Publications (39)81.37 Total impact
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Article: X-ray Binaries and Star Clusters in the Antennae: Optical Cluster Counterparts
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ABSTRACT: We compare the locations of 82 X-ray binaries (XRBs) detected in the merging Antennae galaxies by Zezas et al., based on observations taken with the Chandra X-Ray Observatory, with a catalog of optically selected star clusters presented by Whitmore et al., based on observations taken with the Hubble Space Telescope. Within the 2 sigma positional uncertainty of 0.58", we find 22 XRBs are coincident with star clusters, where only 2-3 chance coincidences are expected. The ages of the clusters were estimated by comparing their UBVI, Halpha colors with predictions from stellar evolutionary models. We find that 14 of the 22 coincident XRBs (64%) are hosted by star clusters with ages of 6 Myr or less. Five of the XRBs are hosted by young clusters with ages 10-100 Myr, while three are hosted by intermediate age clusters with 100-300 Myr. Based on the results from recent N-body simulations, which suggest that black holes are far more likely to be retained within their parent clusters than neutron stars, we suggest that our sample consists primarily of black hole binaries with different ages.09/2012; -
Article: Space-Based UV/Optical Wide-Field Imaging and Spectroscopy: Near-Field Cosmology and Galaxy Evolution Using Globular Clusters in Nearby Galaxies
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ABSTRACT: Star formation plays a central role in the evolution of galaxies and of the Universe as a whole. Studies of star-forming regions in the local universe have shown that star formation typically occurs in a clustered fashion. Building a coherent picture of how star clusters form and evolve is therefore critical to our overall understanding of the star formation process. Most clusters disrupt after they form, thus contributing to the field star population. However, the most massive and dense clusters remain bound and survive for a Hubble time. These globular clusters provide unique observational probes of the formation history of their host galaxies. In particular, the age and metallicity can be determined for each globular cluster individually, allowing the distribution of ages and metallicities within host galaxies to be constrained. We show how space-based UV-to-near-IR imaging covering a wide field of view (>= 20 arcmin per axis) and deep UV/Optical multi-object spectroscopy of globular cluster systems in nearby galaxies would allow one to place important new constraints on the formation history of early-type galaxies and their structural subcomponents (e.g., bulge, halo).09/2012; -
Article: Similarities in Populations of Star Clusters
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ABSTRACT: We compare the observed mass functions and age distributions of star clusters in six well-studied galaxies: the Milky Way, Magellanic Clouds, M83, M51, and Antennae. In combination, these distributions span wide ranges of mass and age: $10^2\lea M/M_{\odot}\lea10^6$ and $10^6\lea\tau/yr \lea10^9$. We confirm that the distributions are well represented by power laws: $dN/dM\propto M^{\beta}$ with $\beta \approx-1.9$ and $dN/d\tau\propto\tau^{\gamma}$ with $\gamma\approx -0.8$. The mass and age distributions are approximately independent of each other, ruling out simple models of mass-dependent disruption. As expected, there are minor differences among the exponents, at a level close to the true uncertainties, $\epsilon_{\beta}\sim\epsilon_{\gamma}\sim$~0.1--0.2. However, the overwhelming impression is the similarity of the mass functions and age distributions of clusters in these different galaxies, including giant and dwarf, quiescent and interacting galaxies. This is an important empirical result, justifying terms such as "universal" or "quasi-universal." We provide a partial theoretical explanation for these observations in terms of physical processes operating during the formation and disruption of the clusters, including star formation and feedback, subsequent stellar mass loss, and tidal interactions with passing molecular clouds. A full explanation will require additional information about the molecular clumps and star clusters in galaxies beyond the Milky Way.06/2012; -
Article: The Resolved Stellar Population in 50 Regions of M83 from HST/WFC3 Early Release Science Observations
