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

CCD CT1 photometry of small angular size candidate star clusters projected on to crowded Small Magellanic Cloud star fields

We present for the first time CCD CT1 photometry for 11 unstudied or poorly studied candidate star clusters. The selected objects are of small angular size, contain a handful of stars, and are projected towards the innermost regions of the Small Magellanic Cloud (SMC). To clean their Colour-Magnitude Diagrams (CMDs) from the unavoidable star field contamination, we use a method which has shown to be able to eliminate stochastic effects in the cluster CMDs caused by the presence of isolated bright stars, as well as, to make a finer cleaning in the most populous CMD regions. The resulting cleaned cluster CMDs suggest that most of the studied candidate star clusters would not appear to be genuine physical systems. Furthermore, we confirm that the ages previously derived for some of them mostly reflect those of the composite stellar populations of the SMC field. On the other hand, if we use the spatial distribution of possible non-clusters to statiscically decontaminate that of the cataloged clusters, we find that there is no clear difference between the expected and the observed cluster spatial distributions. However, if we double the amount of possible non-clusters the difference becomes bigger than 2σ between a ≈ 0.3◦ and 1.2◦ (a = the semi-major axis of an ellipse parallel to the SMC bar and with b/a = 1/2).Presentamos por vez primera fotometría CCD CT1 de 11 objetos no estudiados o poco estudiados, catalogados como cúmulos estelares de la Nube Menor de Magallanes (NMM). Los objetos seleccionados son de pequeño tamaño angular, contienen un puñado de estrellas, y se encuentran proyectados en dirección a las regiones interiores de la NMM. Para limpiar sus diagramas color-magnitud (DCM), utilizamos un método que ha mostrado ser capaz de eliminar efectos estocásticos en los DCM, como así también realizar una limpieza más fina en las regiones más pobladas del DCM. Los DCM de estos objetos, previamente limpiados de la contaminación de estrellas del campo, muestran que no se trata en la mayoría de genuinos sistemas físicos. Encontramos que las edades estimadas previamente para algunos objetos corresponden más bien a la composición de poblaciones estelares del campo. Finalmente, teniendo en cuenta la distribución espacial de los objetos que no resultaron ser cúmulos y la de los cúmulos catalogados, encontramos que no existen diferencias claras entre la distribución observada y la derivada de sustraer los objetos que no son cúmulos. Sin embargo, si duplicamos la cantidad de objetos que posiblemente no son cúmulos, la diferencia resulta mayor que 2σ entre a ≈ 0.3◦ and 1.2◦ (a= semi eje mayor de una elipse paralela a la barra de la NMM y con b/a=1/2)Fil: Piatti, Andres Eduardo. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Bica, E.. Universidade Federal do Rio Grande do Sul; Brasi

Gemini/GMOS photometry of intermediate-age star clusters in the Large Magellanic Cloud

We present Gemini South GMOS g,i photometry of 14 intermediate-age Large Magellanic Cloud (LMC) star clusters, namely: NGC 2155, 2161, 2162, 2173, 2203, 2209, 2213, 2231, 2249, Hodge 6, SL 244, 505, 674, and 769, as part of a continuing project to investigate the extended Main Sequence Turnoff (EMSTO) phenomenon. Extensive artificial star tests were made over the observed field of view. These tests reveal the observed behaviour of photometric errors with magnitude and crowding. The cluster stellar density radial profiles were traced from star counts over the extent of the observed field. We adopt clus- ter radii and build colour-magnitude diagrams (CMDs) with cluster features clearly identified. We used the cluster (g,g-i) CMDs to estimate ages from the matching of theoretical isochrones. The studied LMC clusters are confirmed to be intermediate-age clusters, which range in age 9.10 < log(t) < 9.60. NGC 2162 and NGC 2249 look like new EMSTO candidates, in addition to NGC 2209, on the basis of having dual red clumps.Comment: MNRAS, accepte

Evidence of differential tidal effects in the old globular cluster population of the Large Magellanic Cloud

