Cluster Stellar Population Research Articles

The Clearing Timescale for Infrared-selected Star Clusters in M83 with HST

We present an analysis of Hubble Space Telescope data from Wide Field Camera 3 (WFC3)/Ultraviolet Imaging Spectrometer, WFC3/IR, and the Advanced Camera for Surveys, investigating the young stellar cluster (YSC) population in the face-on spiral galaxy M83. Within the field of view of the IR pointings, we identify 454 sources with compact F814W continuum and Paβ line emission with a S/N ≥ 3 as possible YSC candidates embedded in dust. We refine this selection to 97 candidates based on their spectral energy distributions, multiwavelength morphology, and photometric uncertainties. For sources that are detected in all bands and have mass >102.8 M ⊙ (53 sources), we find that by 2 Myr 75% of IR-selected star clusters have an A V ≤ 1 and that by 3 Myr the fraction rises to ∼82%. This evidence of early clearing implies that presupernova (pre-SN) feedback from massive stars is responsible for clearing the majority of the natal gas and dust that surround IR-selected star clusters in M83. Further, this result is consistent with previous estimates based on WFC3 observations and adds to the growing body of literature suggesting pre-SN feedback to be crucial for YSC emergence in normal star-forming galaxies. Finally, we find a weak correlation between the YSC concentration index and age over the first 10 Myr, which matches previous studies and indicates little or no change in the size of YSCs in M83 during their early evolution.

Dust-buried Compact Sources in the Dwarf Galaxy NGC 4449

Multiwavelength images from the Hubble Space Telescope covering the wavelength range 0.27–1.6 μm show that the central area of the nearby dwarf galaxy NGC 4449 contains several tens of compact sources that are emitting in the hydrogen recombination line Paβ (1.2818 μm) but are only marginally detected in Hα (0.6563 μm) and undetected at wavelengths λ ≤ 0.55 μm. An analysis of the spectral energy distributions (SEDs) of these sources indicates that they are likely relatively young stellar clusters heavily attenuated by dust. The selection function used to identify the sources prevents meaningful statistical analyses of their age, mass, and dust extinction distributions. However, these cluster candidates have ages ∼5–6 Myr and A V > 6 mag, according to their SED fits, and are extremely compact, with typical deconvolved radii of 1 pc. The dusty clusters are located at the periphery of the dark clouds within the galaxy and appear to be partially embedded. Density and pressure considerations indicate that the H ii regions surrounding these clusters may be stalled, and that pre-supernova (pre-SN) feedback has not been able to clear the clusters of their natal cocoons. These findings are in potential tension with existing models that regulate star formation with pre-SN feedback, since pre-SN feedback acts on short timescales, ≲4 Myr, for a standard stellar initial mass function. The existence of a population of dusty stellar clusters with ages >4 Myr, if confirmed by future observations, paints a more complex picture for the role of stellar feedback in controlling star formation.

Introducing EMP-Pathfinder: modelling the simultaneous formation and evolution of stellar clusters in their host galaxies

ABSTRACT The formation and evolution of stellar clusters is intimately linked to that of their host galaxies. To study this connection, we present the emp-Pathfindersuite of cosmological zoom-in Milky Way-mass simulations. These simulations contain a subgrid description for stellar cluster formation and evolution, allowing us to study the simultaneous formation and evolution of stellar clusters alongside their host galaxies across cosmic time. As a key ingredient in these simulations, we include the physics of the multiphase nature of the interstellar medium (ISM), which enables studies of how the presence of a cold, dense ISM affects star cluster formation and evolution. We consider two different star formation prescriptions: a constant star formation efficiency per free-fall time, as well as an environmentally dependent, turbulence-based prescription. We identify two key results drawn from these simulations. First, we find that the tidal shock-driven disruption caused by the graininess of the cold ISM produces old ($\tau \gt 10~\mbox{${\rm Gyr}$}$) stellar cluster populations with properties that are in excellent agreement with the observed populations in the Milky Way and M31. Importantly, the addition of the cold ISM addresses the areas of disagreement found in previous simulations that lacked the cold gas phase. Secondly, we find that the formation of stellar clusters is extremely sensitive to the baryonic physics that govern the properties of the cold, dense gas reservoir in the galaxy. This implies that the demographics of the stellar cluster population represent an important diagnostic tool for constraining baryonic physics models in upcoming galaxy formation simulations that also include a description of the cold ISM.

