arXiv subjects

William Harris

Publications and source records attributed to William Harris.

At least 19 recordsLinked to original sources

arXiv2025

Dynamically Extensible and Retractable Robotic Leg Linkages for Multi-task Execution in Search and Rescue Scenarios

Search and rescue (SAR) robots are required to quickly traverse terrain and perform high-force rescue tasks, necessitating both terrain adaptability and controlled high-force output. Few platforms exist today for SAR, and fewer still have the ability to cover both tasks of terrain adaptability and high-force output when performing extraction. While legged robots offer significant ability to traverse uneven terrain, they typically are unable to incorporate mechanisms that provide variable high-force outputs, unlike traditional wheel-based drive trains. This work introduces a novel concept for a dynamically extensible and retractable robot leg. Leveraging a dynamically extensible and retractable five-bar linkage design, it allows for mechanically switching between height-advantaged and force-advantaged configurations via a geometric transformation. A testbed evaluated leg performance across linkage geometries and operating modes, with empirical and analytical analyses conducted on stride length, force output, and stability. The results demonstrate that the morphing leg offers a promising path toward SAR robots that can both navigate terrain quickly and perform rescue tasks effectively.

cs.RO↗

arXiv2025

Candidate Dark Galaxy-2: Validation and Analysis of an Almost Dark Galaxy in the Perseus Cluster

Candidate Dark Galaxy-2 (CDG-2) is a potential dark galaxy consisting of four globular clusters (GCs) in the Perseus cluster, first identified in Li et al. (2025) through a sophisticated statistical method. The method searched for over-densities of GCs from a \textit{Hubble Space Telescope} (\textit{HST}) survey targeting Perseus. Using the same \textit{HST} images and the new imaging data from the \textit{Euclid} survey, we report the detection of extremely faint but significant diffuse emission around the four GCs of CDG-2. We thus have exceptionally strong evidence that CDG-2 is a galaxy. This is the first galaxy detected purely through its GC population. Under the conservative assumption that the four GCs make up the entire GC population, preliminary analysis shows that CDG-2 has a total luminosity of $L_{V, \mathrm{gal}}= 6.2\pm{3.0} \times 10^6 L_{\odot}$ and a minimum GC luminosity of $L_{V, \mathrm{GC}}= 1.03\pm{0.2}\times 10^6 L_{\odot}$. Our results indicate that CDG-2 is one of the faintest galaxies having associated GCs, while at least $\sim 16.6\%$ of its light is contained in its GC population. This ratio is likely to be much higher ($\sim 33\%$) if CDG-2 has a canonical GC luminosity function (GCLF). In addition, if the previously observed GC-to-halo mass relations apply to CDG-2, it would have a minimum dark matter halo mass fraction of $99.94\%$ to $99.98\%$. If it has a canonical GCLF, then the dark matter halo mass fraction is $\gtrsim 99.99\%$. Therefore, CDG-2 may be the most GC dominated galaxy and potentially one of the most dark matter dominated galaxies ever discovered.

astro-ph.GA↗

arXiv2024

Discovery of Two Ultra-Diffuse Galaxies with Unusually Bright Globular Cluster Luminosity Functions via a Mark-Dependently Thinned Point Process (MATHPOP)

We present \textsc{Mathpop}, a novel method to infer the globular cluster (GC) counts in ultra-diffuse galaxies (UDGs) and low-surface brightness galaxies (LSBGs). Many known UDGs have a surprisingly high ratio of GC number to surface brightness. However, standard methods to infer GC counts in UDGs face various challenges, such as photometric measurement uncertainties, GC membership uncertainties, and assumptions about the GC luminosity functions (GCLFs). \textsc{Mathpop} tackles these challenges using the mark-dependent thinned point process, enabling joint inference of the spatial and magnitude distributions of GCs. In doing so, \textsc{Mathpop} allows us to infer and quantify the uncertainties in both GC counts and GCLFs with minimal assumptions. As a precursor to \textsc{Mathpop}, we also address the data uncertainties coming from the selection process of GC candidates: we obtain probabilistic GC candidates instead of the traditional binary classification based on the color--magnitude diagram. We apply \textsc{Mathpop} to 40 LSBGs in the Perseus cluster using GC catalogs from a \textit{Hubble Space Telescope} imaging program. We then compare our results to those from an independent study using the standard method. We further calibrate and validate our approach through extensive simulations. Our approach reveals two LSBGs having GCLF turnover points much brighter than the canonical value with Bayes' factor being $\sim4.5$ and $\sim2.5$, respectively. An additional crude maximum-likelihood estimation shows that their GCLF TO points are approximately $0.9$~mag and $1.1$~mag brighter than the canonical value, with $p$-value $\sim 10^{-8}$ and $\sim 10^{-5}$, respectively.