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ABSTRACT: We present a multi-wavelength photometric study of ~15,000 resolved stars in the nearby spiral galaxy M83 (NGC5236, D=4.61Mpc) based on Hubble Space Telescope Wide Field Camera 3 observations using four filters: F336W, F438W, F555W, and F814W. We select 50 regions (an average size of 260 pc by 280 pc) in the spiral arm and inter-arm areas of M83, and determine the age distribution of the luminous stellar populations in each region. This is accomplished by correcting for extinction towards each individual star by comparing its colors with predictions from stellar isochrones. We compare the resulting luminosity weighted mean ages of the luminous stars in the 50 regions with those determined from several independent methods, including the number ratio of red-to-blue supergiants, morphological appearance of the regions, surface brightness fluctuations, and the ages of clusters in the regions. We find reasonably good agreement between these methods. We also find that young stars are much more likely to be found in concentrated aggregates along spiral arms, while older stars are more dispersed. These results are consistent with the scenario that star formation is associated with the spiral arms, and stars form primarily in star clusters and then disperse on short timescales to form the field population. The locations of Wolf-Rayet stars are found to correlate with the positions of many of the youngest regions, providing additional support for our ability to accurately estimate ages. We address the effects of spatial resolution on the measured colors, magnitudes, and age estimates. While individual stars can occasionally show measurable differences in the colors and magnitudes, the age estimates for entire regions are only slightly affected.04/2012; -
Article: Analyzing Star Cluster Populations with Stochastic Models: The Hubble Space Telescope/Wide Field Camera 3 Sample of Clusters in M83
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ABSTRACT: The majority of clusters in the universe have masses well below 105 M ☉. Hence, their integrated fluxes and colors can be affected by the presence or absence of a few bright stars introduced by stochastic sampling of the stellar mass function. Specific methods are being developed to extend the analysis of cluster energy distributions into the low-mass regime. In this paper, we apply such a method to real observations of star clusters, in the nearby spiral galaxy M83. We reassess the ages and masses of a sample of 1242 clusters for which UBVIHα fluxes were obtained from observations with the Wide Field Camera 3 instrument on board the Hubble Space Telescope. Synthetic clusters with known properties are used to characterize the limitations of the method (valid range and resolution in age and mass, method artifacts). The ensemble of color predictions of the discrete cluster models are in good agreement with the distribution of observed colors. We emphasize the important role of the Hα data in the assessment of the fraction of young objects, particularly in breaking the age-extinction degeneracy that hampers an analysis based on UBVI data only. We find the mass distribution of the cluster sample to follow a power law of index –2.1 ± 0.2, and the distribution of ages a power law of index –1.0 ± 0.2 for log (M/ M ☉) > 3.5, and ages between 107 and 109 yr. An extension of our main method, which makes full use of the probability distributions of age and mass obtained for the individual clusters of the sample, is explored. It produces similar power-law slopes and will deserve further investigation. Although the properties derived for individual clusters significantly differ from those obtained with traditional, non-stochastic models in about 30% of the objects, the first-order aspect of the age and mass distributions is similar to those obtained previously for this M83 sample in the range of overlap of the studies. We extend the power-law description to lower masses with better mass and age resolution and without most of the artifacts produced by the classical method.The Astrophysical Journal 04/2012; 750(1):60. · 6.02 Impact Factor -
Article: Analysing star cluster populations with stochastic models: the HST/WFC3 sample of clusters in M83
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ABSTRACT: The majority of clusters in the Universe have masses well below 10^5 Msun. Hence their integrated fluxes and colors can be affected by the random presence of a few bright stars introduced by stochastic sampling of the stellar mass function. Specific methods are being developed to extend the analysis of cluster SEDs into the low-mass regime. In this paper, we apply such a method to observations of star clusters, in the nearby spiral galaxy M83. We reassess ages and masses of a sample of 1242 objects for which UBVIHalpha fluxes were obtained with the HST/WFC3 images. Synthetic clusters with known properties are used to characterize the limitations of the method. The ensemble of color predictions of the discrete cluster models are in good agreement with the distribution of observed colors. We emphasize the important role of the Halpha data in the assessment of the fraction of young objects, particularly in breaking the age-extinction degeneracy that hampers an analysis based on UBVI only. We find the mass distribution of the cluster sample to follow a power-law of index -2.1 +/-0.2, and the distribution of ages a power-law of index -1.0 +/-0.2 for M > 10^3.5 Msun and ages between 10^7 and 10^9 yr. An extension of our main method, that makes full use of the probability distributions of age and mass of the individual clusters, is explored. It produces similar power-law slopes and will deserve further investigation. Although the properties derived for individual clusters significantly differ from those obtained with traditional, non-stochastic models in ~30% of the objects, the first order aspect of the age and mass distributions are similar to those obtained previously for this M83 sample in the range of overlap of the studies. We extend the power-law description to lower masses with better mass and age resolution and without most of the artifacts produced by the classical method.02/2012; -
