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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.

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At least 1,603 records · Page 89
All sourcesNASA NTRSDOE OSTI
NASA NTRS2002

A Giant Warm Baryonic Halo for the Coma Cluster

Several deep PSPC observations of the Coma cluster unveil a very large-scale halo of soft X-ray emission, substantially in excess of the well know radiation from the hot intra-cluster medium. The excess emission, previously reported in the central cluster regions through lower-sensitivity EUVE and ROSAT data, is now evident out to a radius of 2.5 Mpc, demonstrating that the soft excess radiation from clusters is a phenomenon of cosmological significance. The spectrum at these large radii cannot be modeled non-thermally, but is consistent with the original scenario of thermal emission at warm temperatures. The mass of this plasma is at least on par with that of the hot X-ray emitting plasma, and significantly more massive if the plasma resides in low-density filamentary structures. Thus the data lend vital support to current theories of cosmic evolution, which predict greater than 50 percent by mass of today's baryons reside in warm-hot filaments converging at clusters of galaxies.

Bonamente, Max↗
NASA NTRS2003

Physics of Galaxy Clusters and How it Affects Cosmological Tests

We have worked on the analysis of the Chandra observations of the distant clusters of galaxies, and on exploring new ways to obtain cosmological constraints from the cluster data. Some of the scientific results are discussed below: (1) Evolution of the cluster scaling relations at z is approximately 0.5; (2) Using gas mass as a proxy for the total cluster mass.

Vikhlinin, Alexey↗
NASA NTRS2006

Gravitational Rocket from the Merging Massive Black Hole Binaries

Coalescing massive black hole binaries are expected to be among the most fascinating gravitational wave sources, observable by the NASA/ESA LISA detector. Not only will the merger events reveal the rich phenomenology of extremely strong and dynamical gravity deep inside the potential wells at the centers of galaxies (thus providing an excellent testing ground for general relativity), it will also make important contributions to the astrophysics of massive black hole evolutions. Typical black hole mergers involve asymmetric radiation of gravitational waves and lose linear momentum as well as energy and angular momentum. As a result, the merger remnant receives a kick from the GW emission: a gravitational rocket effect. High kick velocities (higher than the escape velocities of the host structure) would have a strong impact on our understanding of how massive black holes have evolved over time and, in particular, on the estimates of the merger rate for LISA. The main difficulties in calculations of the kick velocities has been in the last moments of the merger where the full theory of general relativity must be employed to accurately model the black hole dynamics. I describe a recent calculation of the kick velocities from numerical relativity simulations of the merging black hole binaries.

Choi, Dale↗
NASA NTRS2012

Servicing Mission 4 and the Extraordinary Science of the Hubble Space Telescope

Just two years ago, NASA astronauts performed a challenging and flawless final Space Shuttle servicing mission to the orbiting Hubble Space Telescope. With science instruments repaired on board and two new ones installed, the observatory. is more powerful now than ever before. I will show the dramatic highlights of the servicing mission and present some of the early scientific results from the refurbished telescope. Its high sensitivity and multi-wavelength capabilities are revealing the highest redshift galaxies ever seen, as well as details of the cosmic web of intergalactic medium, large scale structure formation, solar system bodies, and stellar evolution. Enlightening studies of dark matter, dark energy, and exoplanet atmospheres add to the profound contributions to astrophysics that are being made with Hubble, setting a critical stage for future observatories such as the James Webb Space Telescope.

Wiseman, Jennifer J.↗
NASA NTRS2013

Hubble Tarantula Treasury Project: Unraveling Tarantula's Web. I. Observational Overview and First Results

The Hubble Tarantula Treasury Project (HTTP) is an ongoing panchromatic imaging survey of stellar populations in the Tarantula Nebula in the Large Magellanic Cloud that reaches into the sub-solar mass regime (<0.5 Stellar Mass). HTTP utilizes the capability of the Hubble Space Telescope to operate the Advanced Camera for Surveys and the Wide Field Camera 3 in parallel to study this remarkable region in the near-ultraviolet, optical, and near-infrared spectral regions, including narrow-band H(alpha) images. The combination of all these bands provides a unique multi-band view. The resulting maps of the stellar content of the Tarantula Nebula within its main body provide the basis for investigations of star formation in an environment resembling the extreme conditions found in starburst galaxies and in the early universe. Access to detailed properties of individual stars allows us to begin to reconstruct the temporal and spatial evolution of the stellar skeleton of the Tarantula Nebula over space and time on a sub-parsec scale. In this first paper we describe the observing strategy, the photometric techniques, and the upcoming data products from this survey and present preliminary results obtained from the analysis of the initial set of near-infrared observations.

