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Lars Hernquist

Publications and source records attributed to Lars Hernquist.

At least 667 records · Page 37Linked to original sources
arXiv1999

Evolution of the Galactic Potential and Halo Streamers with Future Astrometric Satellites

ESA's Global Astrometric Interferometer for Astrophysics (GAIA) holds the promise of mapping out the detailed phase space structure of the Galactic halo by providing unprecedented annual proper motion and parallax of $1-10\muas$ astrometric accuracy (Gilmore et al. 1998). Unlike NASA's Space Interferometry Mission (SIM), which will achieve similar accuracies but is a pointed instrument, GAIA will be able to construct a global catalogue of the halo. Here we study proper motions of giant branch stars in a tidal debris torn from a small satellite system in the halo. We follow the evolution of a cold stream on a polar orbit between 8-50 kpc in a variety of histories of the Galactic potential, and observe the bright ($V<18$mag) members of the debris tail with GAIA accuracy. We simulate effects due to the growing or flipping of the Galactic disk over the past 4 Gyrs or the perturbation from a massive accreted lump such as the progenitor of the Magellanic Clouds. Our simulations suggest that the results of Johnston, Zhao, Spergel & Hernquist (1999) and Helmi, Zhao & de Zeeuw (1999) for static Galactic potentials are likely to be largely generalizable to realistic time-dependent potentials: a tidal debris remains cold in spite of evolution and non-axial symmetry of the potential. GAIA proper motion measurements of debris stars might be used to probe both Galactic structure and Galactic history. We also study several other factors influencing our ability to identify streams, including accuracy of radial velocity and parallax data from GAIA, and contamination from random field stars. We conclude that nearby, cold streams could be detected with GAIA if these cousins of the Sagittarius stream exist.

astro-ph↗
arXiv1999

Do magnetars glitch? : Timing irregularities in anomalous X-ray pulsars

We examine the timing history of several anomalous X-ray pulsars (AXPs), and find that they exhibit spin down irregularities statistically similar to those of radio pulsars. We propose that these irregularities are simply glitches like those of radio pulsars and fit glitching spin down solutions to the data available for 1E~2259+586, 1E~1048.1-5937 and 4U~0142+61. The inferred magnitude of the glitches ($ΔΩ$) is comparable to that exhibited in radio pulsar glitching. With these results we argue that the three AXPs that we studied are isolated neutron stars spinning down by magnetic dipole radiation and powered by a combination of cooling, magnetic field decay and magnetospheric particle bombardment.

astro-ph↗
arXiv1999

Constraining Dark Halo Potentials with Tidal Tails

We present an extensive parameter survey to study the influence of halo mass profiles on the development of tidal tails in interacting disk galaxies. We model the galaxies using a fixed exponential disk with a central bulge and vary the halo potential over a range of parameters using both the Hernquist and NFW mass distributions, probing the effect of the halo mass, extent and concentration. We examine the consistency of the results against both observational and theoretical constraints on halo profiles and comment on the failures and weaknesses of different models. Galaxies with rising or flat rotation curves dominated by the halo are inhibited from forming tidal tails unless the halo is abruptly cut off just beyond the disk edge. Conversely, models with declining rotation curves -- resulting either from compact, low mass halos, or from massive disk components in low concentration dark halos -- produce tidal tails very similar to those observed in well-studied interacting systems. As argued by Springel & White (1998), a unifying, quantitative relation for all cases is that the ratio of the escape velocity to the circular velocity at around the solar radius must be v_e/v_c<2.5 for tidal tails to be produced. The galaxy models which appear to fit most of the observational constraints are those which have disk-dominated rotation curves and low concentration halos. We discuss our results in a cosmological context using recent studies which link halo properties to cosmological models.

astro-ph↗
arXiv1999

Studying Evolution of the Galactic Potential and Halo Streamers with Future Astrometric Satellites