astro-ph.GA↗

arXiv2024

Site-Controlled Purcell-Induced Bright Single Photon Emitters in Hexagonal Boron Nitride

Single photon emitters (SPEs) hosted in hexagonal boron nitride (hBN) are essential elementary building blocks for enabling future on-chip quantum photonic technologies that operate at room temperature. However, fundamental challenges, such as managing non-radiative decay, competing incoherent processes, as well as engineering difficulties in achieving deterministic placement and scaling of the emitters, limit their full potential. In this work, we experimentally demonstrate large-area arrays of plasmonic nanoresonators for Purcell-induced site-controlled SPEs by engineering emitter-cavity coupling and enhancing radiative emission at room temperature. The plasmonic nanoresonator architecture consists of gold-coated silicon pillars capped with an alumina spacer layer, enabling a 10-fold local field enhancement in the emission band of native hBN defects. Confocal photoluminescence and second-order autocorrelation measurements show bright SPEs with sub-30 meV bandwidth and a saturated emission rate of more than 3.8 million counts per second. We measure a Purcell factor of 4.9, enabling average SPE lifetimes of 480 ps, a five-fold reduction as compared to emission from gold-free devices, along with an overall SPE yield of 21%. Density functional theory calculations further reveal the beneficial role of an alumina spacer between defected hBN and gold, as an insulating layer can mitigate the electronic broadening of emission from defects proximal to gold. Our results offer arrays of bright, heterogeneously integrated quantum light sources, paving the way for robust and scalable quantum information systems.

physics.optics↗

arXiv2023

Vacancy-Engineered Phonon Polaritons in a van der Waals Crystal

Phonon-polaritons (PhPs) in low-symmetry van der Waals materials confine mid-infrared electromagnetic radiation well below the diffraction limit for nanoscale optics, sensing, and energy control. However, controlling the PhP dispersion at the nanoscale through intrinsic material properties$-$without external fields, lithography, or intercalants$-$remains elusive. Here, we demonstrate vacancy-engineered tuning of PhPs in $\alpha$-phase molybdenum trioxide ($\alpha$-MoO$_3$) via oxygen vacancy formation and lattice strain. Near-field nanoimaging of PhPs in processed $\alpha$-MoO$_3$ reveals an average polariton wavevector modulation of $\Delta k/k \approx 0.13 $ within the lower Restrahlen band. Stoichiometric analysis, density functional theory, and finite-difference time-domain simulations show agreement with the experimental results and suggest an induced vacancy concentration of $1\% - 2\%$ along with $(1.2\pm 0.2)\%$ compressive strain, resulting in a non-volatile dielectric permittivity modulation of up to $\Delta \varepsilon / \varepsilon \approx 0.15$. Despite these lattice modifications, the lifetimes of thermomechanically tuned PhPs remain high at $1.2 \pm 0.31$ ps. These results establish thermomechanical vacancy engineering as a general strategy to reprogram polaritonic response in vdW crystals, offering a new degree of freedom for embedded, non-volatile nanophotonics.

