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«c ESO 2014 Astronomy & Astrophysics manuscript no. paper1 July 22, 2014 The VMC Survey - XII. Star cluster candidates in the Large Magellanic ...»

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c ESO 2014

Astronomy & Astrophysics manuscript no. paper1

July 22, 2014

The VMC Survey - XII. Star cluster candidates in

the Large Magellanic Cloud⋆

Andr´s E. Piatti1,2, Roald Guandalini3, Valentin D. Ivanov4, Stefano Rubele5,


Maria-Rosa L. Cioni6,7, Richard de Grijs8,9, Bi-Qing For10, Gisella Clementini11,

Vincenzo Ripepi12, Peter Anders13, and Joana M. Oliveira14

arXiv:1407.5471v1 [astro-ph.GA] 21 Jul 2014

Observatorio Astron´mico, Universidad Nacional de C´rdoba, Laprida 854, 5000, o o C´rdoba, Argentina;

o e-mail: andres@oac.uncor.edu Consejo Nacional de Investigaciones Cient´ ıficas y T´cnicas, Av. Rivadavia 1917, e C1033AAJ, Buenos Aires, Argentina Instituut voor Sterrenkunde, Celestijnenlaan 200 D BUS 2401, 3001 Heverlee, Belgium European Southern Observatory, Av. Alonso de C´rdoba 3107, Casilla 19, Santiago, o Chile INAF, Osservatorio Astronomico di Padova, vicolo dell’ Osservatorio 5, I-35122 Padova, Italy University of Hertfordshire, Physics Astronomy and Mathematics, College Lane, Hatfeild AL10 9AB, United Kingdom Leibniz-Institut f¨r Astrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam,

–  –  –

survey of the Magellanic Clouds system (VMC).

Aims. We studied a total of 98 objects of small angular size, typically ∼ 11.6 pc in diameter projected towards both uncrowded tile LMC 8 8 and crowded tile LMC 5 5.

They populate relatively crowded LMC fields with significant fluctuations in the stellar density, the luminosity function, and the colour distribution as well as uncrowded fields. This cluster sample is aimed at actually probing our performance in reaching the CMD features of clusters with different ages in crowded/uncrowded fields.

Methods. We applied a subtraction procedure to statistically clean the cluster CMDs from field star contamination. We then matched theoretical isochrones to the background-subtracted CMDs to determine the ages and metallicities of the clusters.

Results. We estimated the ages of 65 clusters, which resulted to be in the age range

7.3 log(t/yr) 9.55. We also classified as chance grouping of stars 19 previoulsy catalogued clusters, two possible cluster-like asterisms, and one unresolved cluster. For other 8 objects, we could not find a clear star concentration in the Ks images either, so we quoted them as cluster-like asterisms. Finally, we found two previously catalogued single star clusters to be unresolved background galaxies (KMHK747, OGLE366), and one to be a triple cluster system (BSDL 2144).

Key words. techniques: photometric – galaxies: individual: LMC – Magellanic Clouds – galaxies: star clusters.

1. Introduction The Magellanic Clouds are the nearest example of interacting dwarf irregular galaxies.

Because of their distance (50-60 kpc) we can resolve individual stars in the field population and in star clusters. Compared to the Milky Way the Magellanic Clouds have a lower metallicity and host star clusters spanning the entire age range (de Grijs & Anders 2006;

de Grijs & Goodwin 2008). The Magellanic Clouds contain a few thousands star clusters (Bica et al. 2008, hereinafter B08) and represent an important laboratory for studies of stellar evolution. The sample of star clusters with measurements of size, mass and other parameters is, however, modest and corresponds to less than half the number of candidate star clusters (e.g., Hill & Zaritsky 2006, Werchan & Zaritsky 2011, Glatt et al. 2010, Baumgardt et al. 2013, Piatti 2014).

Taking advantage of the high sensitivity and spatial resolution of the VISTA nearinfrared Y JKs survey of the Magellanic Clouds system (VMC; Cioni et al. 2011) we embarked on an homogeneous determination of star cluster parameters. Compared to the wide-scale Magellanic Clouds Photometric Survey data (Zaritsky et al. 2002, 2004) the VMC survey data corresponds to an improvement of about a factor of two in pixel scale and seeing. In addition, the VMC makes use of the near-infrared filters, Y JKs, covers a wider area around each Cloud, and includes the Magellanic Bridge. The VMC covers ∼ 170 deg2 of the entire Magellanic system with 110 individual tiles; each tile covers ∼ 1.5 deg2.

