FEAST: Probing Hierarchical Star Formation with the Spatial Distributions of Young Star Clusters

In this paper, we present photometry for young star clusters in M31, which are selected from Caldwell et al. These star clusters have been observed as part of the Beijing--Arizona--Taiwan--Connecticut (BATC) Multicolor Sky Survey from 1995 February to 2008 March. The BATC images including these star clusters are taken with 15 intermediate-band filters covering 3000--10000 \AA. Combined with photometry in the {\sl GALEX} far- and near-ultraviolet, broad-band $UBVRI$, SDSS $ugriz$, and infrared $JHK_{\rm s}$ of Two Micron All Sky Survey, we obtain their accurate spectral energy distributions (SEDs) from 1538-20000 \AA. We derive these star clusters' ages and masses by comparing their SEDs with stellar population synthesis models. Our results are in good agreement with previous determinations. The mean value of age and mass of young clusters ($<2$ Gyr) is about 385 Myr and $2\times 10^4 {M_\odot}$, respectively. There are two distinct peaks in the age distribution, a highest peak at age $\sim$ 60 Myr and a secondary peak around 250 Myr, while the mass distribution shows a single peak around $10^4 {M_\odot}$. A few young star clusters have two-body relaxation times greater than their ages, indicating that those clusters have not been well dynamically relaxed and therefore have not established the thermal equilibrium. There are several regions showing aggregations of young star clusters around the 10 kpc ring and the outer ring, indicating that the distribution of the young star clusters is well correlated with M31's star-forming regions. The young massive star clusters (age $\leq 100$ Myr and mass $\geq 10^4 {M_\odot}$) show apparent concentration around the ring splitting region, suggesting a recent passage of a satellite galaxy (M32) through M31 disk.

We use the angular two-point correlation function (TPCF) to investigate the hierarchical distribution of young star clusters in 12 local (3–18 Mpc) star-forming galaxies using star cluster catalogs obtained with the Hubble Space Telescope (HST) as part of the Treasury Program Legacy ExtraGalactic UV Survey. The sample spans a range of different morphological types, allowing us to infer how the physical properties of the galaxy affect the spatial distribution of the clusters. We also prepare a range of physically motivated toy models to compare with and interpret the observed features in the TPCFs. We find that, conforming to earlier studies, young clusters ($T \lesssim 10\, \mathrm{Myr}$) have power-law TPCFs that are characteristic of fractal distributions with a fractal dimension D2, and this scale-free nature extends out to a maximum scale lcorr beyond which the distribution becomes Poissonian. However, lcorr, and D2 vary significantly across the sample, and are correlated with a number of host galaxy physical properties, suggesting that there are physical differences in the underlying star cluster distributions. We also find that hierarchical structuring weakens with age, evidenced by flatter TPCFs for older clusters ($T \gtrsim 10\, \mathrm{Myr}$), that eventually converges to the residual correlation expected from a completely random large-scale radial distribution of clusters in the galaxy in $\sim 100 \, \mathrm{Myr}$. Our study demonstrates that the hierarchical distribution of star clusters evolves with age, and is strongly dependent on the properties of the host galaxy environment.

Context. The Vela Molecular Ridge hosts a number of young embedded star clusters in the same evolutionary stage. Aims. The main aim of the present work is testing whether the fraction of members with a circumstellar disk in a sample of clusters in the cloud D of the Vela Molecular Ridge, is consistent with relations derived for larger samples of star clusters with an age spread. Besides, we want to constrain the age of the young embedded star clusters associated with cloud D. Methods. We carried out L (3.78 microns) photometry on images of six young embedded star clusters associated with cloud D of the Vela Molecular Ridge, taken with ISAAC at the VLT. These data are complemented with the available HKs photometry. The 6 clusters are roughly of the same size and appear to be in the same evolutionary stage. The fraction of stars with a circumstellar disk was measured in each cluster by counting the fraction of sources displaying a NIR excess in colour-colour (HKsL) diagrams. Results. The L photometry allowed us to identify the NIR counterparts of the IRAS sources associated with the clusters. The fraction of stars with a circumstellar disk appears to be constant within errors for the 6 clusters. There is a hint that this is lower for the most massive stars. The age of the clusters is constrained to ~1-2 Myr. Conclusions. The fraction of stars with a circumstellar disk in the observed sample is consistent with the relations derived from larger samples of star clusters and with other age estimates for cloud D. The fraction may be lower for the most massive stars. Our results agree with a scenario where all intermediate and low-mass stars form with a disk, whose lifetime is shorter for higher mass stars.