Tarantula↗
DOE OSTI2021

Kiloparsec-scale AGN outflows and feedback in merger-free galaxies

ABSTRACT Recent observations and simulations have challenged the long-held paradigm that mergers are the dominant mechanism driving the growth of both galaxies and supermassive black holes (SMBH), in favour of non-merger (secular) processes. In this pilot study of merger-free SMBH and galaxy growth, we use Keck Cosmic Web Imager spectral observations to examine four low-redshift (0.043 < z < 0.073) disc-dominated ‘bulgeless’ galaxies hosting luminous active galactic nucleus (AGN), assumed to be merger-free. We detect blueshifted broadened [O iii] emission from outflows in all four sources, which the $\mathrm{[O\, \small {III}]}$/$\rm {H}\beta$ ratios reveal are ionized by the AGN. We calculate outflow rates in the range $0.12{-}0.7~\rm {M}_{\odot }~\rm {yr}^{-1}$, with velocities of $675{-}1710~\rm {km}~\rm {s}^{-1}$, large radial extents of $0.6{-}2.4~\rm {kpc}$, and SMBH accretion rates of $0.02{-}0.07~\rm {M}_{\odot }~\rm {yr}^{-1}$. We find that the outflow rates, kinematics, and energy injection rates are typical of the wider population of low-redshift AGN, and have velocities exceeding the galaxy escape velocity by a factor of ∼30, suggesting that these outflows will have a substantial impact through AGN feedback. Therefore, if both merger-driven and non-merger-driven SMBH growth lead to co-evolution, this suggests that co-evolution is regulated by feedback in both scenarios. Simulations find that bars and spiral arms can drive inflows to galactic centers at rates an order of magnitude larger than the combined SMBH accretion and outflow rates of our four targets. This work therefore provides further evidence that non-merger processes are sufficient to fuel SMBH growth and AGN outflows in disc galaxies.

Smethurst, R. J. (ORCID:0000000164177196)↗
DOE OSTI2022

H α emission in local galaxies: star formation, time variability, and the diffuse ionized gas

ABSTRACT The nebular recombination line H α is widely used as a star formation rate (SFR) indicator in the local and high-redshift Universe. We present a detailed H α radiative transfer study of high-resolution isolated Milky-Way and Large Magellanic Cloud simulations that include radiative transfer, non-equilibrium thermochemistry, and dust evolution. We focus on the spatial morphology and temporal variability of the H α emission, and its connection to the underlying gas and star formation properties. The H α and H β radial and vertical surface brightness profiles are in excellent agreement with observations of nearby galaxies. We find that the fraction of H α emission from collisional excitation amounts to fcol ∼ 5–$10{{\ \rm per\ cent}}$, only weakly dependent on radius and vertical height, and that scattering boosts the H α luminosity by $\sim 40{{\ \rm per\ cent}}$. The dust correction via the Balmer decrement works well (intrinsic H α emission recoverable within 25 per cent), though the dust attenuation law depends on the amount of attenuation itself both on spatially resolved and integrated scales. Important for the understanding of the H α–SFR connection is the dust and helium absorption of ionizing radiation (Lyman continuum [LyC] photons), which are about $f_{\rm abs}\approx 28{{\ \rm per\ cent}}$ and $f_{\rm He}\approx 9{{\ \rm per\ cent}}$, respectively. Together with an escape fraction of $f_{\rm esc}\approx 6{{\ \rm per\ cent}}$, this reduces the available budget for hydrogen line emission by nearly half ($f_{\rm H}\approx 57{{\ \rm per\ cent}}$). We discuss the impact of the diffuse ionized gas, showing – among other things – that the extraplanar H α emission is powered by LyC photons escaping the disc. Future applications of this framework to cosmological (zoom-in) simulations will assist in the interpretation of spectroscopy of high-redshift galaxies with the upcoming James Webb Space Telescope.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1995

