arXiv subjects

Lars Hernquist

Publications and source records attributed to Lars Hernquist.

At least 649 records · Page 36Linked to original sources

arXiv2001

Anomalous X-ray Pulsars and Soft gamma-ray Repeaters: Spectral Fits and the Magnetar Model

The energy source powering the X-ray emission from anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) is still uncertain. In one scenario, the presence of an ultramagnetized neutron star, or ``magnetar'', with B on the order of 10^{14} - 10^{15} G is invoked. To investigate this hypothesis, we have analyzed archival ASCA data for several known AXPs and SGRs, and fitted them with a model in which all or part of the X-ray flux originates as thermal emission from a magnetar. Our magnetar spectral model includes the effects of the anisotropy of the heat flow through an ultramagnetized neutron star envelope, reprocessing by a light element atmosphere, and general relativistic corrections to the observed spectrum. We obtain good fits to the data with radii for the emitting areas which are generally consistent with those expected for neutron stars, in contrast to blackbody (BB) fits, which imply much smaller radii. Furthermore, the inclusion of atmospheric effects results in inferred temperatures which are lower than those implied by BB fits, but however still too high to be accounted by thermal cooling alone. An extra source of heating (possibly due to magnetic field decay) is needed. Despite the harder tail in the spectrum produced by reprocessing of the outgoing flux through the atmosphere, spectral fits still require a considerable fraction of the flux to be in a power-law component.

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arXiv2000

Models of Cuspy Triaxial Galaxies

We construct numerical models of mildly triaxial elliptical galaxies with central density cusps. Using a technique we call ``adiabatic squeezing'', we begin with a spherical gamma=1 Hernquist model and apply a drag to the velocities of the particles along each principle axis. The final models are stable in isolation, preserving their density structure and figure shape over many dynamical timescales. The density profile and axial ratios compare well to the observed properties of elliptical galaxies. The orbital structure of these models show a mixture of tubes, boxes, and boxlets, as expected for triaxial systems, with very few chaotic orbits. These N-body realizations of cuspy triaxial galaxies provide a basis for the study of the dynamical evolution of elliptical galaxies.

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arXiv2000

Hydrodynamic Simulation of the Cosmological X-ray Background

(Abridged) We use a hydrodynamic simulation of a LambdaCDM model to predict the extragalactic X-ray background (XRB), focussing on emission from the intergalactic medium (IGM). We also include X-rays from point sources associated with galaxies in the simulation, and make maps of the angular distribution of the emission. We find that filaments in the maps are not evident, being diluted by projection. In the soft (0.5-2 keV) band, the mean intensity of radiation from intergalactic and cluster gas is 2.3*10^-12 ergdeg^-2cm^-2s^-1, 35% of the total soft band emission. This is compatible at the ~1 sigma level with estimates of the unresolved soft background from ROSAT and {\it Chandra}. Only 4% of the hard (2-10 keV) emission is associated with the IGM. Relative to AGN flux, the IGM component peaks at a lower redshift (median z~0.45) so its clustering makes an important contribution to that of the total XRB. The angular correlations on 0.1-10 arcmin scales are significant, with an amplitude roughly consistent with an extrapolation of recent ROSAT results to small scales. A cross-correlation of the XRB against nearby galaxies taken from a simulated redshift survey also yields a strong signal from the IGM. Although some recent papers have argued that the expected soft band intensity from gas in galaxy, group, and cluster halos would exceed XRB limits unless much of the gas is expelled by supernova feedback, we obtain reasonable compatibility with current observations in a simulation that incorporates cooling, star formation, and only modest feedback. A prediction of our model is that the unresolved portion of the soft XRB will remain mostly unresolved.

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arXiv2000

Disks irradiated by beamed radiation from compact objects

We examine the reprocessing of X-ray radiation from compact objects by accretion disks when the X-ray emission from the star is highly beamed. The reprocessed flux for various degrees of beaming and inclinations of the beam axis with respect to the disk is determined. We find that, in the case where the beam is produced by a non-relativistic object, the intensity of the emitted spectrum is highly suppressed if the beam is pointing away from the disk. However, for beams produced by compact objects, general relativistic effects cause only a small reduction in the reradiated flux even for very narrow beams oriented perpendicularly to the disk. This is especially relevant in constraining models for the anomalous X-ray pulsars, whose X-ray emission is highly beamed. We further discuss other factors that can influence the emission from disks around neutron stars.

