Showing papers in "The Astrophysical Journal in 1997"
TL;DR: In this article, the authors used high-resolution N-body simulations to study the equilibrium density profiles of dark matter halos in hierarchically clustering universes, and they found that all such profiles have the same shape, independent of the halo mass, the initial density fluctuation spectrum, and the values of the cosmological parameters.
Abstract: We use high-resolution N-body simulations to study the equilibrium density profiles of dark matter halos in hierarchically clustering universes. We find that all such profiles have the same shape, independent of the halo mass, the initial density fluctuation spectrum, and the values of the cosmological parameters. Spherically averaged equilibrium profiles are well fitted over two decades in radius by a simple formula originally proposed to describe the structure of galaxy clusters in a cold dark matter universe. In any particular cosmology, the two scale parameters of the fit, the halo mass and its characteristic density, are strongly correlated. Low-mass halos are significantly denser than more massive systems, a correlation that reflects the higher collapse redshift of small halos. The characteristic density of an equilibrium halo is proportional to the density of the universe at the time it was assembled. A suitable definition of this assembly time allows the same proportionality constant to be used for all the cosmologies that we have tested. We compare our results with previous work on halo density profiles and show that there is good agreement. We also provide a step-by-step analytic procedure, based on the Press-Schechter formalism, that allows accurate equilibrium profiles to be calculated as a function of mass in any hierarchical model.
9,729 citations
TL;DR: In this paper, the authors derive hydrostatic, radiative equilibrium models for passive disks surrounding T Tauri stars, where each disk is encased by an optically thin layer of superheated dust grains.
Abstract: We derive hydrostatic, radiative equilibrium models for passive disks surrounding T Tauri stars. Each disk is encased by an optically thin layer of superheated dust grains. This layer reemits directly to space about half the stellar energy it absorbs. The other half is emitted inward and regulates the interior temperature of the disk. The heated disk flares. As a consequence, it absorbs more stellar radiation, especially at large radii, than a flat disk would. The portion of the spectral energy distribution contributed by the disk is fairly flat throughout the thermal infrared. At fixed frequency, the contribution from the surface layer exceeds that from the interior by about a factor 3 and is emitted at more than an order of magnitude greater radius. Spectral features from dust grains in the superheated layer appear in emission if the disk is viewed nearly face-on.
1,642 citations
TL;DR: In this article, the authors present a series of nongray calculations of the atmospheres, spectra, colors, and evolution of extrasolar giant planets (EGPs) and brown dwarfs for effective temperatures below 1300 K.
Abstract: We present the results of a new series of nongray calculations of the atmospheres, spectra, colors, and evolution of extrasolar giant planets (EGPs) and brown dwarfs for effective temperatures below 1300 K This theory encompasses most of the mass/age parameter space occupied by substellar objects and is the first spectral study down to 100 K These calculations are in aid of the multitude of searches being conducted or planned around the world for giant planets and brown dwarfs and reveal the exotic nature of the class Generically, absorption by H2 at longer wavelengths and H2O opacity windows at shorter wavelengths conspire to redistribute flux blueward Below 1200 K, methane is the dominant carbon bearing molecule and is a universal diagnostic feature of EGP and brown dwarf spectra We find that the primary bands in which to search are Z (~105 ?m), J (~12 ?m), H (~16 ?m), K (~22 ?m), M (~5 ?m), and N (~10 ?m), that enhancements of the emergent flux over blackbody values, in particular in the near infrared, can be by many orders of magnitude, and that the infrared colors of EGPs and brown dwarfs are much bluer than previously believed In particular, relative to J and H, the K band flux is reduced by CH4 and H2 absorption Furthermore, we conclude that for Teff's below 1200 K most or all true metals may be sequestered below the photosphere, that an interior radiative zone is a generic feature of substellar objects, and that clouds of H2O and NH3 are formed for Teff's below ~400 and ~200 K, respectively This study is done for solar-metallicity objects in isolation and does not include the effects of stellar insulation Nevertheless, it is a comprehensive attempt to bridge the gap between the planetary and stellar realms and to develop a nongray theory of objects from 03MJ (Saturn) to 70MJ (~007 M?) We find that the detection ranges for brown dwarf/EGP discovery of both ground- and space-based telescopes are larger than previously estimated
1,478 citations
TL;DR: In this article, the authors used a light-curve width-corrected magnitudes as a function of redshift of distant (z = 0.35-0.46) supernovae to obtain a global measurement of the mass density.