Among the many mysteries of our Universe, one still unanswered question is how globular clusters form. Globular clusters are very dense agglomerates of hundreds of thousands of stars and they host some of the oldest known stars in our Universe. Since they are luminous, old and found in all massive galaxies, they are a fundamental piece of the puzzle to understand galaxy formation and evolution processes. Traditionally, globular clusters were thought to be simple stellar systems, in which all stars were born at the same time and have the same chemical composition. %Therefore, globular clusters have been considered the perfect laboratory to study how stars evolve. However, in the last few decades, it has been shown that stars within a given globular cluster display inhomogeneities in their chemistry. Every massive old globular cluster located in the Milky Way, for which high precision and deep observations were obtained, was found to host several different stellar populations, i.e. multiple populations. Each stellar population is characterized by specific chemical patterns observed in the atmospheres of individual stars. Only certain elements are found to vary, and they do not do so randomly, but rather the variations are observed to correlate between the elements. The stellar population that has enhanced nitrogen (N) content, also has enhanced sodium and helium abundances but has a depletion in carbon and oxygen, to cite a few examples. At the same time, the iron content is found to be constant among the different populations. Such chemical patterns are often called anomalies. More interestingly, it seems like such chemical anomalies are unique to globular cluster systems, i.e. dense stellar systems, since they are basically not found in other stars located in the field. Knowing how such multiple populations form and how they impact the evolution of globular clusters is crucial to understand the formation of stars and clusters themselves and, more broadly, the formation and evolution of galaxies. Many theoretical scenarios have been proposed to explain the origin of the chemical anomalies in globular clusters. Most models treat the origin of this phenomenon as multiple events of star formation. In such models, a first generation of stars forms from the collapse of a giant molecular cloud which is homogeneous in its chemical composition. The winds of the massive stars from this first generation sink in the centre of the cluster to collapse and provide material for a second generation of stars, which then forms with a different chemical composition. While theoretically straightforward, such scenarios (which involve many types of massive stars) fail in reproducing many of the observed properties of multiple populations in globular clusters. Hence, the formation mechanism for the origin of multiple populations remains an open question. Most studies of multiple populations focused only on ancient globular clusters, aged up to $\sim$13 Gyr. However, many dense and massive younger star clusters are observed in nearby galaxies. Is the multiple populations phenomenon limited to the ancient globular clusters, i.e. could this be a cosmological effect? The goal of this thesis has been expanding the search for multiple populations to star clusters that are significantly younger than the old globular clusters, i.e. up to 10 times younger. Indeed, a compelling line of investigation is to look for multiple populations depending on certain global properties of the clusters, such as age, mass, metallicity. The first major result presented in this work is that multiple populations are found also in the young clusters, down to $\sim$2 Gyr old objects, showing that the phenomenon of multiple populations is not only restricted to the early Universe. Another interesting result I report is that the extent of the multiple populations (in chemical abundance spread) is a strong function of age, with older clusters having larger chemical variations. Additionally, I show that there is no difference in age between the populations in a young star cluster. Such results represent fundamental constraints for the origin of multiple populations and might point towards a new and fresh direction into the onset of this complex phenomenon. An important and related question is whether the young massive star clusters are the same type of stellar systems as the ancient globular clusters, just observed at a different stage of their lifetimes. If confirmed, this could provide important constraints on star cluster formation studies. Therefore, in this thesis I explored clusters at younger ages in order to address the fundamental question whether the star (and cluster) formation conditions were different in the early Universe. The results presented here represent an important hint that ancient and young clusters share the same origin and are only separated in age. I show that star clusters do not require special conditions in which to form, so that they can be used as tracers for the formation and evolution of galaxies.