physics.optics↗

arXiv2023

Symphony: Expressive Secure Multiparty Computation with Coordination

Context: Secure Multiparty Computation (MPC) refers to a family of cryptographic techniques where mutually untrusting parties may compute functions of their private inputs while revealing only the function output. Inquiry: It can be hard to program MPCs correctly and efficiently using existing languages and frameworks, especially when they require coordinating disparate computational roles. How can we make this easier? Approach: We present Symphony, a new functional programming language for MPCs among two or more parties. Symphony starts from the single-instruction, multiple-data (SIMD) semantics of prior MPC languages, in which each party carries out symmetric responsibilities, and generalizes it using constructs that can coordinate many parties. Symphony introduces **first-class shares** and **first-class party sets** to provide unmatched language-level expressive power with high efficiency. Knowledge: Developing a core formal language called $\lambda$-Symphony, we prove that the intuitive, generalized SIMD view of a program coincides with its actual distributed semantics. Thus the programmer can reason about her programs by reading them from top to bottom, even though in reality the program runs in a coordinated fashion, distributed across many machines. We implemented a prototype interpreter for Symphony leveraging multiple cryptographic backends. With it we wrote a variety of MPC programs, finding that Symphony can express optimized protocols that other languages cannot, and that in general Symphony programs operate efficiently. [ full abstract at https://doi.org/10.22152/programming-journal.org/2023/7/14 ]

cs.CR↗

arXiv2020

A Symbolic Approach to Proving Query Equivalence Under Bag Semantics

In database-as-a-service platforms, automated verification of query equivalence helps eliminate redundant computation in the form of overlapping sub-queries. Researchers have proposed two pragmatic techniques to tackle this problem. The first approach consists of reducing the queries to algebraic expressions and proving their equivalence using an algebraic theory. The limitations of this technique are threefold. It cannot prove the equivalence of queries with significant differences in the attributes of their relational operators. It does not support certain widely-used SQL features. Its verification procedure is computationally intensive. The second approach transforms this problem to a constraint satisfaction problem and leverages a general-purpose solver to determine query equivalence. This technique consists of deriving the symbolic representation of the queries and proving their equivalence by determining the query containment relationship between the symbolic expressions. While the latter approach addresses all the limitations of the former technique, it only proves the equivalence of queries under set semantics. However, in practice, database applications use bag semantics. In this paper, we introduce a novel symbolic approach for proving query equivalence under bag semantics. We transform the problem of proving query equivalence under bag semantics to that of proving the existence of a bijective, identity map between tuples returned by the queries on all valid inputs. We implement this symbolic approach in SPES and demonstrate that SPES proves the equivalence of a larger set of query pairs (95/232) under bag semantics compared to the state-of-the-art tools based on algebraic (30/232) and symbolic approaches (67/232) under set and bag semantics, respectively. Furthermore, SPES is 3X faster than the symbolic tool that proves equivalence under set semantics.

cs.DB↗

arXiv2019

Relational Verification via Invariant-Guided Synchronization

Relational properties describe relationships that hold over multiple executions of one or more programs, such as functional equivalence. Conventional approaches for automatically verifying such properties typically rely on syntax-based, heuristic strategies for finding synchronization points among the input programs. These synchronization points are then annotated with appropriate relational invariants to complete the proof. However, when suboptimal synchronization points are chosen the required invariants can be complicated or even inexpressible in the target theory. In this work, we propose a novel approach to verifying relational properties. This approach searches for synchronization points and synthesizes relational invariants simultaneously. Specifically, the approach uses synthesized invariants as a guide for finding proper synchronization points that lead to a complete proof. We implemented our approach as a tool named PEQUOD, which targets Java Virtual Machine (JVM) bytecode. We evaluated PEQUOD by using it to solve verification challenges drawn from the from the research literature and by verifying properties of student-submitted solutions to online challenge problems. The results show that PEQUOD solve verification problems that cannot be addressed by current techniques.

cs.PL↗

arXiv2018

The large-scale structure of the halo of the Andromeda galaxy II. Hierarchical structure in the Pan-Andromeda Archaeological Survey

The Pan-Andromeda Archaeological Survey is a survey of $>400$ square degrees centered on the Andromeda (M31) and Triangulum (M33) galaxies that has provided the most extensive panorama of a $L_\star$ galaxy group to large projected galactocentric radii. Here, we collate and summarise the current status of our knowledge of the substructures in the stellar halo of M31, and discuss connections between these features. We estimate that the 13 most distinctive substructures were produced by at least 5 different accretion events, all in the last 3 or 4 Gyrs. We suggest that a few of the substructures furthest from M31 may be shells from a single accretion event. We calculate the luminosities of some prominent substructures for which previous estimates were not available, and we estimate the stellar mass budget of the outer halo of M31. We revisit the problem of quantifying the properties of a highly structured dataset; specifically, we use the OPTICS clustering algorithm to quantify the hierarchical structure of M31's stellar halo, and identify three new faint structures. M31's halo, in projection, appears to be dominated by two `mega-structures', that can be considered as the two most significant branches of a merger tree produced by breaking M31's stellar halo into smaller and smaller structures based on the stellar spatial clustering. We conclude that OPTICS is a powerful algorithm that could be used in any astronomical application involving the hierarchical clustering of points. The publication of this article coincides with the public release of all PAndAS data products.

