M. Gullieuszik

The VMC survey - IV. The LMC star formation history and disk geometry from four VMC tiles

We derive the star formation history (SFH) for several regions of the Large Magellanic Cloud (LMC), using deep near-infrared data from the VISTA near-infrared Y JKs survey of the Magellanic system (VMC). The regions include three almost-complete 1.4 deg 2 tiles located ∼ 3.5 ◦ away from the LMC centre in distinct directions. They are split into 21.0 ′ × 21.5 ′ (0.12 deg 2 ) subregions, and each of these is analysed independently. To this dataset, we add two 11.3 ′ × 11.3 ′ (0.036 deg 2 ) subregions selected based on their small and uniform extinction inside the 30 Doradus tile. The SFH is derived from the simultaneous reconstruction of two different colour‐magnitude diagrams (CMDs), using the minimization code StarFISH together with a database of “partial models” representing the CMDs of LMC populations of various ages and metallicities, plus a partial model for the CMD of the Milky Way foreground. The distance modulus (m− M)0 and extinction AV is varied within intervals∼ 0.2 and∼ 0.5 mag wide, respectively, within which we identify the best-fitting star formation rate SFR( t) as a function of lookback time t, age‐metallicity relation (AMR), (m− M)0 and AV. Our results demonstrate that VMC data, due to the combination of depth and little sensitivity to differential reddening, allow the derivation of the space-reso lved SFH of the LMC with unprecedented quality compared to previous wide-area surveys. In particular, the data clea rly reveal the presence of peaks in the SFR(t) at ages log(t/yr)≃ 9.3 and 9.7, which appear in most of the subregions. The most recent SFR(t) is found to vary greatly from subregion to subregion, with the general trend of being more intense in the innermost LMC, except for the tile next to the N11 complex. In the bar region, the SFR(t) seems remarkably constant over the time interval from log(t/yr)≃ 8.4 to 9.7. The AMRs, instead, turn out to be remarkably similar across the LMC. Thanks to the accuracy in determining the distance modulus for every subregion ‐ with typical errors of just∼ 0.03 mag ‐ we make a first attempt to derive a spatial model of the LMC disk. The fields studied so far are fit extremel y well by a single disk of inclination i = 26.2± 2.0 ◦ , position angle of the line of nodesθ0 = 129.1± 13.0 ◦ , and distance modulus of (m− M)0 = 18.470± 0.006 mag (random errors only) up to the LMC centre. We show that once the (m− M)0 values or each subregion are assumed to be identical to those derived from this best-fitting plane, systematic errors in t he SFR(t) and AMR are reduced by a factor of about two.

Homogeneous Metallicities and Radial Velocities for Galactic Globular Clusters ⋆ First CaT metallicities for twenty clusters

Well determined radial velocities and abundances are essential for analyzing the properties of the globular cluster system of the Milky Way. However more than 50% of these clusters have no spectroscopic measure of their metallicity. In this context, this work provides new radial velocities and abundances for twenty Milky Way globular clusters which lack or have poorly known values for these quantities. The radial velocities and abundances are derived from spectra obtained at the Ca ii triplet using the FORS2 imager and spectrograph at the VLT, calibrated with spectra of red giants in a number of clusters with well determined abundances. For about half of the clusters in our sample we present significant revisions of the existing velocities or abundances, or both. We also confirm the existence of a sizable abundance spread in the globular cluster M54, which lies at the center of the Sagittarius dwarf galaxy. In addition evidence is provided for the existence of a small intrinsic internal abundance spread (σ[Fe/H]int ≈ 0.11−0.14 dex, similar to that of M54) in the luminous distant globular cluster NGC5824. This cluster thus joins the small number of Galactic globular clusters known to possess internal metallicity ([Fe/H]) spreads.

