skip to main content

Search for: All records

Creators/Authors contains: "Simon, J."

Note: When clicking on a Digital Object Identifier (DOI) number, you will be taken to an external site maintained by the publisher. Some full text articles may not yet be available without a charge during the embargo (administrative interval).
What is a DOI Number?

Some links on this page may take you to non-federal websites. Their policies may differ from this site.

  1. ABSTRACT As the Milky Way and its satellite system become more entrenched in near field cosmology efforts, the need for an accurate mass estimate of the Milky Way’s dark matter halo is increasingly critical. With the second and early third data releases of stellar proper motions from Gaia, several groups calculated full 6D phase-space information for the population of Milky Way satellite galaxies. Utilizing these data in comparison to subhalo properties drawn from the Phat ELVIS simulations, we constrain the Milky Way dark matter halo mass to be ∼1–1.2 × 1012 M⊙. We find that the kinematics of subhaloes drawn from more- or less-massive hosts (i.e. >1.2 × 1012 M⊙ or <1012 M⊙) are inconsistent, at the 3σ confidence level, with the observed velocities of the Milky Way satellites. The preferred host halo mass for the Milky Way is largely insensitive to the exclusion of systems associated with the Large Magellanic Cloud, changes in galaxy formation thresholds, and variations in observational completeness. As more Milky Way satellites are discovered, their velocities (radial, tangential, and total) plus Galactocentric distances will provide further insight into the mass of the Milky Way dark matter halo.
    Free, publicly-accessible full text available May 23, 2023
  2. Identifying the directed connectivity that underlie networked activity between different cortical areas is critical for understanding the neural mechanisms behind sensory processing. Granger causality (GC) is widely used for this purpose in functional magnetic resonance imaging analysis, but there the temporal resolution is low, making it difficult to capture the millisecond-scale interactions underlying sensory processing. Magne- toencephalography (MEG) has millisecond resolution, but only provides low-dimensional sensor-level linear mixtures of neural sources, which makes GC inference challenging. Conventional methods proceed in two stages: First, cortical sources are estimated from MEG using a source localization technique, followed by GC inference among the estimated sources. However, the spatiotemporal biases in estimating sources propagate into the subsequent GC analysis stage, may result in both false alarms and missing true GC links. Here, we introduce the Network Localized Granger Causality (NLGC) inference paradigm, which models the source dynamics as latent sparse multivariate autoregressive processes and estimates their parameters directly from the MEG measurements, integrated with source localization, and employs the resulting parameter estimates to produce a precise statistical characterization of the detected GC links. We offer several theoretical and algorithmic innovations within NLGC and further examine its utility via comprehensive simulations and application to MEGmore »data from an auditory task involving tone processing from both younger and older participants. Our simulation studies reveal that NLGC is markedly robust with respect to model mismatch, network size, and low signal-to-noise ratio, whereas the conventional two-stage methods result in high false alarms and mis-detections. We also demonstrate the advantages of NLGC in revealing the cortical network- level characterization of neural activity during tone processing and resting state by delineating task- and age-related connectivity changes.« less
    Free, publicly-accessible full text available January 1, 2023
  3. Free, publicly-accessible full text available April 1, 2023
  4. Abstract A giant-impact origin for the Moon is generally accepted, but many aspects of lunar formation remain poorly understood and debated. Ćuk et al. proposed that an impact that left the Earth–Moon system with high obliquity and angular momentum could explain the Moon’s orbital inclination and isotopic similarity to Earth. In this scenario, instability during the Laplace Plane transition, when the Moon’s orbit transitions from the gravitational influence of Earth’s figure to that of the Sun, would both lower the system’s angular momentum to its present-day value and generate the Moon’s orbital inclination. Recently, Tian & Wisdom discovered new dynamical constraints on the Laplace Plane transition and concluded that the Earth–Moon system could not have evolved from an initial state with high obliquity. Here we demonstrate that the Earth–Moon system with an initially high obliquity can evolve into the present state, and we identify a spin–orbit secular resonance as a key dynamical mechanism in the later stages of the Laplace Plane transition. Some of the simulations by Tian & Wisdom did not encounter this late secular resonance, as their model suppressed obliquity tides and the resulting inclination damping. Our results demonstrate that a giant impact that left Earth with highmore »angular momentum and high obliquity ( θ > 61°) is a promising scenario for explaining many properties of the Earth–Moon system, including its angular momentum and obliquity, the geochemistry of Earth and the Moon, and the lunar inclination.« less
  5. Free, publicly-accessible full text available January 1, 2023
  6. Abstract We present observations of the dwarf galaxies GALFA Dw3 and GALFA Dw4 with the Advanced Camera for Surveys on the Hubble Space Telescope. These galaxies were initially discovered as optical counterparts to compact H i clouds in the GALFA survey. Both objects resolve into stellar populations which display old red giant branch (RGB), younger helium-burning, and massive main sequence stars. We use the tip of the RGB method to determine the distance to each galaxy, finding distances of 7.61 − 0.29 + 0.28 Mpc and 3.10 − 0.17 + 0.16 Mpc, respectively. With these distances we show that both galaxies are extremely isolated, with no other confirmed objects within ∼1.5 Mpc of either dwarf. GALFA Dw4 is also found to be unusually compact for a galaxy of its luminosity. GALFA Dw3 and Dw4 contain H ii regions with young star clusters and an overall irregular morphology; they show evidence of ongoing star formation through both ultraviolet and H α observations and are therefore classified as dwarf irregulars (dIrrs). The star formation histories of these two dwarfs show distinct differences: Dw3 shows signs of a recently ceased episode of active star formation across the entire dwarf, while Dw4 shows somemore »evidence for current star formation in spatially limited H ii regions. Compact H i sources offer a promising method for identifying isolated field dwarfs in the Local Volume, including GALFA Dw3 and Dw4, with the potential to shed light on the driving mechanisms of dwarf galaxy formation and evolution.« less
    Free, publicly-accessible full text available January 1, 2023
  7. Domain decomposition methods are widely used for the numerical solution of partial differential equations on high performance computers. We develop an adjoint-based a posteriori error analysis for both multiplicative and additive overlapping Schwarz domain decomposition methods. The numerical error in a user-specified functional of the solution (quantity of interest) is decomposed into contributions that arise as a result of the finite iteration between the subdomains and from the spatial discretization. The spatial discretization contribution is further decomposed into contributions arising from each subdomain. This decomposition of the numerical error is used to construct a two stage solution strategy that efficiently reduces the error in the quantity of interest by adjusting the relative contributions to the error.
  8. Free, publicly-accessible full text available December 1, 2022