Ultraviolet radiation from evolved stellar populations. 2: The ultraviolet upturn phenomenon in elliptical galaxies

We discuss the far-ultraviolet upturn phenomenon (UVX) observed in elliptical galaxies and spiral galaxy bulges. Our premise is the UV radiation from these systems emanates primarily from extreme horizontal branch (EHB) stars and their progeny. We derive the broad-band UV colors 1500-V and 2500-V for globular clusters and elliptical galaxies from the available satellite data and investigate color-color and color-line strength correlation. Clusters can be bluer than any galaxy in 15-V and 25-V, implying larger hot star populations, but galaxies are significantly bluer than clusters in 15-25 at a given 15-V. We attribute this primarily to the effect of metal abundance on the mid-UV (2500 A) light. These redder colors of the galaxies also imply that the UVX in galaxies is not produced by metal-poor subpopulations similar to the clusters. We devlop a simple spectral synthesis formulation for all phases of single star evolution from the zero-age main sequence (ZAMS) to the white dwarf cooling track that requires only one or two parameters for each choice of age and abundance. We provide the ingredients necessary for constructing models with arbitrary horizontal branch (HB) morphologies in the age range 2 less than t less than 20 Gyr and for six metallicities in the range -2.26 less than (Fe/H) less than 0.58; we also consider the efect of enhanced Y in metal-rich models. The maximum lifetime UV output is produced by EHB stars with (M(sub env))(sup 0) approximately 0.02 solar mass and can be up to 30 times higher than for post-asymptotic giant branch (P-AGB) stars. The ultraviolet output of old populations is governed primarily by the distribution of (M(sub env))(sup 0)P(M(sub env))(sup 0), on the ZAHB. The UV output is not very sensitive to (Fe/H) or to Y, but it can change very rapidly with (M(sub env))(sup 0). Thus it is extremely sensitive to the precise nature of giant-branch mass loss. Our models use simple descriptions of P(M(sub env))(sup 0) to bracket the colors produced from any real distribution of stars. Our models accurately predict the range of UV colors observed for the globular clusters, given known constraints on their age, abundances, and HB morphologies. We find that models with (Fe/H) greater than or = 0 that do not contain EHB stars cannot reproduce the colors of most of the galaxies. The models also predict that the fraction of the far-UV light from P-AGB stars, which are spatially resolvable in nearby galaxies, is approximately 70% and approximately 20% for moderate UVX and strong UVX systems, respectively. We find that 25-V, but not 15-V, is sensitive to the age and abundance, though these cannot always be cleanly distinguished. The galaxy colors place limits of (Fe/H) greater than -0.5 and less than 15% on the contribution of globular cluster-type populations to the V light. Galaxy colors are consistent with solar-abundance models with ages in the range 6-14 Gyr. We discuss several implications of the observations and the models, including the question of light metal versus iron peak enhancements in galaxies, whether the UV color-Mg(sub 2) correlation is continuous or discrete, effects of helium abundnace on the UVX, and the key question of whether red giant branch mass loss can be large enough to produce the necessary EHB population in the strong UVX galaxies.