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arXiv2000

First Structure Formation: A Simulation of Small Scale Structure at High Redshift

We describe the results of a simulation of collisionless cold dark matter in a LambdaCDM universe to examine the properties of objects collapsing at high redshift (z=10). We analyze the halos that form at these early times in this simulation and find that the results are similar to those of simulations of large scale structure formation at low redshift. In particular, we consider halo properties such as the mass function, density profile, halo shape, spin parameter, and angular momentum alignment with the minor axis. By understanding the properties of small scale structure formation at high redshift, we can better understand the nature of the first structures in the universe, such as Population III stars.

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arXiv2000

Baryons in the Warm-Hot Intergalactic Medium

Approximately 30-40% of all baryons in the present day universe reside in a warm-hot intergalactic medium (WHIM), with temperatures between 10^5<T<10^7 K. This is a generic prediction from six hydrodynamic simulations of currently favored structure formation models having a wide variety of numerical methods, input physics, volumes, and spatial resolutions. Most of these warm-hot baryons reside in diffuse large-scale structures with a median overdensity around 10-30, not in virialized objects such as galaxy groups or galactic halos. The evolution of the WHIM is primarily driven by shock heating from gravitational perturbations breaking on mildly nonlinear, non-equilibrium structures such as filaments. Supernova feedback energy and radiative cooling play lesser roles in its evolution. WHIM gas is consistent with observations of the 0.25 keV X-ray background without being significantly heated by non-gravitational processes because the emitting gas is very diffuse. Our results confirm and extend previous work by Cen & Ostriker and Dave' et al.

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arXiv2000

Cooling Radiation and the Lyman-alpha Luminosity of Forming Galaxies

We examine the cooling radiation from forming galaxies in hydrodynamic simulations of the LCDM model (cold dark matter with a cosmological constant), focusing on the Ly-alpha line luminosities of high-redshift systems. Primordial composition gas condenses within dark matter potential wells, forming objects with masses and sizes comparable to the luminous regions of observed galaxies. As expected, the energy radiated in this process is comparable to the gravitational binding energy of the baryons, and the total cooling luminosity of the galaxy population peaks at z ~= 2. However, in contrast to the classical picture of gas cooling from the \sim 10^6 K virial temperature of a typical dark matter halo, we find that most of the cooling radiation is emitted by gas with T < 20,000 K. As a consequence, roughly 50% of this cooling radiation emerges in the Ly-alpha line. While a galaxy's cooling luminosity is usually smaller than the ionizing continuum luminosity of its young stars, the two are comparable in the most massive systems, and the cooling radiation is produced at larger radii, where the Ly-alpha photons are less likely to be extinguished by dust. We suggest, in particular, that cooling radiation could explain the two large (\sim 100 kpc), luminous (L_{Ly-alpha} \sim 10^{44} erg s^{-1}) ``blobs'' of Ly-alpha emission found in Steidel et al.'s (1999) narrow band survey of a z = 3 proto-cluster. Our simulations predict objects of the observed luminosity at about the right space density, and radiative transfer effects can account for the observed sizes and line widths. We discuss observable tests of this hypothesis for the nature of the Ly-alpha blobs, and we present predictions for the contribution of cooling radiation to the Ly-alpha luminosity function of galaxies as a function of redshift.

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arXiv2000

Consequences of Interstellar Absorption for Models of Anomalous X-ray Pulsars

We examine properties of thermal radiation emitted by strongly magnetized neutron stars (NSs). In particular, we show that the pulsation amplitudes of the energy-integrated flux are an increasing function of the absorption column density to the source. This is especially important for the interpretation of the Anomalous $X$-ray Pulsars (AXPs) as cooling neutron stars with high magnetic fields. We show that the high-pulsation amplitudes observed in these objects are consistent with cooling models, if the large amount of absorption to these sources is taken into account. We also show that cooling models imply inferred radii of the emitting regions on the order of $\sim 5-6$ times smaller than the actual NS radii, again in agreement with observations.

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arXiv2000

The Spin Period, Luminosity and Age Distributions of Anomalous X-Ray Pulsars

We consider the accretion model for anomalous X-ray pulsars proposed recently by Chatterjee, Hernquist and Narayan, in which the emission is powered by accretion from a fossil disk formed by the fallback of material from a supernova explosion. We demonstrate that this model is able to account for the spin period, luminosity and age distributions of the observed population of AXPs for reasonable and broad distributions of the free parameters of the model, namely, the surface magnetic field of the neutron star, the mass of its accretion disk and its initial spin period. In particular, this model is able statistically to account for the puzzlingly narrow observed spin distribution of the AXPs. We show also that if the establishment of fallback accretion disks around isolated neutron stars is a universal phenomenon, then a fairly large minority ($\sim 20%$) of these objects become X-ray bright AXPs or X-ray faint systems spinning down by propeller action, while the rest become radio pulsars.

