arXiv subjects

Lars Hernquist

Publications and source records attributed to Lars Hernquist.

720 records · Page 40Linked to original sources

arXiv1995

Quantifying the Fragility of Galactic Disks in Minor Mergers

We perform fully self-consistent stellar dynamical simulations of the accretion of a companion ("satellite") galaxy by a large disk galaxy to investigate the interaction between the disk, halo, and satellite components of the system during a merger. Our fiducial encounter begins with a satellite in a prograde, circular orbit inclined thirty degrees with respect to the disk plane at a galactocentric distance of six disk scalelengths. The satellite's mass is 10% of the disk's mass and its half-mass radius is about 1.3 kpc. The system is modelled with 500 000 particles, sufficient to mitigate numerical relaxation noise over the merging time. The satellite sinks in only ~1 Gyr and a core containing ~45% of its initial mass reaches the centre of the disk. With so much of the satellite's mass remaining intact, the disk sustains significant damage as the satellite passes through. At the solar circle we find that the disk thickens ~60%, the velocity dispersions increase by $Δ\mbox{\boldmath$σ$} \simeq (10,8,8)$ km/s to $(σ_R, σ_ϕ, σ_z) \simeq (48, 42, 38)$ km/s, and the asymmetric drift is unchanged at ~18 km/s. Although the disk is not destroyed by these events (hence "minor" mergers), its final state resembles a disk galaxy of earlier Hubble type than its initial state, thicker and hotter, with the satellite's core enhancing the bulge. Thus minor mergers continue to be a promising mechanism for driving galaxy evolution.

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arXiv1995

Cosmological Simulations with TreeSPH

We describe numerical methods for incorporating gas dynamics into cosmological simulations and present illustrative applications to the cold dark matter (CDM) scenario. Our evolution code, a version of TreeSPH (Hernquist \& Katz 1989) generalized to handle comoving coordinates and periodic boundary conditions, combines smoothed--particle hydrodynamics (SPH) with the hierarchical tree method for computing gravitational forces. The Lagrangian hydrodynamics approach and individual time steps for gas particles give the algorithm a large dynamic range, which is essential for studies of galaxy formation in a cosmological context. The code incorporates radiative cooling for an optically thin, primordial composition gas in ionization equilibrium with a user-specified ultraviolet background. We adopt a phenomenological prescription for star formation that gradually turns cold, dense, Jeans-unstable gas into collisionless stars, returning supernova feedback energy to the surrounding medium. In CDM simulations, some of the baryons that fall into dark matter potential wells dissipate their acquired thermal energy and condense into clumps with roughly galactic masses. The resulting galaxy population is insensitive to assumptions about star formation; we obtain similar baryonic mass functions and galaxy correlation functions from simulations with star formation and from simulations without star formation in which we identify galaxies directly from the cold, dense gas.

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arXiv1995

The Lyman-alpha Forest in the Cold Dark Matter Model

Cosmological simulations with gas provide a detailed description of the intergalactic medium, making possible predictions of neutral hydrogen absorption in the spectra of background QSOs. We present results from a high-resolution calculation of an $Ω=1$ cold dark matter model. Our simulation reproduces many of the observed properties of the \lya forest surprisingly well. The distribution of HI column densities agrees with existing data to within a factor of $\sim$ two over most of the range from $10^{14}\cm^{-2}$ to $10^{22}\cm^{-2}$; i.e., from unsaturated \lya forest lines to damped \lya systems. The equivalent width distribution matches the observed exponential form with a characteristic width $W_* \approx 0.3$ angstroms. The distribution of b-parameters appears consistent with that derived from QSO spectra. Most of the low column density absorption arises in large, flattened structures of moderate or even relatively low overdensity, so there is no sharp distinction between the \lya forest and the ``Gunn-Peterson'' absorption produced by the smooth intergalactic medium. Our results demonstrate that a \lya forest like that observed develops naturally in a hierarchical clustering scenario with a photoionizing background. Comparison between simulations and high-resolution QSO spectra should open a new regime for testing theories of cosmic structure formation.