We present for the first time extended stellar density and/or surface brightness radial profiles for almost all the known Large Magellanic Cloud (LMC) old globular clusters (GCs). These were built from DECam images and reach out to ∼ 4 times the GCs’ tidal radii. The background subtracted radial profiles reveal that the GCs located closer than ∼ 5 kpc from the LMC centre contain an excess of stars in their outermost regions with respect to the stellar density expected from a King profile. Such a residual amount of stars, not seen in GCs located farther than ∼ 5 kpc from the LMC centre, as well as the GCs’ dimensions, shows a clear dependence with the GCs’ positions in the galaxy, in the sense that, the farther the GC from the centre of the LMC, the larger both the excess of stars in its outskirts and size. Although the masses of GCs located inside and outside ∼ 5 kpc are commensurate, the outermost regions of GCs located closer than ∼ 5 kpc from the LMC centre appear to have dynamically evolved more quickly. These outcomes can be fully interpreted in the light of the known GC radial velocity disc-like kinematics, from which GCs have been somehow mostly experiencing the influence of the LMC gravitational field at their respective mean distances from the LMC centre

New quasars behind the Magellanic Clouds. Spectroscopic confirmation of near-infrared selected candidates

Context. Quasi-stellar objects (quasars) located behind nearby galaxies provide an excellent absolute reference system for astrometric studies, but they are difficult to identify because of fore- and background contamination. Deep wide-field, high angular resolution surveys spanning the entire area of nearby galaxies are needed to obtain a complete census of such quasars. Aims. We embarked on a program to expand the quasar reference system behind the Large and the Small Magellanic Clouds, the Magellanic Bridge, and the Magellanic Stream that connects the Clouds with the Milky Way. Methods. Hundreds of quasar candidates were selected based on their near-infrared colors and variability properties from the ongoing public ESO VISTA Magellanic Clouds survey. A subset of 49 objects was followed up with optical spectroscopy. Results. We confirmed the quasar nature of 37 objects (34 new identifications): four are low redshift objects, three are probably stars, and the remaining three lack prominent spectral features for a secure classification. The bona fide quasars, identified from their broad emisison lines, are located as follows: 10 behind the LMC, 13 behind the SMC, and 14 behind the Bridge. The quasars span a redshift range from z ~ 0.5 to z ~ 4.1. Conclusions. Upon completion the VMC survey is expected to yield a total of ~1500 quasars with Y&lt; 19.32 mag, J&lt; 19.09 mag, and Ks&lt; 18.04 mag

Estudio de cúmulos estelares de la Nube Menor de Magallanes: fotometría de Washington de 14 cúmulos de edad intermedia

We present CCD photometry in the Washington system C and T1 bands of 14 star clusters of the Small Magellanic Cloud (SMC). We determine ages and metallicities using theoretical isochrones, the δT1 parameter and the Red Giant Standard method. Excepting IC 1708, all the observed clusters are aged between 1 and 6.3 Gyr, while their metallicities ([Fe/H]) range between -0.7 and -1.4. Two important cluster formation episodes in the SMC, dated at ∼ 2 Gyr and ∼ 5 Gyr ago, were detected. During approximately the first 4 Gyr in the SMC lifetime, the cluster formation rate turned out to be constant. A detailed version of this work can be seen in Piatti et al. (2011, MNRAS, 417, 1559).Presentamos fotometría CCD en las bandas C y T1 del sistema de Washington de 14 cúmulos estelares de la Nube Menor de Magallanes (NmM). Usando isócronas teóricas, el parámetro δT1 y el método de las Ramas Gigantes Estándar, determinamos edades y metalicidades. Excepto IC 1708, todos los cú- mulos observados tienen edades entre 1000 y 6300 millones de años y valores de [Fe/H] entre -0.7 y -1.4. Constatamos dos importantes episodios de formación de cúmulos en la NmM ocurridos hace ∼ 2 mil y ∼ 5 mil millones de años. Durante los primeros 4 mil millones de años desde que se formaron los cúmulos, la tasa de formación de los mismos se mantuvo constante. Una versión detallada de este trabajo puede verse en Piatti et al. (2011, MNRAS, 417, 1559).Fil: Piatti, Andres Eduardo. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Claria Olmedo, Juan Jose. Universidad Nacional de Cordoba. Observatorio Astronomico de Cordoba; ArgentinaFil: Bica, E.. Universidade Federal do Rio Grande do Sul; BrasilFil: Geisler, D.. Universidad de Concepción; ChileFil: Ahumada, Andrea Veronica. Universidad Nacional de Cordoba. Observatorio Astronomico de Cordoba; ArgentinaFil: Girardi, L.. Observatorio di Padova; Itali

