arXiv subjects

Jeremy J. Webb

Publications and source records attributed to Jeremy J. Webb.

At least 37 records · Page 2Linked to original sources

arXiv2020

The Likelihood of Undiscovered Globular Clusters in the Outskirts of the Milky Way

The currently known Galactic globular cluster population extends out to a maximum galactocentric distance of $\sim$ 145 kpc, with the peculiarity that the outermost clusters predominantly have an inward velocity. Orbit averaging finds that this configuration occurs by chance about $6\%$ of the time, suggesting that several globular clusters with positive radial velocities remain undiscovered. We evaluate the expected number of undiscovered clusters at large distances under the assumption that the cluster population has a smooth radial distribution and is in equilibrium within the Milky Way's virial radius. By comparing the present day kinematic properties of outer clusters to random orbital configurations of the Galactic globular cluster system through orbit averaging, we estimate a likelihood of $73\%$ of there being at least one undiscovered globular cluster within the Milky Way. This estimate assumes the current population is complete out to 50 kpc, and increases to $91\%$ if the population is complete out to 150 kpc. The likelihood of there being two undiscovered clusters is between $60\%$ and $70\%$, with the likelihood of there being three undiscovered clusters being on the order of $50\%$. The most likely scenario is that the undiscovered clusters are moving outwards, which results in the outer cluster population being consistent with an equilibrium state. Searches for distant and possibly quite low concentration and very low metallicity globular clusters will be enabled with upcoming deep imaging surveys.

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arXiv2020

The Initial Properties of Young Star Clusters in M83

The initial sizes and masses of massive star clusters provide information about the cluster formation process and also determine how cluster populations are modified and destroyed, which have implications for using clusters as tracers of galaxy assembly. Young massive cluster populations are often assumed to be unchanged since cluster formation, and therefore their distribution of masses and radii are used as the initial values. However, the first few hundred million years of cluster evolution does change both cluster mass and cluster radius, through both internal and external processes. In this paper, we use a large suite of $N$-body cluster simulations in an appropriate tidal field to determine the best initial mass and initial size distributions of young clusters in the nearby galaxy M83. We find that the initial masses follow a power-law distribution with a slope of -2.7 $\pm$ 0.4, and the half-mass radii follow a log-normal distribution with a mean of 2.57 $\pm$ 0.04 pc and a dispersion of 1.59 $\pm$ 0.01 pc. The corresponding initial projected half-light radius function has a mean of 2.7 $\pm$ 0.3 pc and a dispersion of 1.7 $\pm$ 0.2 pc. The evolution of the initial mass and size distribution functions are consistent with mass loss and expansion due to stellar evolution, independent of the external tidal field and the cluster's initial density profile. Observed cluster sizes and masses should not be used as the initial values, even when clusters are only a few hundred million years old.

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arXiv2020

Radial variation of the stellar mass functions in the globular clusters M15 and M30: clues of a non-standard IMF?

We exploit a combination of high-resolution Hubble Space Telescope and wide-field ESO-VLT observations to study the slope of the global mass function (alphaG) and its radial variation (alpha(r)) in the two dense, massive and post core-collapse globular clusters M15 and M30. The available data-set samples the clusters' Main Sequence down to 0.2 Msun and the photometric completeness allows the study of the mass function between 0.40 Msun and 0.75 Msun from the central regions out to their tidal radii. We find that both clusters show a very similar variation in alpha(r) as a function of clustercentric distance. They both exhibit a very steep variation in alpha(r) in the central regions, which then attains almost constant values in the outskirts. Such a behavior can be interpreted as the result of long-term dynamical evolution of the systems driven by mass-segregation and mass-loss processes. We compare these results with a set of direct N-body simulations and find that they are only able to reproduce the observed values of alpha(r) and alphaG at dynamical ages (t/trh) significantly larger than those derived from the observed properties of both clusters. We investigate possible physical mechanisms responsible for such a discrepancy and argue that both clusters might be born with a non-standard (flatter/bottom-lighter) initial mass function.