Dynamical few-body encounters in the dense cores of young massive star clusters are responsible for the loss of a significant fraction of their massive stellar content. Some of the escaping (runaway) stars move through the ambient medium supersonically and can be revealed via detection of their bow shocks (visible in the infrared, optical or radio). In this paper, which is the second of a series of papers devoted to the search for OB stars running away from young ( < several Myr) Galactic clusters and OB associations, we present the results of the search for bow shocks around the star-forming region NGC 6357. Using the archival data of the Midcourse Space Experiment (MSX) satellite and the Spitzer Space Telescope, and the preliminary data release of the Wide-Field Infrared Survey Explorer (WISE), we discovered seven bow shocks, whose geometry is consistent with the possibility that they are generated by stars expelled from the young (1-2 Myr) star clusters, Pismis 24 and AH03 J1725-34.4, associated with NGC 6357. Two of the seven bow shocks are driven by the already known OB stars, HD 319881 and [N78] 34. Follow-up spectroscopy of three other bow-shockproducing stars showed that they are massive (O-type) stars as well, while the 2MASS photometry of the remaining two stars suggests that they could be B0 V stars, provided that both are located at the same distance as NGC 6357. Detection of numerous massive stars ejected from the very young clusters is consistent with the theoretical expectation that star clusters can effectively lose massive stars at the very beginning of their dynamical evolution (long before the second mechanism for production of runaway stars, based on a supernova explosion in a massive tight binary system, begins to operate) and lends strong support to the idea that probably all field OB stars have been dynamically ejected from their birth clusters. A by-product of our search for bow shocks around NGC 6357 is the detection of three circular shells typical of luminous blue variable and late WN-type Wolf-Rayet stars.

The spectral energy distribution analysis of very young unresolved star clusters challenges our understanding of the cluster formation process. Studies of resolved massive clusters in the Milky Way and in the nearby Magellanic Clouds show us that the contribution from photoionized gas is very important during the first Myr of cluster evolution. We present our models which include both a self-consistent treatment of the photoionized gas and the stellar continuum and quantify the impact of such nebular component on the total flux of young unresolved star clusters. A comparison with other available models is considered. The very young star clusters in the SBS 0335-052E dwarf starburst galaxy are used as a test for our models. Due to the low metallicity of the galactic medium our models predict a longer lasted nebular phase which contributes between 10-40% of the total near infrared (NIR) fluxes at around 10 Myr. We propose thus a possible solution for the observed flux excess in the 6 bright super star clusters of SBS 0335-052E. Reines et al. showed that the observed cluster fluxes, in the red-optical and NIR range, sit irreconcilably above the provided stellar continuum models. We find that in the age range estimated from the H_alpha emission we can explain the red excess in all the 6 super star clusters as due to nebular emission, which at cluster ages around 10 Myr still affects the NIR wavebands substantially.

GW190521 is the most massive binary black hole (BBH) merger observed to date, and its primary component lies in the pair-instability (PI) mass gap. Here, we investigate the formation of GW190521-like systems via three-body encounters in young massive star clusters. We performed 2 × 105 simulations of binary-single interactions between a BBH and a massive $\ge {60}\,$M⊙ black hole (BH), including post-Newtonian terms up to the 2.5 order and a prescription for relativistic kicks. In our initial conditions, we take into account the possibility of forming BHs in the PI mass gap via stellar collisions. If we assume that first-generation BHs have low spins, $\sim {0.17}{{\ \rm per\ cent}}$ of all the simulated BBH mergers have component masses, effective and precessing spin, and remnant mass and spin inside the $90{{\ \rm per\ cent}}$ credible intervals of GW190521. Seven of these systems are first-generation exchanged binaries, while five are second-generation BBHs. We estimate a merger rate density $\mathcal {R}_{\rm GW190521}\sim {0.03}\,$Gpc$^{-3}\,$yr−1 for GW190521-like binaries formed via binary-single interactions in young star clusters. This rate is extremely sensitive to the spin distribution of first-generation BBHs. Stellar collisions, second-generation mergers and dynamical exchanges are the key ingredients to produce GW190521-like systems in young star clusters.