Dorman, Ben↗
NASA NTRS2006

The Evolution of Dust in Extreme Astrophysical Environments

The recent discovery of QSOs located at redshift z greater than 6, with high infrared luminosities (typically 1.E13 solar luminosities), challenges our understanding of dust evolution. Indeed, these high infrared luminosities imply more than 1.E8 solar masses of dust and nearly solar gas-to-dust mass ratios. An interesting question is how do we make so much dust within approx. 400 Myr, which is roughly the age of the galaxy assuming it formed at z-10. During this epoch, AGB stars could not have enriched the ISM, so that all the dust must have formed by supernovae (SN). However, SN blast waves also play an important role in destroying the dust in the ISM. Understanding the formation and evolution of the dust in such objects requires therefore the construction of a detailed model for the chemical evolution of the gas and dust phases of the ISM. We focus on the well-studied quasar SDSS J1148+5251, at z=6.42. Using the IR luminosities as tracers of of the star formation rate (SFR) in these objects implies a rates of 3E3 Msun/yr, and SN rates of about 200/yr. However, this quasar contains an embedded AGN, so that the IR luminosity may not be a good tracer of the SFR in this object. Using the estimated mass of gas as a tracer of the SFR we construct a self-consistent model for the evolution of interstellar dust, which also follows the formation and destruction of the dust by SN. Finally, we discuss the contribution of the AGN to the total infrared luminosity, and its implications for the mass of the black hole that is required to power the observed IR emission.

Dwek, Eli↗
DOE OSTI2022

Cross-correlation of Dark Energy Survey Year 3 lensing data with ACT and P l a n c k thermal Sunyaev-Zel’dovich effect observations. II. Modeling and constraints on halo pressure profiles

Hot, ionized gas leaves an imprint on the cosmic microwave background via the thermal Sunyaev-Zel’dovich (tSZ) effect. The cross-correlation of gravitational lensing (which traces the projected mass) with the tSZ effect (which traces the projected gas pressure) is a powerful probe of the thermal state of ionized baryons throughout the Universe and is sensitive to effects such as baryonic feedback. In a companion paper (Gatti et al. Phys. Rev. D 105, 123525 (2022)), we present tomographic measurements and validation tests of the cross-correlation between Galaxy shear measurements from the first three years of observations of the Dark Energy Survey and tSZ measurements from a combination of Atacama Cosmology Telescope and Planck observations. In this work, we use the same measurements to constrain models for the pressure profiles of halos across a wide range of halo mass and redshift. We find evidence for reduced pressure in low-mass halos, consistent with predictions for the effects of feedback from active Galactic nuclei. We infer the hydrostatic mass bias (B ≡ M 500c /M SZ from our measurements, finding B = 1.8 ± 0.1 when adopting the Planck-preferred cosmological parameters. We additionally find that our measurements are consistent with a nonzero redshift evolution of B, with the correct sign and sufficient magnitude to explain the mass bias necessary to reconcile cluster count measurements with the Planck-preferred cosmology. Our analysis introduces a model for the impact of intrinsic alignments (IAs) of galaxy shapes on the shear-tSZ correlation. We show that IA can have a significant impact on these correlations at current noise levels.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1989

Chemical evolution of viscously evolving galactic discs

The ability of the Lin-Pringle (1987) model of galactic disk formation to reproduce the observed radial distributions of total gas surface density and metals in disk galaxies is investigated. It is found that a satisfactory fit is obtained provided that there exists an outer cut-off to the star-forming disk beyond which gas is allowed to viscously evolve. The metallicity gradient is then established by radial inflow of gas from beyond this cut-off.

Clarke, Catherine J.↗
NASA NTRS1979

Collapse and relaxation of rotating stellar systems

The evolution of initially spherical, uniform-density, and uniform-velocity-dispersion stellar systems is investigated by means of a three-dimensional N-body computer model. Some 100,000 simulation stars are used to follow the collapse and relaxation of the system for various amounts of angular momentum in solid-body rotation. For initially low values of the angular momentum satisfying the Ostriker-Pebbles stability criterion the systems quickly relax to an axisymmetric shape and resemble elliptical galaxies in appearance. The maximum flattening for these systems is equivalent only to an E2 system. For larger values of the initial angular momentum bars develop, and the systems undergo a much more drastic evolution. The apparent rotational and random velocities of the barred systems are very sensitive to the viewing direction. An additional complication is the frequent misalignment of the apparent major axis with the direction that reflects the maximum rotation.

