BS196: an old star cluster far from the SMC main body

We present B and V photometry of the outlying SMC star cluster BS196 with the 4.1-m SOAR telescope. The photometry is deep (to V~25) showing ~3 mag below the cluster turnoff point (TO) at Mv=2.5 (1.03 Msun). The cluster is located at the SMC distance. The CMD and isochrone fittings provide a cluster age of 5.0+-0.5 Gyr, indicating that this is one of the 12 oldest clusters so far detected in the SMC. The estimated metallicity is [Fe/H]=-1.68+-0.10. The structural analysis gives by means of King profile fittings a core radius Rc=8.7+-1.1 arcsec (2.66+-0.14 pc) and a tidal radius Rt=69.4+-1.7 arcsec (21.2+-1.2 pc). BS196 is rather loose with a concentration parameter c=0.90. With Mv=-1.89+-0.39, BS196 belongs to the class of intrinsically fainter SMC clusters, as compared to the well-known populous ones, which starts to be explored.Comment: 8 pages, 10 figures; accepted by MNRA

Probing embedded star clusters in the HII complex NGC 6357 with VVV

NGC 6357 is an active star-forming region located in the Sagittarius arm displaying several star clusters, which makes it a very interesting target to investigate star formation and early cluster evolution. We explore NGC 6357 with the "VISTA Variables in the V\'ia a L\'actea" (VVV) photometry of seven embedded clusters (ECs), and one open cluster (OC) projected in the outskirts of the complex.Photometric and structural properties (age, reddening, distance, core and total radii) of the star clusters are derived. VVV saturated stars are replaced by their 2MASS counterparts. Field-decontaminated VVV photometry is used to analyse Colour-Magnitude Diagrams (CMDs), stellar radial density profiles (RDPs) and determine astrophysical parameters. We report the discovery of four ECs and one intermediate-age cluster in the complex area. We derive a revised distance estimate for NGC 6357 of 1.78$\pm$0.1 kpc based on the cluster CMD morphologies. Among the ECs, one contains the binary star the WR 93, while the remaining ones are dominated by pre-main sequence (PMS) stars, young-stellar objects (YSO) and/or and have a developed main sequence. These features reflect a significant age spread among the clusters. Evidence is found that the relatively populous cluster Pismis 24 hosts two subclusters.Comment: This article will be published in the A&A. 11 pages, 15 figures and 3 table

Discovery of a stellar system in the background of 47 Tucanae

We report on the discovery of a stellar system in the background of the Galactic globular cluster 47 Tucanae (NGC 104), located 14.8' North-West of the cluster center. The object, whose apparent diameter is D~30'', is partially resolved into stars on the available CCD images, reaching a limiting magnitude of V~22.5, and is detected as a significant (more than 5 sigma) overdensity of blue stars (B-V<0.7). The color magnitude diagram of the system, its characteristic projected size and its position in the sky suggest that it is an intermediate-old age cluster belonging to the Small Magellanic Cloud, whose outskirts lie in the background of 47 Tuc. Although less likely, the possibility that the object is an unknown dwarf galaxy in the outskirts of the Local Group cannot completely be ruled out by the present data.Comment: 5 pages, 5 figures, accepted for publication in A&

A Chandra Observation of the Nearby Lenticular Galaxy NGC 5102: Where are the X-ray Binaries?