Future astrometric satellites, such as SIM (NASA's Space Interferometric Mission) and GAIA (ESA's Global Astrometric Interferometer for Astrophysics), hold the promise of mapping out the detailed phase space structure of the Galactic halo by providing unprecedented annual proper motion and parallax of $1-10μ$as astrometric accuracy. Here we show that proper motions of hundred or so giant branch stars in a tidal debris torn from a small satellite (a $10^{5-7}L_\odot$ Galactic dwarf galaxy or globular cluster) in the halo is sensitive to the current Galactic potential and its past evolution. We follow the evolution of a cold (velocity dispersion of 10 km/s) stream on a nearby (between 8-50 kpc) polar orbit in a variety of histories of the potential of the Galaxy, and observe the bright ($V<18$mag) members of the debris tail with GAIA accuracy. We simulate effects due to the growing or flipping of the Galactic disk over the past 4 Gyrs or the perturbation from a massive accreted lump such as the progenitor of the Magellanic Clouds. We study various factors influencing our ability to identify streams, including contamination from field stars, accuracy of radial velocity and distance data and evolution and non-axial symmetry of the potential. Our simulations suggest that nearby, cold streams could be detected with GAIA if these cousins of the Sagittarius stream exist. Results of Johnston, Zhao, Spergel & Hernquist (1999) and Helmi, Zhao & de Zeeuw (1999) for static Galactic potentials are likely to be largely generalizable to moderately time-evolving potentials. SIM and GAIA measurements of debris stars might be used to probe both Galactic structure and Galactic history.

astro-ph↗
arXiv1998

Tidal Streams as Probes of the Galactic Potential

We explore the use of tidal streams from Galactic satellites to recover the potential of the Milky Way. Our study is motivated both by the discovery of the first lengthy stellar stream in the halo (\cite{it98}) and by the prospect of measuring proper motions of stars brighter than 20th magnitude in such a stream with an accuracy of $\sim 4μas/$yr, as will be possible with the Space Interferometry Mission (SIM). We assume that the heliocentric radial velocities of these stars can be determined from supporting ground-based spectroscopic surveys, and that the mass and phase-space coordinates of the Galactic satellite with which they are associated will also be known to SIM accuracy. Using results from numerical simulations as trial data sets, we find that, if we assume the correct form for the Galactic potential, we can predict the distances to the stars as a consequence of the narrow distribution of energy expected along the streams. We develop an algorithm to evaluate the accuracy of any adopted potential by requiring that the satellite and stars recombine within a Galactic lifetime when their current phase-space coordinates are integrated backwards. When applied to a four-dimensional grid of triaxial logarithmic potentials, with varying circular velocities, axis ratios and orientation of the major-axis in the disk plane, the algorithm can recover the parameters used for the Milky Way in a simulated data set to within a few percent using only 100 stars in a tidal stream.

astro-ph↗
arXiv1998

Nonlinear QED Effects in Strong-Field Magnetohydrodynamics

We examine wave propagation and the formation of shocks in strongly magnetized plasmas by applying a variational technique and the method of characteristics to the coupled magnetohydrodynamic (MHD) and quantum-electrodynamic (QED) equations of motion. In sufficiently strong magnetic fields such as those found near neutron stars, not only is the plasma extremely relativistic but the effects of QED must be included to understand processes in the magnetosphere. As Thompson & Blaes [1] find, the fundamental modes in the extreme relativistic limit of MHD coupled with QED are two oppositely directed Alfvén modes and the fast mode. QED introduces nonlinear couplings which affect the propagation of the fast mode such that waves traveling in the fast mode evolve as vacuum electromagnetic ones do in the presence of an external magnetic field [2] (Heyl & Hernquist 1998). The propagation of a single Alfvén mode is unaffected but QED does alter the coupling between the Alfvén modes.

