E. Kontizas

Mass segregation in young Magellanic Cloud star clusters: Four clusters observed with HST

We present the results of our investigation on the phenomenon of mass segregation in young star clusters in the Magellanic Clouds. HST/WFPC2 observations on NGC 1818, NGC 2004 and NGC 2100 in the Large Magellanic Cloud and NGC 330 in the Small Magellanic Cloud have been used for the application of diagnostic tools for mass segregation: i) the radial density profiles of the clusters for various mass groups and ii) their mass functions (MFs) at various radii around their centres. All four clusters are found to be mass segregated, but each one in a different manner. Specifically not all the clusters in the sample show the same dependence of their density profiles on the selected magnitude range, with NGC 1818 giving evidence of a strong relation and NGC 330 showing only a hint of the phenomenon. NGC 2004 did not show any significant signature of mass segregation in its density profiles either. The MFs radial dependence provides clear proof of the phenomenon for NGC 1818, NGC 2100 and NGC 2004, while for NGC 330 it gives only indications. An investigation of the constraints introduced by the application of both diagnostic tools is presented. We also discuss the problems related to the construction of a reliable MF for a cluster and their impact on the investigation of the phenomenon of mass segregation. We find that the MFs of these clusters as they were constructed with two methods are comparable to Salpeters IMF. A discussion is given on the dynamical status of the clusters and a test is applied on the equipartition among several mass groups in them. Both showed that the observed mass segregation in the clusters is of primordial nature.

The Gaia astrophysical parameters inference system (Apsis) - Pre-launch description

The Gaia satellite will survey the entire celestial sphere down to 20th magnitude, obtaining astrometry, photometry, and low resolution spectrophotometry on one billion astronomical sources, plus radial velocities for over one hundred million stars. Its main objective is to take a census of the stellar content of our Galaxy, with the goal of revealing its formation and evolution. Gaias unique feature is the measurement of parallaxes and proper motions with hitherto unparalleled accuracy for many objects. As a survey, the physical properties of most of these objects are unknown. Here we describe the data analysis system put together by the Gaia consortium to classify these objects and to infer their astrophysical properties using the satellites data. This system covers single stars, (unresolved) binary stars, quasars, and galaxies, all covering a wide parameter space. Multiple methods are used for many types of stars, producing multiple results for the end user according to different models and assumptions. Prior to its application to real Gaia data the accuracy of these methods cannot be assessed definitively. But as an example of the current performance, we can attain internal accuracies (RMS residuals) on F,G,K,M dwarfs and giants at G=15 (V=15-17) for a wide range of metallicites and interstellar extinctions of around 100K in effective temperature (Teff), 0.1mag in extinction (A0), 0.2dex in metallicity ([Fe/H]), and 0.25dex in surface gravity (logg). The accuracy is a strong function of the parameters themselves, varying by a factor of more than two up or down over this parameter range. After its launch in November 2013, Gaia will nominally observe for five years, during which the system we describe will continue to evolve in light of experience with the real data.

A Catalogue of carbon stars in the LMC

A catalogue of 7760 carbon stars in the Large Magellenic Cloud is presented. The stars were identified during a systematic survey of objective-prism plates taken with the UK 1.2 m Schmidt Telescope. The catalogue is compared with other lists of carbon stars and the distribution of the carbon stars is discussed.

Observations of the Hubble Deep Field South with the Infrared Space Observatory - II. Associations and star formation rates

We present results from a deep mid-infrared survey of the Hubble Deep Field South (HDF-S) region performed at 6.7 and 15mum with the ISOCAM instrument on board the Infrared Space Observatory (ISO ). We find reliable optical/near-infrared associations for 32 of the 35 sources detected in this field by Oliver et al. (Paper I): eight of them are identified as stars, one is definitely an active galactic nucleus (AGN), a second seems likely to be an AGN too, while the remaining 22 appear to be normal spiral or starburst galaxies. Using model spectral energy distributions (SEDs) of similar galaxies, we compare methods for estimating the star formation rates (SFRs) in these objects, finding that an estimator based on integrated (3-1000mum) infrared luminosity reproduces the model SFRs best. Applying this estimator to model fits to the SEDs of our 22 spiral and starburst galaxies, we find that they are forming stars at rates of ~1-100Msolar yr-1 , with a median value of ~40Msolar yr-1 , assuming an Einstein-de Sitter universe with a Hubble constant of 50kms-1 Mpc-1 , and star formation taking place according to a Salpeter initial mass function (IMF) across the mass range 0.1-100Msolar . We split the redshift range 0.0<=z <=0.6 into two equal-volume bins to compute raw estimates of the star formation rate density, rhod *, contributed by these sources, assuming the same cosmology and IMF as above and computing errors based on estimated uncertainties in the SFRs of individual galaxies. We compare these results with other estimates of rhod * made with the same assumptions, showing them to be consistent with the results of Flores et al. from their ISO survey of the CFRS 1415+52 field. However, the relatively small volume of our survey means that our rhod * estimates suffer from a large sampling variance, implying that our results, by themselves, do not place tight constraints on the global mean star formation rate density.

Observations of the Hubble Deep Field South with the Infrared Space Observatory - I. Observations, data reduction and mid-infrared source counts

We present results from a deep mid-infrared survey of the Hubble Deep Field South (HDF-S) region performed at 6.7 and 15 μm with the ISOCAM instrument on board the Infrared Space Observatory (ISO). The final map in each band was constructed by the co-addition of four independent rasters, registered using bright sources securely detected in all rasters, with the absolute astrometry being defined by a radio source detected at both 6.7 and 15 μm. We sought detections of bright sources in a circular region of radius 2.5 arcmin at the centre of each map, in a manner that simulations indicated would produce highly reliable and complete source catalogues using simple selection criteria. Merging source lists in the two bands yielded a catalogue of 35 distinct sources, which we calibrated photometrically using photospheric models of late-type stars detected in our data. We present extragalactic source count results in both bands, and discuss the constraints that they impose on models of galaxy evolution, given the volume of space sampled by this galaxy population.

