNe). The metal-rich component has [α/Fe] =+0.03 ± 0.04 rms, suggesting that the gas from which it formed was polluted by both type II and type Ia SNe on a longer timescale. Neither of the two populations shows evidence of the [Al/Fe] over [O/Fe] anti-correlation that is typically observed in Galactic GCs. Because these chemical abundance patterns are unique, we propose that Terzan 5 is not a true GC, but a stellar system with a much more complex history of star formation and chemical enrichment.

A Panchromatic Study of the Globular Cluster NGC 1904. I. The Blue Straggler Population

By combining high-resolution (HST/WFPC2) and wide-field ground-based (2.2 m ESO/WFI) and space (GALEX) observations, we have collected a multiwavelength photometric database (ranging from the far-UV to the near infrared) of the galactic globular cluster NGC 1904 (M79). The sample covers the entire cluster extension, from the very central regions up to the tidal radius. In the present paper, such a data set is used to study the BSS population and its radial distribution. A total number of 39 bright (m218 ≤ 19.5) BSSs have been detected, and they have been found to be highly segregated in the cluster core. No significant upturn in the BSS frequency has been observed in the outskirts of NGC 1904, in contrast to other clusters (M3, 47 Tuc, NGC 6752, M5) studied with the same technique. Such evidence, coupled with the large radius of avoidance estimated for NGC 1904 (ravoid ~ 30 core radii), indicates that the vast majority of the cluster heavy stars (binaries) has already sunk to the core. Accordingly, extensive dynamical simulations suggest that BSSs formed by mass transfer activity in primordial binaries evolving in isolation in the cluster outskirts represent only a negligible (0%-10%) fraction of the overall population.

The inner Galactic bar traced by the VVV survey

Aims. We use the VVV survey observations in bulge regions close to the Galactic plane to trace the bar inclination at the Galactic latitude b ∼± 1 and to investigate a distinct structure in the inner regions of the bar that was previously detected at positive latitude (b =+ 1). Methods. We use the (J −Ks) colors of the red clump stars to obtain reddening values on 6 × 6 arcmin scale, minimizing the problems arising from differential extinction. Dereddened magnitudes are then used to build the luminosity function of the bulge in regions of ∼0.4 sq deg to obtain the mean red clump magnitudes. These are used as distance indicators to trace the bar structure. Results. The luminosity function clearly shows the red clump mean magnitude variation with longitude, as expected from a large scale bar oriented towards us at positive Galactic longitude, with a dereddened magnitude varying from Ks0 = 13. 4a tl = −10 ◦ to Ks0 = 12. 4a tl =+ 10 ◦ . We detect a change in the orientation of the bar in the central regions with |l| < 4 ◦ at b = ±1 ◦ , in agreement with results obtained at positive latitudes by other authors. Our results are based on a different dataset and at different latitude, which shows that this change in the bar orientation is real. This suggests that there is an inner structure distinct to the large-scale Galactic bar, with a different orientation angle. This inner structure could be a secondary, inner bar, with a semi-major axis of ∼500 pc that is symmetric with respect to the Galactic plane.

CECI N'EST PAS A GLOBULAR CLUSTER: THE METALLICITY DISTRIBUTION OF THE STELLAR SYSTEM TERZAN 5*

We present new determinations of the iron abundance for 220 stars belonging to the stellar system Terzan 5 in the Galactic bulge. The spectra have been acquired with FLAMES at the Very Large Telescope of the European Southern Observatory and DEIMOS at the Keck II Telescope. This is by far the largest spectroscopic sample of stars ever observed in this stellar system. From this dataset, a subsample of targets with spectra unaffected by TiO bands was extracted and statistically decontaminated from field stars. Once combined with 34 additional stars previously published by our group, a total sample of 135 member stars covering the entire radial extent of the system has been used to determine the metallicity distribution function of Terzan 5. The iron distribution clearly shows three peaks: a super-solar component at [Fe/H]

The GIRAFFE Inner Bulge Survey (GIBS): III. Metallicity distributions and kinematics of 26 Galactic bulge fields

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