arXiv subjects

Rupali Chandar

Publications and source records attributed to Rupali Chandar.

At least 109 records · Page 6Linked to original sources

arXiv2010

The Antennae Galaxies (NGC 4038/4039) Revisited: ACS and NICMOS Observations of a Prototypical Merger

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.

astro-ph.EP↗

arXiv2010

A Comparison of Methods for Determining the Age Distribution of Star Clusters: Application to the Large Magellanic Cloud

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.

astro-ph.GA↗

arXiv2010

New Tests for Disruption Mechanisms of Star Clusters: The Large and Small Magellanic Clouds

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.

astro-ph.GA↗

arXiv2010

SAO-6m Telescope Spectroscopic Observations of Globular Clusters in Nearby Galaxies

We present the results of medium-resolution spectroscopy of 28 globular clusters (GCs) in six nearby galaxies of different luminosities and morphological types, situated in: M33 (15 objects), M31 (3), IC10 (4), UGCA86 (4), HolmbergIX (1), and DDO71 (1) obtained at the Special Astrophysical Observatory 6-meter telescope. Measurements of Lick absorption-line indices and comparison with SSP models enabled us to obtain their spectroscopic ages, metallicities and alpha-element to Fe abundance ratios. We found that all old and intermediate-age GCs in our sample have low metallicities [Z/H] < -0.8 dex. Metal-rich clusters are young and are preferentially found in galaxies more massive than ~10^9 M_sun. The least massive dwarfs of our sample, DDO71 and HolmbergIX, host one massive intermediate-age and one massive young metal-poor GC, respectively. [a/Fe] abundance ratios tend to be enhanced but closer to solar values for dwarf galaxies compared to GCs in more massive galaxies. We analyse the age-metallicity relation for GCs in our galaxy sample and others from the literature, and find, that 1) there is a general trend for GCs in low surface brightness dwarf galaxies to be more metal-poor at a given age than GCs in more massive galaxies; 2) the GC metallicity spread is wider for more massive galaxies; 3) intermediate-age GCs in early-type dwarf galaxies are more metal-rich at any given age than those in irregular galaxies of similar luminosity.

astro-ph.CO↗

arXiv2009

HST's hunt for intermediate-mass black holes in star clusters

Establishing or ruling out, either through solid mass measurements or upper limits, the presence of intermediate-mass black holes (IMBHs) at the centers of star clusters would profoundly impact our understanding of problems ranging from the formation and long-term dynamical evolution of stellar systems, to the nature of the seeds and the growth mechanisms of supermassive black holes. While there are sound theoretical arguments both for and against their presence in today's clusters, observational studies have so far not yielded truly conclusive IMBH detections nor upper limits. We argue that the most promising approach to solving this issue is provided by the combination of measurements of the proper motions of stars at the centers of Galactic globular clusters and dynamical models able to take full advantage of this type of data set. We present a program based on HST observations and recently developed tools for dynamical analysis designed to do just that.

astro-ph.GA↗

arXiv2009

New Tests for Disruption Mechanisms of Star Clusters: Methods and Application to the Antennae Galaxies

We present new tests for disruption mechanisms of star clusters based on the bivariate mass-age distribution g(M,τ). In particular, we derive formulae for g(M,τ) 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: τ\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,τ) \propto M^{-2} τ^{-1}, with power-law mass and age distributions, dN/dM propto M^{-2} and dN/dτ\proptoτ^{-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, τ\la 10^7$ yr; (2) continued stellar mass loss, 10^7 yr \la τ\la 10^8 yr; (3), tidal disturbances by passing molecular clouds, τ\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.

astro-ph.GA↗

arXiv2009

Population Parameters of Intermediate-Age Star Clusters in the Large Magellanic Cloud. I. NGC 1846 and its Wide Main Sequence Turnoff

The Advanced Camera for Surveys on board the Hubble Space Telescope has been used to obtain deep, high-resolution images of the intermediate-age star cluster NGC 1846 in the Large Magellanic Cloud. We present new color-magnitude diagrams (CMDs) based on F435W, F555W, and F814W imaging. We test the previously observed broad main sequence turnoff region for "contamination" by field stars and (evolved) binary star systems. We find that while these impact the number of objects in this region, none can fully account for the large color spread. Our results therefore solidify the recent finding that stars in the main sequence turnoff region of this cluster have a large spread in color which is unrelated to measurement errors or contamination by field stars, and likely due to a ~300 Myr range in the ages of cluster stars. An unbiased estimate of the stellar density distribution across the main sequence turnoff region shows that the spread is fairly continuous rather than strongly bimodal as suggested previously. We fit the CMDs with several different sets of theoretical isochrones, and determine systematic uncertainties for population parameters when derived using any one set of isochrones. We note a degeneracy between age and [alpha/Fe], which can be lifted by matching the shape (curvature) of the full red giant branch in the CMD. We find that stars in the upper part of the main sequence turnoff region are more centrally concentrated than those in any other region of the CMD, including more massive red giant branch and asymptotic giant branch stars. We consider several possible formation scenarios which account for the unusual features observed in the CMD of NGC 1846.