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arXiv1995

Damped Lyman-alpha and Lyman Limit Absorbers in the Cold Dark Matter Model

We study the formation of damped \lya and Lyman limit absorbers in a hierarchical clustering scenario using a gas dynamical simulation of an $Ω= 1$, cold dark matter universe. In the simulation, these high column density systems are associated with forming galaxies. Damped \lya absorption, $N_{HI} \simgt 10^{20.2}\cm^{-2}$, arises along lines of sight that pass near the centers of relatively massive, dense protogalaxies. Lyman limit absorption, $10^{17}\cm^{-2} \simlt N_{HI} \simlt 10^{20.2}\cm^{-2}$, develops on lines of sight that pass through the outer parts of such objects or near the centers of smaller protogalaxies. The number of Lyman limit systems is less than observed, while the number of damped \lya systems is quite close to the observed abundance. Damped absorbers are typically $\sim 10$ kpc in radius, but the population has a large total cross section because the systems are much more numerous than present day $L_*$ galaxies. Our results demonstrate that high column density systems like those observed arise naturally in a hierarchical theory of galaxy formation and that it is now possible to study these absorbers directly from numerical simulations.

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arXiv1995

Using Tidal Tails to Probe Dark Matter Halos

We use simulations of merging galaxies to explore the sensitivity of the morphology of tidal tails to variations of the halo mass distributions in the parent galaxies. Our goal is to constrain the mass of dark halos in well-known merging pairs. We concentrate on prograde encounters between equal mass galaxies which represent the best cases for creating tidal tails, but also look at systems with different relative orientations, orbital energies and mass ratios. As the mass and extent of the dark halo increase in the model galaxies, the resulting tidal tails become shorter and less massive, even under the most favorable conditions for producing these features. Our simulations imply that the observed merging galaxies with long tidal tails ($\sim 50-100$ kpc) such as NGC 4038/39 (the Antennae) and NGC 7252 probably have halo:disk+bulge mass ratios less than 10:1. These results conflict with the favored values of the dark halo mass of the Milky Way derived from satellite kinematics and the timing argument which give a halo:disk+bulge mass ratio of $\sim 30:1$. However, the lower bound of the estimated dark halo mass in the Milky Way (mass ratio $\sim 10:1$) is still consistent with the inferred tidal tail galaxy masses. Our results also conflict with the expectations of $Ω=1$ cosmologies such as CDM which predict much more massive and extended dark halos.

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arXiv1995

Global Properties of Multiple Merger Remnants

Merger remnants of small groups of galaxies are contrasted with relics of mergers of pairs of galaxies to determine which process produces objects that best compare to real ellipticals. In both cases, the progenitors consist of self-gravitating disks, halos, and, sometimes, bulges. Pairs of galaxies merge from orbits that initially have zero--energy. The systems that produce multiple merger remnants are dense, six--member groups in virial equilibrium with low velocity dispersions. Multiple and pair mergers produce remnants which differ in both their spatial and kinematic properties. Multiple merger remnants have small triaxialities and are most likely to appear nearly round from most viewing angles. They possess cores, with sizes of a few tenths of an effective radius, that are more extended than pair remnant cores, even when bulges are included in the progenitors. In multiple mergers, the spin of all components -- halo, disk, and bulge -- increases and, while velocity dispersion dominates in the central regions, $v_r/σ\approx 1$ outside an effective radius in some projections. The angular momentum and minor axis vectors are aligned for multiple merger remnants. During merging of multiple progenitors, about half of the orbital angular momentum in each luminous component is converted into internal rotation in that component. Material is prevented from accumulating in the center of multiple merger remnants as efficiently as it does in pair mergers. In pair mergers that include gas, unrealistically steep surface brightness profiles have been produced in center of the remnants; in multiple mergers the formation of overdense nuclei may be impeded, thus allowing more successful comparison with real elliptical galaxies.

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arXiv1995

A Merger Origin for X-Structures in S0 Galaxies

Using numerical simulation, we study the response of a disk galaxy to a merger involving a low-mass satellite companion. During a prograde satellite accretion, the disk galaxy forms a strong bar in response to the perturbation of the companion. After the accretion event is over, the bar buckles vertically due to a bending instability, sending disk material well out of the disk plane. The material forms into an X-shaped feature when seen edge-on, morphologically similar to X-structures observed in several S0/Sa galaxies. The mechanism described here unites previously suggested accretion and bar instability hypotheses for forming X-structures into a self-consistent scenario for merger-induced formation and evolution of S0 galaxies. To complement these models, we also present observations of the peculiar S0 galaxy Hickson 87a. The thick disk, isophotal warping, and strong X-structure described in the merger model are all evident in Hickson 87a, suggesting this galaxy may be an excellent example of such merger-induced galaxy evolution.

