Sample Of Star Clusters Research Articles

Deriving physical parameters of unresolved star clusters

Context. This study is the sixth of a series that investigates degeneracy and stochasticity problems present in the determination of the age, mass, extinction, and metallicity of partially resolved or unresolved star clusters in external galaxies when using Hubble Space Telescope broadband photometry. In the fifth publication, it was noticed that inconsistencies in cluster colour indices, which arise due to projected foreground and background stars on the apertures, enhance age-metallicity-extinction degeneracies. Aims. In this work we aim to present new aperture photometry results for a sample of star clusters from the M 31 Panchromatic Hubble Andromeda Treasury (PHAT) survey. Methods. We employed two methods of aperture photometry. The first method is ordinary aperture photometry to measure total cluster fluxes. The second method was introduced to avoid the brightest foreground and background stars that project onto large apertures. This method employs smaller apertures, adapted to cover a central part of clusters, and applies an aperture correction, derived for the F475W passband, to other passbands. We note that this procedure is valid only for clusters without prominent systematic gradients of colour indices beyond half-light radii. Results. We present two catalogues of star cluster aperture photometry (produced by applying ordinary aperture photometry and adaptive aperture photometry methods) with estimated uncertainties for a sample of 1181 star clusters from the M 31 PHAT survey. Compared to the M 31 PHAT fundamental star cluster aperture photometry catalogue published by Johnson et al., there are changes made in the following: cluster centre coordinates, aperture sizes, and sky background estimates.

STEP survey – II. Structural analysis of 170 star clusters in the SMC

ABSTRACT We derived surface brightness profiles in the g band for 170 Small Magellanic Cloud (SMC) star clusters (SCs) mainly located in the central region of the galaxy. We provide a set of homogeneous structural parameters obtained by fitting Elson–Fall–Freeman and King models. Through a careful analysis of their colour–magnitude diagrams we also supply the ages for a subsample of 134 SCs. For the first time, such a large sample of SCs in the SMC is homogeneously characterized in terms of their sizes, luminosities, and masses, widening the probed region of the parameter space, down to hundreds of solar masses. We used these data to explore the evolution of the SC’s structural parameters with time. In particular, we confirm the existence of a physical mechanism that induces an increase of the core radius after 0.3–1.0 Gyr. We suggest that cluster mass could be the main parameter driving the inner expansion, as none of the SCs having log (M/M⊙) ≤ 3.5 dex analysed in this work undergoes to such an expansion. We also detected a mass–size relationship almost over the entire range of SCs masses investigated here. Finally, our data suggest that globally the SMC SC system is dynamically evolved.

Astrophysical properties of newly discovered Magellanic Cloud star clusters

New star cluster candidates projected toward the Large and Small Magellanic Clouds (LMC, SMC) have been recently discovered from relatively deep imaging surveys. Here, we conduct a sound analysis of 24 star cluster candidates located in the outer regions of the LMC and SMC using point spread function photometry produced by the Survey of the Magellanic Stellar History. With only one exception, the studied objects were shown to be genuine stellar aggregates. We drew our conclusions on their physical characteristics once their observed color-magnitude diagrams (CMDs) were statistically decontaminated by the presence of field stars. The resulting cleaned CMDs, for stars with assigned membership probabilities higher than 50%, were compared with synthetic CMDs generated for thousands of combinations of ages, distances, metallicities, star cluster masses, and binary fractions. The parameters of the best-matched synthetic CMDs obtained from a likelihood approach were adopted as the star cluster astrophysical properties. The present star cluster sample spans a wide range of distances, from those star clusters located in front of the LMC through those along the onset of the Magellanic Bridge up to those behind the SMC. Their ages reveal different formation episodes that took place over the course of galaxy formation and others as a consequence of interactions among galaxies. From their estimated metallicities and ages, we speculate on the possibility that relatively metal-deficient gaseous flows have existed between these galaxies during nearly the last one Gyr (log(age yr−1) ≈ 9.0), which facilitated the formation of young star clusters in the galaxy peripheries. Despide the LMC-SMC interactions, the studied star clusters are similar or more massive than their counterparts in the Milky Way, suggesting that tidal effects are relatively more important in our Galaxy.

