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Results for “GALACTIC EVOLUTION”

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 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1991

Large-scale gas dynamical processes affecting the origin and evolution of gaseous galactic halos

Observations of galactic halo gas are consistent with an interpretation in terms of the galactic fountain model in which supernova heated gas in the galactic disk escapes into the halo, radiatively cools and forms clouds which fall back to the disk. The results of a new study of several large-scale gas dynamical effects which are expected to occur in such a model for the origin and evolution of galactic halo gas will be summarized, including the following: (1) nonequilibrium absorption line and emission spectrum diagnostics for radiatively cooling halo gas in our own galaxy, as well the implications of such absorption line diagnostics for the origin of quasar absorption lines in galactic halo clouds of high redshift galaxies; (2) numerical MHD simulations and analytical analysis of large-scale explosions ad superbubbles in the galactic disk and halo; (3) numerical MHD simulations of halo cloud formation by thermal instability, with and without magnetic field; and (4) the effect of the galactic fountain on the galactic dynamo.

Shapiro, Paul R.↗
NASA NTRS1990

Spectral evolution of active galactic nuclei Penrose Compton scattering processes and gamma ray emission from Seyfert galaxies

In black hole spectral evolution models for active galactic nuclei (AGN), present epoch Seyfert galaxies evolve from an earlier precursor active galaxy (PAG) stage at redshift z is approximately 7 where they acted as the thermal sources responsible for the residual cosmic x ray background (RCXB). The Seyfert galaxies which emerge in this context emit Penrose Compton Scattering (PCS) gamma ray transients on the order of hours with a kinematic cutoff in the spectrum less than or equal to 3 MeV. The EGRET (Energetic Gamma-Ray Experimental Telescope/ OSSE (Oriented Scintillation Spectrometer Experiment/ COMPTEL (Compton Telescope)/ BATSE (Burst and Transient Source Experiment) instruments on the Gamma Ray Observatory (GRO) are appropriate instruments to carry out further tests of this model by studying: PCS gamma ray transient emission from individual galaxies and, the possibility that present epoch PCS gamma ray emitting Seyfert galaxies contribute observable temporal variability to the excess diffuse gamma ray background component less than or equal to 3 MeV.

Leiter, Darryl↗
NASA NTRS1982

Spectral evolution of active galactic nuclei: A unified description of the X-ray and gamma

A model for spectral evolution is presented whereby active galactic nuclei (AGN) of the type observed individually emerge from an earlier stage at z approx = 4 in which they are the thermal X-ray sources responsible for most of the cosmic X-ray background (CXB). The conjecture is pursued that these precursor objects are initially supermassive Schwarzschild black holes with accretion disks radiating near the Eddington luminosity limit. It is noted that after approx. 10 to the 8th power years these central black holes are spun-up to a canonical Kerr equilibrium state (A/M = 0.998; Thorne 1974) and shown how they then can lead to spectral evolution involving non-thermal emission extending to gamma rays, at the expense of reduced thermal disk radiation. That major portion of the CXB remaining after the contribution of usual AGN are considered, while a superposition of AGN sources at z 1 can account for the gamma ray background. Extensive X-ray measurements carried out with the HEAO 1 and 2 missions as well as gamma ray and optical data are shown to compare favorably with principal features of this model.

Leiter, D.↗
NASA NTRS1990

A simple analytic model for the evolution of captured galactic disks

The general analytic solution developed by Steinman-Cameron and Durisen and published in 1988 for the evolution of dissipative nonplanar disks is applied to captured galactic disks in model galaxies with nonspherical, scale-free, logarithmic gravitational potentials. Such potentials produce flat rotation curves, similar to those seen in real galaxies. In this case, the analytic solution yields a self-similar structure for the warps and twists that develop while the disks is settling. Being scale-free in a simple, analytic form, this solution is completely defined by only a few dimensionless fitting parameters. As a result, it can be utilized as a mathematical tool to fit settling disks in real galaxies. The minimum time it takes for a disk to settle into a steady state orientation is also a scale-free quantity when expressed in units of the precession period or the orbit period. For realistic parameters, settling times are on the order of one-half to two periods. The use of the time-dependent structure of settling disks as a probe of the three-dimensional mass distribution of the host galaxies, including dark halos, is discussed.

Steiman-Cameron, Thomas Y.↗
NASA NTRS1992

Spectral-luminosity evolution of active galactic nuclei (AGN)

The origin of the cosmic X-ray and gamma-ray backgrounds is explained via the mechanism of AGN spectral-luminosity evolution. The spectral evolution of precursor active galaxies into AGN, and Newton-Raphson input and output parameters are discussed.