We present results from a 34 ks Chandra/ACIS-S observation of the nearby (d=3.1 Mpc) lenticular galaxy NGC 5102, previously shown to have an unusually low X-ray luminosity. We detect eleven X-ray point sources within the the $D_{25}$ optical boundary of the galaxy (93% of the light), one third to one half of which are likely to be background AGN. One source is coincident with the optical nucleus and may be a low-luminosity AGN. Only two sources with an X-ray luminosity greater than 10$^{37}$ ergs s$^{-1}$ in the 0.5-5.0 keV band were detected, one of which is statistically likely to be a background AGN. We expected to detect 6 such luminous sources if the XRB population scales linearly with optical magnitude of the host galaxy. NGC 5102 has an unusually low number of XRBs. NGC 5102 is unusually blue for its morphological type, and has undergone at least two recent bursts of star formation. We present the results of optical/UV spectral synthesis analysis and demonstrate that a significant fraction ($>$50%) of the stars in this galaxy are comparatively young ($<3\times10^9$ years old). If the lack of X-ray binaries is related to the relative youth of most of the stars, this would support models of LMXB formation and evolution that require wide binaries to shed angular momentum on a timescale of Gyrs. We find that NGC 5102 has an unusually low specific frequency of globular clusters ($S_N\sim$0.4), which could also explain the lack of LMXBs. We also detect diffuse X-ray emission in the central $\sim$1 kpc of the galaxy. This hot gas is most likely a superbubble created by multiple supernovae of massive stars born during the most recent star burst, and is driving the shock into the ISM which was inferred from optical observations.Comment: 33 pages, 7 figures, 6 tables - Accepted for publication in the Astrophysical Journa

Compact Nuclei in Moderately Redshifted Galaxies

The Hubble Space Telescope WFPC2 is being used to obtain high-resolution images in the V and I bands for several thousand distant galaxies as part of the Medium Deep Survey (MDS). An important scientific aim of the MDS is to identify possible AGN candidates from these images in order to measure the faint end of the AGN luminosity function as well as to study the host galaxies of AGNs and nuclear starburst systems. We are able to identify candidate objects based on morphology. Candidates are selected by fitting bulge+disk models and bulge+disk+point source nuclei models to HST imaged galaxies and determining the best model fit to the galaxy light profile. We present results from a sample of MDS galaxies with I less than 21.5 mag that have been searched for AGN/starburst nuclei in this manner. We identify 84 candidates with unresolved nuclei in a sample of 825 galaxies. For the expected range of galaxy redshifts, all normal bulges are resolved. Most of the candidates are found in galaxies displaying exponential disks with some containing an additional bulge component. 5% of the hosts are dominated by an r^-1/4 bulge. The V-I color distribution of the nuclei is consistent with a dominant population of Seyfert-type nuclei combined with an additional population of starbursts. Our results suggest that 10% +/- 1% of field galaxies at z less than 0.6 may contain AGN/starburst nuclei that are 1 to 5 magnitudes fainter than the host galaxies.Comment: 12 pages AASTeX manuscript, 3 separate Postscript figures, to be published in ApJ Letter

Open Cluster Characterization via Cross-Correlation with Spectral Library

We present a characterization method based on spectral cross-correlation to obtain the physical parameters of the controversial stellar aggregate ESO442-SC04. The data used was obtained with GMOS at Gemini-South telescope including 17 stars in the central region of the ob ject and 6 standard-stars. FXCOR was used in an iterative process to obtain self-consistent radial velocities for the standard-stars and averaged radial velocities for the science spectra. Spectral types, effective temperature, suface gravity and metallicities parameters were determined using FXCOR to correlate cluster spectra with ELODIE spectral library and selecting the best correlation matches using the Tonry and Davis Ratio (TDR). Analysis of the results suggests that the stars in ESO442-SC04 are not bound and therefore they do not constitute a physical system.Comment: 4-page paper from IAU symposium 266. Contains 3 eps figures and IAU document class file 'iau.cls