Comparisons of Different Fitting Methods for the Physical Parameters of a Star Cluster Sample of M33 with Spectroscopy and Photometry

Star clusters are good tracers for formation and evolution of galaxies. We compared different fitting methods by using spectra (or by combining photometry) to determine the physical parameters. We choose a sample of 17 star clusters in M33, which previously lacked spectroscopic observations. The low-resolution spectra were taken with the Xinglong 2.16-m reflector of NAOC. The photometry used in the fitting includes $\rm u_{SC}$ and $\rm v_{SAGE}$ bands from the SAGE survey, as well as the published $UBVRI$ and $ugriz$ photometry. We firstly derived ages and metallicities with the {\sc ULySS} (Vazdekis et al. and {\sc pegase-hr}) SSP model and the Bruzual \& Charlot (2003) (BC03) stellar population synthesis models for the full-spectrum fitting. The fitting results of both the BC03 and {\sc ULySS} models seem consistent with those of previous works as well. Then we add the SAGE $\rm u_{SC}$ and $\rm v_{SAGE}$ photometry in the spectroscopic fitting with the BC03 models. It seems the results become much better, especially for the Padova 2000+Chabrier IMF set. Finally we add more photometry data, $UBVRI$ and $ugriz$, in the fitting and we found that the results do not improve significantly. Therefore, we conclude that the photometry is useful for improving the fitting results, especially for the blue bands ($\lambda <4000$ {\AA}), e.g., $\rm u_{SC}$ and $\rm v_{SAGE}$ band. At last, we discuss the "UV-excess" for the star clusters and we find five star clusters have UV-excess, based on the $GALEX$ FUV, NUV photometry.

The Gaia-ESO Survey: Galactic evolution of lithium at high metallicity

Context. Reconstructing the Galactic evolution of lithium (Li) is the main tool used to constrain the source(s) of Li enrichment in the Galaxy. Recent results have suggested a decline in Li at supersolar metallicities, which may indicate reduced production. Aims. We exploit the unique characteristics of the Gaia-ESO Survey open star cluster sample to further investigate this issue and to better constrain the evolution of Li at high metallicity. Methods. We trace the upper envelope of Li abundance versus metallicity evolution using 18 clusters and considering members that should not have suffered any Li depletion. Results. At variance with previous claims, we do not find any evidence of a Li decrease at high metallicity. The most metal-rich clusters in the sample ([Fe/H] = ∼0.3) actually show the highest Li abundances, with A(Li) &gt; 3.4. Our results clearly show that previous findings, which were based on field stars, were affected by selection effects. The metal-rich population in the solar neighbourhood is composed of relatively old and cool stars that have already undergone some Li depletion; hence, their measured Li does not represent the initial interstellar medium abundance, but a lower limit to it.

Exploring the IMF of star clusters: a joint SLUG and LEGUS effort

AbstractWe present the implementation of a Bayesian formalism within the Stochastically Lighting Up Galaxies (slug) stellar population synthesis code, which is designed to investigate variations in the initial mass function (IMF) of star clusters. By comparing observed cluster photometry to large libraries of clusters simulated with a continuously varying IMF, our formalism yields the posterior probability distribution function (PDF) of the cluster mass, age and extinction, jointly with the parameters describing the IMF. We apply this formalism to a sample of star clusters from the nearby galaxy NGC 628, for which broad-band photometry in five filters is available as part of the Legacy ExtraGalactic UV Survey (LEGUS). After allowing the upper-end slope of the IMF (α3) to vary, we recover PDFs for the mass, age and extinction that are broadly consistent with what is found when assuming an invariant Kroupa IMF. However, the posterior PDF for α3 is very broad due to a strong degeneracy with the cluster mass, and it is found to be sensitive to the choice of priors, particularly on the cluster mass. We find only a modest improvement in the constraining power of α3 when adding Hα photometry from the companion Hα-LEGUS survey. Conversely, Hα photometry significantly improves the age determination, reducing the frequency of multi-modal PDFs. With the aid of mock clusters, we quantify the degeneracy between physical parameters, showing how constraints on the cluster mass that are independent of photometry can be used to pin down the IMF properties of star clusters.

Identifying and Characterizing Stellar Clusters Toward the Galactic Anti-Center from Pan-STARRS1 Survey

AbstractThe current sample of star clusters (SCs) is still incomplete beyond 1.8 kpc. The Pan-STARRS1 with its wide field of views and sensitive cameras provides us an opportunity to identify and characterize SCs as much as possible. By using a star counting algorithm, we obtained a total of 491 stellar density enhancements in a field of 20° × 20° toward the Galactic Anti-Center (GAC), of which 50 are known SCs. The remaining 441 candidates are verified with radii, interstellar reddening, distances, and ages with proper motions and multiple bands photometries. Our results push the completeness of SCs almost near the edge of GAC. With the revised SC sample, the separation between Sagittarius and Perseus arms are about 3.2 ± 0.2 kpc and the widths of the nearby spiral arms—Sagittarius, Orion, and Perseus—with are 1.4 ± 0.1, 1.6 ± 0.1, and 3.3 ± 0.2 kpc, respectively.

