Search NASA⌕ Search

SEARCH · Search NASA

Results for “substructuring”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 757 records · Page 42
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Search FOR extended gamma-ray emission from the Virgo galaxy cluster with Fermi-LAT

Galaxy clusters are one of the prime sites to search for dark matter (DM) annihilation signals. Depending on the substructure of the DM halo of a galaxy cluster and the cross sections for DM annihilation channels, these signals might be detectable by the latest generation of γ-ray telescopes. Here we use three years of Fermi-Large Area Telescope data, which are the most suitable for searching for very extended emission in the vicinity of the nearby Virgo galaxy cluster. Our analysis reveals statistically significant extended emission which can be well characterized by a uniformly emitting disk profile with a radius of 3° that moreover is offset from the cluster center. We demonstrate that the significance of this extended emission strongly depends on the adopted interstellar emission model (IEM) and is most likely an artifact of our incomplete description of the IEM in this region. We also search for and find new point source candidates in the region. We then derive conservative upper limits on the velocity-averaged DM pair annihilation cross section from Virgo. We take into account the potential γ-ray flux enhancement due to DM sub-halos and its complex morphology as a merging cluster. For DM annihilating into b b-bar , assuming a conservative sub-halo model setup, we find limits that are between 1 and 1.5 orders of magnitude above the expectation from the thermal cross section for m{sub DM} ≲ 100 GeV. In a more optimistic scenario, we exclude 〈σv〉∼3×10{sup −26} cm{sup 3} s{sup −1} for m{sub DM} ≲ 40 GeV for the same channel. Finally, we derive upper limits on the γ-ray-flux produced by hadronic cosmic-ray interactions in the inter cluster medium. We find that the volume-averaged cosmic-ray-to-thermal pressure ratio is less than ∼6%.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

Constraints on dark matter annihilation in clusters of galaxies with the Fermi large area telescope

Nearby clusters and groups of galaxies are potentially bright sources of high-energy gamma-ray emission resulting from the pair-annihilation of dark matter particles. However, no significant gamma-ray emission has been detected so far from clusters in the first 11 months of observations with the Fermi Large Area Telescope. We interpret this non-detection in terms of constraints on dark matter particle properties. In particular for leptonic annihilation final states and particle masses greater than ~ 200 GeV, gamma-ray emission from inverse Compton scattering of CMB photons is expected to dominate the dark matter annihilation signal from clusters, and our gamma-ray limits exclude large regions of the parameter space that would give a good fit to the recent anomalous Pamela and Fermi-LAT electron-positron measurements. We also present constraints on the annihilation of more standard dark matter candidates, such as the lightest neutralino of supersymmetric models. The constraints are particularly strong when including the fact that clusters are known to contain substructure at least on galaxy scales, increasing the expected gamma-ray flux by a factor of ~ 5 over a smooth-halo assumption. Here, we also explore the effect of uncertainties in cluster dark matter density profiles, finding a systematic uncertainty in the constraints of roughly a factor of two, but similar overall conclusions. Finally, in this work, we focus on deriving limits on dark matter models; a more general consideration of the Fermi-LAT data on clusters and clusters as gamma-ray sources is forthcoming.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

Implications of the Fermi-LAT diffuse gamma-ray measurements on annihilating or decaying dark matter