Abstract: We have developed a technique to systematically discover and study high-redshift supernovae that can be used to measure the cosmological parameters. We report here results based on the initial seven of more than 28 supernovae discovered to date in the high-redshift supernova search of the Supernova Cosmology Project. We find an observational dispersion in peak magnitudes of ? -->MB=0.27; this dispersion narrows to ?MB, corr=0.19 after correcting the magnitudes using the light-curve width-luminosity relation found for nearby (z ? 0.1) Type Ia supernovae from the Cal?n/Tololo survey (Hamuy et al.). Comparing light-curve width-corrected magnitudes as a function of redshift of our distant (z = 0.35-0.46) supernovae to those of nearby Type Ia supernovae yields a global measurement of the mass density, ?M${r M}$ -->=0.88 -->+ 0.69?0.60 for a ? = 0 cosmology. For a spatially flat universe (i.e., ?M + ?? = 1), we find ?M${r M}$ -->=0.94 -->+ 0.34?0.28 or, equivalently, a measurement of the cosmological constant, ??=0.06 -->+ 0.28?0.34 ( < 0.51 at the 95% confidence level). For the more general Friedmann-Lema?tre cosmologies with independent ?M and ??, the results are presented as a confidence region on the ?M-?? plane. This region does not correspond to a unique value of the deceleration parameter q0. We present analyses and checks for statistical and systematic errors and also show that our results do not depend on the specifics of the width-luminosity correction. The results for ??-versus-?M are inconsistent with ?-dominated, low-density, flat cosmologies that have been proposed to reconcile the ages of globular cluster stars with higher Hubble constant values.
1,272 citations
TL;DR: In this paper, the authors present a self-consistent model of accretion flows around black holes that unifies all of these states except the very high state, which is an extension of the following paradigm applied successfully to the quiescent state.
Abstract: Black hole X-ray binaries (BHXBs) are known to display five distinct spectral states. In order of increasing luminosity these are the quiescent state, low state, intermediate state, high state, and very high state. We present a self-consistent model of accretion flows around black holes that unifies all of these states except the very high state. The model is an extension of the following paradigm, which has been applied successfully to the quiescent state. The accretion flow consists of two zones, an inner advection-dominated accretion flow (ADAF) that extends from the black hole horizon to a transition radius rtr, and an outer thin accretion disk that is present beyond rtr. Above the disk is a hot corona, which is a continuation of the inner ADAF. The model consistently treats the dynamics of the accreting gas, the thermal balance of the ions and electrons in the two-temperature ADAF and corona, and the radiation processes that produce the observed spectrum. At low mass accretion rates, 0.01 (in Eddington units), the inner ADAF zone in the model radiates extremely inefficiently, and the outer thin disk is restricted to large radii (rtr ~ 102-104, in Schwarzschild units). The luminosity therefore is low, and this configuration is identified with the quiescent state. For 0.01 and up to a critical value crit${r crit}$ --> ~0.08, the radiative efficiency of the ADAF increases rapidly and the system becomes fairly luminous. The spectrum is very hard and peaks around 100 keV. This is the low state. The exact value of crit${r crit}$ --> depends on the viscosity parameter α (crit${r crit}$ --> ~1.3α -->2; the paper assumes α = 0.25). For values of > crit${r crit}$ --> and up to a second critical value about 10% higher, the ADAF progressively shrinks in size, the transition radius decreases, and the X-ray spectrum changes continuously from hard to soft. We identify this stage with the intermediate state. Finally, when is sufficiently large, the inner ADAF zone disappears altogether and the thin accretion disk extends down to the marginally stable orbit. The spectrum is dominated by an ultrasoft component with a weak hard tail. This is the high state. Model spectra calculated with this unified scenario agree well with observations of the quiescent, low, intermediate, and high states. Moreover, the model provides a natural explanation for the low state to high state transition in BHXBs. We also make a tentative proposal for the very high state, but this aspect of the model is less secure. An important feature of the model is that it is essentially parameter free. We test the model against observations of the soft X-ray transient Nova Muscae during its 1991 outburst. The model reproduces the observed light curves and spectra surprisingly well and makes a number of predictions that can be tested by observations of other BHXBs.