The role of dust in models of population synthesis

We have employed state-of-the-art evolutionary models of low and intermediate-mass AGB stars, and included the effect of circumstellar dust shells on the spectral energy distribution (SED) of AGB stars, to revise the Padua library of isochrones (Bertelli et al. 1994). The major revision involves the thermally pulsing AGB phase, that is now taken from fully evolutionary calculations by Weiss & Ferguson (2009). Two libraries of about 600 AGB dust-enshrouded SEDs each have also been calculated, one for oxygen-rich M-stars and one for carbon-rich C-stars. Each library accounts for different values of input parameters like the optical depth {\tau}, dust composition, and temperature of the inner boundary of the dust shell. These libraries of dusty AGB spectra have been implemented into a large composite library of theoretical stellar spectra, to cover all regions of the Hertzsprung-Russell Diagram (HRD) crossed by the isochrones. With the aid of the above isochrones and libraries of stellar SEDs, we have calculated the spectro-photometric properties (SEDs, magnitudes, and colours) of single-generation stellar populations (SSPs) for six metallicities, more than fifty ages (from 3 Myr to 15 Gyr), and nine choices of the Initial Mass Function. The new isochrones and SSPs have been compared to the colour-magnitude diagrams (CMDs) of field populations in the LMC and SMC, with particular emphasis on AGB stars, and the integrated colours of star clusters in the same galaxies, using data from the SAGE (Surveying the Agents of Galaxy Evolution) catalogues. We have also examined the integrated colours of a small sample of star clusters located in the outskirts of M31. The agreement between theory and observations is generally good. In particular, the new SSPs reproduce the red tails of the AGB star distribution in the CMDs of field stars in the Magellanic Clouds.

Quantifying the universality of the stellar initial mass function in old star clusters

We present a new technique to quantify cluster-to-cluster variations in the observed present-day stellar mass functions of a large sample of star clusters. Our method quantifies these differences as a function of both the stellar mass and the total cluster mass, and offers the advantage that it is insensitive to the precise functional form of the mass function. We applied our technique to data taken from the ACS Survey for Globular Clusters, from which we obtained completeness-corrected stellar mass functions in the mass range 0.25-0.75 M$_{\odot}$ for a sample of 27 clusters. The results of our observational analysis were then compared to Monte Carlo simulations for globular cluster evolution spanning a range of initial mass functions, total numbers of stars, concentrations, and virial radii. We show that the present-day mass functions of the clusters in our sample can be reproduced by assuming an universal initial mass function for all clusters, and that the cluster-to-cluster differences are consistent with what is expected from two-body relaxation. A more complete exploration of the initial cluster conditions will be needed in future studies to better constrain the precise functional form of the initial mass function. This study is a first step toward using our technique to constrain the dynamical histories of a large sample of old Galactic star clusters and, by extension, star formation in the early Universe.

Structures in surface-brightness profiles of LMC and SMC star clusters: evidence of mergers?

Context. The LMC and SMC are rich in binary star clusters, and some mergers are expected. It is thus important to characterise single clusters, binary clusters and candidates for mergers. Aims. We selected a sample of star clusters in each Cloud with this aim. Surface photometry of 25 SMC and 22 LMC star clusters was carried with the ESO Danish 1.54 m telescope. 23 clusters were observed for the first time for these purposes. Methods. We fitted Elson, Fall and Freeman (EFF) profiles to the data, deriving structural parameters, luminosities and masses. We also use isophotal maps to constrain candidates for cluster interactions. Results. The structural parameters, luminosities and masses presented good agreement with those in the literature. Three binary clusters in the sample have a double profile. Four clusters (NGC 376, K 50, K 54 and NGC 1810) do not have companions and present important deviations from EFF profiles. Conclusions. The present sample contains blue and red Magellanic clusters. Profiles with excess with respect to EFF were detected in some blue clusters. We find evidence that important deviations from the body of EFF profiles might be used as a tool to detect cluster mergers.

A systematic survey for infrared star clusters with b &lt; 20 using 2MASS

We used star density maps obtained from the Two-Micron All-Sky Survey (2MASS) to obtain a sample of star clusters in the entire Galactic Plane with |b| < 20°. A total of 1788 star cluster candidates are identified in this survey. Among those are 681 previously known open clusters and 86 globular clusters. A statistical analysis indicates that our sample of 1021 new cluster candidates has a contamination of about 50 per cent. Star cluster parameters are obtained by fitting a King profile to the star density. These parameters are used to statistically identify probable new globular cluster candidates in our sample. A detailed investigation of the projected distribution of star clusters in the Galaxy demonstrates that they show a clear tendency to cluster on spatial scales in the order of 12–25 pc, a typical size for molecular clouds.