A recent major merger tale for the closest giant elliptical galaxy Centaurus A

ABSTRACT We have used hydrodynamical simulations to model the formation of the closest giant elliptical galaxy Centaurus A. We find that a single major merger event with a mass ratio of up to 1.5, and which has happened ∼2 Gyr ago, is able to reproduce many of its properties, including galaxy kinematics, the inner gas disc, stellar halo ages and metallicities, and numerous faint features observed in the halo. The elongated halo shape is mostly made of progenitor residuals deposited by the merger, which also contribute to stellar shells observed in the Centaurus A halo. The current model also reproduces the measured planetary nebula line-of-sight velocity and their velocity dispersion. Models with a small mass ratio and relatively low gas fraction result in a de Vaucouleurs profile distribution, which is consistent with observations and model expectations. A recent merger left imprints in the age distribution that are consistent with the young stellar and globular cluster populations (2–4 Gyr) found within the halo. We conclude that even if not all properties of Centaurus A have been accurately reproduced, a recent major merger has likely occurred to form the Centaurus A galaxy as we observe it at present day.

Painting a portrait of the Galactic disc with its stellar clusters

Context.The large astrometric and photometric survey performed by theGaiamission allows for a panoptic view of the Galactic disc and its stellar cluster population. Hundreds of stellar clusters were only discovered after the latestGaiadata release (DR2) and have yet to be characterised.Aims.Here we make use of the deep and homogeneousGaiaphotometry down toG= 18 to estimate the distance, age, and interstellar reddening for about 2000 stellar clusters identified withGaiaDR2 astrometry. We use these objects to study the structure and evolution of the Galactic disc.Methods.We relied on a set of objects with well-determined parameters in the literature to train an artificial neural network to estimate parameters from theGaiaphotometry of cluster members and their mean parallax.Results.We obtain reliable parameters for 1867 clusters. Our catalogue confirms the relative lack of old stellar clusters in the inner disc (with a few notable exceptions). We also quantify and discuss the variation of scale height with cluster age, and we detect the Galactic warp in the distribution of old clusters.Conclusions.This work results in a large and homogeneous cluster catalogue, allowing one to trace the structure of the disc out to distances of ∼4 kpc. However, the present sample is still unable to trace the outer spiral arm of the Milky Way, which indicates that the outer disc cluster census might still be incomplete.

The eye of Gaia on globular clusters structure: tidal tails

ABSTRACT I analyse the projected density distribution of member stars over a wide area surrounding 18 Galactic globular clusters using the photometric and astrometric information provided by the second data release of the Gaia mission. A 5D mixture modelling technique has been employed to optimally isolate the signal of the cluster stellar population from the contamination of the Galactic field, taking advantage of its different distribution in the space formed by colours, magnitudes, parallaxes, and proper motions. In 7 clusters I detect collimated overdensities at a >3σ level above the background density extending well beyond the cluster tidal radius, consistent with the distortion expected as a result of the tidal interaction with the Milky Way potential. In five of these clusters (NGC 288, NGC 2298, NGC 5139, NGC 6341, and NGC 7099) spectacular tidal tails extend up to the border of the analysed field of view at 5 deg from the centre. At large distances from the cluster centre, the orientation of the detected overdensities appears to be systematically aligned with the cluster orbital path, in agreement with the predictions of N-body simulations. The fraction of stars contained in the tidal tails of these clusters is also used to determine the first observational estimate of their present-day destruction rates.

