arXiv subjects

Eliot Quataert

Publications and source records attributed to Eliot Quataert.

At least 397 records · Page 22Linked to original sources

arXiv2007

Dissipation-Scale Turbulence in the Solar Wind

We present a cascade model for turbulence in weakly collisional plasmas that follows the nonlinear cascade of energy from the large scales of driving in the MHD regime to the small scales of the kinetic Alfven wave regime where the turbulence is dissipated by kinetic processes. Steady-state solutions of the model for the slow solar wind yield three conclusions: (1) beyond the observed break in the magnetic energy spectrum, one expects an exponential cut-off; (2) the widely held interpretation that this dissipation range obeys power-law behavior is an artifact of instrumental sensitivity limitations; and, (3) over the range of parameters relevant to the solar wind, the observed variation of dissipation range spectral indices from -2 to -4 is naturally explained by the varying effectiveness of Landau damping, from an undamped prediction of -7/3 to a strongly damped index around -4.

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arXiv2007

Luminosity Dependence in the Fundamental Plane Projections of Elliptical Galaxies

We analyze the fundamental plane projections of elliptical galaxies as a function of luminosity, using a sample of approximately 80,000 galaxies drawn from Data Release 4 (DR4) of the Sloan Digital Sky Survey (SDSS). We separate brightest cluster galaxies (BCGs) from our main sample and reanalyze their photometry due to a problem with the default pipeline sky subtraction for BCGs. The observables we consider are effective radius (R_e), velocity dispersion (sigma), dynamical mass (M_dyn ~ R_e sigma2), effective density (sigma2/R_e2), and effective surface brightness (mu_e). With the exception of the L-M_dyn correlation, we find evidence of variations in the slope (i.e. the power-law index) of the fundamental plane projections with luminosity for our normal elliptical galaxy population. In particular, the radius-luminosity and Faber-Jackson relations are steeper at high luminosity relative to low luminosity, and the more luminous ellipticals become progressively less dense and have lower surface brightnesses than lower luminosity ellipticals. These variations can be understood as arising from differing formation histories, with more luminous galaxies having less dissipation. Data from the literature and our reanalysis of BCGs show that BCGs have radius-luminosity and Faber-Jackson relations steeper than the brightest non-BCG ellipticals in our sample, consistent with significant growth of BCGs via dissipationless mergers. The variations in slope we find in the Faber-Jackson relation of non-BCGs are qualitatively similar to that reported in the black hole mass-velocity dispersion (M_BH-sigma) correlation. This similarity is consistent with a roughly constant value of M_BH/M_star over a wide range of early type galaxies, where M_star is the stellar mass.

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arXiv2007

The Effects of Thermal Conduction on Radiatively-Inefficient Accretion Flows

We quantify the effects of electron thermal conduction on the properties of hot accretion flows, under the assumption of spherical symmetry. Electron heat conduction is important for low accretion rate systems where the electron cooling time is longer than the conduction time of the plasma, such as Sgr A* in the Galactic Center. For accretion flows with density profiles similar to the Bondi solution (n[r] ~ r^[-3/2]), we show that heat conduction leads to super-virial temperatures, implying that conduction significantly modifies the dynamics of the accretion flow. We then self-consistently solve for the dynamics of spherical accretion in the presence of saturated conduction and electron heating. We find that the accretion rate onto the central object can be reduced by ~1-3 orders of magnitude relative to the canonical Bondi rate. Electron conduction may thus be an important ingredient in explaining the low radiative efficiencies and low accretion rates inferred from observations of low-luminosity galactic nuclei. The solutions presented in this paper may also describe the nonlinear saturation of the magnetothermal instability in hot accretion flows.