astro-ph.GA↗

arXiv2017

Proofs as Relational Invariants of Synthesized Execution Grammars

The automatic verification of programs that maintain unbounded low-level data structures is a critical and open problem. Analyzers and verifiers developed in previous work can synthesize invariants that only describe data structures of heavily restricted forms, or require an analyst to provide predicates over program data and structure that are used in a synthesized proof of correctness. In this work, we introduce a novel automatic safety verifier of programs that maintain low-level data structures, named LTTP. LTTP synthesizes proofs of program safety represented as a grammar of a given program's control paths, annotated with invariants that relate program state at distinct points within its path of execution. LTTP synthesizes such proofs completely automatically, using a novel inductive-synthesis algorithm. We have implemented LTTP as a verifier for JVM bytecode and applied it to verify the safety of a collection of verification benchmarks. Our results demonstrate that LTTP can be applied to automatically verify the safety of programs that are beyond the scope of previously-developed verifiers.

cs.PL↗

arXiv2017

Simulating radiative feedback and star cluster formation in GMCs: II. Mass dependence of cloud destruction and cluster properties

The process of radiative feedback in Giant Molecular Clouds (GMCs) is an important mechanism for limiting star cluster formation through the heating and ionization of the surrounding gas. We explore the degree to which radiative feedback affects early ($\lesssim$5 Myr) cluster formation in GMCs having masses that range from 10$^{4-6}$ M$_{\odot}$ using the FLASH code. The inclusion of radiative feedback lowers the efficiency of cluster formation by 20-50\% relative to hydrodynamic simulations. Two models in particular --- 5$\times$10$^4$ and 10$^5$ M$_{\odot}$ --- show the largest suppression of the cluster formation efficiency, corresponding to a factor of $\sim$2. For these clouds only, the internal energy, a measure of the energy injected by radiative feedback, exceeds the gravitational potential for a significant amount of time. We find a clear relation between the maximum cluster mass, M$_{cl,max}$, formed in a GMC of mass M$_{GMC}$; M$_{cl,max}\propto$ M$_{GMC}^{0.81}$. This scaling result suggests that young globular clusters at the necessary scale of $10^6 M_{\odot}$ form within host GMCs of masses near $\sim 5 \times 10^7 M_{\odot}$. We compare simulated cluster mass distributions to the observed embedded cluster mass function ($dlog(N)/dlog(M) \propto M^{\beta}$ where $\beta$ = -1) and find good agreement ($\beta$ = -0.99$\pm$0.14) only for simulations including radiative feedback, indicating this process is important in controlling the growth of young clusters. However, the high star formation efficiencies, which range from 16-21\%, and high star formation rates compared to locally observed regions suggest other feedback mechanisms are also important during the formation and growth of stellar clusters.

astro-ph.GA↗

arXiv2017

Solving Constrained Horn Clauses Using Dependence-Disjoint Expansions

Recursion-free Constrained Horn Clauses (CHCs) are logic-programming problems that can model safety properties of programs with bounded iteration and recursion. In addition, many CHC solvers reduce recursive systems to a series of recursion-free CHC systems that can each be solved efficiently. In this paper, we define a novel class of recursion-free systems, named Clause-Dependence Disjoint (CDD), that generalizes classes defined in previous work. The advantage of this class is that many CDD systems are smaller than systems which express the same constraints but are part of a different class. This advantage in size allows CDD systems to be solved more efficiently than their counterparts in other classes. We implemented a CHC solver named Shara. Shara solves arbitrary CHC systems by reducing the input to a series of CDD systems. Our evaluation indicates that Shara outperforms state-of-the-art implementations in many practical cases.