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arXiv2020

Strong chemical tagging with APOGEE: 21 candidate star clusters that have dissolved across the Milky Way disc

Chemically tagging groups of stars born in the same birth cluster is a major goal of spectroscopic surveys. To investigate the feasibility of such strong chemical tagging, we perform a blind chemical tagging experiment on abundances measured from APOGEE survey spectra. We apply a density-based clustering algorithm to the eight dimensional chemical space defined by [Mg/Fe], [Al/Fe], [Si/Fe], [K/Fe], [Ti/Fe], [Mn/Fe], [Fe/H], and [Ni/Fe], abundances ratios which together span multiple nucleosynthetic channels. In a high quality sample of 182,538 giant stars, we detect twenty-one candidate clusters with more than fifteen members. Our candidate clusters are more chemically homogeneous than a population of non-member stars with similar [Mg/Fe] and [Fe/H], even in abundances not used for tagging. Group members are consistent with having the same age and fall along a single stellar-population track in logg vs. Teff space. Each group's members are distributed over multiple kpc, and the spread in their radial and azimuthal actions increases with age. We qualitatively reproduce this increase using N-body simulations of cluster dissolution in Galactic potentials that include transient winding spiral arms. Observing our candidate birth clusters with high-resolution spectroscopy in other wavebands to investigate their chemical homogeneity in other nucleosynthetic groups will be essential to confirming the efficacy of strong chemical tagging. Our initially spatially-compact but now widely dispersed candidate clusters will provide novel limits on chemical evolution and orbital diffusion in the Galactic disc, and constraints on star formation in loosely-bound groups.

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arXiv2020

Searching for Solar Siblings in APOGEE and $Gaia$ DR2 with N-body Simulations

We make use of APOGEE and $Gaia$ data to identify stars that are consistent with being born in the same association or star cluster as the Sun. We limit our analysis to stars that match solar abundances within their uncertainties, as they could have formed from the same Giant Molecular Cloud (GMC) as the Sun. We constrain the range of orbital actions that solar siblings can have with a suite of simulations of solar birth clusters evolved in static and time-dependent tidal fields. The static components of each galaxy model are the bulge, disk, and halo, while the various time-dependent components include a bar, spiral arms, and GMCs. In galaxy models without GMCs, simulated solar siblings all have $J_R < 122$ km $\rm s^{-1}$ kpc, $990 < L_z < 1986$ km $\rm s^{-1}$ kpc, and $0.15 < J_z < 0.58$ km $\rm s^{-1}$ kpc. Given the actions of stars in APOGEE and $Gaia$, we find 104 stars that fall within this range. One candidate in particular, Solar Sibling 1, has both chemistry and actions similar enough to the solar values that strong interactions with the bar or spiral arms are not required for it to be dynamically associated with the Sun. Adding GMCs to the potential can eject solar siblings out of the plane of the disk and increase their $J_z$, resulting in a final candidate list of 296 stars. The entire suite of simulations indicate that solar siblings should have $J_R < 122$ km $\rm s^{-1}$ kpc, $353 < L_z < 2110$ km $\rm s^{-1}$ kpc, and $J_z < 0.8$ km $\rm s^{-1}$ kpc. Given these criteria, it is most likely that the association or cluster that the Sun was born in has reached dissolution and is not the commonly cited open cluster M67.

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arXiv2019

Modelling the Effects of Dark Matter Substructure on Globular Cluster Evolution with the Tidal Approximation