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arXiv2000

High-Redshift Galaxies in Cold Dark Matter Models

We use hydrodynamic cosmological simulations to predict the star formation properties of high-redshift galaxies (z=2-6) in five variants of the inflationary cold dark matter scenario, paying particular attention to z=3, the redshift of the largest "Lyman-break galaxy" (LBG) samples. Because we link the star formation timescale to the local gas density, the rate at which a galaxy forms stars is governed mainly by the rate at which it accretes cooled gas from the surrounding medium. At z=3, star formation in most of the simulated galaxies is steady on 200 Myr timescales, and the instantaneous star formation rate (SFR) is correlated with total stellar mass. However, there is enough scatter in this correlation that a sample selected above a given SFR threshold may contain galaxies with a fairly wide range of masses. The redshift history and global density of star formation in the simulations depend mainly on the amplitude of mass fluctuations in the underlying cosmological model. The three models whose mass fluctuation amplitudes agree with recent analyses of the Lyman-alpha forest also reproduce the observed luminosity function of LBGs reasonably well, though the dynamic range of the comparison is small and the theoretical and observational uncertainties are large. The models with higher and lower amplitudes appear to predict too much and too little star formation, respectively, though they are not clearly ruled out. The intermediate amplitude models predict SFR ~ 30-40 Msun/yr for galaxies with a surface density 1 per arcmin^2 per unit redshift at z=3. They predict much higher surface densities at lower SFR, and significant numbers of galaxies with SFR > 10 Msun/yr at z >= 5.

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arXiv2000

Hotspot Emission from a Freely Precessing Neutron Star

Recent observations of 1E~161348-5055, the neutron-star candidate at the center of the supernova remnant RCW 103, show that a component of its emission varies sinusoidally with a period of approximately six hours. We argue that this period is what one would expect for a freely precessing neutron star with a spin period of about one second. We produce light curves for a freely precessing neutron star with a hotspot. By a suitable choice of parameters, we obtain light curves which are constant with rotational phase when the flux from the star reaches a maximum. At other phases of the precession, the flux varies as the star rotates but the total flux decreases by a factor of several. These models can explain the behavior observed from 1E~161348-5055 and predict that the spin period should be detectable at minimum flux from sufficiently sensitive measurements.

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arXiv2000

Simulating the effects of intergalactic grey dust

Using a high-resolution cosmological hydrodynamic simulation, we present a method to constrain extinction due to intergalactic grey dust based on the observed magnitudes of distant Type IA supernovae. We apply several simple prescriptions to relate the intergalactic dust density to the gas density in the simulation, thereby obtaining dust extinctions that may be directly compared to the observed distribution of supernova magnitudes. Our analysis is sensitive to the spatial distribution of grey dust, but is not dependent on its intrinsic properties such as its opacity or grain size. We present an application of our technique to the supernova data of Perlmutter et al., who find that their high redshift sample is ~0.2 magnitudes fainter than the expectation for a non-accelerating, low-density universe. We find that for grey dust to be responsible, it must be distributed quite smoothly, e.g., tracing intergalactic gas. More realistic dust distributions, such as dust tracing the metal density, are inconsistent with observations at the 1.5-2 sigma level. Upcoming observations and improved modelling of the dust distribution should lead to stronger constraints on intergalactic grey dust extinction.

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arXiv1999

Emission Spectra of Fallback Disks Around Young Neutron Stars

The nature of the energy source powering anomalous X-ray pulsars is uncertain. Proposed scenarios involve either an ultramagnetized neutron star, or accretion onto a neutron star. We consider the accretion model proposed recently by Chatterjee, Hernquist & Narayan, in which a disk is fed by fallback material following a supernova. We compute the optical, infrared, and submillimeter emission expected from such a disk, including both viscous dissipation and reradiation of X-ray flux impinging on the disk from the pulsar. We find that it is possible with current instruments to put serious constraints on this and on other accretion models of AXPs. Fallback disks could also be found around isolated radio pulsars and we compute the corresponding spectra. We show that the excess emission in the R and I bands observed for the pulsar PSR 0656+14 is broadly consistent with emission from a disk.