Purveyors of fine halos

A long-lasting open question in the field of Galactic archeology refers to the size of the contribution from former globular cluster (GC) stars to the formation of the stellar halo of the Milky Way. We contribute to answering this important question by establishing observational links between the present-day halo field star population and GCs. To this end, we combined astrometric information such as space motions and parallaxes from the second data release of theGaiamission (GaiaDR2) with spectroscopic radial velocities and metallicities ([Fe/H]) from the Sloan Digital Sky Survey (SDSS-IV, DR14) to end up with a seven-dimensional chemodynamical information space for more than 3 × 105stars. Moreover, from our previous study, we incorporated the sample of halo giant stars with a distinct chemical signature (strong CN bandheads) that resembles the light-elements anomaly otherwise only seen in the second generation of globular cluster stellar populations. Using three different tagging techniques – among which is the exploration of conservative integrals of motion – we are able to establish unique associations between 151 extratidal stars in the neighborhood of eight GCs, which coincide with earlier findings of stellar envelopes beyond the tidal radius and even beyond (out to several tens of tidal radii). In addition, we trace the possible origin of about 62% of the sample of CN-strong giants to their potential host clusters. We find a connection between several of the involved GCs and the Gaia-Enceladus and Sequoia merger events. By establishing kinematic and chemical connections between 17 CN-strong stars and their surrounding fields, we identify co-moving groups of stars at the same [Fe/H] with a possible cluster origin. Some of these associations contain RR Lyrae variables, which allows meaningful distance inferences to be made. From these, we find strong evidence that four CN-strong stars and their associates are connected to the Sagittarius stream whilst their tightly confined [Fe/H] may hint to a birth site in M 54, the massive cluster in Sagittarius’ core remnant. Finally, by employing the counts of CN-strong and bona-fide CN-normal giants from our novel sample, we provide tentative estimates for the fraction of first-generation cluster stars among all stars lost to the halo. In the immediate cluster vicinity, this value amounts to 50.0 ± 16.7% while the associations in the halo field rather imply 80.2−5.2+4.9%. We speculate that – if proven real by spectroscopic follow-up – the disparity between these numbers could indicate a major contribution of low-mass clusters to the overall number of stars escaped to the halo or could alternatively suggest strong mass loss from the first generation during early cluster dissolution.

The role of cluster age on the onset of multiple populations in stellar clusters

AbstractThe origin of the chemical anomalies in star clusters is still an open question, although much effort has been employed both from a theoretical and observational point of view. The exploration of the dependence of such multiple stellar populations based on certain cluster properties (e.g. mass, age, metallicity) has represented a compelling line of investigation so far. Here I report an overview of the results obtained from our latest surveys aimed at characterising the phenomenon of chemical variations in star clusters that are much younger with respect to the ancient globular clusters. The fundamental question we are asking is whether these abundance patterns are only restricted to the old massive clusters; and if not, is there a difference between young and old objects?

Globular cluster tidal interactions and mergers in the Galactic disc

AbstractThe Galactic globular cluster system went and is still going through dynamical processes that require to be explored in detail. Here we illustrate how primordial massive globular clusters born in the Milky Way’s disc evolved by stripping material from each other or even merging very early during their lives. These processes might explain the puzzling presence of star-by-star spreads in iron content observed in massive globular clusters and should be taken into account when studying globular cluster stellar populations. In this context, we show how the direct comparison between the predictions provided by our direct N-body simulations and observations can shed light on the origin and chemo-dynamical evolution of globular clusters.

The seven sisters DANCe

Context. The photometric and astrometric measurements of the Pleiades DANCe DR2 survey provide an excellent test case for the benchmarking of statistical tools aiming at the disentanglement and characterisation of nearby young open cluster (NYOC) stellar populations. Aims. We aim to develop, test, and characterise of a new statistical tool (intelligent system) for the sifting and analysis of NYOC populations. Methods. Using a Bayesian formalism, with this statistical tool we were able to obtain the posterior distributions of parameters governing the cluster model. It also used hierarchical bayesian models to establish weakly informative priors, and incorporates the treatment of missing values and non-homogeneous (heteroscedastic) observational uncertainties. Results. From simulations, we estimated that this statistical tool renders kinematic (proper motion) and photometric (luminosity) distributions of the cluster population with a contamination rate of 5.8 ± 0.2%. The luminosity distributions and present day mass function agree with the ones found in a recent study, on the completeness interval of the survey. At the probability threshold of maximum accuracy, the classifier recovers ≈90% of the recently published candidate members and finds 10% of new ones. Conclusions. A new statistical tool for the analysis of NYOC is introduced, tested, and characterised. Its comprehensive modelling of the data properties allows it to get rid of the biases present in previous works. In particular, those resulting from the use of only completely observed (non-missing) data and the assumption of homoskedastic uncertainties. Also, its Bayesian framework allows it to properly propagate observational uncertainties into membership probabilities and cluster velocity and luminosity distributions. Our results are in a general agreement with those from the literature, although we provide the most up-to-date and extended list of candidate members of the Pleiades cluster.