We present direct $N$-body simulations of tidally filling 30,000 ${\rm M}_\odot$ star clusters orbiting between 10 kpc and 100 kpc in galaxies with a range of dark matter substructure properties. The time-dependent tidal force is determined based on the combined tidal tensor of the galaxy's smooth and clumpy dark matter components, the latter of which causes fluctuations in the tidal field that can heat clusters. The strength and duration of these fluctuations are sensitive to the local dark matter density, substructure fraction, sub-halo mass function, and the sub-halo mass-size relation. Based on the cold dark matter framework, we initially assume sub-halos are Hernquist spheres following a power-law mass function between $10^5$ and $10^{11} {\rm M}_\odot$ and find that tidal fluctuations are too weak and too short to affect star cluster evolution. Treating sub-halos as point masses, to explore how denser sub-halos affect clusters, we find that only sub-halos with masses greater than $10^{6} {\rm M}_\odot$ will cause cluster dissolution times to decrease. These interactions can also decrease the size of a cluster while increasing the velocity dispersion and tangential anisotropy in the outer regions via tidal heating. Hence increased fluctuations in the tidal tensor, especially fluctuations that are due to low-mass halos, do not necessarily translate into mass loss. We further conclude that the tidal approximation can be used to model cluster evolution in the tidal fields of cosmological simulations with a minimum cold dark matter sub-halo mass of $10^{6} {\rm M}_\odot$, as the effect of lower-mass sub-halos on star clusters is negligible.

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arXiv2019

The evolution of kicked stellar-mass black holes in star cluster environments II. Rotating star clusters

In this paper, we continue our study on the evolution of black holes (BHs) that receive velocity kicks at the origin of their host star cluster potential. We now focus on BHs in rotating clusters that receive a range of kick velocities in different directions with respect to the rotation axis. We perform N-body simulations to calculate the trajectories of the kicked BHs and develop an analytic framework to study their motion as a function of the host cluster and the kick itself. Our simulations indicate that for a BH that is kicked outside of the cluster's core, as its orbit decays in a rotating cluster the BH will quickly gain angular momentum as it interacts with stars with high rotational frequencies. Once the BH decays to the point where its orbital frequency equals that of local stars, its orbit will be circular and dynamical friction becomes ineffective since local stars will have low relative velocities. After circularization, the BH's orbit decays on a longer timescale than if the host cluster was not rotating. Hence BHs in rotating clusters will have longer orbital decay times. The timescale for orbit circularization depends strongly on the cluster's rotation rate and the initial kick velocity, with kicked BHs in slowly rotating clusters being able to decay into the core before circularization occurs. The implication of the circularization phase is that the probability of a BH undergoing a tidal capture event increases, possibly aiding in the formation of binaries and high-mass BHs.

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arXiv2019

A systematic analysis of star cluster disruption by tidal shocks -- I. Controlled N-body simulations and a new theoretical model

Understanding the evolution of stellar clusters in an evolving tidal field is critical for studying the disruption of stellar clusters in a cosmological context. We systematically characterise the response of stellar clusters to tidal shocks using controlled N-body simulations of clusters with various properties that are subjected to different types of shocks. We find that the strength of the shock and the density of the cluster within the half-mass radius are the dominant properties that drive the amount of mass lost by the cluster, with the shape of the cluster profile being of minor influence. When the shock is applied as two separate sub-shocks, the amount of mass loss during the second sub-shock is sensitive to the gap time between them. Clusters that experience successive sub-shocks separated by less than their crossing time attain the same masses and sizes at the end of the simulation. However, clusters subjected to sub-shocks separated by more than a crossing time experience different evolutionary histories. The amount of mass lost in the N-body models and its scaling with shock and cluster properties differs from that predicted by classical tidal disruption theory. We demonstrate that the discrepancy is alleviated by including a dependence on the escape time-scale of unbound stars, analogously to mass loss driven by two-body relaxation. With our new theoretical model for shock-driven mass loss, the predicted relative amounts of mass loss agree with the results of the N-body simulations to ~0.3 dex across the full suite of simulations.