Deep near-infrared photometry of the globular cluster 47Tucanae. Reconciling theory and observations

Context. The Galactic globular cluster 47 Tucanae is central to studies of Galaxy formation, and a test-bed for theoretical models, distance determination and extragalactic age-dating techniques. Independent parallax-based distance determinations in the optical spectral range provide discrepant results; also, star counts along the Red Giant Branch from optical data have disclosed a worrying disagreement with theoretical predictions, that impacts not only the theory of red giant stars, but also the calibration of the age scale of extragalactic systems. Aims. Our new near-infrared data for 47 Tuc set constraints on its distance and test the reliability of theoretical red giant branch star counts, independently of previous conclusions from optical work. Methods. We have obtained deep near-infrared imaging of 47 Tuc using SOFI at the ESO New Technology Telescope. Colour−magnitude diagrams, isochrones and synthetic horizontal branch modelling have been used to determine the distance of 47 Tuc and constrain its age. We have also constructed a luminosity function of red giant stars, which has been compared with theoretical predictions of stellar evolution models. Results. We obtain a distance (m − M)0 = 13.18 ± 0.03 (random) ± 0.04 (systematic), for [Fe/H] = −0.7 ± 0. 1a ndE(B − V) = 0.04±0.02. This supports the shorter end of the range of distances obtained from optical studies. The mean horizontal branch star mass is between 0.65 and 0.66 M� , and its 1σ Gaussian dispersion is between 0.010 and 0.012 M� . The cluster age can only be approximately estimated from the data, and is between ∼10 and ∼13 Gyr. The luminosity function of red giant branch (and early-asymptotic giant branch) stars does not show a statistically significant discrepancy with theory. The brightness of the red giant branch bump in the near-infrared is possibly fainter than the models, although the uncertainty on the spectroscopic metallicity and age prevents to reach a firm conclusion on this issue.

The distance to the Fornax dwarf spheroidal galaxy

ABSTRACT A large multicolour, wide-field photometric database of the Fornax dwarf spheroidalgalaxy has been analysed using three different methods to provide revised distanceestimates based on stellarpopulations in different ageintervals. The distance to Fornaxwas obtained from the Tip of the Red Giant Branch measured by a new method, andusing the luminosity of Horizontal Branch stars and Red Clump stars corrected forstellar population effects. Assuming a reddening E(B − V) = 0.02, the followingdistance moduli were derived: (m− M) 0 = 20.71± 0.07 based on the Tip of the RedGiant Branch, (m − M) 0 = 20.72 ± 0.06 from the level of the Horizontal Branch,and (m − M) 0 = 20.73 ± 0.09 using the Red Clump method. The weighted meandistance modulus to Fornax is (m − M) 0 = 20.72 ± 0.04. All these measurementsagree within the errors, and are fully consistent with previous determinations andwith the distance measurements obtained in a companion paper from near-infraredcolour-magnitude diagrams.Key words: Galaxies: fundamental parameters – Galaxies: distances – Galaxies: in-dividual: Fornax dwarf spheroidal

The blue plume population in dwarf spheroidal galaxies - Genuine blue stragglers or young stellar population?

Aims. Blue stragglers (BSS) in the Milky Way field and globular/open clusters are thought to be the product of either primordial or collisional binary systems. In the context of dwarf spheroidal galaxies it is hard to firmly disentangle a genuine BSS population from young main sequence (MS) stars tracing a ∼1-2 Gyr old star forming episode. Methods. Assuming that their blue plume populations are made of BSS, we estimate the BSS frequency (F BSS HB ; as normalized to the horizontal branch star counts) for 8 Local Group non star-forming dwarf galaxies, using a compilation of ground and space based photometry. Results. (i) The BSS frequency in dwarf galaxies, at any given M v , is always higher than that in globular clusters of similar luminosities; (ii) the BSS frequency for the lowest luminosity dwarf galaxies is in excellent agreement with that observed in the Milky Way halo and open clusters; and most interestingly (iii) derive a statistically significant F BSS HB - M v anti-correlation for dwarf galaxies, similar to that observed in globular clusters. Conclusions. The low density, almost collision-less, environments of our dwarf galaxy sample allow us to infer (i) their very low dynamical evolution; (ii) a negligible production of collisional BSS; and consequently (iii) that their blue plumes are mainly made of primordial binaries. The dwarf galaxies F BSS HB -M v anti-correlation can be used as a discriminator: galaxies obeying the anti-correlation are more likely to possess genuine primordial BSS rather than young main sequence stars.