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arXiv2007

Proto-Neutron Star Winds with Magnetic Fields and Rotation

We solve the 1D neutrino-heated non-relativistic MHD wind problem for conditions that range from slowly rotating (spin period P > 10 ms) protoneutron stars (PNSs) with surface field strengths typical of radio pulsars (B < 10^13 G), to "proto-magnetars" with B ~ 10^14-10^15 G in their hypothesized rapidly rotating initial states (P ~ 1 ms). We use the simulations of Bucciantini et al. (2006) to map our monopole results onto a more physical dipole geometry and to estimate the spindown of PNSs when their winds are relativistic. We then quantify the effects of rotation and magnetic fields on the mass loss, energy loss, and r-process nucleosynthesis in PNS winds. We describe the evolution of PNS winds through the Kelvin-Helmholtz cooling epoch, emphasizing the transition between (1) thermal neutrino-driven, (2) non-relativistic magnetically-dominated, and (3) relativistic magnetically-dominated outflows. We find that proto-magnetars with P ~ 1 ms and B > 10^15 G drive relativistic winds with luminosities, energies, and Lorentz factors (magnetization sigma ~ 0.1-1000) consistent with those required to produce long duration gamma-ray bursts and hyper-energetic supernovae (SNe). A significant fraction of the rotational energy may be extracted in only a few seconds, sufficiently rapidly to alter the asymptotic energy of the SN remnant, its morphology, and, potentially, its nucleosynthetic yield. Winds from PNSs with more modest rotation periods (2 - 10 ms) and with magnetar-strength fields produce conditions significantly more favorable for the r-process than winds from slowly rotating PNSs. Lastly, we show that energy and momentum deposition by convectively-excited waves further increase the likelihood of successful r-process in PNS winds.

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arXiv2006

The Ionization State of Sodium in Galactic Winds

Roughly 80% of Ultraluminous Infrared Galaxies (ULIRGs) show blue shifted absorption in the resonance lines of neutral sodium, indicating that cool winds are common in such objects, as shown by Rupke et al and by Martin. The neutral sodium (NaI) columns indicated by these absorption lines are ~ 10^{13}-3x10^{14}/cm^2, while the bolometric luminosity varies by a factor of only four. We show that the gas in ULIRG outflows is likely to be in photoionization equilibrium. The very small ULIRG sample of Goldader et al. demonstrates that the ratio of ultraviolet flux to far infrared flux varies by a factor $\sim100$ from object to object. While the Goldader sample does not overlap with those of Rupke et al. and Martin, we show that such a large variation in ultraviolet flux will produce a similar variation in the column of neutral sodium for a fixed mass flux and density. However, if the cold gas is in pressure equilibrium with a hot outflow with a mass loss rate similar to the star formation rate, the range of ionization state is significantly smaller. Measurements of the UV flux for objects in the Martin and Rupke et al. catalogs will definitively determine if photoionization effects are responsible for the wide variation seen in the sodium columns. If they are, a determination of the gas density and mass loss rate in the cool winds will follow, with attendant improvements in our understanding of wind driving mechanisms and of the effects of galaxies on their surroundings.

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arXiv2006

Assessing The Starburst Contribution to the Gamma-Ray and Neutrino Backgrounds

If cosmic ray protons interact with gas at roughly the mean density of the interstellar medium in starburst galaxies, then pion decay in starbursts is likely to contribute significantly to the diffuse extra-galactic background in both gamma-rays and high energy neutrinos. We describe the assumptions that lead to this conclusion and clarify the difference between our estimates and those of Stecker (2006). Detection of a single starburst by GLAST would confirm the significant contribution of starburst galaxies to the extra-galactic neutrino and gamma-ray backgrounds.