hep-th↗
arXiv1998

Cosmology with the Lyman-alpha Forest

We outline the physical picture of the high-redshift Ly-alpha forest that has emerged from cosmological simulations, describe statistical characteristics of the forest that can be used to test theories of structure formation, present a preliminary comparison between simulation results and measurements from Keck HIRES spectra, and discuss a recent determination of the slope and amplitude of the linear mass power spectrum P(k) at z=2.5 from moderate resolution spectra. The physical picture is simple if each QSO spectrum is viewed as a continuous non-linear map of the line-of-sight density field rather than a collection of discrete absorption lines. The distribution of flux decrements depends mainly on the amplitude and PDF (Gaussian vs. non-Gaussian) of the primordial density fluctuations. The threshold crossing frequency, analogous to the 3-d genus curve, responds to the shape and amplitude of P(k) and to the values of Omega and Lambda. Open and Lambda-CDM models agree well with the measured flux decrement distribution at smoothing lengths of 25 km/s and 100 km/s and with the threshold crossing frequency at 100 km/s. Discrepancy with the observed threshold crossing frequency at 25 km/s may reflect the combined effects of noise in the data and limited mass resolution of the simulations. The slope of the measured P(k) agrees with the predictions of inflation+CDM models. Combining the amplitude with COBE normalization imposes a constraint on these models of the form Omega h^x n^y Omega_b^z = constant. Assuming Gaussian primordial fluctuations and a power spectrum shape parameter Gamma~0.2, consistency of the measured P(k) with the observed cluster mass function at z=0 requires Omega=0.46^{+0.12}_{-0.10} for an open universe and Omega=0.34^{+0.13}_{-0.09} for a flat universe (1-sigma errors). (Shortened)

astro-ph↗
arXiv1998

Closing in on Omega_0: The Amplitude of Mass Fluctuations from Galaxy Clusters and the Lyman-alpha Forest

We estimate the value of the matter density parameter Ω_0 by combining constraints from the galaxy cluster mass function with Croft et al.'s recent measurement of the mass power spectrum, P(k), from \lya forest data. The key assumption of the method is that cosmic structure formed by gravitational instability from Gaussian primordial fluctuations. For a specified value of Ω_0, matching the observed cluster mass function then fixes the value of σ_8, the rms amplitude of mass fluctuations in 8\hmpc spheres, and it thus determines the normalization of P(k) at z=0. The value of Ω_0 also determines the ratio of P(k) at z=0 to P(k) at z=2.5, the central redshift of the \lya forest data; the ratio is different for an open universe (Λ=0) or a flat universe. Because the \lya forest measurement only reaches comoving scales 2π/k ~ 15-20\hmpc, the derived value of Ω_0 depends on the value of the power spectrum shape parameter Γ, which determines the relative contribution of larger scale modes to σ_8. Adopting Γ=0.2, a value favored by galaxy clustering data, we find Ω_0 = 0.46^{+0.12}_{-0.10} for an open universe and Ω_0=0.34^{+0.13}_{-0.09} for a flat universe (1σerrors, not including the uncertainty in cluster normalization). Cluster-normalized models with Ω_0=1 predict too low an amplitude for P(k) at z=2.5, while models with Ω_0=0.1 predict too high an amplitude. The more general best fit parameter combination is approximately Ω_0 + 0.2Λ_0 = 0.46 + 1.3(Γ-0.2). Analysis of larger, existing samples of QSO spectra could greatly improve the measurement of P(k) from the \lya forest, allowing a determination of Ω_0 by this method with a precision of ~15%, limited mainly by uncertainty in the cluster mass function.

astro-ph↗
arXiv1998

The Power Spectrum of Mass Fluctuations Measured from the Lyman-alpha Forest at Redshift z=2.5

We measure the linear power spectrum of mass density fluctuations at redshift z=2.5 from the \lya forest absorption in a sample of 19 QSO spectra, using the method introduced by Croft et al. (1998). The P(k) measurement covers the range 2π/k ~ 450-2350 km/s (2-12 comoving \hmpc for Ω=1). We examine a number of possible sources of systematic error and find none that are significant on these scales. In particular, we show that spatial variations in the UV background caused by the discreteness of the source population should have negligible effect on our P(k) measurement. We obtain consistent results from the high and low redshift halves of the data set and from an entirely independent sample of nine QSO spectra with mean redshift z=2.1. A power law fit to our measured P(k) yields a logarithmic slope n=-2.25 +/- 0.18 and an amplitude Δ^2(k_p) = 0.57^{+0.26}_{-0.18}, where $Δ^2$ is the contribution to the density variance from a unit interval of lnk and k_p=0.008 (km/s)^{-1}. Direct comparison of our mass P(k) to the measured clustering of Lyman Break Galaxies shows that they are a highly biased population, with a bias factor b~2-5. The slope of the linear P(k), never previously measured on these scales, is close to that predicted by models based on inflation and Cold Dark Matter (CDM). The P(k) amplitude is consistent with some scale-invariant, COBE-normalized CDM models (e.g., an open model with Ω_0=0.4) and inconsistent with others (e.g., Ω=1). Even with limited dynamic range and substantial statistical uncertainty, a measurement of P(k) that has no unknown ``bias factors'' offers many opportunities for testing theories of structure formation and constraining cosmological parameters. (Shortened)