Near-infrared surface brightness fluctuations and optical colours of Magellanic star clusters

This work continues our efforts to calibrate model surface brightness fluctuation luminosities for the study of unresolved stellar populations, through a comparison with the data of Magellanic Cloud star clusters. We present here the relation between absolute Ks-band fluctuation magnitude and (V−I) integrated colour, using data from the Two-Micron All-Sky Survey (2MASS) and the Deep Near-Infrared Southern Sky Survey (DENIS), and from the literature. We compare the star cluster sample with the sample of early-type galaxies and spiral bulges studied by Liu et al. We find that intermediate-age to old star clusters lie along a linear correlation with the same slope, within the errors, of that defined by the galaxies in the versus (V−I) diagram. While the calibration by Liu et al. was determined in the colour range 1.05 < (V−IC)0 < 1.25, ours holds in the interval . This implies, according to Bruzual-Charlot and Mouhcine-Lançon models, that the star clusters and the latest star formation bursts in the galaxies and bulges constitute an age sequence. At the same time, a slight offset between the galaxies and the star clusters [the latter are ~0.7 mag fainter than the former at a given value of (V−I)], caused by the difference in metallicity of roughly a factor of 2, confirms that the versus (V−I) plane may contribute to break the age-metallicity degeneracy in intermediate-age and old stellar populations. The confrontation between models and galaxy data also suggests that galaxies with Ks fluctuation magnitudes that are brighter than predicted, given their (V−I) colour, might be explained in part by longer lifetimes of thermally pulsing asymptotic giant branch stars. A preliminary comparison between the H 2MASS data of the Magellanic star clusters and the sample of 47 early-type galaxies and spiral bulges observed by Jensen et al. through the F160WHubble Space Telescope filter leads to the same basic conclusions: galaxies and star clusters lie along correlations with the same slope, and there is a slight offset between the star cluster sample and the galaxies, caused by their different metallicities. Magellanic star clusters are single populations, while galaxies are composite stellar systems; moreover, the objects analysed live in different environments. Therefore, our findings mean that the relationship between fluctuation magnitudes in the near-infrared, and (V−I) might be a fairly robust tool for the study of stellar population ages and metallicities, could provide additional constraints on star formation histories, and aid in the calibration of near-infrared surface brightness fluctuations for cosmological distance measurements.

Super Star Clusters in NGC 7673 and Mrk 8

We present an analysis of super star cluster populations using WFPC 2 on theHubble Space Telescopefor two starburst galaxies: NGC 7673 and Markarian 8. Both galaxies are luminious and have signatures of massive star formation, such as emission-line dominated optical spectra. Their optical appearances are highly disturbed, featuring prominent knots of intense star formation. Using HST F255W, F555W, and F814W images, we perform aperture photometry for a select sample of star clusters in different regions of each galaxy, and compare the results with evolutionary population synthesis models.

Kinematics, ages and metallicities of star clusters in NGC 1316: a 3-Gyr-old merger remnant

We report on multi-object spectroscopy in the red spectral region of 37 candidate star clusters in an ∼8×8 arcmin2 field centred on the giant early-type radio galaxy NGC 1316 (Fornax A), the brightest galaxy in the Fornax cluster. Out of this sample, 24 targets are found to be genuine star clusters associated with NGC 1316, and 13 targets are Galactic foreground stars. For the star cluster sample, we measure a mean heliocentric velocity vhel = 1698±46 km s−1 and a velocity dispersion σ = 227±33 km s−1 within a galactocentric radius of 24 kpc. Partly responsible for the velocity dispersion is a significant rotation in the star cluster system, with a mean velocity of ∼175±70 km s−1 along a position angle of ∼6°±18°. Using the projected mass estimator and assuming isotropic orbits, the estimated total mass is (6.6±1.7)×1011 M⊙ within a radius of 24 kpc. The mass is uncertain by about a factor of 2, depending on the orbital assumptions. The implied M/LB ratio is in the range 3–6. Four star clusters in our sample are exceptionally luminous (MV<−12.3). This means that (1) at least this many clusters in NGC 1316 are up to an order of magnitude more luminous than the most luminous star cluster in our Galaxy or M31, and that (2) the S/N ratio of their spectra allows us to measure line strengths with good accuracy. By comparing the measured colours and equivalent widths of Hα and the Ca ii triplet (λλ 8498, 8542, 8662 Å) absorption lines for those bright star clusters in our sample with those of single-burst population models (the Bruzual & Charlot models), we find that they are coeval with an age of 3.0±0.5 Gyr. Their metallicities are found to be solar to within ±0.15 dex. We discuss the properties of the main body of NGC 1316, and conclude that they are consistent with having hosted a major merger 3 Gyr ago as well. The presence of intermediate-age globular clusters in NGC 1316 shows once again that globular clusters with near-solar metallicity do form during galactic mergers, and, moreover, that they can survive disruption processes taking place during the merger (e.g., dynamical friction, tidal disruption), as well as evaporation. In this respect, NGC 1316 provides a hitherto ‘missing’ evolutionary link between young merger remnants of age ∼0.5 Gyr such as NGC 3597, 3921 and 7252 on one side and older giant ellipticals featuring bimodal colour distributions on the other side.

