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Caltech Authors (167.915 recursos)

Repository of works by Caltech published authors.

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Mostrando recursos 1 - 20 de 638

  1. The origin of ultra diffuse galaxies: stellar feedback and quenching

    Chan, T. K.; Kereš, D.; Wetzel, A.; Hopkins, P. F.; Faucher-Giguère, C.-A.; El-Badry, K.; Garrison-Kimmel, S.; Boylan-Kolchin, M.
    We test if the cosmological zoom-in simulations of isolated galaxies from the FIRE project reproduce the properties of ultra diffuse galaxies (UDGs). We show that outflows that dynamically heat galactic stars, together with a passively aging stellar population after imposed quenching, naturally reproduce the observed population of red UDGs, without the need for high spin halos, or dynamical influence from their host cluster. We reproduce the range of surface brightness, radius and absolute magnitude of the observed red UDGs by quenching simulated galaxies at a range of different times. They represent a mostly uniform population of dark matter-dominated dwarf galaxies...

  2. Pre-supernova outbursts via wave heating in massive stars – II. Hydrogen-poor stars

    Fuller, Jim; Ro, Stephen
    Pre-supernova (SN) outbursts from massive stars may be driven by hydrodynamical wave energy emerging from the core of the progenitor star during late nuclear-burning phases. Here, we examine the effects of wave heating in stars containing little or no hydrogen, i.e. progenitors of Type IIb/Ib SNe. Because there is no massive hydrogen envelope, wave energy is thermalized near the stellar surface where the overlying atmospheric mass is small but the optical depth is large. Wave energy can thus unbind this material, driving an optically thick, super-Eddington wind. Using 1D hydrodynamic MESAsimulations of ∼5 M⊙ He stars, we find that wave heating...

  3. Principal-component analysis of the cosmic microwave background anisotropies: revealing the tensor degeneracy

    Efstathiou, G.
    A principal-component analysis of cosmic microwave background (CMB) anisotropy measurements is used to investigate degeneracies among cosmological parameters. The results show that a degeneracy with tensor modes — the ‘tensor degeneracy’— dominates uncertainties in estimates of the baryon and cold dark matter densities, ω_b=Ω_bh^2, ω_c=Ω_ch^(2,1) from an analysis of CMB anisotropies alone. The principal-component analysis agrees well with a maximum-likelihood analysis of the observations, identifying the main degeneracy directions and providing an impression of the effective dimensionality of the parameter space.

  4. Covariant magnetoionic theory - II. Radiative transfer

    Broderick, Avery; Blandford, Roger
    Accretion on to compact objects plays a central role in high-energy astrophysics. In these environments, both general relativistic and plasma effects may have significant impacts upon the spectral and polarimetric properties of the accretion flow. In Paper I we presented a fully general relativistic magnetoionic theory, capable of tracing rays in the geometric optics approximation through a magnetized plasma in the vicinity of a compact object. In this paper we discuss how to perform polarized radiative transfer along these rays. In addition we apply the formalism to a barotropic thick-disc model, appropriate for low-luminosity active galactic nuclei. We find that,...

  5. Constraining Alternative Theories of Gravity Using Pulsar Timing Arrays

    Cornish, Neil J.; O’Beirne, Logan; Taylor, Stephen R.; Yunes, Nicolás
    The opening of the gravitational wave window by ground-based laser interferometers has made possible many new tests of gravity, including the first constraints on polarization. It is hoped that, within the next decade, pulsar timing will extend the window by making the first detections in the nanohertz frequency regime. Pulsar timing offers several advantages over ground-based interferometers for constraining the polarization of gravitational waves due to the many projections of the polarization pattern provided by the different lines of sight to the pulsars, and the enhanced response to longitudinal polarizations. Here, we show that existing results from pulsar timing arrays...

  6. Evolution of the magnetized, neutrino-cooled accretion disk in the aftermath of a black hole-neutron star binary merger

    Hossein Nouri, Fatemeh; Duez, Matthew D.; Foucart, Francois; Deaton, M. Brett; Haas, Roland; Haddadi, Milad; Kidder, Lawrence E.; Ott, Christian D.; Pfeiffer, Harald P.; Scheel, Mark A.; Szilagyi, Bela
    Black hole–torus systems from compact binary mergers are possible engines for gamma-ray bursts (GRBs). During the early evolution of the postmerger remnant, the state of the torus is determined by a combination of neutrino cooling and magnetically driven heating processes, so realistic models must include both effects. In this paper, we study the postmerger evolution of a magnetized black hole–neutron star binary system using the Spectral Einstein Code (SpEC) from an initial postmerger state provided by previous numerical relativity simulations. We use a finite-temperature nuclear equation of state and incorporate neutrino effects in a leakage approximation. To achieve the needed...