Article: The Evolution of Stellar Populations in the Outer Disks of Spiral Galaxies
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ABSTRACT: We investigate recent star formation in the extended ultraviolet (XUV) disks of five nearby galaxies (NGC 0628, NGC 2090, NGC 2841, NGC 3621, and NGC 5055) using a long wavelength baseline comprised of ultraviolet and mid-infrared imaging from the Galaxy Evolution Explorer and the Spitzer Infrared Array Camera. We identify 229 unresolved stellar complexes across targeted portions of their XUV disks and utilize spectral energy distribution fitting to measure their stellar ages and masses through comparison with Starburst99 population synthesis models of instantaneous burst populations. We find that the median age of outer-disk associations in our sample is ~100 Myr with a large dispersion that spans the entire range of our models (1 Myr to 1 Gyr). This relatively evolved state for most associations addresses the observed dearth of Hα emission in some outer disks, as Hα can only be observed in star-forming regions younger than ~10 Myr. The large age dispersion is robust against variations in extinction (in the range E(B – V) = 0-0.3 mag) and variations in the upper end of the stellar initial mass function (IMF). In particular, we demonstrate that the age dispersion is insensitive to steepening of the IMF, up to extreme slopes.The Astrophysical Journal 03/2011; 731(1):28. · 6.02 Impact Factor -
Article: Using Hα Morphology and Surface Brightness Fluctuations to Age-date Star Clusters in M83
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ABSTRACT: We use new WFC3 observations of the nearby grand-design spiral galaxy M83 to develop two independent methods for estimating the ages of young star clusters. The first method uses the physical extent and morphology of Hα emission to estimate the ages of clusters younger than τ 10 Myr. It is based on the simple premise that the gas in very young (τ< a few Myr) clusters is largely coincident with the cluster stars, is in a small, ring-like structure surrounding the stars in slightly older clusters since massive star winds and supernovae have had time to push out the natal gas (e.g., τ 5 Myr), and is in a larger ring-like bubble for still older clusters (i.e., 5-10 Myr). If no Hα is associated with a cluster it is generally older than 10 Myr. The second method is based on an observed relation between pixel-to-pixel flux variations within clusters and their ages. This method relies on the fact that the brightest individual stars in a cluster are most prominent at ages around 10 Myr, and fall below the detection limit (i.e., M V < –3.5) for ages older than about 100 Myr. Older clusters therefore have a smoother appearance and smaller pixel-to-pixel variations. The youngest clusters also have lower flux variations, hence the relationship is double valued. This degeneracy in age can be broken using other age indicators such as Hα morphology. These two methods are the basis for a new morphological classification system which can be used to estimate the ages of star clusters based on their appearance. We compare previous age estimates of clusters in M83 determined from fitting UBVIHα measurements using predictions from stellar evolutionary models with our new morphological categories and find good agreement, at the 95% level. The scatter within categories is 0.1 dex in log τ for young clusters (<10 Myr) and 0.5 dex for older (>10 Myr) clusters. A by-product of this study is the identification of 22 "single-star" H II regions in M83, with central stars having ages 4 Myr.The Astrophysical Journal 02/2011; 729(2):78. · 6.02 Impact Factor -
Article: New Constraints on Mass-dependent Disruption of Star Clusters in M51
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ABSTRACT: We use UBVI Hα images of the Whirlpool galaxy, M51, taken with the Advanced Camera for Surveys and WFPC2 cameras on the Hubble Space Telescope (HST) to select star clusters, and to estimate their masses and ages by comparing their observed colors with predictions from population synthesis models. We construct the mass function of intermediate-age (1-4 × 108 yr) clusters, and find that it is well described by a power law, ψ(M) M β, with β = –2.1 ± 0.2, for clusters more massive than M 6 × 103 M ☉. This extends the mass function of intermediate-age clusters in M51 to masses lower by nearly a factor of five over previous determinations. The mass function does not show evidence for curvature at either the high or low mass end. This shape indicates that there is no evidence for the earlier disruption of lower mass clusters compared with their higher mass counterparts (i.e., no mass-dependent disruption) over the observed range of masses and ages, or for a physical upper mass limit MC with which clusters in M51 can form. These conclusions differ from previous suggestions based on poorer-quality HST observations. We discuss their implications for the formation and disruption of the clusters. Ages of clusters in two "feathers," stellar features extending from the outer portion of a spiral arm, show that the feather with a larger pitch angle formed earlier, and over a longer period, than the other.The Astrophysical Journal 01/2011; 727(2):88. · 6.02 Impact Factor -