We analyze the recently published Fermi-LAT diffuse gamma-ray measurements in the context of leptonically annihilating or decaying dark matter (DM) with the aim to explain simultaneously the isotropic diffuse gamma-ray and the PAMELA, Fermi and HESS (PFH) anomalous e{sup ±} data. Five different DM annihilation/decay channels 2e, 2μ, 2τ, 4e, or 4μ (the latter two via an intermediate light particle φ) are generated with PYTHIA. We calculate both the Galactic and extragalactic prompt and inverse Compton (IC) contributions to the resulting gamma-ray spectra. To find the Galactic IC spectra we use the interstellar radiation field model from the latest release of GALPROP. For the extragalactic signal we show that the amplitude of the prompt gamma-emission is very sensitive to the assumed model for the extragalactic background light. For our Galaxy we use the Einasto, NFW and cored isothermal DM density profiles and include the effects of DM substructure assuming a simple subhalo model. Our calculations show that for the annihilating DM the extragalactic gamma-ray signal can dominate only if rather extreme power-law concentration-mass relation C(M) is used, while more realistic C(M) relations make the extragalactic component comparable or subdominant to the Galactic signal. For the decaying DM the Galactic signal always exceeds the extragalactic one. In the case of annihilating DM the PFH favored parameters can be ruled out by gamma-ray constraints only if power-law C(M) relation is assumed. For DM decaying into 2μ or 4μ the PFH favored DM parameters are not in conflict with the gamma-ray data. We find that, due to the (almost) featureless Galactic IC spectrum and the DM halo substructure, annihilating DM may give a good simultaneous fit to the isotropic diffuse gamma-ray and to the PFH e{sup ±} data without being in clear conflict with the other Fermi-LAT gamma-ray measurements.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

Dark matter searches with Cherenkov telescopes: nearby dwarf galaxies or local galaxy clusters?

In this paper, we compare dwarf galaxies and galaxy clusters in order to elucidate which object class is the best target for gamma-ray DM searches with imaging atmospheric Cherenkov telescopes (IACTs). We have built a mixed dwarfs+clusters sample containing some of the most promising nearby dwarf galaxies (Draco, Ursa Minor, Wilman 1 and Segue 1) and local galaxy clusters (Perseus, Coma, Ophiuchus, Virgo, Fornax, NGC 5813 and NGC 5846), and then compute their DM annihilation flux profiles by making use of the latest modeling of their DM density profiles. We also include in our calculations the effect of DM substructure. Willman 1 appears as the best candidate in the sample. However, its mass modeling is still rather uncertain, so probably other candidates with less uncertainties and quite similar fluxes, namely Ursa Minor and Segue 1, might be better options. As for galaxy clusters, Virgo represents the one with the highest flux. However, its large spatial extension can be a serious handicap for IACT observations and posterior data analysis. Yet, other local galaxy cluster candidates with more moderate emission regions, such as Perseus, may represent good alternatives. After comparing dwarfs and clusters, we found that the former exhibit annihilation flux profiles that, at the center, are roughly one order of magnitude higher than those of clusters, although galaxy clusters can yield similar, or even higher, integrated fluxes for the whole object once substructure is taken into account. Even when any of these objects are strictly point-like according to the properties of their annihilation signals, we conclude that dwarf galaxies are best suited for observational strategies based on the search of point-like sources, while galaxy clusters represent best targets for analyses that can deal with rather extended emissions. Finally, we study the detection prospects for present and future IACTs in the framework of the constrained minimal supersymmetric standard model. We find that the level of the annihilation flux from these targets is below the sensitivities of current IACTs and the future CTA.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

Fermi-LAT constraints on dark matter annihilation cross section from observations of the Fornax cluster

We analyze 2.8-yr data of 1–100 GeV photons for clusters of galaxies, collected with the Large Area Telescope onboard the Fermi satellite. By analyzing 49 nearby massive clusters located at high Galactic latitudes, we find no excess gamma-ray emission towards directions of the galaxy clusters. Using flux upper limits, we show that the Fornax cluster provides the most stringent constraints on the dark matter annihilation cross section. Stacking a large sample of nearby clusters does not help improve the limit for most dark matter models. This suggests that a detailed modeling of the Fornax cluster is important for setting robust limits on the dark matter annihilation cross section based on clusters. We therefore perform the detailed mass modeling and predict the expected dark matter annihilation signals from the Fornax cluster, by taking into account effects of dark matter contraction and substructures. By modeling the mass distribution of baryons (stars and gas) around a central bright elliptical galaxy, NGC 1399, and using a modified contraction model motivated by numerical simulations, we show that the dark matter contraction boosts the annihilation signatures by a factor of 4. For dark matter masses around 10 GeV, the upper limit obtained on the annihilation cross section times relative velocity is (σν)∼<(2–3) × 10{sup −25} cm{sup 3} s{sup −1}, which is within a factor of 10 from the value required to explain the dark matter relic density. This effect is more robust than the annihilation boost due to substructure, and it is more important unless the mass of the smallest subhalos is much smaller than that of the Sun.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Galaxy clusters as reservoirs of heavy dark matter and high-energy cosmic rays: constraints from neutrino observations