The recent structural evolution of the SMC

We investigate the spatial distribution of stars of dierent ages towards the main body of the SMC, in an attempt to further understand the nature of the complex structure of the SMC. The old stellar population of the galaxy shows a rather regular and smooth distribution which is typical for a spheroidal body. On the contrary, the distribution of the younger stellar component is highly asymmetric and irregular giving evidence for the severe impact of the SMC during its close encounter with the LMC some 0.2 to 0.4 Gyr ago. In a series of isodensity contour maps of stars within selected ages, the recent structural evolution of the SMC is presented.

OB stellar associations in the Large Magellanic Cloud: Survey of young stellar systems

The method developed by Gouliermis et al. (2000, Paper I), for the detection and classification of stellar systems in the LMC, was used for the identification of stellar associations and open clusters in the central area of the LMC. This method was applied on the stellar catalog produced from a scanned 1.2m UK Schmidt Telescope Plate in U with a field of view almost 6. 5 × 6. 5, centered on the Bar of this galaxy. The survey of the identified systems is presented here followed by the results of the investigation on their spatial distribution and their structural parameters, as were estimated according to our proposed methodology in Paper I. The detected open clusters and stellar associations show to form large filamentary structures, which are often connected with the loci of HI shells. The derived mean size of the stellar associations in this survey was found to agree with the average size found previously by other authors, for stellar associations in different galaxies. This common size of about 80 pc might represent a universal scale for the star formation process, whereas the parameter correlations of the detected loose systems support the distinction between open clusters and stellar associations.

Towards a library of synthetic galaxy spectra and preliminary results of classification and parametrization of unresolved galaxies for Gaia. II

Aims. This paper is the second in a series, implementing a classification system for Gaia observations of unresolved galaxies. Our goals are to determine spectral classes and estimate intrinsic astrophysical parameters via synthetic templates. Here we describe (1) a new extended library of synthetic galaxy spectra; (2) its comparison with various observations; and (3) first results of classification and parametrization experiments using simulated Gaia spectrophotometry of this library. Methods. Using the PEGASE.2 code, based on galaxy evolution models that take account of metallicity evolution, extinction correction, and emission lines (with stellar spectra based on the BaSeL library), we improved our first library and extended it to cover the domain of most of the SDSS catalogue. Our classification and regression models were support vector machines (SVMs). Results. We produce an extended library of 28 885 synthetic galaxy spectra at zero redshift covering four general Hubble types of galaxies, over the wavelength range between 250 and 1050 nm at a sampling of 1 nm or less. The library is also produced for 4 random values of redshift in the range of 0–0.2. It is computed on a random grid of four key astrophysical parameters (infall timescale and 3 parameters defining the SFR) and, depending on the galaxy type, on two values of the age of the galaxy. The synthetic library was compared and found to be in good agreement with various observations. The first results from the SVM classifiers and parametrizers are promising, indicating that Hubble types can be reliably predicted and several parameters estimated with low bias and variance.

The morphology of star clusters in the SMC

The projected ellipticities of 34 populous SMC star clusters have been derived by means of PDS 1010A scans and a computer interactive method of reduction implemented on an Apollo 570 workstation. A pair of J and R plates taken with the 1.2 m UK Schmidt telescope in Australia were used. Radial ellipticity variations within individual globular clusters seem to be a common phenomenon for the SMC clusters, similar to that observed in the LMC clusters where the innerparts are more elliptical than the outer ones in 95 percent of the cases. The derived ellipticities which correspond to the innermost part of the cluster at radial distances near to half-mass radii have been found to be statistically more elliptical than those of the LMC, known to be more elliptical than those of the Galaxy. The dynamical masses of the clusters seem to correlate with ellipticities supporting the hypothesis that, either the gravitational field of the parent galaxy being a dominant factor affect slower the shape of the high mass clusters and/or the most massive clusters, being dynamically younger, retain their original shape. 30 refs.

Structure of the SMC - Stellar component distribution from 2MASS data

Aims. The spatial distribution of the SMC stellar component is investigated from 2MASS data. The morphology of the different age populations is presented. The center of the distribution is calculated and compared with previous estimations. The rotation of the stellar content and possible consequence of the presence of dark matter is discussed. Methods. The different stellar populations are identified through a CMD diagram of the 2MASS data. Isopleth contour maps are produced in each case, to reveal the spatial distribution. The derived density profiles are discussed. Results. The older stellar population follows an exponential profile at projected diameters of about 5 kpc (∼5 ◦ ) for the major axis and ∼4 kpc for the minor axis, centred at RA: 0 h 51 min , Dec: −73 ◦ 7 � (J2000.0). The centre coordinates are found to be the same for all the different age population maps and are in good accordance with the kinematical centre of the SMC. However they are found to be considerably different to the coordinates of the centre of the gas distribution. The fact that the older population is found in an exponential disk suggests that the stellar content is rotating, a possible consequence of dark matter presence. The strong interactions between the MCs and the MilkyWay might explain the difference in the distributions of the stellar and gas components. The lack of an observed velocity element, which implies an absence of rotation and contradicts the consequences of an exponential profile of the stellar component, may also be a result of gravitational interactions.