Espectros integrados EFOSC2/NTT de 10 cúmulos estelares pertenecientes a las Nubes de Magallanes

We present integrated spectra in the optical range for 10 concentrated Magellanic Clouds’ (MCs) stellar clusters, six of them belonging to the Large Magellanic Cloud, and the other four to the Small Magellanic Cloud. The spectra were obtained with the EFOSC2 spectrograph (NTT-ESO, La Silla, Chile). We estimate simultaneously age and foreground reddening by comparing the continuum distribution and line strengths of the cluster spectra with those of template spectra. The present cluster sample complements previous ones, in an effort to create a spectral library for the MCs with several clusters per age bin.Se presentan espectros integrados en el rango optico obtenidos con el espectrografo EFOSC2 (NTT-ESO, La Silla, Chile) de 6 cumulos estelares (CEs) de la Nube Mayor de Magallanes (NMM) y 4 de la Nube Menor (NmM). Las edades y enrojecimientos se determinaron comparando la distribucion del continuo y la intensidad de lıneas espectrales con estas mismas caracterısticas en espectros patrones (templates). Los 10 cumulos estudiados abarcan un amplio rango de edad y estan levemente enrojecidos. Estos espectros seran de gran utilidad para actualizar las bases de templates actualmente existentes para la metalicidad tıpicamente subsolar de las Nubes de Magallanes (NMs).Fil: Ahumada, Andrea Veronica. Universidad Nacional de Cordoba. Observatorio Astronomico de Cordoba; ArgentinaFil: Benítez Llambay, Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Astronomia Teórica y Experimental. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba. Instituto de Astronomia Teórica y Experimental; ArgentinaFil: Santos, J. F. C.. Universidade Federal do Minas Gerais; BrasilFil: Claria Olmedo, Juan Jose. Universidad Nacional de Cordoba. Observatorio Astronomico de Cordoba; ArgentinaFil: Bica, E.. Universidade Federal do Rio Grande do Sul; BrasilFil: Piatti, Andres Eduardo. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentin

New old star clusters in the Small Magellanic Cloud

Using Ca triplet spectroscopy and P SF photometry performed on data taken with the “Very Large Telescope” (Chile), we derived reliable ages and metallicities of 15 Small Magellanic Cloud (SMC) star clusters. Three of them were found to be very old clusters so that the determination of their fundamental properties allows us to improve our current knowledge on the SMC chemical evolution during the earliest epochs. Based on our new data, we further discuss the age-metallicity relation. The procedures and results included in this article will be presented in a more extensive and detailed way in Parisi et al. (2011 - in preparation).Usando espectroscopıa del Triplete del Calcio Ionizado y fotometrıa P SF realizada sobre datos tomados con el “Very Large Telescope” (Chile), derivamos edades y metalicidades confiables de 15 cumulos estelares de la Nube Menor de Magallanes (NmM). Encontramos que 3 de ellos son cumulos muy viejos, por lo que la determinacion de sus propiedades fundamentales permite mejorar nuestro actual conocimiento sobre la evolucion quımica de la NmM durante su etapa mas temprana. Basados en nuestros nuevos datos, rediscutimos la relacion edad-metalicidad en esta galaxia. Los procedimientos y resultados incluidos en este artıculo, seran presentados de manera mas extensa y detallada en Parisi et al. (2011 - en preparacion).Fil: Parisi, M. C.. Universidad Nacional de Cordoba. Observatorio Astronomico de Cordoba; ArgentinaFil: Geisler, D.. Universidad de Concepción; ChileFil: Grocholski, A. J.. University of Florida; Estados UnidosFil: Claria Olmedo, Juan Jose. Universidad Nacional de Cordoba. Observatorio Astronomico de Cordoba; ArgentinaFil: Carraro, G.. European Southern Observatory; ChileFil: Costa, E.. Universidad de Chile; ChileFil: Piatti, Andres Eduardo. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Sarajedini, A.. University of Florida; Estados UnidosFil: Leiton, R.. Universidad de Concepción; Chil