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arXiv1995

The Disruption of the Sagittarius Dwarf Galaxy.

Numerical simulations of dwarf spheroidal galaxies undergoing several close encounters with the Milky Way are described. By comparing our models to observed properties of the recently discovered dwarf galaxy in Sagittarius (Sgr), we discuss implications of our results for the formation and evolution of the Milky Way system. We find that existing observations are not sufficient to allow us to place precise limits on either the orbit or the initial state of the dwarf. Debris from the ongoing tidal stripping of the Sagittarius galaxy are expected to form moving groups in the halo of the Galaxy and the discovery of such stars would strongly constrain the history and dynamical state of the dwarf. Furthermore, if Sgr is presently being disrupted, we predict that its remains will be detectable as a moving group in the halo for more than 1 Gyr. Thus, if similar accretion events have occurred in the recent history of the Galaxy, their aftereffects may still be observable.

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arXiv1995

Excitation of Activity in Galaxies by Minor Mergers

Mergers between gas--rich disks and less--massive dwarf galaxies are studied using numerical simulation. As the orbit of a satellite decays through dynamical friction, the primary disk develops large-amplitude spirals in response to its tidal forcing. While these features arise in both the stars and the gas in the disk, the non--axisymmetric structures in the gas differ slightly from those in the stars. In particular, as a consequence of the formation of strong shocks in the gas and the effects of radiative cooling, the gas response tends to lead the stellar response, enabling the stars to strongly torque the gas. These torques deprive the gas of its angular momentum, forcing a significant fraction of it into the inner regions of the disk. The radial inflows induced by these mergers accumulate large quantities of interstellar gas in the nuclear regions of the host disks. In some cases, nearly half of all the gas initially distributed throughout the disk winds up in a dense ``cloud'' several hundred parsecs in extent. The models reported here do not include star formation and, so, we cannot determine the ultimate fate of the gas. Nevertheless, given the high densities in the nuclear gas, it is plausible to identify these concentrations of dense gas in the remnants with those accompanying intense starbursts in some active galaxies. Therefore, the calculations here provide a framework for interpreting the origin of nuclear activity in otherwise quiescent disk galaxies. To the extent that galaxy formation is a chaotic process in which large structures are built up by the accretion of smaller fragments, our models may also be relevant to starbursts and the onset of nuclear activity in proto--galaxies at high redshifts.

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arXiv1995

A Parallel Self--Consistent Field Code

We describe a version of an algorithm for evolving self-gravitating collections of particles that should be nearly ideal for parallel architectures. Our method is derived from the ``self-consistent field'' (SCF) approach suggested previously by Clutton-Brock and others. Owing to the use of a global description of the gravitational field, the particles in an SCF simulation do not interact with one another directly, minimizing communications overhead between nodes in a parallel implementation. Ideal load-balancing is achieved since precisely the same number of operations are needed to compute the acceleration for each particle. Consequently, the SCF technique is perfectly scalable and the size of feasible applications will grow in simple proportion to advances in computational hardware. We describe an SCF code developed for and tested on a Connection Machine 5. Empirical tests demonstrate the efficient and scalable nature of the algorithm. Depending on the application, simulations with particle numbers in the range $N\sim 10^7 - 10^{8.5}$ are now possible. Larger platforms should make simulations with billions of particles feasible in the near future. Specific astrophysical applications are discussed in the context of collisionless dynamics.