astro-ph.CO↗

arXiv2008

Young star clusters in interacting galaxies - NGC 1487 and NGC 4038/4039

We estimate the dynamical masses of several young (~10 Myr) massive star clusters in two interacting galaxies, NGC 4038/4039 ("The Antennae") and NGC 1487, under the assumption of virial equilibrium. These are compared with photometric mass estimates from K-band photometry and assuming a standard Kroupa IMF. The clusters were selected to have near-infrared colors dominated by red supergiants, and hence to be old enough to have survived the earliest phases of cluster evolution when the interstellar medium is rapidly swept out from the cluster, supported by the fact that there is no obvious Halpha emission associated with the clusters. All but one of the Antennae clusters have dynamical and photometric mass estimates which are within a factor ~2 of one another, implying both that standard IMFs provide a good approximation to the IMF of these clusters, and that there is no significant extra-virial motion, as would be expected if they were rapidly dispersing. These results suggest that almost all of the Antennae clusters in our sample have survived the gas removal phase as bound or marginally bound objects. Two of the three NGC 1487 clusters studied here have M_dyn estimates which are significantly larger than the photometric mass estimates. At least one of these two clusters, and one in the Antennae, may be actively in the process of dissolving. The process of dissolution contributes a component of non-virial motion to the integrated velocity measurements, resulting in an estimated M_dyn which is too high relative to the amount of measured stellar light. The dissolution candidates in both galaxies are amongst the clusters with the lowest pressures/densities measured in our sample.

astro-ph↗

arXiv2007

Density Dependence of the Mass Function of Globular Star Clusters in the Sombrero Galaxy and its Dynamical Implications

We have constructed the mass function of globular star clusters in the Sombrero galaxy in bins of different internal half-mass density rho_h and projected galactocentric distance R. This is based on the published measurements of the magnitudes and effective radii of the clusters by Spitler et al. (2006) in BVR images taken with the ACS on HST. We find that the peak of the mass function M_p increases with rho_h by a factor of about 4 but remains nearly constant with R. Our results are almost identical to those presented recently by McLaughlin & Fall (2007) for globular clusters in the Milky Way. The mass functions in both galaxies agree with a simple, approximate model in which the clusters form with a Schechter initial mass function and evolve subsequently by stellar escape driven by internal two-body relaxation. These findings therefore undermine recent claims that the present peak of the mass function of globular clusters must have been built into the initial conditions.

astro-ph↗

arXiv2006

Star Cluster Demographics. I. A General Framework and Application to the Antennae Galaxies

We present a framework for understanding the demographics of star cluster systems, and develop a toy model which incorporates a universal initial power law mass function, selected formation histories, selected disruption laws, and a convolution with common artifacts and selection effects found in observational data. The model confirms that the observed correlation between the brightest young cluster in a galaxy and the total number of young clusters can be understood as a statistical size-of-sample effect, rather than a difference in the physical process responsible for the formation of the clusters. A comparison is made between different cluster disruption laws and it is shown that the break in the dN/dτdiagram used to determine the parameters in the Boutloukos & Lamers model may be produced by incompleteness near the breakpoint. A model of the Antennae galaxies is developed and compared with the observational data. An important component of our model is the use of a "two-stage" disruption process, with a very high "infant mortality" rate for the clusters with ages less than \approx10^8 yrs (i.e., roughly 80%-90% are lost each factor of ten in time, τ, independent of mass), and two-body relaxation, which becomes the dominant disruption mechanism at older ages, preferentially removing the lower mass clusters. Hence, in our model, stars from the dissolved clusters form the field population. We note that a 90% infant mortality rate for each factor of ten in τ(i.e., dN/dτ\propto τ^{-1}) is consistent with all measured young cluster populations, including those in the Antennae, Small Magellanic Cloud, and the Milky Way.

astro-ph↗

arXiv2006

Connection between the Age Distributions of Star Clusters and Field Stars: A First Application to the Small Magellanic Cloud