Article: Ultraviolet Spectroscopy of Circumnuclear Star Clusters in M83
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ABSTRACT: We analyze archival HST/STIS/FUV-MAMA imaging and spectroscopy of 13 compact star clusters within the circumnuclear starburst region of M83, the closest such example. We compare the observed spectra with semi-empirical models, which are based on an empirical library of Galactic O and B stars observed with IUE, and with theoretical models, which are based on a new theoretical UV library of hot massive stars computed with WM-Basic. The models were generated with Starburst99 for metallicities of Z=0.020 and Z=0.040, and for stellar IMFs with upper mass limits of 10, 30, 50, and 100 M_sol. We estimate the ages and masses of the clusters from the best fit model spectra, and find that the ages derived from the semi-empirical and theoretical models agree within a factor of 1.2 on average. A comparison of the spectroscopic age estimates with values derived from HST/WFC3/UVIS multi-band photometry shows a similar level of agreement for all but one cluster. The clusters have a range of ages from about 3 to 20 Myr, and do not appear to have an age gradient along M83's starburst. Clusters with strong P-Cygni profiles have masses of a few times 10^4 M_sol, seem to have formed stars more massive than 30 M_sol, and are consistent with a Kroupa IMF from 0.1-100 M_sol. Field regions in the starburst lack P-Cygni profiles and are dominated by B stars. Comment: 38 pages, 13 figures, accepted for publication in the ApJ11/2010; -
Article: The Massive Star Content of Circumnuclear Star Clusters in M83
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ABSTRACT: The circumnuclear starburst of M83 (NGC 5236), the nearest such example (4.6 Mpc), constitutes an ideal site for studying the massive star IMF at high metallicity (12+log[O/H]=9.1$\pm$0.2, Bresolin & Kennicutt 2002). We analyzed archival HST/STIS FUV imaging and spectroscopy of 13 circumnuclear star clusters in M83. We compared the observed spectra with two types of single stellar population (SSP) models, semi-empirical models, which are based on an empirical library of Galactic O and B stars observed with IUE (Robert et al. 1993), and theoretical models, which are based on a new theoretical UV library of hot massive stars described in Leitherer et al. (2010) and computed with WM-Basic (Pauldrach et al. 2001). The models were generated with Starburst99 (Leitherer & Chen 2009). We derived the reddenings, the ages, and the masses of the clusters from model fits to the FUV spectroscopy, as well as from optical HST/WFC3 photometry. Comment: 4 pages, 2 figures, Up2010 conference proceedings11/2010; -
Article: A New Method for Measuring the Upper End of the IMF
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ABSTRACT: A method is presented here for investigating variations in the upper end of the stellar Initial Mass Function (IMF) by probing the production rate of ionizing photons in unresolved, compact star clusters with ages<10 Myr and covering a range of masses. We test this method on the young cluster population in the nearby galaxy M51a, for which multi-wavelength observations from the Hubble Space Telescope are available. Our results indicate that the proposed method can probe the upper end of the IMF in galaxies located out to at least 10 Mpc, i.e., a factor 200 further away than possible by counting individual stars in young compact clusters. Our results for this galaxy show no obvious dependence of the upper mass end of the IMF on the mass of the star cluster, down to ~1000 M_sun, although more extensive analyses involving lower mass clusters and other galaxies are needed to confirm this conclusion. Comment: 7 pages, 1 figure; to appear in the proceedings of the conference `UP: Have Observations Revealed a Variable Upper End of the Stellar Initial Mass Function?', ASP Conference Series10/2010; -
Article: The Luminosity, Mass, and Age Distributions of Compact Star Clusters in M83 Based on HST/WFC3 Observations