We report on a multiwavelength study of the relationship between young star clusters in the Antennae galaxies (NGC 4038/9) and their interstellar environment, with the goal of understanding the formation and feedback effects of star clusters in merging galaxies. This is possible for the first time because various new observations (from X-rays to radio wavelengths) have become available in the past several years. Quantitative comparisons are made between the positions of the star clusters (broken into three age groups) and the properties of the interstellar medium by calculating the two-point correlation functions. We find that young star clusters are distributed in a clustered fashion, demonstrated by power-law angular autocorrelation functions with slopes in the range -0.8 to -1.0. The young embedded clusters (ages ~5 Myr) are found to be more associated with long-wavelength radiation (mid-infrared and longer), while clusters with ages ~10 Myr or older are more associated with short-wavelength radiation (e.g., far-UV and X-ray). The youngest star clusters are associated with molecular cloud complexes with characteristic radii of about 1 kpc. In addition, there is a weak tendency for them to be found in regions with higher H I velocity dispersions. There is some evidence that both cloud-cloud collisions and shocks from recent star formation can trigger star cluster formation, but no dominant triggering mechanism is identified for the majority of the clusters in the Antennae. Feedback from young bright cluster complexes reveals itself in the form of large Hα bubbles and Hα velocity gradients in shells around the complexes. We estimate the current star formation rate to be ≈20 M☉ yr-1 and the gas consumption timescale to be ~700 Myr. The latter is comparable to the merging timescale and indicates that star formation has been enhanced by the merger event. Finally, we find that the Schmidt law, with index N ≈ -1.4, is also a good description of the cluster formation triggered by merging in the Antennae. There is some evidence that feedback effects may modify the Schmidt law at scales below 1 kpc.

In the hierarchical view of star formation, giant molecular clouds (GMCs) undergo fragmentation to form small-scale structures made up of stars and star clusters. Here we study the connection between young star clusters and cold gas across a range of extragalactic environments by combining the high resolution (1″) PHANGS–ALMA catalogue of GMCs with the star cluster catalogues from PHANGS–HST. The star clusters are spatially matched with the GMCs across a sample of 11 nearby star-forming galaxies with a range of galactic environments (centres, bars, spiral arms, etc.). We find that after 4 − 6 Myr the star clusters are no longer associated with any gas clouds. Additionally, we measure the autocorrelation of the star clusters and GMCs as well as their cross-correlation to quantify the fractal nature of hierarchical star formation. Young (≤10 Myr) star clusters are more strongly autocorrelated on kpc and smaller spatial scales than the $\gt \, 10$ Myr stellar populations, indicating that the hierarchical structure dissolves over time.

Young star clusters like R136 in the Large Magellanic Cloud and NGC 3603, Westerlund 1, and 2 in the Milky Way are dynamically more evolved than expected based on their current relaxation times. In particular, the combination of a high degree of mass segregation, a relatively low central density, and the large number of massive runaway stars in their vicinity are hard to explain with the monolithic formation of these clusters. Young star clusters can achieve such a mature dynamical state if they formed through the mergers of a number of less massive clusters. The shorter relaxation times of less massive clusters cause them to dynamically evolve further by the time they merge, and the merger product preserves the memory of the dynamical evolution of its constituent clusters. With a series of $N$-body simulations, we study the dynamical evolution of single massive clusters and those that are assembled through merging smaller clusters together. We find that the formation of massive star clusters through the mergers of smaller clusters can reproduce the currently observed spatial distribution of massive stars, the density, and the characteristics (number and mass distribution) of the stars ejected as runaways from young dense clusters. We therefore conclude that these clusters and possibly other young massive star clusters formed through the mergers of smaller clusters.