We present the age distributions for star clusters and individual stars in the Small Magellanic Cloud (SMC) based on data from the Magellanic Clouds Photometric Survey by Zaritsky and collaborators. The age distribution of the SMC clusters shows a steep decline, dN_{cluster}/dt \propto t^{-0.85\pm0.15}, over the period 10^7 < t <10^9 yr. This decline is essentially identical to that observed previously for more massive clusters in the merging Antennae galaxies, and also for lower-mass embedded clusters in the solar neighborhood. The SMC cluster age distribution therefore provides additional evidence for the rapid disruption of star clusters (``infant mortality''). These disrupted clusters deliver their stars to the general field population, implying that the field star age distribution, dN_{fld star}/dt, should have an inverse relation to dN_{cluster}/dt if most stars form initially in clusters. We make specific predictions for dN_{fldstar}/dt based on our cluster disruption models, and compare them with current data available for stars in the SMC. While these data do not extend to sufficiently young ages for a definitive test, they are consistent with a scenario wherein most SMC stars formed in clusters. Future analyses of dN_{fldstar}/dt that extend down to ages of approximately few million years are needed to verify the age relationship between stars residing in clusters and in the field.

astro-ph↗

arXiv2006

A Database of 2MASS Near-Infrared Colors of Magellanic Cloud Star Clusters

The (rest-frame) near-IR domain contains important stellar population diagnostics and is often used to estimate masses of galaxies at low as well as high redshifts. However, many stellar population models are still relatively poorly calibrated in this part of the spectrum. To allow an improvement of this calibration we present a new database of integrated near-infrared JHKs magnitudes for 75 star clusters in the Magellanic Clouds, using the 2-Micron All-Sky Survey (2MASS). The majority of the clusters in our sample have robust age and metallicity estimates from color-magnitude diagrams available in the literature, and populate a range of ages from 10 Myr to 15 Gyr and a range in [Fe/H] from -2.17 to +0.01 dex. A comparison with matched star clusters in the 2MASS Extended Source Catalog (XSC) reveals that the XSC only provides a good fit to the unresolved component of the cluster stellar population. We also compare our results with the often-cited single-channel JHK photometry of Persson and collaborators, and find significant differences, especially for their 30"-diameter apertures up to ~2.5 mag in the K-band, more than 1 mag in J-K, and up to 0.5 mag in H-K. Using simulations to center apertures based on maximum light throughput (as performed by Persson et al, we show that these differences can be attributed to near-IR-bright cluster stars (e.g., Carbon stars) located away from the true center of the star clusters. The wide age and metallicity coverage of our integrated JHKs photometry sample constitutes a fundamental dataset for testing population synthesis model predictions, and for direct comparison with near-IR observations of distant stellar populations.

astro-ph↗

arXiv2006

A Genuine Intermediate-Age Globular Cluster in M33

We present deep integrated-light spectroscopy of nine M33 globular clusters taken with the Hectospec instrument at the MMT Observatory. Based on our spectroscopy and previous deep color-magnitude diagrams obtained with HST/WFPC2, we present evidence for the presence of a genuine intermediate-age globular cluster in M33. The analysis of Lick line indices indicates that all globular clusters are metal-poor ([Z/H] <~ -1.0) and that cluster M33-C38 is about 5-8 Gyr younger than the rest of the sample M33 star clusters. We find no evidence for a population of blue horizontal branch stars in the CMD of M33-C38, which rules out the possibility of an artificially young spectroscopic age due to the presence of hot stars. We infer a total mass of 5-9 x 10^4 M_sol for M33-C38, which implies that M33-C38 has survived ~2-3 times longer than some dynamical evolution model predictions for star clusters in M33, although it is not yet clear to which dynamical component of M33 - thin disk, thick disk, halo - the cluster is associated.

astro-ph↗

arXiv2006

Hierarchical Star Formation in the Spiral Galaxy NGC 628

The distributions of size and luminosity for star-forming regions in the nearby spiral galaxy NGC 628 are studied over a wide range of scales using progressively blurred versions of an image from the Hubble Space Telescope Advanced Camera for Surveys. Four optical filters are considered for the central region, including Halpha. Two filters are used for an outer region. The features in each blurred image are counted and measured using SExtractor. The cumulative size distribution is found to be a power law in all passbands with a slope of about -1.5 over 1.8 orders of magnitudes. The luminosity distribution is approximately a power law too, with a slope of about -1 for logarithmic intervals of luminosity. The results suggest a scale-free nature for stellar aggregates in a galaxy disk. Fractal models of thin disks reproduce the projected size distribution and suggest a projected mass distribution slope of about -0.5 for these extended regions. This mass slope converts to the observed luminosity slope if we account for luminosity evolution and longer lifetimes in larger regions.