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ABSTRACT: The newly installed Wide Field Camera 3 (WFC3) on the Hubble Space Telescope has been used to obtain multi-band images of the nearby spiral galaxy M83. These new observations are the deepest and highest resolution images ever taken of a grand-design spiral, particularly in the near ultraviolet, and allow us to better differentiate compact star clusters from individual stars and to measure the luminosities of even faint clusters in the U band. We find that the luminosity function for clusters outside of the very crowded starburst nucleus can be approximated by a power law, dN/dL \propto L^{alpha}, with alpha = -2.04 +/- 0.08, down to M_V ~ -5.5. We test the sensitivity of the luminosity function to different selection techniques, filters, binning, and aperture correction determinations, and find that none of these contribute significantly to uncertainties in alpha. We estimate ages and masses for the clusters by comparing their measured UBVI,Halpha colors with predictions from single stellar population models. The age distribution of the clusters can be approximated by a power-law, dN/dt propto t^{gamma}, with gamma=-0.9 +/- 0.2, for M > few x 10^3 Msun and t < 4x10^8 yr. This indicates that clusters are disrupted quickly, with ~80-90% disrupted each decade in age over this time. The mass function of clusters over the same M-t range is a power law, dN/dM propto M^{beta}, with beta=-1.94 +/- 0.16, and does not have bends or show curvature at either high or low masses. Therefore, we do not find evidence for a physical upper mass limit, M_C, or for the earlier disruption of lower mass clusters when compared with higher mass clusters, i.e. mass-dependent disruption. We briefly discuss these implications for the formation and disruption of the clusters. Comment: 36 pages, 13 figures, 1 table; accepted for publication in the Astrophysical Journal07/2010; -
Article: The Luminosity, Mass, and Age Distributions of Compact Star Clusters in M83 Based on Hubble Space Telescope/Wide Field Camera 3 Observations
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ABSTRACT: The newly installed Wide Field Camera 3 (WFC3) on the Hubble Space Telescope has been used to obtain multi-band images of the nearby spiral galaxy M83. These new observations are the deepest and highest resolution images ever taken of a grand-design spiral, particularly in the near-ultraviolet, and allow us to better differentiate compact star clusters from individual stars and to measure the luminosities of even faint clusters in the U band. We find that the luminosity function (LF) for clusters outside of the very crowded starburst nucleus can be approximated by a power law, dN/dL L α, with α = –2.04 ± 0.08, down to MV –5.5. We test the sensitivity of the LF to different selection techniques, filters, binning, and aperture correction determinations, and find that none of these contribute significantly to uncertainties in α. We estimate ages and masses for the clusters by comparing their measured UBVI, Hα colors with predictions from single stellar population models. The age distribution of the clusters can be approximated by a power law, dN/dτ τγ, with γ = –0.9 ± 0.2, for M few × 103 M ☉ and τ 4 × 108 yr. This indicates that clusters are disrupted quickly, with 80%-90% disrupted each decade in age over this time. The mass function of clusters over the same M-τ range is a power law, dN/dM M β, with β = –1.94 ± 0.16, and does not have bends or show curvature at either high or low masses. Therefore, we do not find evidence for a physical upper mass limit, MC , or for the earlier disruption of lower mass clusters when compared with higher mass clusters, i.e., mass-dependent disruption. We briefly discuss these implications for the formation and disruption of the clusters.The Astrophysical Journal 07/2010; 719(1):966. · 6.02 Impact Factor -
Dataset: Galaxy Wars: Stellar Populations and Star Formation in Interacting Galaxies ASP Conference Series Galaxy Mergers and Star Clusters: An HST Perspective
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ABSTRACT: One trademark of gas rich galaxy mergers is a firestorm of star formation. Many (and possibly most) of the stars in these systems form in com-pact star clusters, some of which reach extremely high masses. The spectacular images returned by the Hubble Space Telescope (HST) allow us to detect and study star clusters in nearby mergers in detail. We summarize recent results on the cluster systems in four nearby mergers: the Antennae, NGC 520, NGC 2623, and NGC 3256, based on multi-band imaging taken with HST . -
Article: The Antennae Galaxies (NGC 4038/4039) Revisited: Advanced Camera for Surveys and NICMOS Observations of a Prototypical Merger