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arXiv1999

An Accretion Model for Anomalous X-Ray Pulsars

We present a model for the anomalous X-ray pulsars (AXPs) in which the emission is powered by accretion from a fossil disk, established from matter falling back onto the neutron star following its birth. The time-dependent accretion drives the neutron star towards a ``tracking'' solution in which the rotation period of the star increases slowly, in tandem with the declining accretion rate. For appropriate choices of disk mass, neutron star magnetic field strength and initial spin period, we demonstrate that a rapidly rotating neutron star can be spun down to periods characteristic of AXPs on timescales comparable to the estimated ages of these sources. In other cases, accretion onto the neutron star switches off after a short time, and the star becomes an ordinary radio pulsar. Thus, in our picture, radio pulsars and AXPs are drawn from the same underlying population, in contrast to models involving neutron stars with ultrastrong magnetic fields, which require a new population of stars with very different properties.

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arXiv1999

The Nature of Lyman Break Galaxies in Cosmological Hydrodynamic Simulations

What type of objects are being detected as $z\sim 3$ "Lyman break galaxies"? Are they predominantly the most massive galaxies at that epoch, or are many of them smaller galaxies undergoing a short-lived burst of merger-induced star formation? We attempt to address this question using high-resolution cosmological hydrodynamic simulations including star formation and feedback. Our $Λ$CDM simulation, together with Bruzual-Charlot population synthesis models, reproduces the observed number density and luminosity function of Lyman break galaxies when dust is incorporated. The inclusion of dust is crucial for this agreement. In our simulation, these galaxies are predominantly the most massive objects at this epoch, and have a significant population of older stars. Nevertheless, it is possible that our simulations lack the resolution and requisite physics to produce starbursts, despite having a physical resolution of $\la 700$ pc at z=3. Thus we cannot rule out merger-induced starburst galaxies also contributing to the observed population of high-redshift objects.

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arXiv1999

Evolution of Clustering and Bias in a Lambda-CDM Universe

We determine the evolution from $z=3\to 0$ of the galaxy and mass correlation functions and bias factor in a $50h^{-1}$Mpc $Λ$CDM hydrodynamic simulation with $10h^{-1}$kpc resolution. The mass correlation function grows with time, but the galaxy correlation function shows little evolution and is well described by a power law. At early times, galaxies are biased traces of mass, with bias being higher on smaller scales. By z=0, galaxies trace the mass, and the bias shows little scale dependence.

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arXiv1999

Theoretical Modeling of the High Redshift Galaxy Population

We review theoretical approaches to the study of galaxy formation, with emphasis on the role of hydrodynamic simulations in modeling the high redshift galaxy population. We present new predictions for the abundance of star-forming galaxies in the Lambda + cold dark matter model (Omega_m=0.4, Omega_L=0.6), combining results from several simulations to probe a wide range of redshift. At a threshold density of one object per arcmin^2 per unit z, these simulations predict galaxies with star formation rates of 2 msun/yr (z=10), 5 msun/yr (z=8), 20 msun/yr (z=6), 70-100 msun/yr (z=4-2), and 30 msun/yr (z=0.5). For galaxies selected at a fixed comoving space density n=0.003 h^3 Mpc^{-3], a (50 Mpc/h)^3 simulation predicts a galaxy correlation function (r/5 Mpc/h)^{-1.8} in comoving coordinates, essentially independent of redshift from z=4 to z=0.5. Different cosmological models predict global histories of star formation that reflect their overall histories of mass clustering, but robust numerical predictions of the comoving space density of star formation are difficult because the simulations miss the contribution from galaxies below their resolution limit. The LCDM model appears to predict a star formation history with roughly the shape inferred from observations, but it produces too many stars at low redshift, predicting Omega_* ~ 0.015 at z=0. We conclude with a brief discussion of this discrepancy and three others that suggest gaps in our current theory of galaxy formation: small disks, steep central halo profiles, and an excess of low mass dark halos. While these problems could fade as the simulations or observations improve, they could also guide us towards a new understanding of galactic scale star formation, the spectrum of primordial fluctuations, or the nature of dark matter.

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arXiv1999

On the Formation of Boxy and Disky Elliptical Galaxies

The origin of boxy and disky elliptical galaxies is investigated. The results of two collisionless N-body simulations of spiral-spiral mergers with mass ratios of 1:1 and 3:1 are discussed and the projected properties of the merger remnants are investigated. It is shown that the equal-mass merger leads to an anisotropic, slowly rotating system with preferentially boxy isophotes and significant minor axis rotation. The unequal-mass merger results in the formation of a rotationally supported elliptical with disky isophotes and small minor axis rotation. The observed scatter in the kinematical and isophotal properties of both classes of elliptical galaxies can be explained by projection effects.

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