Star Clusters in M33. I. Detection, Morphology, and Photometry

In this paper we present the discovery of 60 star clusters in 20 multiband Hubble Space Telescope (HST) WFPC2 fields in M33. The fields sample a variety of environments, from outer regions to spiral arms and central regions, as well as a range of galactocentric distances. The HST spatial resolution allowed us to penetrate the crowded, spiral arm regions of M33 yielding the first unbiased, representative sample of star clusters for this galaxy. We discuss the separation of clusters from stellar sources, and from other extended sources such as star-forming regions, H II regions, and supernova remnants. For the clusters we present multiband images and discuss morphology, location, and integrated photometry. Measured cluster colors and magnitudes are presented. The cluster density in our sample as a function of galactocentric distance yields an estimate of 690 total clusters in M33.

Star Clusters in M33. II. Global Properties

We discuss the global physical properties of stellar clusters in M33 and compare them with LMC cluster properties. In a companion paper (Paper I) we have compiled the first unbiased, representative sample of star clusters for this galaxy from Hubble Space Telescope WFPC2 multiband images of 20 fields in M33, sampling a variety of environments from outer regions to spiral arms and central regions, as well as a range of Galactocentric distances. In this paper we fit single-mass King models to cluster surface brightness profiles and find that clusters at all ages have well-relaxed density profiles. Core radii are found to be smaller on average than those of Galactic globular clusters and LMC populous clusters at similar magnitudes. Measurements of integrated photometric properties for 60 clusters are also found in Paper I. By comparing the integrated photometric measurements with evolutionary models and theoretical M/LV ratios, we find that clusters formed continuously in M33 from ~4 × 106-1010 yr and have masses between ~4 × 102 and 3 × 105 M⊙. There is a correlation between the ages and masses of our clusters, the oldest objects being the most massive. The global properties of the M33 clusters are compared with cluster systems of other Local Group galaxies. By studying the photometric properties (magnitudes and colors) of M33 clusters and comparing them with those of LMC clusters, a global picture of the star formation history in these two galaxies emerges.

Bar star clusters in the LMC - Formation history from UBV integrated photometry

The sample of star clusters in the LMC Bar region with integrated UBV photometry was enlarged by approximately a factor 4, totalling 129 objects. The (B-V) histogram gap between blue and red clusters disappears with this deeper sample. Age groups in terms of equivalent SWB types were derived and their spatial distribution studied. Clusters younger than t≃200 Myr are not homogeneously distributed through the bar. In particular a strong star forming event at t≃100 Myr was detected in the eastern part of the Bar, consisting of a compact grouping of seven coeval clusters around NGC 2058 and NGC 2065

STAR CLUSTERS IN THE SMALL MAGELLANIC CLOUD: III. THEIR SPACE DISTRIBUTION

In Papers I and II, star clusters in the Small Magellanic Cloud were identified, tabulated, and described.1 Their properties as individual clusters were discussed and compared with analogous properties of clusters of the Galaxy. It is the purpose of this paper, the last of the series, to discuss some of the features of the star clusters as a group. Such a discussion is of particular significance for the Small Magellanic Cloud, which offers an opportunity to analyze the properties of an entire population of star clusters associated with a fair-sized galaxy. It is reasonably certain also that neither the completeness of the sample nor the photometry of the individual clusters can be greatly affected by local absorption. In these respects the Small Cloud is unique among the known local group galaxies that contain both stellar populations, and that are large enough to include a statistically significant sample of star clusters. Serial numbers of clusters used here are taken from Paper I, which also discusses the properties, limitations, and weaknesses of the data.