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ABSTRACT: The Advanced Camera for Surveys and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) have been used to obtain new Hubble Space Telescope images of NGC 4038/4039 ("The Antennae"). These new observations allow us to better differentiate compact star clusters from individual stars, based on both size and color. We use this ability to extend the cluster luminosity function (LF) by approximately 2 mag over our previous WFPC2 results, and find that it continues as a single power law, dN/dL L α with α = –2.13 ± 0.07, down to the observational limit of MV –7. Similarly, the mass function (MF) is a single power law dN/dM M β with β = –2.10 ± 0.20 for clusters with ages <3 × 108 yr, corresponding to lower mass limits that range from 104 to 105 M ☉, depending on the age range of the subsample. Hence, the power-law indices for the luminosity and MFs are essentially the same. The LF for intermediate-age clusters (i.e., ~100-300 Myr old objects found in the loops, tails, and outer areas) shows no bend or turnover down to MV –6, consistent with relaxation-driven cluster disruption models which predict the turnover should not be observed until MV –4. An analysis of individual ~0.5 kpc sized areas over diverse environments shows good agreement between values of α and β, similar to the results for the total population of clusters in the system. There is tentative evidence that the values of both α and β are flatter for the youngest clusters in some areas, but it is possible that this is caused by observational biases. Several of the areas studied show evidence for age gradients, with somewhat older clusters appearing to have triggered the formation of younger clusters. The area around Knot B is a particularly interesting example, with a ~10-50 Myr old cluster of estimated mass ~106 M ☉ having apparently triggered the formation of several younger, more massive (up to 5 × 106 M ☉) clusters along a dust lane. A comparison with new NICMOS observations reveals that only 16% ± 6% of the IR-bright clusters in the Antennae are still heavily obscured, with values of AV >3 mag.The Astronomical Journal 05/2010; 140(1):75. · 4.03 Impact Factor -
Article: The Antennae Galaxies (NGC 4038/4039) Revisited: ACS and NICMOS Observations of a Prototypical Merger
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ABSTRACT: The ACS and NICMOS have been used to obtain new HST images of NGC 4038/4039 ("The Antennae"). These new observations allow us to better differentiate compact star clusters from individual stars, based on both size and color. We use this ability to extend the cluster luminosity function by approximately two magnitudes over our previous WFPC2 results, and find that it continues as a single power law, dN/dL propto L^alpha with alpha=-2.13+/-0.07, down to the observational limit of Mv~-7. Similarly, the mass function is a single power law dN/dM propto M^beta with beta=-2.10+/-0.20 for clusters with ages t<3x10^8 yr, corresponding to lower mass limits that range from 10^4 to 10^5 Msun, depending on the age range of the subsample. Hence the power law indices for the luminosity and mass functions are essentially the same. The luminosity function for intermediate-age clusters (i.e., ~100-300 Myr old objects found in the loops, tails, and outer areas) shows no bend or turnover down to Mv~-6, consistent with relaxation-driven cluster disruption models which predict the turnover should not be observed until Mv~-4. An analysis of individual ~0.5-kpc sized areas over diverse environments shows good agreement between values of alpha and beta, similar to the results for the total population of clusters in the system. Several of the areas studied show evidence for age gradients, with somewhat older clusters appearing to have triggered the formation of younger clusters. The area around Knot B is a particularly interesting example, with an ~10-50 Myr old cluster of estimated mass ~10^6 Msun having apparently triggered the formation of several younger, more massive (up to 5x10^6 Msun) clusters along a dust lane. Comment: 84 pages, 9 tables, 31 figures; ApJ accepted05/2010; -
Article: A Comparison of Methods for Determining the Age Distribution of Star Clusters: Application to the Large Magellanic Cloud
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ABSTRACT: The age distribution of star clusters in nearby galaxies plays a crucial role in evaluating the lifetimes and disruption mechanisms of the clusters. Two very different results have been found recently for the age distribution chi(t) of clusters in the Large Magellanic Cloud (LMC). We found that chi(t) can be described approximately by a power law chi(t) propto t^{gamma}, with gamma -0.8, by counting clusters in the mass-age plane, i.e., by constructing chi(t) directly from mass-limited samples. Gieles & Bastian inferred a value of gamma~, based on the slope of the relation between the maximum mass of clusters in equal intervals of log t, hereafter the M_max method, an indirect technique that requires additional assumptions about the upper end of the mass function. However, our own analysis shows that the M_max method gives a result consistent with our direct counting method for clusters in the LMC, namely chi(t) propto t^-0.8 for t<10^9 yr. The reason for the apparent discrepancy is that our analysis includes many massive (M>1.5x10^3 M_sol), recently formed (t<10^7 yr) clusters, which are known to exist in the LMC, whereas Gieles & Bastian are missing such clusters. We compile recent results from the literature showing that the age distribution of young star clusters in more than a dozen galaxies, including dwarf and giant galaxies, isolated and interacting galaxies, irregular and spiral galaxies, has a similar declining shape. We interpret this approximately "universal" shape as due primarily to the progressive disruption of star clusters over their first ~few x 10^8 yr, starting soon after formation, and discuss some observational and physical implications of this early disruption for stellar populations in galaxies. Comment: 21 pages, 5 figures, published in the Astrophysical Journal, volume 713, page 134302/2010; -
