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At least 793 records · Page 44

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NASA NTRS2022

A Candle in the Wind: A Radio Filament in the Core of the A3562 Galaxy Cluster

Using a MeerKAT observation of the galaxy cluster A3562 (a member of the Shapley Supercluster), we have discovered a narrow, long and straight, very faint radio filament, which branches out at a straight angle from the tail of a radio galaxy located in projection near the core of the cluster. The radio filament spans 200 kpc and aligns with a sloshing cold front seen in the X-rays, staying inside the front in projection. The radio spectral index along the filament appears uniform (within large uncertainties) atα' −1.5. We propose that the radio galaxy is located outside the cold front, but dips its tail under the front. The tangential wind that blows there may stretch the radio plasma from the radio galaxy into a filamentary structure. Some reacceleration is needed in this scenario to maintain the radio spectrum uniform. Alternatively, the cosmic ray electrons from that spot in the tail can spread along the cluster magnetic field lines, straightened by that same tangential flow, via anomalously fast diffusion. Our radio filament can provide constraints on this process. We also uncover a compact radio source at the Brightest Cluster Galaxy (BCG) that is 2–3 orders of magnitude

S Giacintucci↗

NASA NTRSDate not supplied

Cooperative Clustering Techniques Applied to Contact Graph Routing

Routing in the space internet has to face many unique challenges - from unplanned disconnections and interruptions to predictable intermittent connectivity due to high network mobility and long propagation delays. NASA’s current approach to such routing is Contact Graph Routing (CGR), using a graph formed of prescheduled communication contacts to compute routes through the network. While this approach manages to tackle issues of connectivity and propagation delays, it is a global approach that requires continuous knowledge of the entire network. In a potential future Solar Space Internet (SSI) such an approach on its own cannot scale to large networks with thousands of members. In this presentation we propose clustering as a solution to CGR scalability. Clustering has been used in many networking problems as a way to subdivide the network and allow for localized routing and better scalability. Using techniques from graph theory and game theory, we explore various existing clustering algorithms and adapt them to the Contact Graph Routing setting. Finally, we propose a way to combine multiple algorithms to create a Delay Tolerant Clustering Protocol.

Yael Kirkpatrick↗

NASA NTRS2023

Is the M81 fast radio burst host globular cluster special?

We use multiband archival Hubble Space Telescope (HST) observations to measure the photometric and structural parameters of the M81 globular cluster (GC) that hosts the fast radio burst FRB 20200120E. Our best-fitting King model has an effective radius rh = 3.06 pc with a moderate King model concentration of c = 53, and an inferred core radius of 0.81 pc. We revisit the exact astrometric location of the FRB within the cluster, and find that FRB 20200120E is located 1.92 pc from the center, but within the projected half-light radius. We estimate the relative encounter rate of the FRB host, along with the corresponding rates of 210 other GCs in M81, and compare these values with the encounter rates of Galactic GCs. The FRB resides in a GC with an encounter rate that is moderately higher than the median stellar encounter rate in our two comparison samples. While the estimated encounter rate of the FRB host cluster (e.g., ∼50% of a cluster like 47 Tuc) is sufficient to allow the possibility that the FRB formed dynamically, our results do not place strong constraints on this scenario due to the limitations of the available HST data and the possible systematic uncertainties and selection effects in the comparison data.

Kristen C. Dage↗

NASA NTRS2005

Constrained spectral clustering under a local proximity structure assumption

This work focuses on incorporating pairwise constraints into a spectral clustering algorithm. A new constrained spectral clustering method is proposed, as well as an active constraint acquisition technique and a heuristic for parameter selection. We demonstrate that our constrained spectral clustering method, CSC, works well when the data exhibits what we term local proximity structure.

domain knowledge↗

NASA NTRS2006

When is Constrained Clustering Beneficial, and Why?

