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

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

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

The Dusty Starburst Nucleus of M33

We have thoroughly characterized the ultraviolet to near-infrared (0.15 - 2.2 micron) spectral energy distribution (SED) of the central parsec of the M33 nucleus through new infrared photometry and optical/near-infrared spectroscopy, combined with ultraviolet/optical observations from the literature and the HST archive. The SED shows evidence for a significant level of attenuation, which we model through a Monte Carlo radiative transfer code as a shell of clumpy Milky Way-type dust (tau_V ~ 2 +/- 1). The discovery of Milky Way-type dust (with a strong 2175 A bump) internal to the M33 nucleus is different from previous work which has found SMC-like dust (no bump) near starburst regions. The amount by which dust can be processed may be related to the mass and age of the starburst as well as the extent to which the dust can shield itself. Our starburst models include the effects of this dust and can fit the SED if the nucleus was the site of a moderate (~10^8 L_sun at 10 Myrs) episode of coeval star formation about 70 Myrs ago. This result is quite different from previous studies which resorted to multiple stellar populations (between 2 and 7) attenuated by either no or very little internal dust. The M33 nuclear starburst is remarkably similar to an older version (70 Myr versus 10 Myr) of the ultra-compact starburst in the center of the Milky Way.Comment: 29 pages, 9 embedded figures, ApJ, in pres

The Central Regions of M31 in the 3 - 5 micron Wavelength Region

Images obtained with NIRI on the Gemini North telescope are used to investigate the photometric properties of the central regions of M31 in the 3 - 5 micron wavelength range. The light distribution in the central arcsecond differs from what is seen in the near-infrared in the sense that the difference in peak brigh tness between P1 and P2 is larger in M' than in K'; no obvious signature of P3 is dete cted in M'. These results can be explained if there is a source of emission that contributes ~ 20% of the peak M' light of P1 and has an effective temperature of no more than a few hundred K that is located between P1 and P2. Based on the red K-M' color of this source, it is suggested that the emission originates in a circumstellar dust shell surrounding a single bright AGB star. A similar bright source that is ~ 8 arcsec from the center of the galaxy is also detected in M'. Finally, the (L', K-L') color-magnitude diagram of unblended stars shows a domin ant AGB population with photometric characteristics that are similar to those of the most luminous M giants in the Galactic bulge.Comment: To appear in the Astronomical Journa

The Star Clusters in the Irregular Galaxy NGC 4449

We examine the star clusters in the irregular galaxy NGC 4449. We use a near-infrared spectrum and broad-band images taken with the HST to place a limit of 8--15 Myrs on the age of the bright central ojbect in NGC 4449. Its luminosity and size suggest that it is comparable to young super star clusters. However, there is a peculiar nucleated-bar structure at the center of this star cluster, and we suggest that this structure is debris from the interaction that has produced the counter-rotating gas systems and extended gas streamers in the galaxy. From the images we identify 60 other candidate compact star clusters in NGC 4449. Fourteen of these could be background elliptical galaxies or old globular star clusters. Of the star clusters, three, in addition to the central object, are potentially super star clusters, and many others are comparable to the populous clusters found in the LMC. The star clusters span a large range in ages with no obvious peak in cluster formation that might be attributed to the interaction that the galaxy has experienced.Comment: To be published in PASP, Feb. 2001; also attainable from ftp.lowell.edu, cd pub/dah/n4449pape

Ages and Luminosities of Young SMC/LMC Star Clusters and the recent Star Formation History of the Clouds

In this paper we discuss the age and spatial distribution of young (age$<$1Gyr) SMC and LMC clusters using data from the Magellanic Cloud Photometric Surveys. Luminosities are calculated for all age-dated clusters. Ages of 324 and 1193 populous star clusters in the Small and the Large Magellanic Cloud have been determined fitting Padova and Geneva isochrone models to their resolved color-magnitude diagrams. The clusters cover an age range between 10Myr and 1Gyr in each galaxy. For the SMC a constant distance modulus of $(m-M)_0$ = 18.90 and a metallicity of Z = 0.004 were adopted. For the LMC, we used a constant distance modulus of $(m-M)_0$ = 18.50 and a metallicity of Z = 0.008. For both galaxies, we used a variable color excess to derive the cluster ages. We find two periods of enhanced cluster formation in both galaxies at 160Myr and 630Myr (SMC) and at 125Myr and 800Myr (LMC). We present the spatially resolved recent star formation history of both Clouds based on young star clusters. The first peak may have been triggered by a close encounter between the SMC and the LMC. In both galaxies the youngest clusters reside in the supergiant shells, giant shells, the inter-shell regions, and toward regions with a high H$\alpha$ content, suggesting that their formation is related to expansion and shell-shell interaction. Most of the clusters are older than the dynamical age of the supergiant shells. No evidence for cluster dissolution was found. Computed V band luminosities show a trend for fainter magnitudes with increasing age as well as a trend for brighter magnitudes with increasing apparent cluster radii.Comment: 17 pages, 14 figures, Accepted for publication in A&

