Star complexes and stellar populations in NGC 6822 - Comparison with the Magellanic Clouds

The star complexes (large scale star forming regions) of NGC 6822 were traced and mapped and their size distribution was compared with the size distribution of star complexes in the Magellanic Clouds (MCs). Furthermore, the spatial distributions of different age stellar populations were compared with each other. The star complexes of NGC 6822 were determined by using the isopleths, based on star counts, of the young stars of the galaxy, using a statistical cutoff limit in density. In order to map them and determine their geometrical properties, an ellipse was fitted to every distinct region satisfying this minimum limit. The Kolmogorov-Smirnov statistical test was used to study possible patterns in their size distribution. Isopleths were also used to study the stellar populations of NGC 6822. The star complexes of NGC 6822 were detected and a list of their positions and sizes was produced. Indications of hierarchical star formation, in terms of spatial distribution, time evolution and preferable sizes were found in NGC 6822 and the MCs. The spatial distribution of the various age stellar populations has indicated traces of an interaction in NGC 6822, dated before 350 +/- 50 Myr.Comment: 10 pages, 7 figures, accepted by A&A; minor typeface correction

Structure of the SMC - Stellar component distribution from 2MASS data

The spatial distribution of the SMC stellar component is investigated from 2MASS data. The morphology of the different age populations is presented. The center of the distribution is calculated and compared with previous estimations. The rotation of the stellar content and possible consequence of dark matter presence are discussed. The different stellar populations are identified through a CMD diagram of the 2MASS data. Isopleth contour maps are produced in every case, to reveal the spatial distribution. The derived density profiles are discussed. The older stellar population follows an exponential profile at projected diameters of about 5 kpc (~5 deg) for the major axis and ~4 kpc for the minor axis, centred at RA: 0h:51min, Dec: -73deg 7' (J2000.0). The centre coordinates are found the same for all the different age population maps and are in good accordance with the kinematical centre of the SMC. However they are found considerably different from the coordinates of the centre of the gas distribution. The fact that the older population found on an exponential disk, gives evidence that the stellar content is rotating, with a possible consequence of dark matter presence. The strong interactions between the MCs and the MilkyWay might explain the difference in the distributions of the stellar and gas components. The lack in the observed velocity element, that implies absence of rotation, and contradicts with the consequences of exponential profile of the stellar component, may also be a result of the gravitational interactions.Comment: 7 Pages, 6 figures, accepted for publication in A&

Spatial distribution of stellar populations in the Magellanic Clouds: Implementation to Gaia

The main goal of our project is to investigate the spatial distribution of different stellar populations in the Magellanic Clouds. The results from modelling the Magellanic Clouds can be useful, among others, for simulations during the Gaia mission preparation. Isodensity contour maps have been used in order to trace the morphology of the different stellar populations and estimate the size of these structures. Moreover, star density maps are constructed through star counts and projected radial density profiles are obtained. Fitting exponential disk and King law curves to the spatial distribution allows us to derive the structural parameters that describe these profiles. The morphological structure and spatial distributions of various stellar components in the Magellanic Clouds (young and intermediate age stars, carbon stars) along with the overall spatial distribution in both Clouds are provided.Comment: 12 pages, 9 figures, to be published in Astronomy & Astrophysics; typos and language correcte

Star Formation History in two fields of the Small Magellanic Cloud Bar

The Bar is the most productive region of the Small Magellanic Cloud in terms of star formation but also the least studied one. In this paper we investigate the star formation history of two fields located in the SW and in the NE portion of the Bar using two independent and well tested procedures applied to the color-magnitude diagrams of their stellar populations resolved by means of deep HST photometry. We find that the Bar experienced a negligible star formation activity in the first few Gyr, followed by a dramatic enhancement from 6 to 4 Gyr ago and a nearly constant activity since then. The two examined fields differ both in the rate of star formation and in the ratio of recent over past activity, but share the very low level of initial activity and its sudden increase around 5 Gyr ago. The striking similarity between the timing of the enhancement and the timing of the major episode in the Large Magellanic Cloud is suggestive of a close encounter triggering star formation.Comment: 30 pages, 22 figures, accepted for publication in Ap

Age-metallicity relation in the Magellanic Clouds clusters

Aims. We study small open star clusters, using Strömgren photometry to investigate a possible dependence between age and metallicity in the Magellanic Clouds (MCs). Our goals are to trace evidence of an age metallicity relation (AMR) and correlate it with the mutual interactions of the two MCs and to correlate the AMR with the spatial distribution of the clusters. In the Large Magellanic Cloud (LMC), the majority of the selected clusters are young (up to 1 Gyr), and we search for an AMR at this epoch, which has not been much studied. Methods. We report results for 15 LMC and 8 Small Magellanic Cloud (SMC) clusters, scattered all over the area of these galaxies, to cover a wide spatial distribution and metallicity range. The selected LMC clusters were observed with the 1.54 m Danish Telescope in Chile, using the Danish Faint Object Spectrograph and Camera (DFOSC) with a single 2k × 2k CCD. The SMC clusters were observed with the ESO 3.6 m Telescope, also in Chile, using the ESO Faint Object Spectrograph and Camera (EFOSC). The obtained frames were analysed with the conventional DAOPHOT and IRAF software. We used Strömgren filters in order to achieve reliable metallicities from photometry. Isochrone fitting was used to determine the ages and metallicities. Results. The AMR for the LMC displays a metallicity gradient, with higher metallicities for the younger ages. The AMR for LMC-SMC star clusters shows a possible jump in metallicity and a considerable increase at about 6 × 108 yr. It is possible that this is connected to the latest LMC-SMC interaction. The AMR for the LMC also displays a metallicity gradient with distance from the centre. The metallicities in SMC are lower, as expected for a metal-poor host galaxy.Facultad de Ciencias Astronómicas y Geofísica