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arXiv2019

The orbital anisotropy profiles of nearby globular clusters from Gaia Data Release 2

Gaia Data Release 2 provides a wealth of data to study the internal structure of nearby globular clusters. We use this data to investigate the internal kinematics of 11 nearby globular clusters, with a particular focus on their poorly-studied outer regions. We apply a strict set of selection criteria to remove contaminating sources and create pure cluster-member samples over a significant fraction of the radial range of each cluster. We confirm previous measurements of rotation (or a lack thereof) in the inner regions of several clusters, while extending the detection of rotation well beyond where it was previously measured and finding a steady decrease in rotation with radius. We also determine the orbital anisotropy profile and determine that clusters have isotropic cores, are radially anisotropic out to $\approx$ 4 half-light radii or $35\%$ of their limiting radii, and are then isotropic out to the limits of our datasets. We detect for the first time the presence of radial anisotropy in M 22, while confirming previous detections of radial anisotropy in 47 Tuc, M 3, M 13, M 15, and $ω$ Cen's innermost regions. The implications of these measurements are that clusters can be separated into two categories: 1) clusters with observed radial anisotropy that likely formed tidally under-filling or are dynamically young, and 2) clusters that are primarily isotropic that likely formed tidally filling or are dynamically old.

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arXiv2019

Searching for the GD-1 Stream Progenitor in Gaia DR2 with Direct N-body Simulations

We perform a large suite of direct N-body simulations aimed at revealing the location of the progenitor, or its remnant, of the GD-1 stream. Data from \gaia\ DR2 reveals the GD-1 stream extends over $\approx 100^\circ$, allowing us to determine the stream's leading and trailing ends. Our models suggest the length of the stream is consistent with a dynamical age of between 2-3 Gyr and the exact length, width and location of the GD-1 stream correspond to the stream's progenitor being located between $-30^\circ < ϕ_{1,\mathrm{pro}} < -45^{\circ}$ in the standard GD-1 coordinate system. The model stream density profiles reveal that intact progenitors leave a strong over-density, recently-dissolved progenitors appear as gaps in the stream as escaped stars continue to move away from the remnant progenitor's location, and long-dissolved progenitors leave no observational signature on the remaining stream. Comparing our models to the GD-1 stream yields two possible scenarios for its progenitor's history: a) the progenitor reached dissolution approximately 500 Myr ago during the cluster's previous perigalactic pass and is both located at and responsible for the observed gap at $ϕ_1=-40^{\circ}$ or b) the progenitor reached dissolution over 2.5 Gyr ago, the fully-dissolved remnant is at $-30^\circ < ϕ_1 < -45^{\circ}$, and an observational signature of its location no longer exists. That the dissolved progenitor is in the range $-30^\circ < ϕ_1 < -45^{\circ}$ implies that density fluctuations outside of this range, e.g., a deep gap at $ϕ_1 \approx -20^\circ$, are likely produced by compact baryonic or dark-matter perturbers.

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arXiv2019

Rediscovering the Tidal Tails of NGC 288 with Gaia DR2

NGC 288 is a Galactic globular cluster having observed extra tidal structure, without confirmed tidal tails. $Gaia$ DR2 provides photometric and astrometric data for many of the stars in NGC 288 and its extra tidal structure. To compare with $Gaia$ data, we simulate an $N$-body model of a star cluster with the same orbit as NGC 288 in a Milky Way potential. The simulation shows that the cluster forms tidal tails that are compressed along the cluster's orbit when it is at apocentre and are expected to be a diffuse bipolar structure. In this letter, we present a comparison between the simulation and observations from $Gaia$ DR2. We find that both the simulation and the observations share comparable trends in the position on the sky and proper motions of the extra-tidal stars, supporting the presence of tidal tails around NGC 288.

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arXiv2018

Spatial mixing of binary stars in multiple-population globular clusters

We present the results of a study aimed at investigating the effects of dynamical evolution on the spatial distribution and mixing of primordial binary stars in multiple-population globular clusters. Multiple stellar population formation models predict that second-generation (SG) stars form segregated in the inner regions of a more extended first-generation (FG) cluster. Our study, based on the results of a survey of N-body simulations, shows that the spatial mixing process for binary stars is more complex than that of single stars since additional processes such as binary ionization, recoil and ejection following binary-single and binary-binary interactions play a key role in determining the spatial distribution of the population of surviving binaries. The efficiency and relative importance of these additional effects depends on the binary binding energy and determines the timescale of the spatial mixing of FG and SG binaries. Our simulations illustrate the role of ionization, recoil and ejection combined with the effects of mass segregation driven by two-body relaxation and show that the complex interplay of all these processes results in a significant extension of the time needed for the complete spatial mixing of FG and SG binaries compared to that of single stars. Clusters in which FG and SG single stars have already reached complete spatial mixing might be characterized by a significant radial gradient in the ratio of the FG-to-SG binary fraction. The implications of the delayed mixing of FG and SG binaries for the differences between the kinematics of the two populations are discussed.