Several researchers have shown that constraints can improve the results of a variety of clustering algorithms. However, there can be a large variation in this improvement, even for a fixed number of constraints for a given data set. We present the first attempt to provide insight into this phenomenon by characterizing two constraint set properties: informativeness and coherence. We show that these measures can help explain why some constraint sets are more beneficial to clustering algorithms than others. Since they can be computed prior to clustering, these measures can aid in deciding which constraints to use in practice.

clustering algorithms↗

NASA NTRS2006

Value, Cost, and Sharing: Open Issues in Constrained Clustering

Clustering is an important tool for data mining, since it can identify major patterns or trends without any supervision (labeled data). Over the past five years, semi-supervised (constrained) clustering methods have become very popular. These methods began with incorporating pairwise constraints and have developed into more general methods that can learn appropriate distance metrics. However, several important open questions have arisen about which constraints are most useful, how they can be actively acquired, and when and how they should be propagated to neighboring points. This position paper describes these open questions and suggests future directions for constrained clustering research.

constraints↗

NASA NTRS2016

Mapping the Gas Turbulence in the Coma Cluster: Predictions for Astro-H

Astro-H will be able for the first time to map gas velocities and detect turbulence in galaxy clusters. One of the best targets for turbulence studies is the Coma cluster, due to its proximity, absence of a cool core, and lack of a central active galactic nucleus. To determine what constraints Astro-H will be able to place on the Coma velocity field, we construct simulated maps of the projected gas velocity and compute the second-order structure function, an analog of the velocity power spectrum. We vary the injection scale, dissipation scale, slope, and normalization of the turbulent power spectrum, and apply measurement errors and finite sampling to the velocity field. We find that even with sparse coverage of the cluster, Astro-H will be able to measure the Mach number and the injection scale of the turbulent power spectrum-the quantities determining the energy flux down the turbulent cascade and the diffusion rate for everything that is advected by the gas (metals, cosmic rays, etc.). Astro-H will not be sensitive to the dissipation scale or the slope of the power spectrum in its inertial range, unless they are outside physically motivated intervals. We give the expected confidence intervals for the injection scale and the normalization of the power spectrum for a number of possible pointing configurations, combining the structure function and velocity dispersion data. Importantly, we also determine that measurement errors on the line shift will bias the velocity structure function upward, and show how to correct this bias.

clusters: intracluster medium↗

NASA NTRS2003

Improved models for the evolution of the coma cluster of galaxies

The analysis by Fitchett & Webster of the observations of the Coma Cluster of galaxies has demonstrated that the center of the Coma Cluster consists of two subclusters. Therefore, it is important to construct realistic dynamical models of a galaxy cluster with two mass centers.

dark↗

NASA NTRSDate not supplied

Clustering Approach To Identifying Low Lunar Frozen Orbits In A High-Fidelity Model

Low lunar frozen orbits are of continued interest in the astrodynamics community for trajectory design and space domain awareness. This paper presents a data-driven approach to analyzing a wide variety of numerically-generated lunar trajectories in a 100 × 100 lunar gravity model with the point mass gravity of the Earth and Sun. First, clustering is used to extract a summary of these trajectories: within each cluster, trajectories possess a geometrically similar evolution of perilune but varying drift and lifetimes. Within some clusters, trajectories with a bounded perilune evolution are also identified to produce candidates for lunar frozen orbits of distinct geometries.

lunar frozen orbit gravity model data clustering↗

NASA NTRS2024

DNA Break Clustering as a Predictor of Cell Death across Various Radiation Qualities: Influence of Cell Size, Cell Asymmetry, and Beam Orientation

Cosmic radiation, composed of high charge and energy (HZE) particles, causes cellular DNA damage that can result in cell death or mutation that can evolve into cancer. In this work, a cell death model is applied to several cell lines exposed to HZE ions spanning a broad range of linear energy transfer (LET) values. We hypothesize that chromatin movement leads to the clustering of multiple double strand breaks (DSB) within one radiation-induced foci (RIF). The survival probability of a cell population is determined by averaging the survival probabilities of individual cells, which is function of the number of pairwise DSB interactions within RIF. The simulation code RITCARD was used to compute DSB. Two clustering approaches were applied to determine the number of RIF per cell. RITCARD outputs were combined with experimental data from four normal human cell lines to derive the model parameters and expand its predictions in response to ions with LET ranging from ∼0.2keV/μmto∼3000keV/μm. Spherical and ellipsoidal nuclear shapes and two ion beam orientations were modeled to assess the impact of geometrical properties on cell death. The calculated average number of RIF per cell reproduces the saturation trend for high doses and high-LET values that is usually experimentally observed. The cell survival model generates the recognizable bell shape of LET dependence for the relative biological effectiveness (RBE). At low LET, smaller nuclei have lower survival due to increased DNA density and DSB clustering. At high LET, nuclei with a smaller irradiation area either because of a smaller size or a change in beam orientation have a higher survival rate due to a change in the distribution of DSB/RIF per cell. If confirmed experimentally, the geometric characteristics of cells would become a significant factor in predicting radiation-induced biological effects.