Hohl, F.↗
DOE OSTI2022

Dark Energy Survey Year 3 Results: Three-point shear correlations and mass aperture moments

We present high signal-to-noise measurements of three-point shear correlations and the third moment of the mass aperture statistic using the first 3 years of data from the Dark Energy Survey. We additionally obtain the first measurements of the configuration and scale dependence of the four three-point shear correlations which carry cosmological information. With the third-order mass aperture statistic, we present tomographic measurements over angular scales of 4 to 60 arcminutes with a combined statistical significance of 15.0 σ . Using the tomographic information and measuring also the second-order mass aperture, we additionally obtain a skewness parameter and its redshift evolution. We find that the amplitudes and scale-dependence of these shear 3pt functions are in qualitative agreement with measurements in a mock galaxy catalog based on N-body simulations, indicating promise for including them in future cosmological analyses. We validate our measurements by showing that B-modes, parity-violating contributions and PSF modeling uncertainties are negligible, and determine that the measured signals are likely to be of astrophysical and gravitational origin.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

R-process-rich Stellar Streams in the Milky Way*

We present high-resolution Magellan/MIKE spectra of 22 bright (9 < V < 13.5) metal-poor stars (-3.18 < [Fe/H] < -1.37) in three different stellar streams, the Helmi debris stream, the Helmi trail stream, and the ω Centauri progenitor stream. We augment our Helmi debris sample with results for 10 stars by Roederer et al. for a total of 32 stars. Detailed chemical abundances of light elements as well as heavy neutron-capture elements have been determined for our 22 stars. All three streams contain carbon-enhanced stars. For 13 stars, neutron-capture element lines were detectable, and they all show signatures in agreement with the scaled solar r -process pattern, albeit with a large spread of -0.5 < [Eu/Fe] < +1.3. Eight of these stars show an additional small s -process contribution superposed onto their r -process pattern. This could be discerned because of the relatively high signal-to-noise ratio of the spectra given that the stars are close by in the halo. Our results suggest that the progenitors of these streams experienced one or more r -process events early on, such as a neutron star merger or another prolific r -process source. This widely enriched these host systems before their accretion by the Milky Way. The small s -process contribution suggests the presence of asymptotic giant branch stars and associated local (inhomogeneous) enrichment as part of the ongoing chemical evolution by low-mass stars. Stars in stellar streams may thus be a promising avenue for studying the detailed history of large dwarf galaxies and their role in halo assembly with easily accessible targets for high-quality spectra of many stars.

Astronomy & Astrophysics↗
NASA NTRS1988

The red envelope and the age of the universe

The photometric evolution of old stellar populations is slow in the optical/IR. Consequently, observational error is a strongly limiting factor in the accuracy possible for determining galaxy ages or estimating q0 at high redshifts from indices such as the 4000-A break. Existing observations of the red envelope at z above about 0.3 are consistent with a wide range of evolutionary histories. The middle-UV spectral region (rest wavelengths = 2000-3300 A) appears to promise less ambiguous results.

O'Connell, Robert W.↗
NASA NTRS1991

Do clusters of galaxies affect the spectrum of the microwave background?

The distortion, averaged over the sky, due to the Compton scattering of background photons with electrons in the hot gas in clusters of galaxies is calculated. Using an existing sample of X-ray clusters, various values of the density parameter Omega, and plausible models for cluster evolution, Monte Carlo realizations of the microwave sky are generated. The spatial structure of these simulations shows a network of discrete sources whose properties can be a strong function of both evolution and Omega. The amount of spectral distortion in the models is greatest for models characterized by self-similar cluster evolution in an open universe and is within an order of magnitude of the current upper limits. Thus, improved observational sensitivity must inevitably detect some deviation from a blackbody spectrum.

Markevitch, M.↗
NASA NTRS1993

Heating of galactic disks by mergers

The paper considers dynamical aspects of mergers between disks and small satellite companions, with special attention given to: the self-consistent response of disks to satellite mergers, which generalizes an earlier investigation by Quinn & Goodman (1986); the evolution of satellite orbits and debris; and the long-term structure and kinematics of thickened disks as compared with observed properties of disk galaxies. For satellites with masses in the range 0.04-0.2 M sub disk on initially circular inclined prograde orbits merging with disks similar to the thin disk of the Galaxy, it is found that the dynamical coupling between disks and satellites leads to orbital decay. Disks respond to mergers by spreading both radially and vertically. The radial heating is driven largely by the spiral response of the dynamically cool disks to satellite perturbations.

Quinn, P. J.↗