astro-ph↗

arXiv2005

Properties of Resolved Star Clusters in M51

We present a study of compact star clusters in the nearby pair of interacting galaxies NGC 5194/95 (M51), based on multifilter Hubble Space Telescope WFPC2 archival images. We have detected ~400 isolated clusters. Our requirement that clusters be detected based only on their morphology results in the selection of relatively isolated objects, and we estimate that we are missing the majority (by a factor 4-6) of <10 Myr clusters due to crowding. Hence we focus on the cluster population older than 10 Myr. An age distribution shows a broad peak between 100-500 Myr, which is consistent with the crossing times of NGC 5195 through the NGC 5194 disk estimated in both single and multiple-passage dynamical models. We estimate that the peak contains approximately 2.2-2.5 times more clusters than expected from a constant rate of cluster formation over this time interval. We estimate the effective radii of our sample clusters and find a median value of 3-4 pc. Additionally, we see correlations of increasing cluster size with cluster mass (with a best fit slope of 0.14\pm0.03) at the 4sigma level, and with cluster age (0.06\pm0.02) at the 3sigma level. Finally, we report for the first time the discovery of faint, extended star clusters in the companion, NGC 5195, an SB0 galaxy. These have red [(V-I)>1.0] colors, effective radii >7 pc, and are scattered over the disk of NGC 5195. Our results indicate that NGC 5195 is therefore currently the third known barred lenticular galaxy to have formed so-called "faint fuzzy" star clusters. (abridged)

astro-ph↗

arXiv2005

The Age Distribution of Massive Star Clusters in the Antennae Galaxies

We determine the age distribution of star clusters in the Antennae galaxies (NGC 4038/9) for two mass-limited samples (M > 3 x 10^4 M_{\odot} and M > 2 x 10^5 M_{\odot}). This is based on integrated broadband UBVI and narrowband H-alpha photometry from deep images taken with the Hubble Space Telescope. We find that the age distribution of the clusters declines steeply, approximately as dN/dτ\propto τ^{-1}. The median age of the clusters is ~10^7 yr, which we interpret as evidence for rapid disruption ("infant mortality"). It is very likely that most of the young clusters are not gravitationally bound and were disrupted near the times they formed by the energy and momentum input from young stars to the interstellar matter of the protoclusters. At least 20% and possibly all stars form in clusters and/or associations, including those that are unbound and short-lived.

astro-ph↗

arXiv2005

STIS Spectroscopy of Young Star Clusters in "The Antennae" Galaxies (NGC 4038/4039)

Long-slit spectra of several dozen young star clusters have been obtained at three positions in the Antennae galaxies with the Space Telescope Imaging Spectrograph (STIS) and its 52"x0.2" slit. Based on H-alpha emission-line measurements, the average cluster-to-cluster velocity dispersion in 7 different cluster aggregates ("knots") is <10 \kms. The fact that this upper limit is similar to the velocity dispersion of gas in the disks of typical spiral galaxies suggests that the triggering mechanism for the formation of young massive compact clusters ("super star clusters") is unlikely to be high velocity cloud-cloud collisions. On the other hand, models where preexisting giant molecular clouds in the disks of spiral galaxies are triggered into cluster formation are compatible with the observed low velocity dispersions. These conclusions are consistent with those reached by Zhang et al. (2001) based on comparisons between the positions of the clusters and the velocity and density structure of the nearby interstellar medium. We find evidence for systematically lower values of the line ratios [NII]/H-alpha and [SII]/H-alpha in the bright central regions of some of the knots, relative to their outer regions. This suggests that the harder ionizing photons are used up in the regions nearest the clusters, and the diffuse ionized gas farther out is photoionized by 'leakage' of the leftover low-energy photons. The low values of the [SII]/H-alpha line ratio, typically [SII]/H-alpha<0.4, indicates that the emission regions are photoionized rather than shock heated. The absence of evidence for shock-heated gas is an additional indication that high velocity cloud-cloud collisions are not playing a major role in the formation of the young clusters.

astro-ph↗

arXiv2005

The Stellar Content of Nearby Star-Forming Galaxies. III. Unravelling the Nature of the Diffuse Ultraviolet Light

We investigate the nature of the diffuse intra-cluster ultraviolet light seen in twelve local starburst galaxies, using long-slit ultraviolet spectroscopy obtained with the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope (HST). We take this faint intra-cluster light to be the field in each galaxy, and compare its spectroscopic signature with STARBURST99 evolutionary synthesis models and with neighboring star clusters. Our main result is that the diffuse ultraviolet light in eleven of the twelve starbursts lacks the strong O-star wind features that are clearly visible in spectra of luminous clusters in the same galaxies. The difference in stellar features dominating cluster and field spectra indicate that the field light originates primarily from a different stellar population, and not from scattering of UV photons leaking out of the massive clusters. We consider several scenarios to understand the lack of observed O stars in the field, and their implications for the origin of the field stellar population. If the field stellar populations formed in situ, the field must either have an IMF which is steeper than Salpeter (3.0-3.5) or a Salpeter slope with an upper mass cutoff of 30-50 solar masses. If star formation occurs primarily in star clusters, the field could be composed of older, faded clusters, and/or a population which is coeval with the luminous clusters but lower in mass. If the field is composed of older, dissolving clusters, we estimate that star clusters (regardless of mass) need to dissolve on timescales 7-10 Myr to create the field (abridged).

astro-ph↗