On Cepheid Distance Scale Bias Due to Stellar Companions and Cluster Populations

Abstract State-of-the-art photometric measurements of extragalactic Cepheids account for the mean additional light due to chance superposition of Cepheids on crowded backgrounds through the use of artificial star measurements. However, light from stars physically associated with Cepheids may bias relative distance measurements if the changing spatial resolution along the distance ladder significantly alters the amount of associated blending. We have identified two regimes where this phenomenon may occur: Cepheids in wide binaries and in open clusters. We estimate stellar association bias using the photometric passbands and reddening-free Wesenheit magnitudes used to set up the distance scale. For wide binaries, we rely on Geneva stellar evolution models in conjunction with detailed statistics on intermediate-mass binary stars. For the impact of cluster stars, we have compiled information on the clustered Cepheid fraction and measured the typical cluster contribution in M31 via deep Hubble Space Telescope imaging provided by the Panchromatic Hubble Andromeda Treasury project. We find that the dominant effect on the distance scale comes from Cepheids in clusters, even though cluster Cepheids are a relatively rare phenomenon. Wide binaries have a negligible effect of 0.004% on H 0 for long-period Cepheids observed in the near-infrared or when considering Wesenheit magnitudes. We estimate that blending due to cluster populations has previously resulted in a 0.23% overestimate of H 0. Correcting for this bias, we obtain , which remains in 3.3σ tension with the Planck value. We conclude that stellar association bias does not constitute a limit for measuring H 0 with an accuracy of 1%.

Age as a major factor in the onset of multiple populations in stellar clusters

It is now well established that globular clusters (GCs) exhibit star-to-star light-element abundance variations (known as multiple stellar populations, MPs). Such chemical anomalies have been found in (nearly) all the ancient GCs (more than 10 Gyr old) of our Galaxy and its close companions, but so far no model for the origin of MPs is able to reproduce all the relevant observations. To gain new insights into this phenomenon, we have undertaken a photometric Hubble Space Telescope survey to study clusters with masses comparable to that of old GCs, where MPs have been identified, but with significantly younger ages. Nine clusters in the Magellanic Clouds with ages between $\sim$ 1.5-11 Gyr have been targeted in this survey. We confirm the presence of multiple populations in all clusters older than 6 Gyr and we add NGC 1978 to the group of clusters for which MPs have been identified. With an age of $\sim$ 2 Gyr, NGC 1978 is the youngest cluster known to host chemical abundance spreads found to date. We do not detect evident star-to-star variations for slightly younger massive clusters ($\sim$ 1.7 Gyr), thus pointing towards an unexpected age dependence for the onset of multiple populations. This discovery suggests that the formation of MPs is not restricted to the early Universe and that GCs and young massive clusters share common formation and evolutionary processes.

Young LMC clusters: the role of red supergiants and multiple stellar populations in their integrated light and CMDs