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arXiv1994

Using X-rays to Determine Which Compact Groups are Illusory

If the large-scale galaxy distribution is filamentary, as suggested by some observations and recent hydrodynamical simulations, then lengthwise views of filaments will apparently produce compact groups (CGs) that are in reality stretched out along the line of sight. This possibility has been advocated recently by Hernquist, Katz \& Weinberg (1994). Here, we propose a test for this hypothesis using X--ray emission from CGs. The observable quantity $Q \equiv L_{x} a_{p}^{3}/L_{g}^2 T_{x}^{1/2}$ should be proportional to the axis ratio of the group, $a/c$, where $a$ and $c$ are the long and short axis of a prolate distribution, $a_p$ is the radius of the group projected onto the sky, $L_{x}$ is the bolometric X-ray luminosity, $L_{g}$ is the group blue luminosity, and $T_{x}$ is the gas temperature. We find that the distribution of $Q$ is consistent with the notion that many spiral-rich CGs with unusually small values of ($a/c)$ are frauds, i.e. that the values of $Q$ are anomalously small. An alternative possibility is that CGs are gas-poor relative to rich clusters; however, this can be tested using the Sunyaev-Zeldovich effect. If the groups have a normal ratio of gas-to-total mass, but are simply stretched out along the line of sight, a Sunyaev-Zeldovich signal should be detectable.

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arXiv1994

Dense Stellar Cores in Merger Remnants

We use numerical models which include star formation to analyze the mass profiles of remnants formed by mergers of disk galaxies. During a merger, dissipation in gas and ensuing star formation leave behind a dense stellar core in the remnant. Rather than joining smoothly onto a de Vaucouleurs profile, the starburst population leads to a sharp break in the surface density profile at a few percent the effective radius. While our results are preliminary, the lack of such signatures in most elliptical galaxies suggests that mergers of gas-rich disk galaxies may not have contributed greatly to the population of present-day ellipticals.

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arXiv1994

Models of Galaxies with Central Black Holes: Simulation Methods

We present a method for simulating numerically the effect of the adiabatic growth of black holes on the structure of elliptical galaxies. Using a parallel self--consistent field code, we add black holes to N--body realizations of model distribution functions for spherical galaxies, using a continuous mass--spectrum. The variable particle mass, combined with a simple multiple timestep integration scheme, makes it possible to evolve the models for many dynamical times with $N \sim 10^6-10^8$, allowing high spatial and mass resolution. This paper discusses verification of the code using analytic models for spherical galaxies, comparing our numerical results of the effect of central black holes on the structure of the galaxies with previously published models. The intrinsic and projected properties of the final particle distribution, including higher order moments of the velocity distribution, permit comparison with observed characteristics of real galaxies, and constrain the masses of any central black holes present in those galaxies. Our technique is promising and is easily extended to axisymmetric and triaxial galaxies.

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arXiv1994

Physically Detached "Compact Groups"

A small fraction of galaxies appear to reside in dense compact groups, whose inferred crossing times are much shorter than a Hubble time. These short crossing times have led to considerable disagreement about the dynamical state of these systems. We suggest that many of the observed groups are not physically bound but are chance projections of galaxies well-separated along the line of sight. Unlike earlier similar proposals, ours does not require that the galaxies in the compact group be members of a more diffuse, but physically bound entity. The probability of physically separated galaxies projecting into an apparent compact group is non-negligible if most galaxies are distributed, as observed, in thin filaments. We illustrate this general point with a specific example: a simulation of a cold dark matter universe, in which hydrodynamic effects are included to identify galaxies. The simulated galaxy distribution is filamentary, and end-on views of these filaments produce apparent galaxy associations that have sizes and velocity dispersions similar to those of observed compact groups. The frequency of such projections can explain the observed space-density of groups in the Hickson catalog. We discuss the implications of our proposal for the formation and evolution of groups and elliptical galaxies. The proposal can be tested by using redshift-independent distance estimators to measure the line-of-sight spatial extent of nearby compact groups.

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arXiv1994

Models of Galaxies with Central Black Holes: Adiabatic Growth in Spherical Galaxies

We add black holes to nonrotating, spherical galaxy models, with the assumption that the black-hole growth is slow compared with the dynamical time but fast compared with the relaxation time. The outcome differs depending on whether the core of the initial galaxy does or does not resemble that of an isothermal sphere. For the isothermal case the previously-known results are confirmed and sharpened: the black hole induces cusps in the density ($ρ\sim r^{-3/2}$) and velocity dispersion ($v^2\sim r^{-1}$), and a tangential anisotropy in the velocity distribution away from the center. For the non-isothermal case the induced density cusp is steeper, and the induced anisotropy is larger and penetrates right to the center. The cusp around the black hole is insensitive to anisotropy in the initial velocity distribution, and also to the origin of the black hole, unless its mass comes exclusively from the stars of lowest angular momentum, in which case the cusp is suppressed. We discuss the implications for the interpretation of evidence for massive black holes in galactic nuclei.