Galaxy Clusters (GCs) are the largest reservoirs of both dark matter and cosmic rays (CRs). Dark matter self-annihilation can lead to a high luminosity in gamma rays and neutrinos, enhanced by a strong degree of clustering in dark matter substructures. Hadronic CR interactions can also lead to a high luminosity in gamma rays and neutrinos, enhanced by the confinement of CRs from cluster accretion/merger shocks and active galactic nuclei. We show that IceCube/KM3Net observations of high-energy neutrinos can probe the nature of GCs and the separate dark matter and CR emission processes, taking into account how the results depend on the still-substantial uncertainties. Neutrino observations are relevant at high energies, especially at ∼>10 TeV. Our results should be useful for improving experimental searches for high-energy neutrino emission. Neutrino telescopes are sensitive to extended sources formed by dark matter substructures and CRs distributed over large scales. Recent observations by Fermi and imaging atmospheric Cherenkov telescopes have placed interesting constraints on the gamma-ray emission from GCs. We also provide calculations of the gamma-ray fluxes, taking into account electromagnetic cascades inside GCs, which can be important for injections at sufficiently high energies. This also allows us to extend previous gamma-ray constraints to very high dark matter masses and significant CR injections at very high energies. Using both neutrinos and gamma rays, which can lead to comparable constraints, will allow more complete understandings of GCs. Neutrinos are essential for dark matter annihilation channels like χχ→μ{sup +}μ{sup −}, where the neutrino signals are larger than the gamma-ray signals, and for hadronic instead of electronic CRs, because only the first leads to neutrinos. Our results suggest that the multi-messenger observations of GCs will be able to give useful constraints on specific models of dark matter and CRs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Is Sextans dwarf galaxy in a scalar field dark matter halo?

The Bose-Einstein condensate/scalar field dark matter model, considers that the dark matter is composed by spinless-ultra-light particles which can be described by a scalar field. This model is an alternative model to the Λ-cold dark matter paradigm, and therefore should be studied at galactic and cosmological scales. Dwarf spheroidal galaxies have been very useful when studying any dark matter theory, because the dark matter dominates their dynamics. In this paper we study the Sextans dwarf spheroidal galaxy, embedded in a scalar field dark matter halo. We explore how the dissolution time-scale of the stellar substructures in Sextans, constrain the mass, and the self-interacting parameter of the scalar field dark matter boson. We find that for masses in the range (0.12< m{sub φ}<8) ×10{sup -22} eV, scalar field dark halos without self-interaction would have cores large enough to explain the longevity of the stellar substructures in Sextans, and small enough mass to be compatible with dynamical limits. If the self-interacting parameter is distinct to zero, then the mass of the boson could be as high as m{sub φ}≈2×10{sup -21} eV, but it would correspond to an unrealistic low mass for the Sextans dark matter halo . Therefore, the Sextans dwarf galaxy could be embedded in a scalar field/BEC dark matter halo with a preferred self-interacting parameter equal to zero.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Power spectrum tomography of dark matter annihilation with local galaxy distribution