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arXiv2006

The Starburst Contribution to the Extra-Galactic Gamma-Ray Background

Cosmic ray protons interacting with gas at the mean density of the interstellar medium in starburst galaxies lose energy rapidly via inelastic collisions with ambient nuclei. The resulting pions produce secondary electrons and positrons, high-energy neutrinos, and gamma-ray photons. We estimate the cumulative gamma-ray emission from starburst galaxies. We find a total integrated background above 100 MeV of F_gamma ~ 10^{-6} GeV/cm^2/s/sr and a corresponding specific intensity at GeV energies of nuI_nu ~ 10^{-7} GeV/cm^2/s/sr. Starbursts may thus account for a significant fraction of the extra-galactic $γ$-ray background. We show that the FIR-radio correlation provides a strong constraint on the gamma-ray emission from starburst galaxies because pions decay into both gamma-rays and radio-emitting electron/positron pairs. We identify several nearby systems where the potential for observing gamma-ray emission is the most favorable (M82, NGC 253, and IC 342), predict their fluxes, and predict a linear FIR-gamma-ray correlation for the densest starbursts. If established, the FIR-gamma-ray correlation would provide strong evidence for the ``calorimeter'' theory of the FIR-radio correlation and would imply that cosmic rays in starburst galaxies interact with gas at approximately the mean density of the interstellar medium (ISM), thereby providing an important constraint on the physics of the ISM in starbursts.

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arXiv2006

Astrophysical Gyrokinetics: Basic Equations and Linear Theory

Magnetohydrodynamic (MHD) turbulence is encountered in a wide variety of astrophysical plasmas, including accretion disks, the solar wind, and the interstellar and intracluster medium. On small scales, this turbulence is often expected to consist of highly anisotropic fluctuations with frequencies small compared to the ion cyclotron frequency. For a number of applications, the small scales are also collisionless, so a kinetic treatment of the turbulence is necessary. We show that this anisotropic turbulence is well described by a low frequency expansion of the kinetic theory called gyrokinetics. This paper is the first in a series to examine turbulent astrophysical plasmas in the gyrokinetic limit. We derive and explain the nonlinear gyrokinetic equations and explore the linear properties of gyrokinetics as a prelude to nonlinear simulations. The linear dispersion relation for gyrokinetics is obtained and its solutions are compared to those of hot-plasma kinetic theory. These results are used to validate the performance of the gyrokinetic simulation code {\tt GS2} in the parameter regimes relevant for astrophysical plasmas. New results on global energy conservation in gyrokinetics are also derived. We briefly outline several of the problems to be addressed by future nonlinear simulations, including particle heating by turbulence in hot accretion flows and in the solar wind, the magnetic and electric field power spectra in the solar wind, and the origin of small-scale density fluctuations in the interstellar medium.

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arXiv2006

Red Mergers and the Assembly of Massive Elliptical Galaxies: the Fundamental Plane and its Projections

Several recent observations suggest that gas-poor (dissipationless) mergers of elliptical galaxies contribute significantly to the build-up of the massive end of the red sequence. We perform a series of major merger simulations to investigate the spatial and velocity structure of the remnants of such mergers. Regardless of orbital energy or angular momentum, we find that the stellar remnants lie on the fundamental plane defined by their progenitors, a result of virial equilibrium with a small tilt due to an increasing central dark matter fraction. However, the locations of merger remnants in the projections of the fundamental plane -- the Faber-Jackson and R_e-M_* relations -- depend strongly on the merger orbit, and the relations steepen significantly from the canonical scalings (L sigma^4 and R_e M_*^0.6) for mergers on radial orbits. Our results imply that the projections of the fundamental plane -- but not necessarily the plane itself -- provide a powerful way of investigating the assembly history of massive elliptical galaxies, including the brightest cluster galaxies at or near the centers of galaxy clusters. We argue that most massive ellipticals are formed by anisotropic merging and that their fundamental plane projections should thus differ noticeably from those of lower mass ellipticals even though they should lie on the same fundamental plane. Current observations are consistent with this conclusion. The steepening in the L-sigma relation for luminous ellipticals may also be reflected in a corresponding steepening in the M_BH-sigma relation for massive black holes.