astro-ph↗
arXiv1998

Multidimensional thermal structure of magnetized neutron star envelopes

Recently launched x-ray telescopes have discovered several candidate isolated neutron stars. The thermal radiation from these objects may potentially constrain our understanding of nuclear physics in a realm inaccessible to terrestrial experiments. To translate the observed fluxes from neutron stars into constraints, one needs precise calculations of the heat transfer through the thin insulating envelopes of neutron stars. We describe models of the thermal structure of the envelopes of neutron stars with magnetic fields up to 10^{14} G. Unlike earlier work, we infer the properties of envelope models in two dimensions and precisely account for the quantization of the electron phase space. Both dipole and uniformly magnetized envelopes are considered.

astro-ph↗
arXiv1998

The Low Redshift Lyman Alpha Forest in Cold Dark Matter Cosmologies

We study the physical origin of the low-redshift Lyman alpha forest in hydrodynamic simulations of four CDM cosmologies. Our main conclusions are insensitive to the cosmological model but depend on our assumption that the UV background declines at low redshift. We find that the expansion of the universe drives rapid evolution of dN/dz (the number of absorbers per unit z) at z > 1.7, but that at lower redshift the fading of the UV background counters the influence of expansion, leading to slow evolution. At every redshift, weaker lines come primarily from moderate fluctuations of the diffuse, unshocked IGM, and stronger lines originate in shocked or radiatively cooled gas of higher overdensity. However, the neutral hydrogen column density associated with structures of fixed overdensity drops as the universe expands, so an absorber at z = 0 is dynamically analogous to an absorber with neutral hydrogen column density 10 to 50 times higher at z = 2-3. We find no clear distinction between lines arising in "galaxy halos" and lines arising in larger scale structures; however, galaxies tend to lie near the dense regions of the IGM that produce strong Lyman alpha lines. The simulations provide a unified physical picture that accounts for the most distinctive observed properties of the low redshift Lyman alpha forest: (1) a sharp transition in the evolution of dN/dz at z ~ 1.7, (2) stronger evolution for absorbers of higher equivalent width, (3) a correlation of increasing Lyman alpha equivalent width with decreasing galaxy impact parameter, and (4) a tendency for stronger lines to arise in close proximity to galaxies while weaker lines trace more diffuse large scale structure. (Abridged)

astro-ph↗
arXiv1998

The Clustering of High Redshift Galaxies in the Cold Dark Matter Scenario

We investigate the clustering of high redshift galaxies in five variants of the cold dark matter (CDM) scenario, using hydrodynamic cosmological simulations that resolve the formation of systems with circular velocities $v_c \geq 100 \kms$ ($Ω=1$) or $v_c \geq 70 \kms$ ($Ω=0.4$). Although the five models differ in their cosmological parameters and in the shapes and amplitudes of their mass power spectra, they predict remarkably similar galaxy clustering at $z=2$, 3, and 4. The galaxy correlation functions show almost no evolution over this redshift range, even though the mass correlation functions grow steadily in time. Despite the fairly low circular velocity threshold of the simulations, the high redshift galaxies are usually highly biased tracers of the underlying mass distribution; the bias factor evolves with redshift and varies from model to model. Predicted correlation lengths for the resolved galaxy population are $2-3\hmpc$ (comoving) at $z=3$. More massive galaxies tend to be more strongly clustered. These CDM models have no difficulty in explaining the strong observed clustering of Lyman-break galaxies, and some may even predict excessive clustering. Because the effects of bias obscure differences in mass clustering, it appears that Lyman-break galaxy clustering will not be a good test of cosmological models but will instead provide a tool for constraining the physics of galaxy formation.