Leiter, Darryl↗
NASA NTRS1992

Spectral-luminosity evolution of active galactic nuclei and the cosmic X- and gamma ray background

Coherent electromagnetic dynamo acceleration processes, which act on charge particles within the context of black hole accretion disk scenarios, are generally regarded as the underlying central power source for active galactic nuclei (AGN). If the precursor active galaxies (PAG) for such AGN are formed at high redshift and contain initial seed black holes with mass approximately equal to 10(exp 4) solar masses, then the Eddington limited X-ray radiation emitted during their lifetime will undergo the phenomenon of 'spectral-luminosity evolution'. When accretion disks are first formed at the onset of galaxy formation the accretion rate occurs at very high values of luminosity/size compactness parameter L/R greater than 10(exp 30) erg/cm-sec. In the absence of extended structure, such high values of L/R generate dynamic constraints which suppress coherent, black hole/accretion disk dynamo particle acceleration processes. This inhibits nonthermal radiation processes and causes the spectrum of X-radiation emitted by PAG to be predominantly thermal. A superposition of PAG sources at z is greater than or equal to 6 can account for the residual cosmic X-ray background (CXB) obtained from the total CXB after subtraction of foreground AGN sources associated with present epoch Seyfert galaxies. The manner in which the PAG undergo spectral-luminosity evolution into Seyfert galaxies is investigated.

Leiter, Darryl↗
NASA NTRS1991

Spectral-luminosity evolution of active galactic nuclei and the cosmic X-ray background

Black hole accretion disk dynamo processes are generally regarded as the central power source for AGN (Rees, 1984). If the precursor active galaxies for such AGN are formed at redshift z greater than about 4 and contain initial central seed black holes about 10 exp 6 solar masses, then the Eddington limited X-ray radiation emitted during their lifetime will undergo the phenomenon of 'spectral-luminosity evolution'. When accretion disks are first formed at the onset of galaxy formation, the accretion rate occurs at high values of luminosity/size compactness parameter L/R greater than 10 exp 30 erg/cm-sec. Such high values of L/R generate dynamic constraints which suppress nonthermal black hole accretion disk dynamo processes in favor of thermal processes. This causes the spectrum of X-radiation emitted by early AGN to be predominantly thermal. A superposition of such thermal, comptonized PAG sources can account for the residual cosmic X-ray background and can act as a source of X-ray heating of the intergalactic medium for z greater than about 4.

Leiter, Darryl↗
NASA NTRS1988

Evolution of inclined galactic gas disks. I - A cloud-fluid approach

In this paper, numerical models are developed to follow the time-dependent behavior of inclined gas disks in nonspherical galaxies. Differential equations are developed which describe the time rate of change of the orbits of mass elements composing a thin, smoothly warping and twisting gas disk. These equations include multiple moments due to the nonsphericity of the galactic mass distribution, Coriolis effects arising from determination of orbits in a reference frame which tumbles with the surface figure of the galaxy, and viscous forces arising from cloud-cloud collisions. A cloud-fluid approach is used in determining the viscous forces. Physical arguments are used to determine the maximum permissible local value of the coefficient of kinematic viscosity. For small initial inclinations, analytic expressions are obtained for the inclination as a function of time and for the settling time, and the features of the solution are discussed.

Steiman-Cameron, Thomas Y.↗
NASA NTRS1988

Evolution of heavy-element abundances in the Galactic halo and disk

The constraints on the universal energy density and cosmological constant from cosmochronological ages and the Hubble age are reviewed. Observational evidence for the galactic chemical evolution of the heavy-element chronometers is descirbed in the context of numerical models. The viability of the recently discovered Th/Nd stellar chronometer is discussed, along with the suggestion that high r-process abundances in metal-poor stars may have resulted from a primordial r-process, as may be required by some inhomogeneous cosmologies.

Mathews, G. J.↗
DOE OSTI2024

Chemical Doppelgangers in GALAH DR3: The Distinguishing Power of Neutron-capture Elements among Milky Way Disk Stars

The observed chemical diversity of Milky Way stars places important constraints on Galactic chemical evolution and the mixing processes that operate within the interstellar medium. Recent works have found that the chemical diversity of disk stars is low. For example, the Apache Point Observatory Galactic Evolution Experiment (APOGEE) "chemical doppelganger rate," or the rate at which random pairs of field stars appear as chemically similar as stars born together, is high, and the chemical distributions of APOGEE stars in some Galactic populations are well-described by two-dimensional models. However, limited attention has been paid to the heavy elements (Z > 30) in this context. In this work, we probe the potential for neutron-capture elements to enhance the chemical diversity of stars by determining their effect on the chemical doppelganger rate. We measure the doppelganger rate in GALactic Archaeology with HERMES DR3, with abundances rederived using The Cannon, and find that considering the neutron-capture elements decreases the doppelganger rate from ~2.2% to 0.4%, nearly a factor of 6, for stars with –0.1 < [Fe/H] < 0.1. While chemical similarity correlates with similarity in age and dynamics, including neutron-capture elements does not appear to select stars that are more similar in these characteristics. Our results highlight that the neutron-capture elements contain information that is distinct from that of the lighter elements and thus add at least one dimension to Milky Way abundance space. This work illustrates the importance of considering the neutron-capture elements when chemically characterizing stars and motivates ongoing work to improve their atomic data and measurements in spectroscopic surveys.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1989