With a statistical sample of characterised star clusters as complete as possible we will be able to answer some key open questions in star cluster studies, such as: Has the field exBased on observations made with VISTA at the Paranal Observatory under programme ID


perienced the same star formation history as the cluster stellar population? What is the distribution of star clusters as a function of age and metallicity? What galaxy structure is defined by star clusters with different ages? Is there a relation between the age and size of star clusters?

This is the first VMC paper that provides information on the star clusters of the Magellanic system. Some preliminary results from the analysis of star clusters in a tile covering the South Ecliptic Pole (SEP) region are published in Cioni et al. (2011). The complete study of star clusters in the SEP tile (tile LMC 8 8) is presented here along with analysis of clusters in tile LMC 5 5 covering the LMC bar. Study of star clusters in other tiles will follow. We plan to study known star clusters identified in previous studies, and included in the B08’s catalogue, and to search for new star clusters based on the stellar surface density method (Ivanov et al. 2002; Borissova et al. 2003). Fig. 1 depicts the spatial distribution of B08’s catalog of star clusters, wherein black points and green circles represent the B08’s catalogued star clusters and those with age estimates available. The VMC tile distribution is superimposed.

This paper is organised as follows. VMC observations and data reduction are presented in Section 2. The star cluster sample is described in Section 3 while the cleaning of the colour-magnitude diagrams (CMDs) and the derivation of the cluster parameters (size, age, metallicity) are presented in Sections 4 and 5, respectively. Finally, we discuss the results in Section 6 and draw our main conclusions of this analysis in Section 7.

2. Data collection and reduction

The VMC survey strategy involves repeated observations of tiles across the Magellanic system, where one tile covers uniformly an area of ∼ 1.5 deg2, as a result of the mosaic of six paw-print images, in a given waveband with 3 epochs at Y and J, and 12 epochs at Ks. Individual epochs have exposure times of 800 s (Y and J) and 750 s (Ks ). The average quality of the VMC data analysed here corresponds to 0.34′′ pixel size, 0.90′′ FWHM, and

0.06 ellipticity.

To date eleven tiles in the LMC are completely observed, three of them are located in the innermost region of the LMC, nominally the tiles LMC 6 6, 6 4 and 5 5. The tile LMC 6 6 (30 Doradus) is a high rate star formation region affected by large differential extinction (see for example Rubele et al 2012, Tatton et al. 2013), the tile LMC 6 4 is located in the centre of the LMC Bar with high levels of crowding that could affect the capability of our tools to detect stars in clusters and decontaminate them by the LMC background stars.

Star clusters in these central tiles will be analysed separately.

The tile LMC 5 5 is located towards the LMC outer Bar/Bar region (centred at RA = 05 : 24 : 30, DEC = −70 : 48 : 34 (J2000)), contains 77 catalogued clusters with a noticeable field star crowding level and moderate extinction. It was completed early in the course of the survey, and we obtained PSF photometry of the clusters in this tile. Consequently, we can probe our performance in reaching the Main Sequence Turnoffs (MSTOs) of intermediate-age and relatively old clusters in crowded fields. Our previous experience (Cioni et al. 2011; Rubele et al. 2012) shows that the widest colour range of the A.E. Piatti et al.: LMC star clusters Y − Ks colour is best for cluster studies because it makes clearer distinguishing different cluster Main Sequences (MSs) -particularly their turnoff regime- and the Red Giant phases, as well as having a higher sensitivity to reddening and metallicity than the Y − J, and J − Ks colours. Therefore, we mainly rely the present analysis on the Ks versus Y − Ks CMDs; the J versus Y − J and Ks versus J − Ks CMDs being useful in order to confirm our results.

The tile LMC 8 8 was one of the first two fully completed VMC survey tiles, and it

overlaps with the SEP field. The tile is centered at RA = 05 : 59 : 23, Dec = −66 :

20 : 28 (J2000) and includes 23 catalogued clusters, out of which two are binary clusters (KMHK 1552 + BSDL 3190, and KMHK 1519 + BSDL 3174; Dieball et al. 2002). The clusters catalogued by B08 located within the limits of tiles LMC 5 5 and 8 8 are listed in Table 1 (see also Fig. 1).

The tile LMC 5 5 and 8 8 data refers to observations acquired from November 2009 to December 2012 under homogeneous sky conditions since it was obtained in service mode when the sky quality met the requested VMC criteria (see Cioni et al. (2011)). The data were reduced with the VISTA Data Flow System pipeline, version 1.1 (VDFS; Irwin et al.

