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

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

Population synthesis of HII galaxies

We study the stellar population of galaxies with active star formation, determining ages of the stellar components by means of spectral population synthesis of their absorption spectra. The data consist of optical spectra of 185 nearby ($z \leq 0.075$) emission line galaxies. They are mostly HII galaxies, but we also include some Starbursts and Seyfert 2s, for comparison purposes. They were grouped into 19 high signal-to-noise ratio template spectra, according to their continuum distribution, absorption and emission line characteristics. The templates were then synthesized with a star cluster spectral base. The synthesis results indicate that HII galaxies are typically age-composite stellar systems, presenting important contribution from generations up to as old as 500 Myr. We detect a significant contribution of populations with ages older than 1 Gyr in two groups of HII galaxies. The age distributions of stellar populations among Starbursts can vary considerably despite similarities in the emission line spectra. In the case of Seyfert 2 groups we obtain important contributions of old population, consistent with a bulge. From the diversity of star formation histories, we conclude that typical HII galaxies in the local universe are not systems presently forming their first stellar generation.Comment: 12 pages, 4 figures, MNRAS in pres

UKIRT follow-up observations of the old open cluster FSR0358 (Kirkpatrick1)

We aim to characterise the properties of the stellar clusters in the Milky Way. Utilising an expectation-maximisation method we determined that the cluster FSR0358, originally discovered by J.D.Kirkpatrick, is the most likely real cluster amongst the cluster candidates from Froebrich et al.. Here we present new deep high resolution near infrared imaging of this object obtained with UKIRT. The analysis of the data reveals that FSR0358 (Kirkpatrick1) is a 5+-2Gyr old open cluster in the outer Milky Way. Its age, metallicity of Z=0.008 and distance from the Galactic Centre of 11.2kpc are typical for the known old open galactic clusters. So far six of the FSR cluster candidates have been identified as having an age above 5Gyr. This shows the significance of this catalogue in enhancing our knowledge of the oldest open clusters in the Galaxy.Comment: 8 pages, 1 table, 5 figures, accepted for publication by MNRAS, a version with higher resolution figures can be found at http://astro.kent.ac.uk/~df

The frequency of nuclear star-formation in Seyfert 2 galaxies

We investigate the detectability of starburst signatures in the nuclear spectrum of Seyfert 2 galaxies by constructing spectral models in the wavelength range 3500-4100A, combining the spectrum of a bulge population (of age ~10Gyr) with that of younger stellar populations, spanning ages from ~3 Myr to 1 Gyr. We also construct models combining the bulge template with a power-law (PL) continuum, which is observed in some Seyfert 2's in polarized light, contributing with typically 10-40% of the flux at 4020A. We conclude that such continuum cannot be distinguished from that of a very young stellar population (age < 10 Myr), contributing with less than ~0.02% of the mass of the bulge. The models are compared with nuclear spectra - corresponding to a radius of 200-300 pc at the galaxy - of 20 Seyfert 2 galaxies, in which we specifically look for the signatures above of young to intermediate age stellar populations. We find them in ten galaxies, thus 50% of the sample. But only in six cases (30% of the sample) they can be attributed to young stars (age < 500 Myr): Mrk 1210, ESO 362-G8, NGC 5135, NGC 5643, NGC 7130 and NGC 7582. In the remaining four cases, the signatures are due to intermediate age stars (~1 Gyr). We find a tendency for the young stars to be found more frequently among the late type Seyfert's. This tendency is supported by a comparison between the equivalent widths (W) of absorption lines of the nuclear spectra of the Seyfert 2's with those of normal galaxies of the same Hubble type.Comment: 18 figures, revised version published in ApJ, December 2000, vol. 544, p. 74

Fast Integrated Spectra Analyzer: A New Computational Tool For Age and Reddening Determination of Small Angular Diameter Open Clusters

We present a new algorithm called 'Fast Integrated Spectra Analyzer" (FISA) that permits fast and reasonably accurate age and reddening determinations for small angular diameter open clusters by using their integrated spectra in the (3600-7400) \AA \ range and currently available template spectrum libraries. This algorithm and its implementation help to achieve astrophysical results in shorter times than from other methods. A brief review is given of the integrated spectroscopic technique applied to the study of open clusters as well as the basic assumptions that justify its use. We describe the numerical algorithm employed in detail, show examples of its application, and provide a link to the code. Our method has successfully been applied to integrated spectroscopy of open clusters, both in the Galaxy and in the Magellanic Clouds, to determine ages and reddenings.Comment: 27 Pages, 7 Figures, 1 table. Accepted to PAS

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&

Probing Mass Segregation in NGC 6397

In this study, we present a detailed study of mass segregation in the globular clister NGC 6397. First, we carry out a photometric analysis of projected ESO-VLT data (between 1 and 10 arcmin from the cluster centre), presenting the luminosity function corrected by completeness. The luminosity function shows a higher density of bright stars near the central region of the data, with respect to the outer region. We calculate a deprojected model (covering the whole cluster) estimating a total number of stars of 193000 +- 19000. The shapes of the surface brightness and density-number profiles versus the radial coordinate r (instead of the projected coordinate R) lead to a decreasing luminosity for an average star, and thus of mass, up to 1 arcmin, quantifying the mass segregation. The deprojected model does not show evidence of mass segregation outside this region

Aggregation of biological particles under radial directional guidance

AbstractMany biological environments display an almost radially-symmetric structure, allowing proteins, cells or animals to move in an oriented fashion. Motivated by specific examples of cell movement in tissues, pigment protein movement in pigment cells and animal movement near watering holes, we consider a class of radially-symmetric anisotropic diffusion problems, which we call thestar problem. The corresponding diffusion tensorD(x) is radially symmetric with isotropic diffusion at the origin. We show that the anisotropic geometry of the environment can lead to strong aggregations and blow-up at the origin. We classify the nature of aggregation and blow-up solutions and provide corresponding numerical simulations. A surprising element of this strong aggregation mechanism is that it is entirely based on geometry and does not derive from chemotaxis, adhesion or other well known aggregating mechanisms. We use these aggregate solutions to discuss the process of pigmentation changes in animals, cancer invasion in an oriented fibrous habitat (such as collagen fibres), and sheep distributions around watering holes.JTB classification:21.050, 21.160, 52.250, 71.060</jats:sec

Aggregation of biological particles under radial directional guidance

Many biological environments display an almost radially-symmetric structure, allowing proteins, cells or animals to move in an oriented fashion. Motivated by specific examples of cell movement in tissues, pigment protein movement in pigment cells and animal movement near watering holes, we consider a class of radially-symmetric anisotropic diffusion problems, which we call the star problem. The corresponding diffusion tensor D(x) is radially symmetric with isotropic diffusion at the origin. We show that the anisotropic geometry of the environment can lead to strong aggregations and blow-up at the origin. We classify the nature of aggregation and blow-up solutions and provide corresponding numerical simulations. A surprising element of this strong aggregation mechanism is that it is entirely based on geometry and does not derive from chemotaxis, adhesion or other well known aggregating mechanisms. We use these aggregate solutions to discuss the process of pigmentation changes in animals, cancer invasion in an oriented fibrous habitat (such as collagen fibres), and sheep distributions around watering holes