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arXiv2018

The Structural and Kinematic Evolution of Central Star Clusters in Dwarf Galaxies and Their Dependence on Dark Matter Halo Profiles

Through a suite of direct N-body simulations, we explore how the structural and kinematic evolution of a star cluster located at the center of a dwarf galaxy is affected by the shape of its host's dark matter density profile. The stronger central tidal fields of cuspier halos minimize the cluster's ability to expand in response to mass loss due to stellar evolution during its early evolutionary stages and during its subsequent long-term evolution driven by two-body relaxation. Hence clusters evolving in cuspier dark matter halos are characterized by more compact sizes, higher velocity dispersions and remain approximately isotropic at all clustercentric distances. Conversely, clusters in cored halos can expand more and develop a velocity distribution profile that becomes increasingly radially anisotropic at larger clustercentric distances. Finally, the larger velocity dispersion of clusters evolving in cuspier dark matter profiles results in them having longer relaxation times. Hence clusters in cuspy galaxies relax at a slower rate and, consequently, they are both less mass segregated and farther from complete energy equipartition than cluster's in cored galaxies. Application of this work to observations allows for star clusters to be used as tools to measure the distribution of dark matter in dwarf galaxies and to distinguish isolated star clusters from ultra-faint dwarf galaxies.

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arXiv2018

Evolution of the Stellar Mass Function in Multiple-Population Globular Clusters

We present the results of a survey of N-body simulations aimed at studying the effects of the long-term dynamical evolution on the stellar mass function (MF) of multiple stellar populations in globular clusters. Our simulations show that if first-(1G) and second-generation (2G) stars have the same initial MF (IMF), the global MFs of the two populations are affected similarly by dynamical evolution and no significant differences between the 1G and the 2G MFs arise during the cluster's evolution. If the two populations have different IMFs, dynamical effects do not completely erase memory of the initial differences. Should observations find differences between the global 1G and 2G MF, these would reveal the fingerprints of differences in their IMFs. Irrespective of whether the 1G and 2G populations have the same global IMF or not, dynamical effects can produce differences between the local (measured at various distances from the cluster centre) 1G and 2G MFs; these differences are a manifestation of the process of mass segregation in populations with different initial structural properties. In dynamically old and spatially mixed clusters, however, differences between the local 1G and 2G MFs can reveal differences between the 1G and 2G global MFs. In general, for clusters with any dynamical age, large differences between the local 1G and 2G MFs are more likely to be associated with differences in the global MF. Our study also reveals a dependence of the spatial mixing rate on the stellar mass, another dynamical consequence of the multiscale nature of multiple-population clusters.

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arXiv2017

The Evolution of Kicked Stellar-Mass Black Holes in Star Cluster Environments

We consider how dynamical friction acts on black holes that receive a velocity kick while located at the center of a gravitational potential, analogous to a star cluster, due to either a natal kick or the anisotropic emission of gravitational waves during a black hole-black hole merger. Our investigation specifically focuses on how well various Chandrasekhar-based dynamical friction models can predict the orbital decay of kicked black holes with $m_{bh} \lesssim 100 M_\odot$ due to an inhomogeneous background stellar field. In general, the orbital evolution of a kicked black hole follows that of a damped oscillator where two-body encounters and dynamical friction serve as sources of damping. However, we find models for approximating the effects of dynamical friction do not accurately predict the amount of energy lost by the black hole if the initial kick velocity $v_{k}$ is greater than the stellar velocity dispersion $σ$. For all kick velocities, we also find that two-body encounters with nearby stars can cause the energy evolution of a kicked BH to stray significantly from standard dynamical friction theory as encounters can sometimes lead to an energy gain. For larger kick velocities, we find the orbital decay of a black hole departs from classical theory completely as the black hole's orbital amplitude decays linearly with time as opposed to exponentially. Therefore, we have developed a linear decay formalism which scales linearly with black hole mass and $\frac{v_{k}}σ$ in order to account for the variations in the local gravitational potential.