Hierarchical structures in the Large and Small Magellanic Clouds

We investigate the degree of spatial correlation among extended structures in the LMC and SMC. To this purpose we work with sub-samples characterised by different properties such as age and size, taken from the updated catalogue of Bica et al. or gathered in the present work. The structures are classified as star clusters or non-clusters (basically, nebular complexes and their stellar associations). The radius distribution functions follow power-laws ($dN/dR\propto R^{-\alpha}$) with slopes and maximum radius ($R_{max}$) that depend on object class (and age). Non-clusters are characterised by $\alpha\approx1.9$ and R_{max}\la472 pc, while young clusters (age \la10 Myr) have $\alpha\approx3.6$ and R_{max}\la15 pc, and old ones (age \ga600 Myr) have $\alpha\approx2.5$ and R_{max}\la40 pc. Young clusters present a high degree of spatial self-correlation and, especially, correlate with star-forming structures, which does not occur with the old ones. This is consistent with the old clusters having been heavily mixed up, since their ages correspond to several LMC and SMC crossing times. On the other hand, with ages corresponding to fractions of the respective crossing times, the young clusters still trace most of their birthplace structural pattern. Also, small clusters ($R<10$ pc), as well as small non-clusters ($R<100$ pc), are spatially self-correlated, while their large counterparts of both classes are not. The above results are consistent with a hierarchical star-formation scenario for the LMC and SMC.Comment: Accepted by MNRA

<i>Gaia</i> Data Release 1. Summary of the astrometric, photometric, and survey properties

Context. At about 1000 days after the launch of Gaia we present the first Gaia data release, Gaia DR1, consisting of astrometry and photometry for over 1 billion sources brighter than magnitude 20.7. Aims. A summary of Gaia DR1 is presented along with illustrations of the scientific quality of the data, followed by a discussion of the limitations due to the preliminary nature of this release. Methods. The raw data collected by Gaia during the first 14 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium (DPAC) and turned into an astrometric and photometric catalogue. Results. Gaia DR1 consists of three components: a primary astrometric data set which contains the positions, parallaxes, and mean proper motions for about 2 million of the brightest stars in common with the HIPPARCOS and Tycho-2 catalogues – a realisation of the Tycho-Gaia Astrometric Solution (TGAS) – and a secondary astrometric data set containing the positions for an additional 1.1 billion sources. The second component is the photometric data set, consisting of mean G-band magnitudes for all sources. The G-band light curves and the characteristics of ∼3000 Cepheid and RR-Lyrae stars, observed at high cadence around the south ecliptic pole, form the third component. For the primary astrometric data set the typical uncertainty is about 0.3 mas for the positions and parallaxes, and about 1 mas yr−1 for the proper motions. A systematic component of ∼0.3 mas should be added to the parallax uncertainties. For the subset of ∼94 000 HIPPARCOS stars in the primary data set, the proper motions are much more precise at about 0.06 mas yr−1. For the secondary astrometric data set, the typical uncertainty of the positions is ∼10 mas. The median uncertainties on the mean G-band magnitudes range from the mmag level to ∼0.03 mag over the magnitude range 5 to 20.7. Conclusions. Gaia DR1 is an important milestone ahead of the next Gaia data release, which will feature five-parameter astrometry for all sources. Extensive validation shows that Gaia DR1 represents a major advance in the mapping of the heavens and the availability of basic stellar data that underpin observational astrophysics. Nevertheless, the very preliminary nature of this first Gaia data release does lead to a number of important limitations to the data quality which should be carefully considered before drawing conclusions from the data

Physical properties of star clusters in the outer LMC as observed by the DES

The Large Magellanic Cloud (LMC) harbours a rich and diverse system of star clusters, whose ages, chemical abundances and positions provide information about the LMC history of star formation. We use Science Verification imaging data from the Dark Energy Survey (DES) to increase the census of known star clusters in the outer LMC and to derive physical parameters for a large sample of such objects using a spatially and photometrically homogeneous data set. Our sample contains 255 visually identified cluster candidates, of which 109 were not listed in any previous catalogue. We quantify the crowding effect for the stellar sample produced by the DES Data Management pipeline and conclude that the stellar completeness is <10 per cent inside typical LMC cluster cores. We therefore reanalysed the DES co-add images around each candidate cluster and remeasured positions and magnitudes for their stars. We also implement a maximum-likelihood method to fit individual density profiles and colour-magnitude diagrams. For 117 (from a total of 255) of the cluster candidates (28 uncatalogued clusters), we obtain reliable ages, metallicities, distance moduli and structural parameters, confirming their nature as physical systems. The distribution of cluster metallicities shows a radial dependence, with no clusters more metal rich than [Fe/H] ~= -0.7 beyond 8 kpc from the LMC centre. The age distribution has two peaks at ~=1.2 and ~=2.7 Gyr