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

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

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

On the physical nature of globular cluster candidates in the Milky Way bulge

We present results from 2MASS JKs photometry on the physical reality of recently reported globular cluster (GC) candidates in the Milky Way (MW) bulge. We relied our analysis on photometric membership probabilities that allowed us to distinguish real stellar aggregates from the composite field star population. When building colour-magnitude diagrams and stellar density maps for stars at different membership probability levels, the genuine GC candidate populations are clearly highlighted. We then used the tip of the red giant branch (RGB) as distance estimator, resulting heliocentric distances that place many of the objects in regions near of the MW bulge where no GC had been previously recognised. Some few GC candidates resulted to be MW halo/disc objects.Metallicities estimated from the standard RGB method are in agreement with the values expected according to the position of the GC candidates in the Galaxy. We finally derived from the first time their structural parameters. We found that the studied objects have core, half-light and tidal radii in the ranges spanned by the population of known MW GCs. Their internal dynamical evolutionary stages will be described properly when their masses are estimated.Comment: 11 pages, 7 figures. Accepted for publication in Monthly Notices of the Royal Astronomical Societ

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

Towards a comprehensive knowledge of the open cluster Haffner 9

We turn our attention to Haffner 9, aMilkyWay open cluster whose previous fundamental parameter estimates are far from being in agreement. In order to provide with accurate estimates, we present high-quality Washington CT1 and Johnson BVI photometry of the cluster field.We put particular care in statistically cleaning the colour-magnitude diagrams (CMDs) from field star contamination, which was found a common source in previous works for the discordant fundamental parameter estimates. The resulting cluster CMD fiducial features were confirmed from a proper motion membership analysis. Haffner 9 is a moderately young object (age ~350 Myr), placed in the Perseus arm - at a heliocentric distance of ~3.2 kpc -, with a lower limit for its present mass of ~160 M ⊙ and of nearly metal solar content. The combination of the cluster structural and fundamental parameters suggest that it is in an advanced stage of internal dynamical evolution, possibly in the phase typical of those with mass segregation in their core regions. However, the cluster still keeps its mass function close to that of the Salpeter's law.Fil: Piatti, Andres Eduardo. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba; Argentin

New fundamental parameters of the Galactic open clusters Berkeley 26, Czernik 27, Melotte 72, NGC 2479 and BH 37

We have obtained CCD UBVI_{KC} photometry down to V ~ 21.0 for the open clusters Berkeley 26, Czernik 27, Melotte 72, NGC 2479 and BH 37. The latter has never been studied before. Cluster stellar density profiles were obtained from star counts in appropriate-sized boxes distributed throughout the entire observed fields. Based on different measured indices, we estimate the ages of Berkeley 26, Melotte 72 and NGC 2479. On the other hand, we indicate possible solutions for the cluster fundamental parameters by matching theoretical isochrones which reasonably reproduce the main cluster features in their CMDs. In the case of NGC 2479, the cluster E(B-V) and E(V-I) colour excesses and apparent distance modulus were estimated from the fit of the Zero-Age Main Sequence (ZAMS) to the colour-colour and colour-magnitude diagrams, respectively.Comment: MNRAS accepte