The VMC survey - XV : The Small Magellanic Cloud-Bridge connection history as traced by their star cluster populations

Date of Acceptance: 19/03/2015We present results based on YJKs photometry of star clusters located in the outermost, eastern region of the Small Magellanic Cloud (SMC). We analysed a total of 51 catalogued clusters whose colour-magnitude diagrams (CMDs), having been cleaned from field-star contamination, were used to assess the clusters' reality and estimate ages of the genuine systems. Based on CMD analysis, 15 catalogued clusters were found to be possible non-genuine aggregates. We investigated the properties of 80 per cent of the catalogued clusters in this part of the SMC by enlarging our sample with previously obtained cluster ages, adopting a homogeneous scale for all. Their spatial distribution suggests that the oldest clusters, log(t/yr) ≥ 9.6, are in general located at greater distances to the galaxy's centre than their younger counterparts - 9.0 ≤ log(t/yr) ≤ 9.4 - while two excesses of clusters are seen at log(t/yr) ~9.2 and log(t yr-1) ˜ 9.7. We found a trail of younger clusters which follow the wing/bridge components. This long spatial sequence does not only harbour very young clusters, log(t yr-1) ~7.3, but it also hosts some of intermediate ages, log(t/yr) ~9.1. The derived cluster and field-star formation frequencies as a function of age are different. The most surprising feature is an observed excess of clusters with ages of log(t/yr) < 9.0, which could have been induced by interactions with the LMC.Peer reviewedFinal Accepted Versio

A new Extended Main Sequence Turnoff star cluster in the Large Magellanic Cloud

We present results on the age and metallicity estimates of the poorly studied LMC cluster SL 529, from CCD SDSS gr photometry obtained at the Gemini South telescope with the GMOS attached. The cluster MSTO region possesses an extended structure, with an age spread (~ 0.5 Gyr) bigger than the mean age width of known EMSTO LMC clusters. We report for the first time a mean cluster age of 2.25 Gyr and a mean cluster metallicity of Z=0.004, which place it as the most metal-poor and oldest cluster in the EMSTO LMC cluster group. In addition, the cluster RC appears to be formed by two concentrations of stars - although it is not clear whether this feature can be caused, in part, by binary interactions and mergers -, whereas the cluster core radius of 4.2 pc is in excellent agreement with those determined for the previously 12 known EMSTO LMC clusters.Comment: 6 pages, 6 figures, MNRAS (accepted

CCD SDSS Photometry of Poorly Studied Star Clusters in the Large Magellanic Cloud

We present for the first time CCD SDSS푔푟photometry, obtained at the Gemini South telescope with the GMOS attached, of stars inthe field of the poorly studied star clusters NGC1768, HS 85, SL 676, NGC2107, NGC2190, and SL 866, which are distributed in themain body of the Large Magellanic Cloud. We applied a subtraction procedure to statistically clean the cluster CMDs from field starcontamination. In order to disentangle cluster features from those belonging to their surrounding fields, we applied a subtractionprocedure which makes use of variable cells to reproduce the field star color-magnitude diagrams (CMDs) as closely as possible. Wethentraced theirstellardensity radialprofilesfromstarcountsperformedover thecleaned field starsdataset and derivedtheir radii.Using the cleaned cluster CMDs, we estimated ages and metallicities from matching theoretical isochrones computed for the SDSSsystem. The studied star clusters have ages from 0.1 up to 2.0Gyr and are of slightly metal-poor metal content ([Fe/H]≈−0.4dex).Fil: Piatti, Andres Eduardo. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Cordoba. Observatorio Astronomico de Cordoba; Argentin

Discovery of a loose star cluster in the Large Magellanic Cloud

We present results for an up-to-date uncatalogued star cluster projected towards the Eastern side of the Large Magellanic Cloud (LMC) outer disc. The new object was discovered from a search of loose star cluster in the Magellanic Clouds' (MCs) outskirts using kernel density estimators on Washington CT1 deep images. Contrarily to what would be commonly expected, the star cluster resulted to be a young object (log(t /yr) = 8.45) with a slightly subsolar metal content (Z = 0.013) and a total mass of 650Mo. Its core, half-mass and tidal radii also are within the frequent values of LMC star clusters. However, the new star cluster is placed at the Small Magellanic Cloud distance and at 11.3 kpc from the LMC centre. We speculate with the possibility that it was born in the inner body of the LMC and soon after expeled into the intergalactic space during the recent Milky Way/MCs interaction. Nevertheless, radial velocity and chemical abundance measurements are needed to further understand its origin, as well as extensive search for loose star clusters in order to constrain the effectiveness of star cluster scattering during galaxy interactions.Comment: MNRAS Letters, 5 pages, 2 figures (in press

Disentangling the physical reality of star cluster candidates projected towards the inner disc of the Large Magellanic Cloud