Article: New Tests for Disruption Mechanisms of Star Clusters: The Large and Small Magellanic Clouds
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ABSTRACT: We compare the observed bivariate distribution of masses(M) and ages(t) of star clusters in the LMC with the predicted distributions g(M,t) from 3 idealized models for the disruption of star clusters: (1)sudden mass-dependent disruption;(2)gradual mass-dependent disruption; and (3)gradual mass-independent disruption. The model with mass-{\em in}dependent disruption provides a good, first-order description of these cluster populations, with g(M,t) propto M^{beta} t^{gamma}, beta=-1.8+/-0.2 and gamma=-0.8+/-0.2, at least for clusters with ages t<10^9 yr and masses M<10^3 M_sol (more specifically, t<10^7(M/10^2 M_sol)^{1.3} yr). This model predicts that the clusters should have a power-law luminosity function, dN/dL propto L^-1.8, in agreement with observations. The first two models, on the other hand, fare poorly when describing the observations, refuting previous claims that mass-dependent disruption of star clusters is observed in the LMC over the studied M-t domain. Clusters in the SMC can be described by the same g(M,t) distribution as for the LMC, but with smaller samples and hence larger uncertainties. The successful g(M,t) model for clusters in the Magellanic Clouds is virtually the same as the one for clusters in the merging Antennae galaxies, but extends the domain of validity to lower masses and to older ages. This indicates that the dominant disruption processes are similar in these very different galaxies over at least t<10^8 yr and possibly t<10^9 yr. The mass functions for young clusters in the LMC are power-laws, while that for ancient globular clusters is peaked. We show that the observed shapes of these mass functions are consistent with expectations from the simple evaporation model presented by McLaughlin & Fall. Comment: 46 pages, 17 figures, published ApJ, vol 711, page 126302/2010; -
Article: New Tests for Disruption Mechanisms of Star Clusters: Methods and Application to the Antennae Galaxies
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ABSTRACT: We present new tests for disruption mechanisms of star clusters based on the bivariate mass-age distribution g(M,\tau). In particular, we derive formulae for g(M,\tau) for two idealized models in which the rate of disruption depends on the masses of the clusters and one in which it does not. We then compare these models with our Hubble Space Telescope observations of star clusters in the Antennae galaxies over the mass-age domain in which we can readily distinguish clusters from individual stars: \tau\la10^7(M/10^4 M_{\odot})^{1.3} yr. We find that the models with mass-dependent disruption are poor fits to the data, even with complete freedom to adjust several parameters, while the model with mass-independent disruption is a good fit. The successful model has the simple form g(M,\tau) \propto M^{-2} \tau^{-1}, with power-law mass and age distributions, dN/dM propto M^{-2} and dN/d\tau\propto\tau^{-1}. The predicted luminosity function is also a power law, dN/dL \propto L^{-2}, in good agreement with our observations of the Antennae clusters. The similarity of the mass functions of star clusters and molecular clouds indicates that the efficiency of star formation in the clouds is roughly independent of their masses. The age distribution of the massive young clusters is plausibly explained by the following combination of disruption mechanisms: (1) removal of interstellar material by stellar feedback, \tau \la 10^7$ yr; (2) continued stellar mass loss, 10^7 yr \la \tau \la 10^8 yr; (3), tidal disturbances by passing molecular clouds, \tau \ga 10^8 yr. None of these processes is expected to have a strong dependence on mass, consistent with our observations of the Antennae clusters. We speculate that this simple picture also applies--at least approximately--to the clusters in many other galaxies. Comment: 30 pages, 15 figures; Published in the Asrophysical Journal, volume 704, pages 453-46810/2009;
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Institutions
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2010–2012
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University of Toledo
- Department of Physics and Astronomy
Toledo, OH, USA
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2007–2009
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Johns Hopkins University
- Department of Physics and Astronomy
Baltimore, MD, USA
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2005–2008
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Space Telescope Science Institute
Baltimore, MD, USA
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