The optical integrated spectra of three LMC young stellar clusters (NGC 1984, NGC 1994 and NGC 2011) exhibit concave continua and prominent molecular bands which deviate significantly from the predictions of single stellar population (SSP) models. In order to understand the appearance of these spectra, we create a set of young stellar population (MILES) models, which we make available to the community. We use archival International Ultraviolet Explorer integrated UV spectra to independently constrain the cluster masses and extinction, and rule out strong stochastic effects in the optical spectra. In addition, we also analyze deep colour-magnitude diagrams of the clusters to provide independent age determinations based on isochrone fitting. We explore hypotheses including age-spreads in the clusters, a top-heavy initial mass function, different SSP models and the role of red supergiant stars (RSG). We find that the strong molecular features in the optical spectra can only be reproduced by modeling an increased fraction of about 20 per cent by luminosity of RSG above what is predicted by canonical stellar evolution models. Given the uncertainties in stellar evolution at Myr ages, we cannot presently rule-out the presence of Myr age-spreads in these clusters. Our work combines different wavelengths as well as different approaches (resolved data as well as integrated spectra for the same sample) in order to reveal the complete picture. We show that each approach provides important information but in combination can we better understand the cluster stellar populations.

A tight relation between the age distributions of stellar clusters and the properties of the interstellar medium in the host galaxy

The age distributions of stellar cluster populations have long been proposed to probe the recent formation history of the host galaxy. However, progress is hampered by the limited understanding of cluster disruption by evaporation and tidal shocks. We study the age distributions of clusters in smoothed particle hydrodynamics simulations of isolated disc galaxies, which include a self-consistent, physical model for the formation and dynamical evolution of the cluster population and account for the variation of cluster disruption in time and space. We show that the downward slope of the cluster age distribution due to disruption cannot be reproduced with a single functional form, because the disruption rate exhibits systematic trends with cluster age (the `cruel cradle effect'). This problem is resolved by using the median cluster age to trace cluster disruption. Across 120 independent galaxy snapshots and simulated cluster populations, we perform two-dimensional power law fits of the median cluster age to various macroscopic physical quantities and find that it scales as $t_{\rm med}\propto \Sigma^{-0.51\pm0.03}\sigma_{\rm 1D}^{-0.85\pm0.10}M_{\rm min}^\gamma$, for the gas surface density $\Sigma$, gas velocity dispersion $\sigma_{\rm 1D}$, and minimum cluster mass $M_{\rm min}$. This scaling accurately describes observed cluster populations and indicates disruption by impulsive tidal shocks from the interstellar medium. The term $M_{\rm min}^\gamma$ provides a model-independent way to measure the mass dependence of the cluster disruption time $\gamma$. Finally, the ensemble-average cluster lifetime depends on the gas density less strongly than the instantaneous disruption time of single clusters. These results reflect the variation of cluster disruption in time and space. We provide quantitative ways of accounting for these physics in cluster population studies.

The complex star cluster system of NGC 1316 (Fornax A)

This paper presents Gemini-$gri'$ high quality photometry for cluster candidates in the field of NGC 1316 (Fornax A) as part of a study that also includes GMOS spectroscopy. A preliminary discussion of the photometric data indicates the presence of four stellar cluster populations with distinctive features in terms of age, chemical abundance and spatial distribution. Two of them seem to be the usually old (metal poor and metal rich) populations typically found in elliptical galaxies. In turn, an intermediate-age (5 Gyr) globular cluster population is the dominant component of the sample (as reported by previous papers). We also find a younger cluster population with a tentative age of $\approx$ 1 Gyr.

THE SPATIAL DISTRIBUTION OF THE YOUNG STELLAR CLUSTERS IN THE STAR-FORMING GALAXY NGC 628

We present a study of the spatial distribution of the stellar cluster populations in the star forming galaxy NGC 628. Using Hubble Space Telescope broad band WFC3/UVIS UV and optical images from the Treasury Program LEGUS (Legacy ExtraGalactic UV Survey), we have identified 1392 potential young (<100 Myr) stellar clusters within the galaxy, identified from a combination of visual inspection and automatic selection. We investigate the clustering of these young stellar clusters and quantify the strength and change of clustering strength with scale using the two-point correlation function. We also investigate how image boundary conditions and dust lanes affect the observed clustering. The distribution of the clusters is well fit by a broken power law with negative exponent $\alpha$. We recover a weighted mean index of $\alpha$ ~ -0.8 for all spatial scales below the break at 3".3 (158 pc at a distance of 9.9 Mpc) and an index of $\alpha$ ~ -0.18 above 158 pc for the accumulation of all cluster types. The strength of the clustering increases with decreasing age and clusters older than 40 Myr lose their clustered structure very rapidly and tend to be randomly distributed in this galaxy whereas the mass of the star cluster has little effect on the clustering strength. This is consistent with results from other studies that the morphological hierarchy in stellar clustering resembles the same hierarchy as the turbulent interstellar medium.