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arXiv2017

Modelling the Observed Stellar Mass Function and its Radial Variation in Galactic Globular Clusters

We measure how the slope $α$ of the stellar mass function (MF) changes as a function of clustercentric distance $r$ in five Galactic globular clusters and compare $α(r)$ to predictions from direct $N$-body star cluster simulations. Theoretical studies predict that $α(r)$ (which traces the degree of mass segregation in a cluster) should steepen with time as a cluster undergoes two-body relaxation and that the amount by which the global MF can evolve from its initial state due to stellar escape is directly linked to $α(r)$. We find that the amount of mass segregation in M10, NGC 6218, and NGC 6981 is consistent with their dynamical ages, but only the global MF of M10 is consistent with its degree of mass segregation as well. NGC 5466 and NGC 6101 on the other hand appear to be less segregated than their dynamical ages would indicate. Furthermore, despite the fact that the escape rate of stars in non-segregated clusters is independent of stellar mass, both NGC 5466 and NGC 6101 have near-flat MFs. We discuss various mechanisms which could produce non-segregated clusters with near-flat MFs, including higher mass-loss rates and black hole retention, but argue that for some clusters (NGC 5466 and NGC 6101) explaining the present-day properties might require either a non-universal IMF or a much more complex dynamical history.

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arXiv2017

The Early Evolution of Star Clusters in Compressive and Extensive Tidal Fields

We present N-body simulations of star clusters that initially evolve within a strong compressive tidal field and then transition into extensive tidal fields of varying strengths. While subject to compressive tides, clusters can undergo significant heating due to two-body interactions and mass loss due to stellar evolution. When the cluster transitions into an extensive tidal field it is super-virialized, which leads to a rapid expansion and significant mass loss before the cluster reaches virial equilibrium. After the transition, clusters are significantly less massive, more extended and therefore more tidally filling than clusters which have spent their entire lifetime in a similar extensive tidal field.

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arXiv2016

On the Link Between Energy Equipartition and Radial Variation in the Stellar Mass Function of Star Clusters

We make use of $N$-body simulations to determine the relationship between two observable parameters that are used to quantify mass segregation and energy equipartition in star clusters. Mass segregation can be quantified by measuring how the slope of a cluster's stellar mass function $α$ changes with clustercentric distance r, and then calculating $δ_α= \frac{d α(r)}{d ln(r/r_m)}$ where $r_m$ is the cluster's half-mass radius. The degree of energy equipartition in a cluster is quantified by $η$, which is a measure of how stellar velocity dispersion $σ$ depends on stellar mass m via $σ(m) \propto m^{-η}$. Through a suite of $N$-body star cluster simulations with a range of initial sizes, binary fractions, orbits, black hole retention fractions, and initial mass functions, we present the co-evolution of $δ_α$ and $η$. We find that measurements of the global $η$ are strongly affected by the radial dependence of $σ$ and mean stellar mass and the relationship between $η$ and $δ_α$ depends mainly on the cluster's initial conditions and the tidal field. Within $r_m$, where these effects are minimized, we find that $η$ and $δ_α$ initially share a linear relationship. However, once the degree of mass segregation increases such that the radial dependence of $σ$ and mean stellar mass become a factor within $r_m$, or the cluster undergoes core collapse, the relationship breaks down. We propose a method for determining $η$ within $r_m$ from an observational measurement of $δ_α$. In cases where $η$ and $δ_α$ can be measured independently, this new method offers a way of measuring the cluster's dynamical state.

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