A Search for Old Star Clusters in the Large Magellanic Cloud

We report the first results of a color-magnitude diagram survey of 25 candidate old LMC clusters. For almost all of the sample, it was possible to reach the turnoff region, and in many clusters we have several magnitudes of the main sequence. Age estimates based on the magnitude difference $\delta T_1$ between the giant branch clump and the turnoff revealed that no new old clusters were found. The candidates turned out to be of intermediate age (1-3 Gyr) We show that the apparently old ages as inferred from integrated UBV colors can be explained by a combination of stochastic effects produced by bright stars and by photometric errors for faint clusters lying in crowded fields. The relatively metal poor candidates from the CaII triplet spectroscopy also turned out to be of intermediate age. This, combined with the fact that they lie far out in the disk, yields interesting constraints regarding the formation and evolution of the LMC disk. We also study the age distribution of intermediate age and old clusters This homogeneous set of accurate relative ages allows us to make an improved study of the history of cluster formation/destruction for ages $>1$Gyr. We confirm previous indications that there was apparently no cluster formation in the LMC during the period from 3-8 Gyr ago, and that there was a pronounced epoch of cluster formation beginning 3 Gyrs ago that peaked at about 1.5 Gyrs ago. Our results suggest that there are few, if any, genuine old clusters in the LMC left to be found.Comment: LaTeX, to be published in Nov. 1997 Astronomical Journa

Synthetic spectra of H Balmer and HeI absorption lines. II: Evolutionary synthesis models for starburst and post-starburst galaxies

We present evolutionary stellar population synthesis models to predict the spectrum of a single-metallicity stellar population, with a spectral sampling of 0.3 A in five spectral regions between 3700 and 5000 A. The models, which are optimized for galaxies with active star formation, synthesize the profiles of the hydrogen Balmer series (Hb, Hg, Hd, H8, H9, H10, H11, H12 and H13) and the neutral helium absorption lines (HeI 4922, HeI 4471, HeI 4388, HeI 4144, HeI 4121, HeI 4026, HeI 4009 and HeI 3819) for a burst with an age ranging from 1 to 1000 Myr, and different assumptions about the stellar initial mass function. Continuous star formation models lasting for 1 Gyr are also presented. The input stellar library includes NLTE absorption profiles for stars hotter than 25000 K and LTE profiles for lower temperatures. The temperature and gravity coverage is 4000 K <Teff< 50000 K and 0.0< log g$< 5.0, respectively. The models can be used to date starburst and post-starburst galaxies until 1 Gyr. They have been tested on data for clusters in the LMC, the super-star cluster B in the starburst galaxy NGC 1569, the nucleus of the dwarf elliptical NGC 205 and a luminous "E+A" galaxy. The full data set is available for retrieval at http://www.iaa.es/ae/e2.html and at http://www.stsci.edu/science/starburst/, or on request from the authors at [email protected]: To be published in ApJS. 48 pages and 20 figure

Spectral evolution of star clusters in the Large Magellanic Cloud: I. Blue concentrated clusters in the age range 40-300 Myr

Integrated spectroscopy of a sample of 17 blue concentrated Large Magellanic Cloud (LMC) clusters is presented and its spectral evolution studied. The spectra span the range ~3600-6800A with a resolution of ~14A FWHM, being used to determine cluster ages and, in connection with their spatial distribution, to explore the LMC structure and cluster formation history. Cluster reddening values were estimated by interpolation, using the available extinction maps. We used two methods to derive cluster ages: (i) template matching, in which line strengths and continuum distribution of the cluster spectra were compared and matched to those of template clusters with known astrophysical properties, and (ii) equivalent width (EW) method, in which new age/metallicity calibrations were used together with diagnostic diagrams involving the sum of EWs of selected spectral lines (KCaII, G band (CH), MgI, Hdelta, Hgamma and Hbeta). The derived cluster ages range from 40Myr (NGC2130 and SL237) to 300Myr (NGC1932 and SL709), a good agreement between the results of the two methods being obtained. Combining the present sample with additional ones indicates that cluster deprojected distances from the LMC center are related to age in the sense that inner clusters tend to be younger. Spectral libraries of star clusters are useful datasets for spectral classifications and extraction of parameter information for target star clusters and galaxies. The present cluster sample complements previous ones, in an effort to gather a spectral library with several clusters per age bin.Comment: 13 pages, 22 figures. Accepted for publication in A&