2004), and calibrated into the VISTA photometric system, which is close to the Vegamag system; we extracted it from the VISTA Science Archive (VSA; Cross et al. 2012).

For this work we perform our point-spread function (PSF) photometry on a homogenized VMC deep tile image, that was created starting from the paw-print VMC images.

The PSF homogenized methodology consists in a convolution with a kernel of the original paw-print images to turn different PSF shapes into a more constant and uniform PSF model on the paw-print images. The purpose of degrading the PSF on paw-print images, to a unique PSF model, is to produce deep tile images with a uniform and homogeneous PSF. As a matter of fact the paw-print images are stacked single exposures reaching a continuous observation time of hundreds seconds, therefore variations of the seeing occurring over these time scales could affect the PSF shape on the final deep tile, as a function of the position.

We performed PSF photometry on the homogenized deep tile image -created as described in Rubele et al. (2012)- of VMC tiles LMC 5 5 and 8 8, using the IRAF DAOPHOT packages (Stetson 1987). The PSF model was created using ∼2500 stars uniformly distributed and with magnitude close to the saturation limit + 1.5 mag. (for the VMC survey the single paw-print saturation limits are 12.9 mag, 12.7 mag, 11.4 mag in Y, J, and Ks, respectively). Subsequently we used the ALLSTAR routine to perform the final PSF photometry on all the three filters images, and correlated the resulting catalogs using a one arcsec radius. We checked and corrected our PSF photometry to the aperture effect using catalogs retrieved from the VSA (Lewis et al. 2010; Cross et al. 2012)1, for the bulk of the observed stars. We ran a large number of artificial star tests (ASTs) to estimate the incompleteness and error distribution of our data for each tile and throughout the CMD.

For each region we ran ∼ 20×106 ASTs as described in Rubele et al. (2012), using a spatial grid with 25 pixels width and with a magnitude distribution proportional to the square http://horus.roe.ac.uk/vsa/ of the magnitude. This latter choice allows us to better map completeness and errors in the less complete regions of the CMD. Fig. 2 depicts CMDs for both tiles with error bars coloured according to the colour scale of the completeness level. Photometric errors of 0.10 mag were derived for stars with Y = 19.95 mag, J = 19.78 mag, and Ks = 19.27 mag in the tile LMC 5 5, and Y = 21.52 mag, J = 21.23 mag, and Ks = 20.43 mag in the tile LMC 8 8. As for the photometry completeness we found that our data set is 50% complete at Y = 20.6 mag, J = 20.3 mag, and Ks = 19.9 mag in the tile LMC 5 5, and at Y = 22.1 mag, J = 21.7 mag, and Ks = 20.6 mag in the tile LMC 8 8.

3. The cluster sample

We analysed a total of 98 (75 in LMC 5 5 and 23 in LMC 8 8) candidate clusters spread over the area covered by the tiles considered. They are all the objects catalogued by B08 which overlap the tile areas, in addition to BSDL 2147 and BSDL 2221 in LMC 5 5, which we discarded because they fall in a small tile region affected by dead pixels and on the edge of the tile, respectively. The studied objects range from intermediate-age cluster candidates (age ≤ 6 Gyr) to very young clusters (age ∼ 20 Myrs). We also confirmed that some of the previously catalogued clusters are not indeed real stellar aggregates, but possible clusterlike asterisms (see section 6). Furthermore, the angular resolution of VMC made it apparent that two catalogued clusters (KMHK747, OGLE366 in LMC 5 5) are most likely compact galaxies. Consequently, the analysis of the candidate clusters allowed us to attain a more robust cluster sample with genuine physical systems in this particular field. We also refer the reader to the work by Piatti (2014) for a discussion about the completeness of the presently known star cluster population.

The confirmed clusters with ages larger than 1 Gyr will allow us to explore overall features related to the star formation and chemical evolution history of the LMC. For instance, an important burst of cluster formation took place ∼ 2 Gyr ago after a cluster age gap (Piatti 2011; Piatti et al. 2002). On the other hand, younger clusters have been studied in the context of a variety of different astrophysical issues, like the initial mass function, the recent star formation rate, the early star cluster disruption (Da Rio et al.

2009; Indu & Subramaniam 2011; de Grijs et al. 2013), among others. The accuracy of the astropysical properties derived of clusters covering a wide age range allows us to assess the ability of the VMC survey in dealing with such a variety of objects, particularly those of relatively small angular size and projected towards crowded regions such as in the LMC outer Bar.

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