I present spectroscopic imaging of the high-velocity system (HVS) towards the central cD galaxy (NGC 1275) in the Perseus Cluster at a high spectral resolution for the first time. Previous observation suggests that the HVS is a highly inclined dusty and gas-rich galaxy moving towards the center of NGC 1275 at a high speed of 3000 km/s relative to the systemic velocity of NGC 1275 through the hot intracluster medium (ICM). If this is the case, then the HVS should be undergoing intense ram-pressure stripping. However, there is tentative evidence for ram-pressure stripping in the HVS, and furthermore confined to a small region of the galaxy. Previous observations also point out that at the location where the HVS is seen, there are many star clusters seen towards the inner region of NGC 1275. The separation of young star clusters between those belong to NGC 1275 and those belong to the HVS is, however, not clearly defined.&#13;\n&#13;\nThe primary scientific objectives are to (i) search for evidence for ram-pressure stripping in the HVS, as well as signs of tidal interactions between the HVS and NGC 1275; and (ii) separate the numerous young star clusters seen towards the entire NGC 1275 into those associated with the HVS and those associated with NGC 1275. NGC 1275 and the HVS were observed simultaneously with the use of Potsdam Multi-Aperture Spectrophotometer. The main emission lines being studied are the Hα &amp; [NII]λ6548,6483 lines in NGC 1275 and the HVS. I present maps of intensity distribution, velocity field and velocity dispersion of the Hαemission of the HVS, as well as the line ratio of the [NII] doublets lines to the Hα line in the HVS.&#13;\n&#13;\nI find that the line ratio of [NII]/Hα is less than 0.1 throughout the entire body of the HVS, indicating metallicity is low in the HVS. I also find that the metallicity is decreasing with distance from the center, just like other normal spiral galaxies. I demonstrate that a large fraction of the young star clusters seen towards the inner regions of NGC 1275 are closely associated with bright Hα-emitting regions in the HVS, and trace the overall Hα-emitting body of the HVS, suggesting that some young star clusters are associated to the HVS. I find that there are two distributions of young star clusters in color-color space, providing a way to separate out the star clusters likely belong to the HVS. I present evidence that the HVS is experiencing intense ram-pressure stripping and also evidence suggesting that the HVS is possibly tidally interacting with NGC 1275.&#13;\n&#13;\nThe results demonstrate that the HVS is a dusty, gas-rich, low-metallicity galaxy that has been disrupted by ram-pressure stripping and possibly also tidal interactions. I show that the HVS exhibit widespread and vigorous (~3.6 MM_⊙ yr^(-1)) star formation over the last at least ~0.1 Gyr. The vigorous SFR of the HVS is in contrast to what suggested by the observed low metallicity (suggestive of relatively weak star-formation activity over the recent history). The SFR of the HVS is likely to be triggered by the same process that produces global distortion on the HVS, here ram pressure stripping and possibly tidal interaction are in consideration.

Aims. To understand whether there is a difference in the dispersion of discs around stars in high-density young stellar clusters like the Orion Nebula Cluster (ONC) according to the mass of the star. Methods. Two types of simulations were combined – N -body simulations of the dynamics of the stars in the ONC and mass loss results from simulations of star-disc encounters, where the disc mass loss of all stars is determined as a function of time. Results. We find that in the Trapezium, the discs around high-mass stars are dispersed much more quickly and to a larger degree by their gravitational interaction than for intermediate-mass stars. This is consistent with recent observations of IC 348, where a higher disc frequency was found around solar mass stars than for more massive stars, suggesting that this might be a general trend in large young stellar clusters.