  7. The Extended Baryonic Halo of NGC 3923

    Miller, Bryan W.; Ahumada, Tomás; Puzia, Thomas H.; Candlish, Graeme N.; McGaugh, Stacy S.; Mihos, J. Christopher; Sanderson, Robyn E.; Schirmer, Mischa; Smith, Rory; Taylor, Matthew A.
    Galaxy halos and their globular cluster systems build up over time by the accretion of small satellites. We can learn about this process in detail by observing systems with ongoing accretion events and comparing the data with simulations. Elliptical shell galaxies are systems that are thought to be due to ongoing or recent minor mergers. We present preliminary results of an investigation of the baryonic halo—light profile, globular clusters, and shells/streams—of the shell galaxy NGC 3923 from deep Dark Energy Camera (DECam) g and i-band imaging. We present the 2D and radial distributions of the globular cluster candidates out to...

  8. xGASS: total cold gas scaling relations and molecular-to-atomic gas ratios of galaxies in the local Universe

    Catinella, Barbara; Saintonge, Amélie; Janowiecki, Steven; Cortese, Luca; Davé, Romeel; Lemonias, Jenna J.; Cooper, Andrew P.; Schiminovich, David; Hummels, Cameron B.; Fabello, Silvia; Geréb, Katinka; Kilborn, Virginia; Wang, Jing
    We present the extended GALEX Arecibo SDSS Survey (xGASS), a gas fraction-limited census of the atomic hydrogen (H I) gas content of 1179 galaxies selected only by stellar mass (M⋆ = 10^9–10^(11.5) M_⊙) and redshift (0.01 < z < 0.05). This includes new Arecibo observations of 208 galaxies, for which we release catalogues and H I spectra. In addition to extending the GASS H I scaling relations by one decade in stellar mass, we quantify total (atomic+molecular) cold gas fractions and molecular-to-atomic gas mass ratios, R_(mol), for the subset of 477 galaxies observed with the IRAM 30 m telescope. We find that atomic...

  9. Pressure-anisotropy-induced nonlinearities in the kinetic magnetorotational instability

    Squire, J.; Quataert, E.; Kunz, M. W.
    In collisionless and weakly collisional plasmas, such as hot accretion flows onto compact objects, the magnetorotational instability (MRI) can differ significantly from the standard (collisional) MRI. In particular, pressure anisotropy with respect to the local magnetic-field direction can both change the linear MRI dispersion relation and cause nonlinear modifications to the mode structure and growth rate, even when the field and flow perturbations are very small. This work studies these pressure-anisotropy-induced nonlinearities in the weakly nonlinear, high-ion-beta regime, before the MRI saturates into strong turbulence. Our goal is to better understand how the saturation of the MRI in a low-collisionality...

  10. A variational formalism for tidal excitation: non-rotating, homentropic stars

    Rathore, Yasser; Broderick, Avery E.; Blandford, Roger
    We present a variational formalism for describing the dynamical evolution of an oscillating star with a point-mass companion in the linear, non-relativistic regime. This includes both the excitation of normal modes and the back-reaction of the modes on the orbit. The general formalism for arbitrary fluid configurations is presented, and then specialized to a homentropic potential flow. Our formalism explicitly identifies and conserves both energy and angular momentum. We also consider corrections to the orbit up to 7/2 post-Newtonian order.

  11. Pair cascade models of gamma-ray blazars

    Levinson, Amir; Blandford, Roger
    The detection of bright, variable γ-ray emission from about 40 blazars by the EGRET instrument on CGRO and the nondetection of radio quite AGN or extended radio sources, has lent additional support to the unified model of AGN. It is now widely believed that the γ-ray emission seen originates from a relativistic jet pointing along our line of sight, and several models involving X-and γ-ray production in jets have been proposed recently. Below we consider a specific class of models involving pair-cascade processes, and discuss some of the observational consequences of these models.

  12. On particle energization in accretion flows

    Blackman, Eric G.
    Two-temperature advection-dominated accretion flow (ADAF) or hot ion tori (HIT) models help to explain low-luminosity stellar and galactic accreting sources and may complement observational support for black holes in nature. However, low radiative efficiencies demand that ions receive a fraction η ≥ 99 per cent of energy dissipated in the turbulent accretion. The η value depends on the ratio of particle to magnetic pressure. If modes of dissipation involving compressions at least perpendicular to the magnetic field (like magnetic mirroring) dominate, then even when the pressure ratio is O(1), the required large η can be attained. However, the relative importance...

  13. Are AGN broad emission lines formed by discrete clouds? Analysis of Keck high-resolution spectroscopy of NGC 4151

    Arav, Nahum; Barlow, Tom A.; Laor, Ari; Sargent, Wallace L. W.; Blandford, Roger D.
    We search for a direct signature of discrete ‘clouds’ in the broad-line region (BLR) of the Seyfert galaxy NGC 4151. For this purpose we apply cross-correlation (CC) analysis to a high-resolution Keck spectrum of the galaxy. No such signature is found in the data. In order for cloud models to be compatible with this result, there must be at least ∼3×10^7 emitting clouds in the BLR, where the limit is based on simulation of a homogeneous cloud population. More realistic distributions increase the lower limit to above 10^8. These numbers are an order-of-magnitude improvement on our previous limit from Mrk...