cell survival↗

NASA NTRS1996

IC 1257: A New Globular Cluster in the Galactic Halo

New CCD photometry of the faint, compact star cluster IC 1257 (L = 17? = +/- 15?obtained with the Palomar 5m telescope, reveals that it is a highly reddened globular cluster well beyond the Galactic center.

ccd photometry stars clusters star cluster globula↗

NASA NTRS1972

Ultraviolet photometry of globular clusters

Filter photometry measurements of the integrated light from several globular clusters between 1500 and 4000 A are discussed. The techniques and problems associated with carrying out photometry of faint extended sources with the OAO-2 instrumentation are described first. The energy distribution for globular clusters determined from these data are corrected for interstellar extinction on the basis of B-V color excesses and a mean ultraviolet extinction curve. The results are compared with synthetic cluster calculations. The ultraviolet energy distribution is primarily determined by the number of stars at the blue end of the horizontal branch.

Welch, G. A.↗

NASA NTRS1972

X-ray emission from rich clusters of galaxies.

Evaluation of the newly published Uhuru catalog, revealing significant number of rich clusters associated with X-ray sources. Combined with earlier observations of X-ray emission from the Virgo, Coma, and Perseus clusters, this suggests that most, if not all, rich clusters include an X-ray emission region of large size and of net luminosity from 10 to the 43rd to 10 to the 44th ergs/sec.

Gursky, H.↗

NASA NTRS1972

The structure of the coma cluster of galaxies.

Analysis of radial velocities in the coma cluster (many of them new) in conjunction with previous data on the density distribution. The cluster extends more than 200 min in radius, with bright and faint galaxies distributed in nearly the same way. Some spirals are members. Several different determinations of M/L give values around 250. The cluster is stabilized by a 'missing mass' some seven times the mass of the galaxies, which must be distributed in the same way as the galaxies.

Rood, H. J.↗

NASA NTRS1973

Editing ERTS-1 data to exclude land aids cluster analysis of water targets

The author has identified the following significant results. It has been determined that an increase in the number of spectrally distinct coastal water types is achieved when data values over the adjacent land areas are excluded from the processing routine. This finding resulted from an automatic clustering analysis of ERTS-1 system corrected MSS scene 1002-18134 of 25 July 1972 over Monterey Bay, California. When the entire study area data set was submitted to the clustering only two distinct water classes were extracted. However, when the land area data points were removed from the data set and resubmitted to the clustering routine, four distinct groupings of water features were identified. Additionally, unlike the previous separation, the four types could be correlated to features observable in the associated ERTS-1 imagery. This exercise demonstrates that by proper selection of data submitted to the processing routine, based upon the specific application of study, additional information may be extracted from the ERTS-1 MSS data.

Erb, R. B.↗

NASA NTRS1972

Observations of the extended X-ray sources in the Perseus and Coma clusters from Uhuru.

The X-ray source in Perseus identified as NGC 1275 is found to have a finite angular extent of about 35 arc minutes. The improved location of the center of the emission is consistent with NGC 1275. An improved location for the center of the Coma X-1 source is consistent both with the kinematic center of the cluster and with NGC 4874. These extended sources - Coma, Perseus, and the source in the Virgo cluster - may be a new class of X-ray objects associated with active galaxies in rich clusters.

Forman, W.↗

NASA NTRS1973

Shakhbazian I - A distant cluster of compact galaxies.

Shakhbazian (1957) reported a cluster of red objects which he interpreted to be a cluster of stars in Ursa Major at a distance of 130 kpc. Spectra obtained with an image-tube scanner for 5 members of this cluster show that they are all galaxies with a redshift corresponding to a radial velocity 29.700 km/sec. The remaining 12 members of the group appear to be diffuse, and are probably also galaxies.

Robinson, L. B.↗

NASA NTRS1973

Mariner 9 ultraviolet spectrometer experiment - Upper limits on the Lyman-alpha flux from clusters of galaxies.

Data from the ultraviolet spectrometer on the Mariner 9 Mars orbiter have been used to set upper limits on the redshifted Lyman-alpha flux from the Perseus and Pegasus I (NGC 7619) clusters of galaxies. The limit for the Perseus cluster implies that the temperature of the intergalactic gas required for gravitational binding must be greater than 100,000 K, if it exists. A less severe limit is set for the Pegasus I cluster.

Bohlin, R. C.↗