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arXiv2006

Magnetic Fields in Starburst Galaxies and The Origin of the FIR-Radio Correlation

We estimate minimum energy magnetic fields (B_min) for a sample of galaxies with measured gas surface densities, spanning from normal spirals to starbursts. We show that the ratio of the minimum energy magnetic pressure to the total pressure in the ISM decreases substantially with increasing surface density; for Arp 220 this ratio is ~10^-4. Therefore, if the minimum energy estimate is applicable, magnetic fields in starbursts are dynamically weak compared to gravity, in contrast to normal spiral galaxies. We argue, however, that rapid cooling of relativistic electrons in starbursts invalidates the minimum energy estimate. We critically assess a number of independent constraints on the magnetic field strength in starbursts. In particular, we argue that the existence of the FIR-radio correlation implies that the synchrotron cooling timescale for cosmic ray electrons is much shorter than their escape time from the galactic disk; this in turn implies that the true magnetic field in starbursts is significantly larger than B_min. The strongest argument against such large fields is that one might expect starbursts to have steep radio spectra indicative of strong synchrotron cooling, which is not observed. We show, however, that ionization and bremsstrahlung losses can flatten the nonthermal spectra of starburst galaxies even in the presence of rapid cooling, providing much better agreement with observed spectra. We further demonstrate that ionization and bremsstrahlung losses are likely to be important in shaping the radio spectra of most starbursts at GHz frequencies, thereby preserving the linearity of the FIR-radio correlation. We thus conclude that magnetic fields in starbursts are significantly larger than B_min. We highlight several observations that can test this conclusion.

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arXiv2005

Thermal X-Ray Iron Line Emission from the Galactic Center Black Hole Sagittarius A*

We model thermal X-ray emission from the accreting supermassive black hole Sagittarius A* at the Galactic Center. For the region inside $1.^{\prime\prime}5$ of the center, we use a {generalized} radiatively inefficient accretion flow (RIAF) model, and for the region out to $10^{\prime\prime}$ we use published models of the ambient hot gas. We calculate the equivalent widths of Hydrogen-like and Helium-like emission lines of various elements, notably iron. We predict that a strong Helium-like iron line with an equivalent width $\sim1$ keV should be emitted by both the external medium and the RIAF. The equivalent width in the external medium is sensitive to the metallicity $Z$ of the gas as well as the mean temperature. For reasonable choices of these parameters, the calculated results agree with Chandra's detection of an iron line with an equivalent width of 1.3 keV within $10^{\prime\prime}$. The emission from within $1.^{\prime\prime}5$ is not sensitive to the external temperature, but is sensitive to the density and, especially, temperature profile inside the Bondi radius. For the range of profiles we consider, we calculate the equivalent width of the iron line to be $\sim0.6-1.5 (Z/Z_\odot)$ keV, where $Z_\odot$ is the solar metallicity. We present a new Chandra spectrum of the quiescent emission within $1.^{\prime\prime}5$ of Sgr A*. The measured equivalent width of the iron line is 0.7 keV. Although this measurement has a large uncertainty, it is consistent with our predictions, provided the metallicity of the gas is approximately solar.

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arXiv2005

Nonthermal THz to TeV Emission from Stellar Wind Shocks in the Galactic Center

The central parsec of the Galaxy contains dozens of massive stars with a cumulative mass loss rate of ~ 10^{-3} solar masses per year. Shocks among these stellar winds produce the hot plasma that pervades the central part of the galaxy. We argue that these stellar wind shocks also efficiently accelerate electrons and protons to relativistic energies. The relativistic electrons inverse Compton scatter the ambient ultraviolet and far infrared radiation field, producing high energy gamma-rays with a roughly constant luminosity from \~ GeV to ~ 10 TeV. This can account for the TeV source seen by HESS in the Galactic Center. Our model predicts a GLAST counterpart to the HESS source with a luminosity of ~ 10^{35} ergs/s and cooling break at ~ 4 GeV. Synchrotron radiation from the same relativistic electrons should produce detectable emission at lower energies, with a surface brightness ~ 10^{32} B^2_{-3} ergs/s/arcsec^2 from ~ THz to ~ keV, where B_{-3} is the magnetic field strength in units of mG. The observed level of diffuse thermal X-ray emission in the central parsec requires B < 300 micro-G in our models. Future detection of the diffuse synchrotron background in the central parsec can directly constrain the magnetic field strength, providing an important boundary condition for models of accretion onto Sgr A*.