EVOLUTION OF THERMALLY PULSING ASYMPTOTIC GIANT BRANCH STARS. IV. CONSTRAINING MASS LOSS AND LIFETIMES OF LOW MASS, LOW METALLICITY AGB STARS*

The evolution and lifetimes of thermally pulsating asymptotic giant branch (TP-AGB) stars suffer from significant uncertainties. In this work, we analyze the numbers and luminosity functions of TP-AGB stars in six quiescent, low metallicity ([Fe/H] –0.86) galaxies taken from the ACS Nearby Galaxy Survey Treasury sample, using Hubble Space Telescope (HST) photometry in both optical and near-infrared filters. The galaxies contain over 1000 TP-AGB stars (at least 60 per field). We compare the observed TP-AGB luminosity functions and relative numbers of TP-AGB and red giant branch (RGB) stars, N TP-AGB/N RGB, to models generated from different suites of TP-AGB evolutionary tracks after adopting star formation histories derived from the HST deep optical observations. We test various mass-loss prescriptions that differ in their treatments of mass loss before the onset of dust-driven winds (pre-dust). These comparisons confirm that pre-dust mass loss is important, since models that neglect pre-dust mass loss fail to explain the observed N TP-AGB/N RGB ratio or the luminosity functions. In contrast, models with more efficient pre-dust mass loss produce results consistent with observations. We find that for [Fe/H] –0.86, lower mass TP-AGB stars (M 1 M ☉) must have lifetimes of ~0.5 Myr and higher masses (M 3 M ☉) must have lifetimes 1.2 Myr. In addition, assuming our best-fitting mass-loss prescription, we show that the third dredge-up has no significant effect on TP-AGB lifetimes in this mass and metallicity range.The evolution and lifetimes of thermally pulsating asymptotic giant branch (TP-AGB) stars suffer from significant uncertainties. We present a detailed framework for constraining model luminosity functions of TP-AGB stars using resolved stellar populations. We show an example of this method that compares various TP-AGB mass-loss prescriptions that differ in their treatments of mass loss before the onset of dust-driven winds (pre-dust). We find that models with more efficient pre-dust driven mass loss produce results consistent with observations, as opposed to more canonical mass-loss models. Efficient pre-dust driven mass-loss predicts for [Fe/H] < -1.2, lower mass TP-AGB stars (M < 1 Msun) must have lifetimes less than about 1.2 Myr.

Variable Stars in the Fornax dSph Galaxy. II. Pulsating Stars below the Horizontal Branch

We have carried out an intensive survey of the northern region of the Fornax dwarf spheroidal galaxy with the aim of detecting the galaxys short-period pulsating stars (P < 0.25 days). Observations collected over three consecutive nights with the Wide Field Imager of the 2.2 m MPI telescope at ESO allowed us to detect 85 high-amplitude (0.20-1.00 mag in B light) variable stars with periods in the range from 0.046 to 0.126 days, similar to SX Phoenicis stars in Galactic metal-poor stellar populations. The plots of the observed periods vs. the B and V magnitudes show a dispersion largely exceeding the observational errors. To disentangle the matter, we separated the first-overtone from the fundamental-mode pulsators and tentatively identified a group of subluminous variables, about 0.35 mag fainter than the others. Their nature as either metal-poor intermediate-age stars or stars formed by the merging of close binary systems is discussed. The rich sample of the Fornax variables also led us to reconstruct the period-luminosity relation for short-period pulsating stars. An excellent linear fit, MV = − 1.83(± 0.08) − 3.65(± 0.07) log PF, was obtained using 153 δ Scuti and SX Phoenicis stars in a number of different stellar systems.

The evolved stars of Leo II dSph galaxy from near-infrared UKIRT/WFCAM observations

We present a study of the evolved stellar populations in the dwarf spheroidal galaxy Leo II, based on JHK s observations obtained with the near-infrared array WFCAM at the UKIRT telescope. Combining the new data with optical data, we derived photometric estimates of the distribution of global metallicity [M/H] of individual red giant stars from their V-K s colours. Our results are consistent with the metallicities of red giant branch (RGB) stars obtained from Can triplet spectroscopy, once the age effects are considered. The photometric metallicity distribution function has a peak at [M/H] =-1.74 (uncorrected) or [M/H] = - 1.64 ± 0.06 (random) ±0.17 (systematic) after correction for the mean age of Leo II stars (9 Gyr). The distribution is similar to a Gaussian with a [M/H] = 0.19 dex, corrected for instrumental errors. We used the new data to derive the properties of a nearly complete sample of asymptotic giant branch (AGB) stars in Leo II. Using a near-infrared two-colour diagram, we were able to obtain a clean separation from Milky Way foreground stars and discriminate between carbon- and oxygen-rich AGB stars, which allowed us to study their distribution in K s -band luminosity and colour. We simulate the JHK s data with the TRILEGAL population synthesis code together with the most updated thermally pulsing AGB models, and using the star formation histories derived from independent work based on deep Hubble Space Telescope photometry. After scaling the mass of Leo II models to the observed number of upper RGB stars, we find that present models predict too many O-rich thermally pulsing AGB (TP-AGB) stars of higher luminosity due to a likely underestimation of either their mass-loss rates at low metallicity, and/or their degree of obscuration by circumstellar dust. On the other hand, the TP-AGB models are able to reproduce the observed number and luminosities of carbon stars satisfactorily well, indicating that in this galaxy the least massive stars that became carbon stars should have masses as low as ∼1 M ⊙ .