Cross-correlating the gamma-ray background with local galaxy catalogs potentially gives stringent constraints on dark matter annihilation. We provide updated theoretical estimates of sensitivities to the annihilation cross section from gamma-ray data with Fermi telescope and 2MASS galaxy catalogs, by elaborating the galaxy power spectrum and astrophysical backgrounds, and adopting the Markov-Chain Monte Carlo simulations. In particular, we show that taking tomographic approach by dividing the galaxy catalogs into more than one redshift slice will improve the sensitivity by a factor of a few to several. If dark matter halos contain lots of bright substructures, yielding a large annihilation boost (e.g., a factor of ∼100 for galaxy-size halos), then one may be able to probe the canonical annihilation cross section for thermal production mechanism up to masses of ∼700 GeV. Even with modest substructure boost (e.g., a factor of ∼10 for galaxy-size halos), on the other hand, the sensitivities could still reach a factor of three larger than the canonical cross section for dark matter masses of tens to a few hundreds of GeV.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Tidal streams from axion miniclusters and direct axion searches

In some axion dark matter models a dominant fraction of axions resides in dense small-scale substructures, axion miniclusters. A fraction of these substructures is disrupted and forms tidal streams where the axion density may still be an order of magnitude larger than the average. We discuss implications of these streams for the direct axion searches. We estimate the fraction of disrupted miniclusters and the parameters of the resulting streams, and find that stream-crossing events would occur at a rate of about 1/(20 yr) for 2–3 days, during which the signal in axion detectors would be amplified by a factor ∼ 10. These estimates suggest that the effect of the tidal disruption of axion miniclusters may be important for direct axion searches and deserves a more thorough study.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Heating of galactic gas by dark matter annihilation in ultracompact minihalos

The existence of substructure in halos of annihilating dark matter would be expected to substantially boost the rate at which annihilation occurs. Ultracompact minihalos of dark matter (UCMHs) are one of the more extreme examples of this. The boosted annihilation can inject significant amounts of energy into the gas of a galaxy over its lifetime. Here we determine the impact of the boost factor from UCMH substructure on the heating of galactic gas in a Milky Way-type galaxy, by means of N-body simulation. If 1% of the dark matter exists as UCMHs, the corresponding boost factor can be of order 10{sup 5}. For reasonable values of the relevant parameters (annihilation cross section 3×10{sup −26} cm{sup 3} s{sup −1}, dark matter mass 100 GeV, 10% heating efficiency), we show that the presence of UCMHs at the 0.1% level would inject enough energy to eject significant amounts of gas from the halo, potentially preventing star formation within ∼1 kpc of the halo centre.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Fast inference of deep neural networks in FPGAs for particle physics

Here, recent results at the Large Hadron Collider (LHC) have pointed to enhanced physics capabilities through the improvement of the real-time event processing techniques. Machine learning methods are ubiquitous and have proven to be very powerful in LHC physics, and particle physics as a whole. However, exploration of the use of such techniques in low-latency, low-power FPGA (Field Programmable Gate Array) hardware has only just begun. FPGA-based trigger and data acquisition systems have extremely low, sub-microsecond latency requirements that are unique to particle physics. We present a case study for neural network inference in FPGAs focusing on a classifier for jet substructure which would enable, among many other physics scenarios, searches for new dark sector particles and novel measurements of the Higgs boson. While we focus on a specific example, the lessons are far-reaching. A companion compiler package for this work is developed based on High-Level Synthesis (HLS) called hls4ml to build machine learning models in FPGAs. The use of HLS increases accessibility across a broad user community and allows for a drastic decrease in firmware development time. We map out FPGA resource usage and latency versus neural network hyperparameters to identify the problems in particle physics that would benefit from performing neural network inference with FPGAs. For our example jet substructure model, we fit well within the available resources of modern FPGAs with a latency on the scale of 100 ns.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2018

Orientation-dependent fatigue damage in planar slip metals

Shear banding as a ubiquitous phenomenon is mostly observed in planar slip metals under complex plastic deformation conditions, especially in cyclic deformation. The deformation substructures and local damages during fatigue vary from grain to grain, which depends greatly on the crystallographic orientation. Despite previous extensive studies, the orientation-dependent deformation substructure is still elusive due to the complexity of damage evolution and the limitation of spatial resolution in both reciprocal and real sample spaces by conventional characterization methods. In this study, we conducted synchrotron-based X-ray micro-diffraction (mu XRD) experiments to explore the dependence of localized elastic strains on grain orientation. It is demonstrated that the typical intersection of slip shear bands (SSBs) with Lomer-Cottrell locks (LCLs) are the potential damage locations, which are most likely to form in the near [001]//LD grains. The advanced. XRD with ability of spatially quantify the localized strains/fine orientations opens new opportunities for understanding the microscopic mechanisms of damage and failure in bulk materials.