  14. Resonant excitation of white dwarf oscillations in compact object binaries - I. The no back reaction approximation

    Rathore, Yasser; Blandford, Roger D.; Broderick, Avery E.
    We consider the evolution of white dwarfs with compact object companions (specifically black holes with masses up to ∼10^6 M_⊙, neutron stars, and other white dwarfs). We suppose that the orbits are initially quite elliptical and then shrink and circularize under the action of gravitational radiation. During this evolution, the white dwarfs will pass through resonances when harmonics of the orbital frequency match the stellar oscillation eigenfrequencies. As a star passes through these resonances, the associated modes will be excited and can be driven to amplitudes that are so large that there is a back reaction on the orbit which,...

  15. Probing the dark ages with metal absorption lines

    Oh, S. Peng
    Recent observations of high-redshift quasars at z∼ 6 have finally revealed complete Gunn–Peterson absorption. However, this at best constrains the volume-weighted and mass-weighted neutral fractions to be x^V_(HI) ≥ 10^(−3) and x^M_(HI) ≥ 10^(−2) respectively; stronger constraints are not possible because of the high optical depth for hydrogen Lyman transitions. Here I suggest certain metal lines as tracers of the hydrogen neutral fraction. These lines should cause unsaturated absorption when the intergalactic medium is almost fully neutral, if it is polluted to metallicities Z∼ 10^(−3.5) – 10^(−2.5) Z_⊙. Such a minimal level of metal pollution is inevitable in the middle...

  16. Foregrounds for 21-cm observations of neutral gas at high redshift

    Oh, S. Peng; Mack, Katherine J.
    We investigate a number of potential foregrounds for an ambitious goal of future radio telescopes such as the Square Kilometer Array (SKA) and the Low Frequency Array (LOFAR): spatial tomography of neutral gas at high redshift in 21-cm emission. While the expected temperature fluctuations due to unresolved radio point sources is highly uncertain, we point out that free–free emission from the ionizing haloes that reionized the Universe should define a minimal bound. This emission is likely to swamp the expected brightness temperature fluctuations, making proposed detections of the angular patchwork of 21-cm emission across the sky unlikely to be viable....

  17. The early afterglow

    Sari, R.; Piran, T.
    We calculate the expected spectrum and light curves of the early afterglow. For short GRBs the peak of the afterglow will be delayed, typically, by few dozens of seconds after the burst. The X-ray and γ-ray characteristics of this delayed emission provide a way to discriminate between late internal shocks emission (part of the GRB) and the early afterglow signal. Detection of this delayed emission will prove the internal shock scenario as producing the GRB, and will pinpoint the initial Lorentz factor . In the optical band, the dominant emission arises from the reverse shock. This shock, carries a comparable...

  18. Deformation of extremal black holes from stringy interactions

    Chen, Baoyi; Stein, Leo C.
    Black holes are a powerful setting for studying general relativity and theories beyond GR. However, analytical solutions for rotating black holes in beyond-GR theories are difficult to find because of the complexity of such theories. In this paper, we solve for the deformation to the near-horizon extremal Kerr metric due to two example string-inspired beyond-GR theories: Einstein-dilaton-Gauss-Bonnet and dynamical Chern-Simons theory. We accomplish this by making use of the enhanced symmetry group of NHEK and the weak-coupling limit of EdGB and dCS. We find that the EdGB metric deformation has a curvature singularity, while the dCS metric is regular. From...

  19. Turbulence in core-collapse supernovae

    Radice, David; Abdikamalov, Ernazar; Ott, Christian D.; Mösta, Philipp; Couch, Sean M.; Roberts, Luke F.
    Multidimensional simulations show that non-radial, turbulent, fluid motion is a fundamental component of the core-collapse supernova explosion mechanism. Neutrino-driven convection, the standing accretion shock instability, and relic-perturbations from advanced nuclear burning stages can all impact the outcome of core collapse in a qualitative and quantitative way. Here, we review the current understanding of these phenomena and their role in the explosion of massive stars. We also discuss the role of protoneutron star convection and of magnetic fields in the context of the delayed neutrino mechanism.

  20. The gravitational wave background from cosmological compact binaries

    Farmer, Alison J.; Phinney, E. S.
    We use a population synthesis approach to characterize, as a function of cosmic time, the extragalactic close binary population descended from stars of low to intermediate initial mass. The unresolved gravitational wave (GW) background due to these systems is calculated for the 0.1–10 mHz frequency band of the planned Laser Interferometer Space Antenna (LISA). This background is found to be dominated by emission from close white dwarf–white dwarf (WD–WD) pairs. The spectral shape can be understood in terms of some simple analytic arguments. To quantify the astrophysical uncertainties, we construct a range of evolutionary models which produce populations consistent with...

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