Integrated parameters of star clusters: A comparison of theory and observations

(Abridged) This paper presents integrated magnitude and colours for synthetic clusters. The integrated parameters have been obtained for the whole cluster population as well as for the main-sequence (MS) population of star clusters. We have also estimated observed integrated magnitudes and colours of MS population of galactic open clusters, LMC and SMC star clusters. It is found that the colour evolution of MS population of star clusters is not affected by the stochastic fluctuations, however these fluctuations significantly affect the colour evolution of the whole cluster population. The fluctuations are maximum in $(V-I)$ colour in the age range 6.7 $<$ log (age) $<$ 7.5. Evolution of integrated colours of MS population of the clusters in the Milky Way, LMC and SMC, obtained in the present study are well explained by the present synthetic cluster model. The observed integrated $(B-V)$ colours of MS population of LMC star clusters having age $\geq$ 500 Myr seem to be distributed around $Z=$ 0.004 model, whereas $(V-I)$ colours are found to be more bluer than those predicted by the $Z=$ 0.004 model. $(V-I)$ vs $(B-V)$ two-colour diagram for the MS population of the Milky Way star clusters shows a fair agreement between the observations and present model, however the diagrams for LMC and SMC clusters indicate that observed $(V-I)$ colours are relatively bluer. Possible reasons for this anomaly have been discussed.Comment: 35 pages, 20 figs, accepted for publication in MNRA

Star formation and figure rotation in the early-type galaxy NGC2974

We present Galaxy Evolution Explorer (GALEX) far (FUV) and near (NUV) ultraviolet imaging of the nearby early-type galaxy NGC2974, along with complementary ground-based optical imaging. In the ultraviolet, the galaxy reveals a central spheroid-like component and a newly discovered complete outer ring of radius 6.2kpc, with suggestions of another partial ring at an even larger radius. Blue FUV-NUV and UV-optical colours are observed in the centre of the galaxy and from the outer ring outward, suggesting young stellar populations (< 1Gyr) and recent star formation in both locations. This is supported by a simple stellar population model which assumes two bursts of star formation, allowing us to constrain the age, mass fraction and surface mass density of the young component pixel by pixel. Overall, the mass fraction of the young component appears to be just under 1per cent (lower limit, uncorrected for dust extinction). The additional presence of a nuclear and an inner ring (radii 1.4 and 2.9kpc, respectively), as traced by [OIII] emission, suggests ring formation through resonances. All three rings are consistent with a single pattern speed of $78\pm6$ km/s/kpc, typical of S0 galaxies and only marginally slower than expected for a fast bar if traced by a small observed surface brightness plateau. This thus suggests that star formation and morphological evolution in NGC2974 at the present epoch are primarily driven by a rotating asymmetry (probably a large-scale bar), despite the standard classification of NGC2974 as an E4 elliptical.Comment: 13 pages, 10 figures, Changed content, Accepted for publication in MNRA

Kinematics of the Galactic Globular Cluster System: New Radial Velocities for Clusters in the Direction of the Inner Galaxy

HIRES on the Keck I telescope has been used to measure the first radial velocities for stars belonging to eleven, heavily-reddened globular clusters in the direction of the inner Galaxy. The question of kinematic substructuring among the Galactic globular cluster system is investigated using an updated catalog of globular cluster distances, metallicities and velocities. It is found that the population of metal-rich globular clusters shows significant rotation at all Galactocentric radii. For the metal-rich clusters within 4 kpc of the Galactic center, the measured rotation velocity and line-of-sight velocity dispersion are similar to those of bulge field stars. We investigate claims that the metal-rich clusters are associated with the central Galactic bar by comparing the kinematics of the innermost clusters to that of the atomic hydrogen in the inner Galaxy. The longitude-velocity diagram of both metal-rich and metal-poor clusters bears a remarkable similarity to that of the gas, including the same non-circular motions which have traditionally been interpreted as evidence for a Galactic bar, or, alternatively, a non-axisymmetric bulge. However, uncertainties in the existing three-dimensional Galactocentric positions for most of the clusters do not yet allow an unambiguous discrimination between the competing scenarios of membership in a rigidly rotating bar, or in a bulge which is an oblate isotropic rotator. We conclude that the majority of metal-rich clusters within the central 4 kpc of the Galaxy are probably associated with the bulge/bar, and not the thick disk. (ABRIDGED)Comment: 18 pages, including 7 of 13 postscript figures. Figures 1-6 available at http://astro.caltech.edu/~pc. Accepted for publication in the Astronomical Journa