We have used Washington photometry for 90 star cluster candidates of small angular size -typically ~ 11" in radius- distributed within nine selected regions in the inner disc of the Large Magellanic Cloud (LMC) to disentangle whether they are genuine physical system, and to estimate the ages for the confirmed clusters. In order to avoid a misleading interpretation of the cluster colour-magnitude diagrams (CMDs), we applied a subtraction procedure to statistically clean them from field star contamination. Out of the 90 candidate clusters studied, 61 of them resulted to be genuine physical systems, whereas the remaining ones were classified as possible non- clusters since either their CMDs and/or the distribution of stars in the respective fields do not resemble those of stellar aggregates. We statistically show that ~ (13 +- 6)% of the catalogued clusters in the inner disc could be possible non-clusters, independently of their deprojected distances. We derived the ages for the confirmed clusters from the fit of theoretical isochrones to the cleaned cluster CMDs. The derived ages resulted to be in the age range 7.8 < log(t) < 9.2. Finally, we built cluster frequencies for the different studied regions and found that there exists some spatial variation of the LMC CF throughout the inner disc. Particularly, the innermost field contains a handful of clusters older than ~ 2 Gyr, while the wider spread between different CFs has taken place during the most recent 50 Myr of the galaxy lifetime.Comment: MNRAS, accepte

The age-metallicity relationship in the Small Magellanic Cloud periphery

We present results from Washington CT1 photometry for eleven star fields located in the western outskirts of the Small Magellanic Cloud (SMC), which cover angular distances to its centre from 2 up to 13 degrees (~ 2.2 - 13.8 kpc). The colour- magnitude diagrams, cleaned from the unavoidable Milky Way (MW) and background galaxy signatures, reveal that the most distant dominant main sequence (MS) stellar populations from the SMC centre are located at an angular distance of ~ 5.7 deg (6.1 kpc); no sign of farther clear SMC MS is visible other than the residuals from the MW/background field contamination. The derived ages and metallicities for the dominant stellar populations of the western SMC periphery show a constant metallicity level ([Fe/H] = -1.0 dex) and an approximately constant age value (~ 7-8 Gyr). Their age-metallicity relationship (AMR) do not clearly differ from the most comprehensive AMRs derived for almost the entire SMC main body. Finally, the range of ages of the dominant stellar populations in the western SMC periphery confirms that the major stellar mass formation activity at the very early galaxy epoch peaked ~ 7-8 Gyr ago.Comment: 20 pages, 7 figures, MNRAS (accepted

The star cluster frequency throughout the Large Magellanic Cloud

We address the issue about the variation of the star cluster frequency (CF) in the Large Magellanic Cloud (LMC) in terms of the cluster spatial distribution. We adopted the LMC regions traced by Harris & Zaritsky (2009) and used an updated version of the cluster database compiled by Baumgardt et al. (2013). The CFs were produced by taking into account an appropriate selection of age bins. Since the uncertainty in a cluster's age can be large compared to the size of the age bins, we account for the fact that a cluster could actually reside in one of a few adjacent age bins. We confirm that there exist some variations of the LMC CFs in terms of their spatial distributions, although some caveats should be pointed out. 30 Doradus resulted to be the region with the highest relative frequency of youngest clusters, while the log($t$) = 9-9.5 (1-3 Gyr) age range is featured by cluster formation at a higher rate in the inner regions than in the outer ones. We compared the observed CFs to theoretical CFs, which are based on the star formation histories of the field stars in each region of the LMC, and found the former predicting more or fewer clusters than observed depending on the field and age range considered.Comment: 27 pages, 14 figures, MNRAS, accepte

Bruck 88 : a young star cluster with an old age resemblance in the outskirts of the Small Magellanic Cloud

We present spectroscopic and photometric results for the Small Magellanic Cloud (SMC) cluster Bruck 88. From the comparison of the cluster integrated spectrum with template cluster spectra we found that the Milky Way globular cluster template spectra are the ones which best resemble it. However, the extracted cluster colour magnitude diagram reveals that Bruck 88 is a young cluster (log(t) = 8.1 +- 0.1). The derived cluster age is compatible with the presence of a Bright Red Giant (BRG) star located ~ 2.6 arcsec in the sky from the cluster centre. We serendipitously observed HW 33, a star cluster located ~ 3 arcmin to the south-east from Bruck 88. We obtained for the cluster the same age than Bruck 88 and surprisingly, a BRG star located within the cluster radius also appears to be compatible with the cluster age. We estimated the MK type of the BRG star in the Bruck 88 field to be in the range G9 II/Ib - K1 III. By combining the spectrum of a star within this MK type range with a 100-150 Myr template cluster integrated spectrum, we found that a proportion 85/15 in the sense BRG/template results in a spectrum which best resembles that of Bruck 88. This result confirms that a BRG star dominates the cluster integrated spectrum, so that it causes the globular cluster appearance of its integrated light.Comment: MNRAS, accepte