Li, R G↗
DOE OSTI2011

ANALYZING THE FLUX ANOMALIES OF THE LARGE-SEPARATION LENSED QUASAR SDSS J1029+2623

Using a high-resolution radio image, we successfully resolve the two-fold image components B and C of the quasar lens system SDSS J1029+2623. The flux anomalies associated with these two components in the optical regime persist, albeit less strongly, in our radio observations suggesting that the cluster must be modeled by something more than a single central potential. We argue that placing substructure close to one of the components can account for a flux anomaly with negligible changes in the component positions. Our best-fit model has a substructure mass of {approx}10{sup 9} M{sub sun} up to the mass-sheet degeneracy, located roughly 0.''1 west and 0.''1 north of component B. We demonstrate that a positional offset between the centers of the source components can explain the differences between the optical and radio flux ratios.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

A SPITZER INFRARED SPECTROGRAPH DETECTION OF CRYSTALLINE SILICATES IN A PROTOSTELLAR ENVELOPE

We present the Spitzer Space Telescope Infrared Spectrograph spectrum of the Orion A protostar HOPS-68. The mid-infrared spectrum reveals crystalline substructure at 11.1, 16.1, 18.8, 23.6, 27.9, and 33.6 {mu}m superimposed on the broad 9.7 and 18 {mu}m amorphous silicate features; the substructure is well matched by the presence of the olivine end-member forsterite (Mg{sub 2}SiO{sub 4}). Crystalline silicates are often observed as infrared emission features around the circumstellar disks of Herbig Ae/Be stars and T Tauri stars. However, this is the first unambiguous detection of crystalline silicate absorption in a cold, infalling, protostellar envelope. We estimate the crystalline mass fraction along the line of sight by first assuming that the crystalline silicates are located in a cold absorbing screen and secondly by utilizing radiative transfer models. The resulting crystalline mass fractions of 0.14 and 0.17, respectively, are significantly greater than the upper limit found in the interstellar medium ({approx}<0.02-0.05). We propose that the amorphous silicates were annealed within the hot inner disk and/or envelope regions and subsequently transported outward into the envelope by entrainment in a protostellar outflow.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

KINEMATIC DISCOVERY OF A STELLAR STREAM LOCATED IN PISCES

We report the kinematic discovery of the Pisces Stellar Stream (PSS), at Galactic longitude l Almost-Equal-To 135 Degree-Sign and -39 Degree-Sign < b < -36 Degree-Sign . We originally identified this halo substructure from velocities of red giant branch stars in the Sloan Digital Sky Survey (SDSS) Data Release 8, and confirmed its presence in turnoff stars from SDSS photometric data. The PSS is a narrow, kinematically cold tidal stream, with {sigma}{sub v,0} Almost-Equal-To 8 km s{sup -1}. Its metallicity is [Fe/H] Almost-Equal-To -2.2, with {approx}0.3 dex dispersion. The color-magnitude signature of the stream turnoff, combined with our measured metallicity, places the PSS at a distance of 35 {+-} 3 kpc. The PSS is the same as the previously announced ''Triangulum stream'' and part of the proposed ''stream a''. We rule out an association of the PSS with other previously known Milky Way substructures in the same region of the sky.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