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arXiv2005

Shearing Box Simulations of the MRI in a Collisionless Plasma

We describe local shearing box simulations of turbulence driven by the magnetorotational instability (MRI) in a collisionless plasma. Collisionless effects may be important in radiatively inefficient accretion flows, such as near the black hole in the Galactic Center. The MHD version of ZEUS is modified to evolve an anisotropic pressure tensor. A fluid closure approximation is used to calculate heat conduction along magnetic field lines. The anisotropic pressure tensor provides a qualitatively new mechanism for transporting angular momentum in accretion flows (in addition to the Maxwell and Reynolds stresses). We estimate limits on the pressure anisotropy due to pitch angle scattering by kinetic instabilities. Such instabilities provide an effective ``collision'' rate in a collisionless plasma and lead to more MHD-like dynamics. We find that the MRI leads to efficient growth of the magnetic field in a collisionless plasma, with saturation amplitudes comparable to those in MHD. In the saturated state, the anisotropic stress is comparable to the Maxwell stress, implying that the rate of angular momentum transport may be moderately enhanced in a collisionless plasma.

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arXiv2005

Dissipationless Mergers of Elliptical Galaxies and the Evolution of the Fundamental Plane

We carry out numerical simulations of dissipationless major mergers of elliptical galaxies using initial galaxy models that consist of a dark matter halo and a stellar bulge with properties consistent with the observed fundamental plane. By varying the density profile of the dark matter halo (standard NFW versus adiabatically contracted NFW), the global stellar to dark matter mass ratio, and the orbit of the merging galaxies, we are able to assess the impact of each of these factors on the structure of the merger remnant. Our results indicate that the properties of the remnant bulge depend primarily on the angular momentum and energy of the orbit; for a cosmologically motivated orbit, the effective radius and velocity dispersion of the remnant bulge remain approximately on the fundamental plane. This indicates that the observed properties of elliptical galaxies are consistent with significant growth via late dissipationless mergers. We also find that the dark matter fraction within the effective radius of our remnants increases after the merger, consistent with the hypothesis that the tilt of the fundamental plane from the virial theorem is due to a varying dark matter fraction as a function of galaxy mass.

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arXiv2005

Radiation Pressure Supported Starburst Disks and AGN Fueling

We consider the structure of marginally Toomre-stable starburst disks under the assumption that radiation pressure on dust grains provides the dominant vertical support against gravity. This is particularly appropriate when the disk is optically thick to its own IR radiation, as in the central regions of ULIRGs. Because the disk radiates at its Eddington limit, the Schmidt-law for star formation changes in the optically-thick limit, with the star formation rate per unit area scaling as Sigma_g/kappa, where Sigma_g is the gas surface density and kappa is the mean opacity. We show that optically thick starburst disks have a characteristic flux and dust effective temperature of F ~ 10^{13} L_sun/kpc^2 and T_eff ~ 90K, respectively. We compare our predictions with observations and find good agreement. We extend our model from many-hundred parsec scales to sub-parsec scales and address the problem of fueling AGN. We assume that angular momentum transport proceeds via global torques rather than a local viscosity. We account for the radial depletion of gas due to star formation and find a strong bifurcation between two classes of disk models: (1) solutions with a starburst on large scales that consumes all of the gas with little fueling of a central AGN and (2) models with an outer large-scale starburst accompanied by a more compact starburst on 1-10 pc scales and a bright central AGN. The luminosity of the latter models is in many cases dominated by the AGN. We show that the vertical thickness of the starburst disk on pc scales can approach h ~ r, perhaps accounting for the nuclear obscuration in some Type 2 AGN. We also argue that the disk of young stars in the Galactic Center may be the remnant of such a compact nuclear starburst.