Evolution of self-gravitating accretion disks in active galactic nuclei

The evolution of self-gravitating gaseous disks in active galactic nuclei on scales of about 10-1000 pc is investigated. Star formation is a plausible outcome of the Jeans instability operating in a disk which violates the criterion for local stability. Even a low efficiency of star formation would deplete the gaseous disk on a short time scale and create a flat stellar system. These systems can evolve (sphericalize) secularly by means of stellar encounters but this process appears to be too slow to be important. Such flattened stellar systems may be common in the circumnuclear regions of disk galaxies. Conventional viscosities are inefficient in building anew the accretion process even in a cosmological time. Strongly self-gravitating disks are unstable to global nonaxisymmetric modes, which can induce radial inflow of gas in a short dynamical time. The latter effect is studied in a separate paper.

Shlosman, Isaac↗
NASA NTRS1992

Possible sources of the Population I lithium abundance and light-element evolution

One-zone numerical models of Galactic chemical evolution of the light elements (lithium, beryllium, boron, and deuterium) with a broad sample of possible stellar lithium production sites and star formation histories, including the multiple merger model of Mathews and Schramm (1992), are examined. Models with high primordial lithium are constrained by observations of lithium and potassium in the interstellar medium of the LMC to have Li abundances close to the Population I value of about 10 exp -9. Li-7 production in intermediate- or high-mass stars (greater than 4 solar masses) is found to fit observations somewhat better than production in low-mass (1-5 solar masses) stars. Since elevated levels of lithium are commonly observed in intermediate-mass stars in the LMC, it is argued that this is indeed the major source of the Population I Li-7 abundance.

Brown, Lawrence E.↗
NASA NTRS1980

Star counts as an indicator of galactic structure and quasar evolution

A detailed model of the stellar content of the Galaxy is described briefly. Illustrative applications of the model are made, using existing data, to indicate how star counts can be used to determine some parameters of galactic structure, to detect a massive (stellar) halo, and to constrain models of quasar evolution.

Bahcall, J. N.↗
NASA NTRS1965

The helium content of the galaxy.

Problem of large solar and stellar helium content examined in regard to nucleosynthesis theory that initial composition of galaxy was hydrogen

HELIUM↗
DOE OSTI2025

Inference of Multichannel r -process Element Enrichment in the Milky Way Using Binary Neutron Star Merger Observations

Observations of GW170817 strongly suggest that binary neutron star (BNS) mergers produce rapid neutron-capture nucleosynthesis ( r -process) elements. However, it remains an open question whether these mergers can account for all the r -process element enrichment in the Milky Way’s history. Here, we constrain the contributions of the BNS channel using astrophysical neutron star observations. The rate and mass distributions are constrained by LIGO/Virgo/Kagra through the latest catalog GWTC-3, the neutron star equation of state by gravitational-wave, radio, and X-ray observations, and the delay time distribution by short gamma-ray burst (GRB) host galaxy associations. We present a Bayesian framework to consistently combine these observations with abundance information to quantify the contribution and uncertainties of single and multiple astrophysical enrichment sources, and obtain a distribution of per-event BNS r -process element yields consistent with geophysical and astrophysical abundance constraints. We then adopt a Galactic chemical evolution model assuming an instantaneous and fixed amount of Fe enrichment from core-collapse supernovae, and show that BNS-only enrichment scenarios remain inconsistent with the observed r-process abundance trend of disk stars in the Galaxy even with the uncertainties in BNS merger observations. Using stellar abundance observations instead of the short GRB constraints, we can infer a shorter BNS delay time distribution with power-law index α ≤ −2.0 and minimum delay time ${t}_{{\rm{\min }}}\leqslant 40$ Myr at 90% confidence, consistent with detailed Galactic chemical evolution models. Such delay times are in tension with those predicted by standard BNS formation models. Alternatively, we confirm that a two-channel scenario, in which the second channel tracks the star formation history without significant delay, can account for both Galactic stellar and short GRB observations. We estimate that 45%–90% of the r -process abundance in the Milky Way today would have been produced by this star formation-tracking channel, rather than BNS mergers with significant delay times.

gravitational wave astronomy↗