astro-ph↗
arXiv1998

Electromagnetic Shocks in Strong Magnetic Fields

We examine the propagation of electromagnetic radiation through a strong magnetic field using the method of characteristics. Owing to nonlinear effects associated with vacuum polarization, such waves can develop discontinuities analogous to hydrodynamical shocks. We derive shock jump conditions and discuss the physical nature of these non-linear waves

hep-ph↗
arXiv1998

Secular Evolution of Barred Galaxies with Massive Central Black Holes

The influence of central black holes on the dynamical evolution of bars in disk galaxies is examined. In particular, we use numerical simulation to estimate the minimum mass black hole (BH) needed to destroy a bar. Initially, bars form in the disks via dynamical instability. Thereafter, once a bar is fully developed, a BH is adiabatically added at the center of the disk. To mitigate the global effects of gravitational softening, Poisson's equation for the disk is solved by expanding the density and potential of the galaxy in a set of basis functions. Our results indicate that a bar can be completely destroyed in a time much smaller than a Hubble time if the central mass exceeds about 0.5% of the disk mass. Since the implied minimum BH mass for bar destruction is of order 10^{8.5} solar masses for a typical disk galaxy, this process should not be a rare phenomenon. The bar amplitude decreases gradually with time after the BH is added, and the rate at which the bar is destroyed increases with increasing BH mass. This suggests that bar destruction arises from scattering of stars that support the bar as they pass close to the center.

astro-ph↗
arXiv1998

Triaxial Halos and Cusps

We present $N$--body models for triaxial elliptical galaxies or halos of galaxies, which are fully self--gravitating, have near constant axis ratio as a function of radius and a $r^{-1}$ central density cusp. Preliminary investigation suggests the model are stable and orbit analysis shows no indication of chaotic orbits. The models provide a starting point for investigations into the evolution of triaxial figures of equilibrium, response of triaxial figures to central black holes, external perturbations and interactions.

astro-ph↗
arXiv1998

Measuring Mass Loss Rates from Galactic Satellites

We present the results of a study that uses numerical simulations to interpret observations of tidally disturbed satellites around the Milky Way. When analysing the simulations from the viewpoint of an observer, we find a break in the slope of the star count and velocity dispersion profiles in our models at the location where unbound stars dominate. We conclude that `extra-tidal' stars and enhanced velocity dispersions observed in the outskirts of Galactic satellites are due to contamination by stellar debris from the tidal interaction with the Milky Way. However, a significant bound population can exist beyond the break radius and we argue that it should not be identified with the tidal radius of the satellite. We also develop and test a method for determining the mass loss rate from a Galactic satellite using its extra-tidal population. We apply this method to observations of globular clusters and dwarf spheroidal satellites of the Milky Way, and conclude that a significant fraction of both satellite systems are likely be destroyed within the next Hubble time. Finally, we demonstrate that this mass loss estimate allows us to place some limits on the initial mass function (IMF) of stars in a cluster from the radial dependence of its present day mass function (PDMF).

astro-ph↗
arXiv1998

Almost Analytic Models of Ultramagnetized Neutron Star Envelopes

Recent ROSAT measurements show that the x-ray emission from isolated neutron stars is modulated at the stellar rotation period. To interpret these measurements, one needs precise calculations of the heat transfer through the thin insulating envelopes of neutron stars. We present nearly analytic models of the thermal structure of the envelopes of ultramagnetized neutron stars. Specifically, we examine the limit in which only the ground Landau level is filled. We use the models to estimate the amplitude of modulation expected from non-uniformities in the surface temperatures of strongly magnetized neutron stars. In addition, we estimate cooling rates for stars with fields $B \sim 10^{15}-10^{16}$ G which are relevant to models that invoke ``magnetars'' to account for soft $γ$-ray emission from some repeating sources.

astro-ph↗