FOSSIL EVIDENCE FOR THE TWO-PHASE FORMATION OF ELLIPTICAL GALAXIES

Massive early-type galaxies (ETGs) have undergone dramatic structural evolution over the last 10 Gyr. A companion paper shows that nearby elliptical galaxies with M{sub *} {>=} 1.3 Multiplication-Sign 10{sup 11} M{sub Sun} generically contain three photometric subcomponents: a compact inner component with effective radius R{sub e} {approx}< 1 kpc, an intermediate-scale middle component with R{sub e} Almost-Equal-To 2.5 kpc, and an extended outer envelope with R{sub e} Almost-Equal-To 10 kpc. Here we attempt to relate these substructures with the properties of ETGs observed at higher redshifts. We find that a hypothetical structure formed from combining the inner and middle components of local ellipticals follows a strikingly tight stellar mass-size relation, one that resembles the distribution of ETGs at z Almost-Equal-To 1. Outside of the central kpc, the median stellar mass surface density profiles of this composite structure agree closest with those of massive galaxies that have similar cumulative number density at 1.5 < z < 2.0 within the uncertainty. We propose that the central substructures in nearby ellipticals are the evolutionary descendants of the ''red nuggets'' formed under highly dissipative (''wet'') conditions at high redshifts, as envisioned in the initial stages of the two-phase formation scenario recently advocated for massive galaxies. Subsequent accretion, plausibly through dissipationless (''dry'') minor mergers, builds the outer regions of the galaxy identified as the outer envelope in our decomposition. The large scatter exhibited by this component on the stellar mass-size plane testifies to the stochastic nature of the accretion events.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

DISCOVERY OF A NEW FAINT DWARF GALAXY ASSOCIATED WITH NGC 253

We report the discovery of a new faint dwarf galaxy, which we dub Scl-MM-Dw1, at a projected distance of ∼65 kpc from the spiral galaxy NGC 253. The discovery results from the Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS), a program with the Magellan/Megacam imager to study faint substructure in resolved stellar light around massive galaxies outside of the Local Group. We measure a tip of the red giant branch distance to Scl-MM-Dw1 of D = 3.9 ± 0.5 Mpc, consistent with that of NGC 253, making their association likely. The new dwarf's stellar population is complex, with an old, metal-poor red giant branch (≳10 Gyr, [Fe/H] ∼ –2), and an asymptotic giant branch with an age of ∼500 Myr. Scl-MM-Dw1 has a half-light radius of r{sub h} = 340 ± 50 pc and an absolute magnitude of M{sub V} = –10.3 ± 0.6 mag, comparable to the Milky Way's satellites at the same luminosity. Once complete, our imaging survey of NGC 253 and other nearby massive galaxies will provide a census of faint substructure in halos beyond the Local Group, both to put our own environment into context and to confront models of hierarchical structure formation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Antlia B: a faint dwarf galaxy member of the NGC 3109 association

We report the discovery of Antlia B, a faint dwarf galaxy at a projected distance of ∼72 kpc from NGC 3109 (M{sub V} ∼ −15 mag), the primary galaxy of the NGC 3109 dwarf association at the edge of the Local Group. The tip of the red giant branch distance to Antlia B is D = 1.29 ± 0.10 Mpc, which is consistent with the distance to NGC 3109. A qualitative analysis indicates the new dwarf's stellar population has both an old, metal-poor red giant branch (≳10 Gyr, [Fe/H] ∼ −2), and a younger blue population with an age of ∼200–400 Myr, analogous to the original Antlia dwarf, another likely satellite of NGC 3109. Antlia B has H i gas at a velocity of v{sub helio,H I} = 376 km s{sup −1}, confirming the association with NGC 3109 (v{sub helio} = 403 km s{sup −1}). The H i gas mass (M{sub H i} = 2.8 ± 0.2 × 10{sup 5} M{sub ⊙}), stellar luminosity (M{sub V} = −9.7 ± 0.6 mag) and half light radius (r{sub h} = 273 ± 29 pc) are all consistent with the properties of dwarf irregular and dwarf spheroidal galaxies in the Local Volume, and is most similar to the Leo P dwarf galaxy. The discovery of Antlia B is the initial result from a Dark Energy Camera survey for halo substructure and faint dwarf companions to NGC 3109 with the goal of comparing observed substructure with expectations from the Λ+Cold Dark Matter model in the sub-Milky Way regime.

79 ASTRONOMY AND ASTROPHYSICS↗