ABSTRACTWe explore hierarchical black hole (BH) mergers in nuclear star clusters (NSCs), globular clusters (GCs) and young star clusters (YSCs), accounting for both original and dynamically assembled binary BHs (BBHs). We find that the median mass of both first- and nth-generation dynamical mergers is larger in GCs and YSCs with respect to NSCs because the lighter BHs are ejected by supernova kicks from the lower mass clusters. Also, first- and nth-generation BH masses are strongly affected by the metallicity of the progenitor stars: the median mass of the primary BH of a nth-generation merger is ∼24–38 M⊙ (∼9–15 M⊙) in metal-poor (metal-rich) NSCs. The maximum BH mass mainly depends on the escape velocity: BHs with mass up to several thousand M⊙ form in NSCs, while YSCs and GCs host BHs with mass up to several hundred M⊙. Furthermore, we calculate the fraction of mergers with at least one component in the pair-instability mass gap (fPI) and in the intermediate-mass BH regime (fIMBH). In the fiducial model for dynamical BBHs with metallicity Z = 0.002, we find fPI ≈ 0.05, 0.02 and 0.007 (fIMBH ≈ 0.01, 0.002 and 0.001) in NSCs, GCs and YSCs, respectively. Both fPI and fIMBH drop by at least one order of magnitude at solar metallicity. Finally, we investigate the formation of GW190521 by assuming that it is either a nearly equal-mass BBH or an intermediate-mass ratio inspiral.

We exploit the superb resolution of the new HST/ACS mosaic image of M51 to select a large sample of young (< 1 Gyr) star clusters in the spiral disk, based on their sizes. The image covers the entire spiral disk in B, V, I and H_alpha, at a resolution of 2 pc per pixel. The surface density distribution of 4357 resolved clusters shows that the clusters are more correlated with clouds than with stars, and we find a hint of enhanced cluster formation at the corotation radius. The radius distribution of a sample of 769 clusters with more accurate radii suggests that young star clusters have a preferred effective radius of ~3 pc, which is similar to the preferred radius of the much older GCs. However, in contrast to the GCs, the young clusters in M51 do not show a relation between radius and galactocentric distance. This means that the clusters did not form in tidal equilibrium with their host galaxy, nor that their radius is related to the ambient pressure.

Context. The environments of young star clusters are shaped by the interactions of the powerful winds of massive stars and their feedback on the cluster birth cloud. Several young star clusters show diffuse γ-ray emission on the degree scale, which hints at ongoing particle acceleration. Aims. To date, particle acceleration and transport in star-cluster environments are not well understood. A characterisation of magnetic fields and flow structures is necessary to progress towards physical models. Previous work has largely focused on 100 pc scale feedback or detailed modelling of wind interaction of just a few stars. We aim to bridge this gap. We focus in particular on compact clusters in order to study collective effects arising from stellar-wind interaction. Objects in this class include Westerlund 1 and R136. Methods. We performed 3D ideal-magnetohydrodynamics simulations of compact young massive star clusters. We kinetically injected stellar winds for 46 individual very massive stars (M &gt; 40 M⊙) distributed in a spherical region of radius ≤1 pc. We included a sub-population of five magnetic stars with increased dipole field strengths of 0.1–1 kG, and we studied the evolving superbubble over several hundred thousand years. Results. The bulk flow and magnetic fields show an intricate non-uniform morphology that is critically impacted by the relative position of individual stars. The cluster wind terminates in a strong shock that is non-spherical, and similar to the flow, it has non-uniform properties. The magnetic field is composed of both highly tangled sections and coherent quasi-radial field-line bundles. Steep particle spectra in the teraelectronvolt domain arise naturally from the variation of magnetic field magnitude over the cluster-wind termination shock. This finding is consistent with γ-ray observations. We deem the scenario of petaelectronvolt particle acceleration as unlikely.