STEP: the VST survey of the SMC and the Magellanic Bridge - I : Overview and first results

STEP (the SMC in Time: Evolution of a Prototype interacting late-type dwarf galaxy) is a Guaranteed Time Observation survey being performed at the VST (the ESO VLT Survey Telescope). STEP will image an area of 74 sq. deg. covering the main body of the Small Magellanic Cloud (32 sq. deg.), the Bridge that connects it to the Large Magellanic Cloud (30 sq. deg.) and a small part of the Magellanic Stream (2 sq. deg.). Our g, r, i, Hα photometry is able to resolve individual stars down to magnitudes well below the main-sequence turn-off of the oldest populations. In this first paper, we describe the observing strategy, the photometric techniques and the upcoming data products of the survey. We also present preliminary results for the first two fields for which data acquisition is completed, including some detailed analysis of the two stellar clusters IC 1624 and NGC 419.Peer reviewedFinal Accepted Versio

The VMC survey - VIII : First results for anomalous Cepheids

The VISTA near-infrared YJKs survey of the Magellanic Clouds System (VMC, PI M.-R. L. Cioni) is collecting deep Ks-band time-series photometry of the pulsating variable stars hosted in the system formed by the two Magellanic Clouds and the Bridge connecting them. In this paper, we present for the first time Ks-band light curves for anomalous Cepheid (AC) variables. In particular, we have analysed a sample of 48 Large Magellanic Cloud ACs, for which identification and optical magnitudes were obtained from the OGLE III and IV catalogues. The VMC Ks-band light curves for ACs are well sampled, with the number of epochs ranging from 8 to 16, and allowing us to obtain very precise mean Ks magnitudes with errors on average of the order of 0.01 mag. The values were used to build the first period-luminosity and period-Wesenheit relations in the near-infrared for fundamental mode and first overtone ACs. At the same time we exploited the optical (V, I) OGLE data to build accurate period-luminosity, period-luminosity-colour and period-Wesenheit relations both for fundamental mode and first overtone ACs. For the first time, these relations were derived from a sample of pulsators which uniformly cover the whole AC instability strip. The application of the optical period-Wesenheit relation to a sample of dwarf galaxies hosting a significant population of ACs revealed that this relation is a valuable tool for deriving distances within the Local Group. Due to its lower dispersion, we expect the Ks period-Wesenheit relations first derived in this paper to represent a valuable tool for measuring accurate distances to galaxies hosting ACs when more data in near-infrared filters become available.Peer reviewe

Spatially resolved LMC star formation history: I. Outside in evolution of the outer LMC disk

We study the evolution of three fields in the outer LMC disk Rgc=3.5-6.2 Kpc. Their star formation history indicates a stellar populations gradient such that younger stellar populations are more centrally concentrated. We identify two main star forming epochs, separated by a period of lower activity between ~7 and ~4 Gyr ago. Their relative importance varies from a similar amount of stars formed in the two epochs in the innermost field, to only 40% of the stars formed in the more recent epoch in the outermost field. The young star forming epoch continues to the present time in the innermost field, but lasted only till ~0.8 and 1.3 Gyr ago at Rgc=5.5 degrees and 7.1 degrees, respectively. This gradient is correlated with the measured HI column density and implies an outside-in quenching of the star formation, possibly related to a variation of the size of the HI disk. This could either result from gas depletion due to star formation or ram-pressure stripping, or from to the compression of the gas disk as ram-pressure from the Milky Way halo acted on the LMC interstellar medium. The latter two situations may have occurred when the LMC first approached the Milky Way.Comment: 15 pages, 13 figures, 4 tables. MNRAS, in pres

Constraining the thermally pulsing asymptotic giant branch phase with resolved stellar populations in the Large Magellanic Cloud

Reliable models of the thermally pulsing asymptotic giant branch (TP-AGB) phase are of critical importance across astrophysics, including our interpretation of the spectral energy distribution of galaxies, cosmic dust production, and enrichment of the interstellar medium. With the aim of improving sets of stellar isochrones that include a detailed description of the TP-AGB phase, we extend our recent calibration of the AGB population in the Small Magellanic Cloud (SMC) to the more metal-rich Large Magellanic Cloud (LMC). We model the LMC stellar populations with the trilegal code, using the spatially resolved star formation history derived from the VISTA survey. We characterize the efficiency of the third dredge-up by matching the star counts and the Ks-band luminosity functions of the AGB stars identified in the LMC. In line with previous findings, we confirm that, compared to the SMC, the third dredge-up in AGB stars of the LMC is somewhat less efficient, as a consequence of the higher metallicity. The predicted range of initial mass of C-rich stars is between Mi 48 1.7 and 3 M 99 at Zi = 0.008. We show how the inclusion of new opacity data in the carbon star spectra will improve the performance of our models. We discuss the predicted lifetimes, integrated luminosities, and mass-loss rate distributions of the calibrated models. The results of our calibration are included in updated stellar isochrones publicly available

The VMC survey - XIV. First results on the look-back time star formation rate tomography of the Small Magellanic Cloud

We analyse deep images from the VISTA survey of the Magellanic Clouds in the YJK(s) filters, covering 14 deg(2) (10 tiles), split into 120 subregions, and comprising the main body and Wing of the Small Magellanic Cloud (SMC). We apply a colour-magnitude diagram reconstruction method that returns their best-fitting star formation rate SFR(t), age-metallicity relation (AMR), distance and mean reddening, together with 68 per cent confidence intervals. The distance data can be approximated by a plane tilted in the East-West direction with a mean inclination of 39 degrees, although deviations of up to +/- 3 kpc suggest a distorted and warped disc. After assigning to every observed star a probability of belonging to a given age-metallicity interval, we build high-resolution population maps. These dramatically reveal the flocculent nature of the young star-forming regions and the nearly smooth features traced by older stellar generations. They document the formation of the SMC Wing at ages <0.2 Gyr and the peak of star formation in the SMC Bar at similar to 40 Myr. We clearly detect periods of enhanced star formation at 1.5 and 5 Gyr. The former is possibly related to a new feature found in the AMR, which suggests ingestion of metal-poor gas at ages slightly larger than 1 Gyr. The latter constitutes a major period of stellar mass formation. We confirm that the SFR(t) was moderately low at even older ages