The incidence of binaries in globular cluster stellar populations

Binary fraction and orbital characteristics provide indications on the conditions of star formation, as they shed light on the environment they were born in. Multiple systems are more common in low density environments rather than in higher density ones. In the current debate about the formation of Globular Clusters and their multiple populations, studying the binary incidence in the populations they host offers a crucial piece of information on the environment of their birth and their subsequent dynamical evolution. Through a multi-year observational campaign using FLAMES at VLT, we monitored the radial velocity of 968 Red-Giant Branch stars located around the half-light radii in a sample of 10 Galactic Globular Clusters. We found a total of 21 radial velocity variables identified as {\it bona fide} binary stars, for a binary fraction of 2.2%$\pm$0.5%. When separating the sample into first generation and second generation stars, we find a binary fraction of 4.9%$\pm$1.3% and 1.2%$\pm$0.4% respectively. Through simulations that take into account possible sources of bias in detecting radial velocity variations in the two populations, we show that the difference is significant and only marginally affected by such effects. Such a different binary fraction strongly suggests different conditions in the environment of formation and evolution of first and second generations stars, with the latter being born in a much denser environment. Our result hence strongly supports the idea that the second generation forms in a dense subsystem at the center of the loosely distributed first generation, where (loose) binaries are efficiently destroyed.

Open clusters

Fil: Aidelman, Yael Judith. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico la Plata. Instituto de Astrofisica de la Plata; Argentina. Universidad Nacional de la Plata. Facultad de Ciencias Astronomicas y Geofisicas; Argentina

The SLUGGS survey: globular cluster stellar population trends from weak absorption lines in stacked spectra

As part of the SAGES Legacy Unifying Globulars and GalaxieS (SLUGGS) survey, we stack 1137 Keck DEIMOS (Deep Imaging Multi-Object Spectrograph) spectra of globular clusters from 10 galaxies to study their stellar populations in detail. The stacked spectra have median signal-to-noise ratios of ∼90 Å−1. Besides the calcium triplet, we study weaker sodium, magnesium, titanium and iron lines as well as the Hα and higher order Paschen hydrogen lines. In general, the stacked spectra are consistent with old ages and a Milky Way-like initial mass function. However, we see different metal line index strengths at fixed colour and magnitude, and differences in the calcium triplet–colour relation from galaxy to galaxy. We interpret this as strong evidence for variations in the globular cluster colour–metallicity relation between galaxies. Two possible explanations for the colour–metallicity relation variations are that the average ages of globular clusters vary from galaxy to galaxy or that the average abundances of light elements (i.e. He, C, N and O) differ between galaxies. Stacking spectra by magnitude, we see that the colours become redder and metal line indices stronger with brighter magnitudes. These trends are consistent with the previously reported ‘blue tilts’ being mass–metallicity relations.

Probing the nuclear star cluster of galaxies with extremely large telescopes

The unprecedented sensitivity and spatial resolution of next generation ground-based extremely large telescopes (ELTs) will open a completely new window on the study of resolved stellar populations. In this paper we study the feasibility of the analysis of nuclear star cluster (NSC) stellar populations with ELTs. To date, NSC stellar population studies are based on the properties of their integrated light. NSC are in fact observed as unresolved sources even with the HST. We explore the possibility to obtain direct estimates of the age of NSC stellar populations from photometry of main-sequence turn-off stars. We simulated ELT observations of NSCs at different distances and with different stellar populations. Photometric measurements on each simulated image were analysed in detail and results about photometric accuracy and completeness are reported here. We found that the main-sequence turn-off is detectable -and therefore the age of stellar populations can be directly estimated- up to 2Mpc for old, up to 3 Mpc for intermediate-age and up to 4-5 Mpc for young stellar populations. We found that for this particular science case the performances of TMT and E-ELT are of comparable quality.