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arXiv1994

Star Forming Galaxy Models: Blending Star Formation into TREESPH

We have incorporated star formation algorithms into a hybrid N-body/smoothed particle hydrodynamics code (TREESPH) in order to describe the star forming properties of disk galaxies over timescales of a few billion years. The models employ a Schmidt law of index $n\sim 1.5$ to calculate star formation rates, and explicitly include the energy and metallicity feedback into the ISM. Modeling the newly formed stellar population is achieved through the use of hybrid SPH/young star particles which gradually convert from gaseous to collisionless particles, avoiding the computational difficulties involved in creating new particles. The models are shown to reproduce well the star forming properties of disk galaxies, such as the morphology, rate of star formation, and evolution of the global star formation rate and disk gas content. As an example of the technique, we model an encounter between a disk galaxy and a small companion which gives rise to a ring galaxy reminiscent of the Cartwheel (AM 0035-35). The primary galaxy in this encounter experiences two phases of star forming activity: an initial period during the expansion of the ring, and a delayed phase as shocked material in the ring falls back into the central regions.

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arXiv1994

Ultraluminous Starbursts in Major Mergers

We use numerical simulation to investigate the triggering of starbursts in merging disk galaxies. The properties of the merger-driven starbursts are sensitive to the structure of the progenitor galaxies; specifically, to the amount of material in a dense central bulge. Galaxies without bulges develop bars shortly after their first close passage, driving significant gas inflow and subsequent starbursts in the centers of the galaxies. These starbursts significantly deplete the star-forming gas, so that only relatively weak starbursts arise during the final merger. By contrast, models of galaxies with central bulges show that a bulge acts to stabilize the galaxies against inflow and starbursts until the galaxies actually merge. At this time, strong dissipation leads to the formation of a massive central gas mass and an ensuing star formation rate two orders of magnitude greater than that in our isolated disk models. These starbursts are very short in duration, typically $\sim$ 50 Myr, suggesting that the rarity of ultraluminous infrared galaxies is a result of their being in a very short evolutionary phase, rather than special and rare formation conditions. The fact that these mergers display many of the properties of ultraluminous infrared galaxies -- tidal features, double nuclei, massive compact gas concentrations, and extreme levels of starburst activity -- suggests that merger-driven starbursts can explain the emission from many ultraluminous infrared galaxies without an active nucleus.

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arXiv1994

A Comparison of Cosmological Hydrodynamic Codes

We present a detailed comparison of the simulation results of various cosmological hydrodynamic codes. Starting with identical initial conditions based on the Cold Dark Matter scenario for the growth of structure, we integrate from redshift $z=20$ to $z=0$ to determine the physical state within a representative volume of size $L^3$ where $L=64 h^{-1} {\rm Mpc}$. Five independent codes are compared: three of them Eulerian mesh based and two variants of the Smooth Particle Hydrodynamics "SPH" Lagrangian approach. The Eulerian codes were run at $N^3=(32^3,~64^3,~128^3,~{\rm and},~256^3)$ cells, the SPH codes at $N^3= 32^3$ and $64^3$ particles. Results were then rebinned to a $16^3$ grid with the expectation that the rebinned data should converge, by all techniques, to a common and correct result as $N \rightarrow \infty$. We find that global averages of various physical quantities do, as expected, tend to converge in the rebinned model, but that uncertainties in even primitive quantities such as $\langle T \rangle$, $\langle ρ^2\rangle^{1/2}$ persists at the 3\%-17\% level after completion of very large simulations. The two SPH codes and the two shock capturing Eulerian codes achieve comparable and satisfactory accuracy for comparable computer time in their treatment of the high density, high temperature regions as measured in the rebinned data; the variance among the five codes (at highest resolution) for the mean temperature (as weighted by $ρ^2$) is only 4.5\%. Overall the comparison allows us to better estimate errors, it points to ways of improving this current generation of hydrodynamic codes and of suiting their use to problems which exploit their individually best features.

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