cs.LO↗

arXiv2017

Completely Automated Equivalence Proofs

Verifying partial (i.e., termination-insensitive) equivalence of programs has significant practical applications in software development and education. Conventional equivalence verifiers typically rely on a combination of given relational summaries and suggested synchronization points; such information can be extremely difficult for programmers without a background in formal methods to provide for pairs of programs with dissimilar logic. In this work, we propose a completely automated verifier for determining partial equivalence, named Pequod. Pequod automatically synthesizes expressive proofs of equivalence conventionally only achievable via careful, manual constructions of product programs To do so, Pequod syntheses relational proofs for selected pairs of program paths and combines the per-path relational proofs to synthesize relational program invariants. To evaluate Pequod, we implemented it as a tool that targets Java Virtual Machine bytecode and applied it to verify the equivalence of hundreds of pairs of solutions submitted by students for problems hosted on popular online coding platforms, most of which could not be verified by existing techniques.

cs.PL↗

arXiv2016

Bayesian Mass Estimates of the Milky Way: including measurement uncertainties with hierarchical Bayes

We present a hierarchical Bayesian method for estimating the total mass and mass profile of the Milky Way Galaxy. The new hierarchical Bayesian approach further improves the framework presented by Eadie, Harris, & Widrow (2015) and Eadie & Harris (2016) and builds upon the preliminary reports by Eadie et al (2015a,c). The method uses a distribution function $f(\mathcal{E},L)$ to model the galaxy and kinematic data from satellite objects such as globular clusters (GCs) to trace the Galaxy's gravitational potential. A major advantage of the method is that it not only includes complete and incomplete data simultaneously in the analysis, but also incorporates measurement uncertainties in a coherent and meaningful way. We first test the hierarchical Bayesian framework, which includes measurement uncertainties, using the same data and power-law model assumed in Eadie & Harris (2016), and find the results are similar but more strongly constrained. Next, we take advantage of the new statistical framework and incorporate all possible GC data, finding a cumulative mass profile with Bayesian credible regions. This profile implies a mass within $125$kpc of $4.8\times10^{11}M_{\odot}$ with a 95\% Bayesian credible region of $(4.0-5.8)\times10^{11}M_{\odot}$. Our results also provide estimates of the true specific energies of all the GCs. By comparing these estimated energies to the measured energies of GCs with complete velocity measurements, we observe that (the few) remote tracers with complete measurements may play a large role in determining a total mass estimate of the Galaxy. Thus, our study stresses the need for more remote tracers with complete velocity measurements.

astro-ph.GA↗

arXiv2016

Bayesian Mass Estimates of the Milky Way II: The dark and light sides of parameter assumptions

We present mass and mass profile estimates for the Milky Way Galaxy using the Bayesian analysis developed by Eadie et al (2015b) and using globular clusters (GCs) as tracers of the Galactic potential. The dark matter and GCs are assumed to follow different spatial distributions; we assume power-law model profiles and use the model distribution functions described in Evans et al. (1997); Deason et al (2011, 2012a). We explore the relationships between assumptions about model parameters and how these assumptions affect mass profile estimates. We also explore how using subsamples of the GC population beyond certain radii affect mass estimates. After exploring the posterior distributions of different parameter assumption scenarios, we conclude that a conservative estimate of the Galaxy's mass within 125kpc is $5.22\times10^{11} M_{\odot}$, with a $50\%$ probability region of $(4.79, 5.63) \times10^{11} M_{\odot}$. Extrapolating out to the virial radius, we obtain a virial mass for the Milky Way of $6.82\times10^{11} M_{\odot}$ with $50\%$ credible region of $(6.06, 7.53) \times 10^{11} M_{\odot}$ ($r_{vir}=185^{+7}_{-7}$kpc). If we consider only the GCs beyond 10kpc, then the virial mass is $9.02~(5.69, 10.86) \times 10^{11} M_{\odot}$ ($r_{vir}=198^{+19}_{-24}$kpc). We also arrive at an estimate of the velocity anisotropy parameter $\beta$ of the GC population, which is $\beta=0.28$ with a $50\%$ credible region (0.21, 0.35). Interestingly, the mass estimates are sensitive to both the dark matter halo potential and visible matter tracer parameters, but are not very sensitive to the anisotropy parameter.