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arXiv2004

Synchrotron Radiation From Radiatively Inefficient Accretion Flow Simulations: Applications to Sgr A*

We calculate synchrotron radiation in three-dimensional pseudo-Newtonian magnetohydrodynamic simulations of radiatively inefficient accretion flows. We show that the emission is highly variable at optically thin frequencies, with order of magnitude variability on time-scales as short as the orbital period near the last stable orbit; this emission is linearly polarized at the 20-50 % level due to the coherent toroidal magnetic field in the flow. At optically thick frequencies, both the variability amplitude and polarization fraction decrease significantly with decreasing photon frequency. We argue that these results are broadly consistent with the observed properties of Sgr A* at the Galactic Center, including the rapid infrared flaring.

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arXiv2004

Viscosity and Rotation in Core-Collapse Supernovae

We construct models of core-collapse supernovae in one spatial dimension, including rotation, angular momentum transport, and viscous dissipation employing an alpha-prescription. We compare the evolution of a fiducial 11 M_sun non-rotating progenitor with its evolution including a wide range of imposed initial rotation profiles (1.25<P_0<8 s, where P_0 is the initial, approximately solid-body, rotation period of the iron core). This range of P_0 covers the region of parameter space from where rotation begins to modify the dynamics (P_0~8 s) to where angular velocities at collapse approach Keplerian (P_0~1 s). Assuming strict angular momentum conservation, all models in this range leave behind neutron stars with spin periods <10 ms, shorter than those of most radio pulsars, but similar to those expected theoretically for magnetars at birth. A fraction of the gravitational binding energy of collapse is stored in the free energy of differential rotation. This energy source may be tapped by viscous processes, providing a mechanism for energy deposition that is not strongly coupled to the mass accretion rate through the stalled supernova shock. This effect yields qualitatively new dynamics in models of supernovae. We explore several potential mechanisms for viscosity in the core-collapse environment: neutrino viscosity, turbulent viscosity caused by the magnetorotational instability (MRI), and turbulent viscosity by entropy- and composition-gradient-driven convection. We argue that the MRI is the most effective. We find that for rotation periods in the range P_0<~5 s, and a range of viscous stresses, that the post-bounce dynamics is significantly effected by the inclusion of this extra energy deposition mechanism; in several cases we obtain strong supernova explosions.

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arXiv2004

The distribution and cosmic evolution of massive black hole spins

We study the expected distribution of massive black hole (MBH) spins and its evolution with cosmic time in the context of hierarchical galaxy formation theories. Our model uses Monte Carlo realizations of the merger hierarchy in a LCDM cosmology, coupled to semi-analytical recipes, to follow the merger history of dark matter halos, the dynamics of the MBHs they host, and their growth via gas accretion and binary coalescences. The coalescence of comparable mass holes increases the spin of MBHs, while the capture of smaller companions in randomly-oriented orbits acts to spin holes down. We find that, given the distribution of MBH binary mass ratios in hierarchical models, binary coalescences alone do not lead to a systematic spin-up or spin-down of MBHs with time: the spin distribution retains memory of its initial conditions. By contrast, because of the Bardeen-Petterson effect, gas accretion via a thin disk tends to spin holes up even if the direction of the spin axis changes randomly in time. In our models, accretion dominates over black hole captures and efficiently spins holes up. The spin distribution is heavily skewed towards fast-rotating Kerr holes, is already in place at early epochs, and does not change much below redshift 5. If accretion is via a thin disk, about 70% of all MBHs are maximally rotating and have radiative efficiencies approaching 30% (assuming a "standard'' spin-efficiency conversion). Even in the conservative case where accretion is via a geometrically thick disk, about 80% of all MBHs have spin parameters a/m > 0.8 and accretion efficiencies > 12%. Rapidly spinning holes with high radiative efficiencies may satisfy constraints based on comparing the local MBH mass density with the mass density inferred from luminous quasars (Soltan's argument).

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