WINGS Data Release: a database of galaxies in nearby clusters

Context. To effectively investigate galaxy formation and evolution, it is of paramount importance to exploit homogeneous data for large samples of galaxies in different environments. Aims. The WIde-field Nearby Galaxy-cluster Survey (WINGS) project aim is to evaluate physical properties of galaxies in a complete sample of low redshift clusters to be used as reference sample for evolutionary studies. The WINGS survey is still ongoing and the original dataset will be enlarged with new observations. This paper presents the entire collection of WINGS measurements obtained so far. Methods. We decided to make use of the Virtual Observatory (VO) tools to share the WINGS database (that will be updated regularly) with the community. In the database each object has one unique identification (WINGSID). Each subset of estimated properties is accessible using a cone search (including wide-field images). Results. We provide the scientific community with the entire set of wide-field images. Furthermore, the published database contains photometry of 759 024 objects and surface brightness analysis for 42 275 and 41 463 galaxies in the V and B band, respectively. The completeness depends on the image quality, and on the cluster redshift, reaching on average 90% at V 21.7. Near-infrared photometric catalogs for 26 (in K) and 19 (in J) clusters are part of the database and the number of sources is 962 344 in K and 628 813 in J. Here again the completeness depends on the data quality, but it is on average higher than 90% for J 20. 5a ndK 19.4. The IR subsample with a Sersic fit comprises 71 687 objects. A morphological classification is available for 39 923 galaxies. We publish spectroscopic data, including 6132 redshifts, 5299 star formation histories, and 4381 equivalent widths. Finally, a calculation of local density is presented and implemented in the VO catalogs for 66 164 galaxies. The latter is presented here for the first time.

Near-infrared observations of the Fornax dwarf galaxy. I. The red giant branch ⋆

Aims. We present a study of the evolved stellar populations in the dwarf spheroidal galaxy Fornax based on wide-area near-infrared observations, aimed at obtaining new independent estimates of its distance and metallicity distribution. Assessing the reliability of near-infrared methods is most important in view of future space- and ground-based deep near-infrared imaging of resolved stellar systems. Methods. We have obtained JHK imaging photometry of the stellar populations in Fornax. The observations cover an 18.5× 18. 5a rcmin 2 central area with a mosaic of SOFI images at the ESO NTT. Our data sample all the red giant branch (RGB) for the whole area. Deeeper observations reaching the red clump of helium-burning stars have also been obtained for a 4.5 × 4. 5a rcmin 2 region. Results. Near-infrared photometry led to measurements of the distance to Fornax based on the K-band location of the RGB tip and the red clump. Once corrected for the mean age of the stellar populations in the galaxy, the derived distance modulus is (m − M)0 = 20.74 ± 0.11, corresponding to a distance of 141 Kpc, in good agreement with estimates from optical data. We have obtained a photometric estimate of the mean metallicity of red giant stars in Fornax from their (J − K )a nd (V − K) colors, using several methods. The effect of the age-metallicity degeneracy on the combined optical-infrared colors is shown to be less important than for optical or infrared colors alone. By taking age effects into account, we have derived a distribution function of global metallicity [M/H] from optical-infrared colors of individual stars. Our photometric Metallicity Distribution Function covers the range −2.0 < [M/H] < −0.6, with a main peak at [M/H] �− 0.9 and a long tail of metal-poor stars, and less metal-rich stars than derived by recent spectroscopy. If metallicities from Ca ii triplet lines are correct, this result confirms a scenario of enhanced metal enrichment in the last 1–4 Gyr.