astro-ph.GA↗

arXiv2016

Simulating radiative feedback and star cluster formation in GMCs: I. Dependence on gravitational boundedness

Radiative feedback is an important consequence of cluster formation in Giant Molecular Clouds (GMCs) in which newly formed clusters heat and ionize their surrounding gas. The process of cluster formation, and the role of radiative feedback, has not been fully explored in different GMC environments. We present a suite of simulations which explore how the initial gravitational boundedness, and radiative feedback, affect cluster formation. We model the early evolution ($<$ 5 Myr) of turbulent, 10$^6$ M$_{\odot}$ clouds with virial parameters ranging from 0.5 to 5. To model cluster formation, we use cluster sink particles, coupled to a raytracing scheme, and a custom subgrid model which populates a cluster via sampling an IMF with an efficiency of 20\% per freefall time. We find that radiative feedback only decreases the cluster particle formation efficiency by a few percent. The initial virial parameter plays a much stronger role in limiting cluster formation, with a spread of cluster formation efficiencies of 37\% to 71\% for the most unbound to the most bound model. The total number of clusters increases while the maximum mass cluster decreases with an increasing initial virial parameter, resulting in steeper mass distributions. The star formation rates in our cluster particles are initially consistent with observations but rise to higher values at late times. This suggests that radiative feedback alone is not responsible for dispersing a GMC over the first 5 Myr of cluster formation.

astro-ph.GA↗

arXiv2015

Dark Matter Halos in Galaxies and Globular Cluster Populations. II: Metallicity and Morphology

An increasing body of data reveals a one-to-one linear correlation between galaxy halo mass and the total mass in its globular cluster (GC) population, M_{GCS} ~ M_h^{1.03 \pm 0.03}, valid over 5 orders of magnitude. We explore the nature of this correlation for galaxies of different morphological types, and for the subpopulations of metal-poor (blue) and metal-rich (red) GCs. For the subpopulations of different metallicity we find M_{GCS}(blue) ~ M_h^{0.96 \pm 0.03} and M_{GCS}(red) ~ M_h^{1.21 \pm 0.03} with similar scatter. The numerical values of these exponents can be derived from the detailed behavior of the red and blue GC fractions with galaxy mass and provide a self-consistent set of relations. In addition, all morphological types (E, S0, S/Irr) follow the same relation, but with a second-order trend for spiral galaxies to have a slightly higher fraction of metal-rich GCs for a given mass. These results suggest that the amount of gas available for GC formation at high redshift was in nearly direct proportion to the dark-matter halo potential, in strong contrast to the markedly nonlinear behavior of total stellar mass versus halo mass. Of the few available theoretical treatments that directly discuss the formation of GCs in a hierarchical merging framework,the model of Kravtsov & Gnedin (2005) best matches these observations. They find that the blue, metal-poor GCs formed in small halos at $z > 3$ and did so in nearly direct proportion to halo mass. Similar models addressing the formation rate of the red, metal-richer GCs in the same detail and continuing to lower redshift are still needed for a comprehensive picture.

astro-ph.GA↗

arXiv2009

A Blue Tilt in the Globular Cluster System of the Milky Way-like Galaxy NGC 5170

Here we present HST/ACS imaging, in the B and I bands, of the edge-on Sb/Sc galaxy NGC 5170. Excluding the central disk region region, we detect a 142 objects with colours and sizes typical of globular clusters (GCs). Our main result is the discovery of a `blue tilt' (a mass-metallicity relation), at the 3sigma level, in the metal-poor GC subpopulation of this Milky Way like galaxy. The tilt is consistent with that seen in massive elliptical galaxies and with the self enrichment model of Bailin & Harris. For a linear mass-metallicity relation, the tilt has the form Z ~ L^{0.42 +/- 0.13}. We derive a total GC system population of 600 +/- 100, making it much richer than the Milky Way. However when this number is normalised by the host galaxy luminosity or stellar mass it is similar to that of M31. Finally, we report the presence of a potential Ultra Compact Dwarf of size ~ 6 pc and luminosity M_I ~ -12.5, assuming it is physically associated with NGC 5170.

astro-ph.CO↗