Calibrating the thermally-pulsing asymptotic giant branch phase through resolved stellar populations in nearby galaxies

Most of the physical processes driving the Thermally-Pulsing Asymptotic Giant Branch (TP-AGB) evolution are not yet fully understood and they need to be modelled with parametrised descriptions. The uncertainties of the models affect the interpretation of the spectrophotometric properties of galaxies up to high-redshift. In the framework of the ERC - STARKEY project, the aim of this Thesis is to constrain the uncertain parameters, i.e. third dredge-up and mass-loss, that still affect the TP-AGB models. To this purpose, I perform detailed simulations of AGB star populations in the Small Magellanic Cloud (SMC) based on robust measurements of the space-resolved star formation history as derived from the deep near-infrared photometry of the VISTA survey of the Magellanic Clouds. I compare the resulting synthetic catalogues with high-quality observations of resolved stellar populations in the infrared passbands of 2MASS and Spitzer. A large grid of TP-AGB evolutionary tracks is computed with several combinations of third dredge-up and mass-loss prescriptions. By requiring the models to reproduce the star counts and the luminosity functions of the observed Oxygen-, Carbon-rich and extreme-AGB stars, I put quantitative constraints on the efficiencies of the third dredge-up and mass-loss. The observed luminosity functions in all the available infrared photometric filters are successfully reproduced by two set of models, one with a relatively high mass-loss efficiency for Oxygen-rich stars and the second with a lower mass-loss efficiency and a lower efficiency of the third dredge-up for the more massive TP-AGB stars, i.e. initial masses larger than three solar masses. On the basis of the best-fitting model I present a complete characterisation of the AGB population in terms of stellar parameters, including the predicted mass-loss rates, initial masses, and Carbon-to-Oxygen ratio. I use the TP-AGB models calibrated in the SMC to model the population of Long Period Variables (LPVs) in the Large Magellanic Cloud as observed by Gaia. The remarkable agreement between models and observations allows us to guide the interpretation of a new observational diagram that is able to photometrically distinguish the evolutionary stages, the initial masses and the chemical type of these stars. In the context of the Large Synoptic Survey Telescope (LSST) science collaboration, I produce catalogues containing the synthetic photometry of the Magellanic Clouds in the Gaia and LSST filters. These catalogues, together with the all-sky simulations of the Milky Way will be made available to the community through the NOAO Data Lab to help defining the observing strategy of the LSST mini-surveys. In addition, I simulate samples of AGB stars in Local Group dwarf galaxies and find a general agreement with the data. However, to properly consider these objects in the TP-AGB models calibration, the simulations should be improved to take into account the crowding effects and the different areas used for the star formation histories derivation and the AGB stars identification. Finally, the products of this work, namely calibrated stellar isochrones and pulsation periods of LPVs, will be publicly available and ready to use for the interpretation of the data coming from present and future observing facilities. The calibrated TP-AGB models may be included in population synthesis models used to probe the integrated light of galaxies in the extragalactic Universe

IAU Symposium 241 - Stellar Populations as Building Blocks of Galaxies

Stellar populations, building blocks of galaxies, are direct tracers of the star formation history, the chemical enrichment and the assembly of galaxies in the Universe. They therfore allow us to understand how galaxies formed and evolved. This last decade has witnessed a revolution in our observations of galaxies; with larger telescopes and new instruments we are not only able to look deeper in the Universe, we can also study nearby galaxies with greater detail. The fact that now is becoming possible to resolve stars up to the distance of Virgo Cluster allows us to rigorously compare and calibrate the analysis of the integrated light with resolved stellar populations. These Proceedings report the considerable progress made in recent years in this topic. Theorists and observers, researchers of resolved and unresolved stellar populations, discussed the ingredients of stellar population models, and rigorously compared them to new data, forcing theorists to develop more refined models and methods to derive the physical parameters of the stellar populations. New results from the Milky Way, the Local Group, and nearby and distant galaxies were presented.Comment: This is the table of contents of the upcoming proceedings of IAU Symposium 241. The book will appear in September, from Cambridge University Press, and will also be available electronically at http://www.journals.cambridge.org/action/displayJournal?jid=IA

Toward an internally consistent astronomical distance scale

Accurate astronomical distance determination is crucial for all fields in astrophysics, from Galactic to cosmological scales. Despite, or perhaps because of, significant efforts to determine accurate distances, using a wide range of methods, tracers, and techniques, an internally consistent astronomical distance framework has not yet been established. We review current efforts to homogenize the Local Group's distance framework, with particular emphasis on the potential of RR Lyrae stars as distance indicators, and attempt to extend this in an internally consistent manner to cosmological distances. Calibration based on Type Ia supernovae and distance determinations based on gravitational lensing represent particularly promising approaches. We provide a positive outlook to improvements to the status quo expected from future surveys, missions, and facilities. Astronomical distance determination has clearly reached maturity and near-consistency.Comment: Review article, 59 pages (4 figures); Space Science Reviews, in press (chapter 8 of a special collection resulting from the May 2016 ISSI-BJ workshop on Astronomical Distance Determination in the Space Age