1,257 citations
TL;DR: In this article, the authors presented a catalog of 138,665 radio sources derived from the initial 1550 deg2 of the FIRST survey, which they used to construct a log N-log S relation for 20 cm radio sources over four decades of flux density, finding excellent agreement with previous determinations from smaller surveys.
Abstract: We present a catalog of 138,665 radio sources derived from the initial 1550 deg2 of the FIRST survey. The survey parameters are reviewed, and a map depicting the coverage for the first two observing sessions is presented. We then describe in detail our algorithm for radio source detection and parameterization, as well as our procedures for constructing the final catalog. The results of extensive tests for astrometric and photometric accuracy, as well as for uncertainties in source extent and morphological characterization, are presented. Source positions are all good to better than 1'', and the flux density scale is accurate to 5%; our elliptical Gaussian fitting procedure is shown both to provide a high-fidelity representation of source morphology for most objects and to characterize correctly bright sources extended on scales down to ~ the synthesized beam size of 54. We use the catalog to construct a log N-log S relation for 20 cm radio sources over four decades of flux density, finding excellent agreement with previous determinations from smaller surveys. As one example of the catalog's utility, we present comparisons with the Guide Star Catalog, the IRAS Faint Source catalog, and the ROSAT WGA catalog, identifying radio counterparts to thousands of the objects. Complete instructions for access to this FIRST catalog and subsequent updates thereto are provided.
1,211 citations
TL;DR: In this article, the authors discuss the evolution of cosmological gamma-ray burst remnants, consisting of the cooling and expanding fireball ejecta together with any swept-up external matter.
Abstract: We discuss the evolution of cosmological gamma-ray burst remnants, consisting of the cooling and expanding fireball ejecta together with any swept-up external matter, after the gamma-ray event. We show that significant optical emission is predicted, which should be measurable for timescales of hours after the event, and in some cases radio emission may be expected days to weeks after the event. The flux at optical, X-ray, and other long wavelengths decays as a power of time, and the initial value of the flux or magnitude, as well as the value of the time-decay exponent, should help to distinguish between possible types of dissipative fireball models.
1,210 citations
TL;DR: In this paper, the authors presented the first results of a submillimeter survey of distant clusters using the new Submillimeter Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope.
Abstract: We present the first results of a submillimeter survey of distant clusters using the new Submillimeter Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope. We have mapped fields in two massive, concentrated clusters, A370 at z=0.37 and Cl 2244-02 at z=0.33, at wavelengths of 450 and 850 μm. The resulting continuum maps cover a total area of about 10 arcmin2 to 1 σ noise levels less than 14 and 2 mJy beam-1 at the two wavelengths, 2-3 orders of magnitude deeper than was previously possible. We have concentrated on lensing clusters to exploit the amplification of all background sources by the cluster, improving the sensitivity by a factor of 1.3-2 as compared with a blank-field survey. A cumulative source surface density of (2.4±1.0)×103 deg-2 is found to a 50% completeness limit of ~4 mJy at 850 μm. The submillimeter spectral properties of these sources indicate that the majority lie at high redshift, z>1. Without correcting for lens amplification, our observations limit the blank-field counts at this depth. The surface density is 3 orders of magnitude greater than the expectation of a nonevolving model using the local IRAS 60 μm luminosity function. The observed source counts thus require a substantial increase in the number density of strongly star-forming galaxies in the high-redshift universe and suggest that optical surveys may have substantially underestimated the star formation density in the distant universe. Deeper submillimeter surveys with SCUBA should detect large numbers of star-forming galaxies at high redshift and so provide strong constraints on the formation of normal galaxies.
1,178 citations
TL;DR: In this paper, the authors used traditional morphological classifications of galaxies in 10 intermediate-redshift (z similar to 0.5) clusters observed with WFPC2 on the Hubble Space Telescope, and derived relations between morphology and local galaxy density similar to that found by Dressier for low-Redshift clusters.
Abstract: Using traditional morphological classifications of galaxies in 10 intermediate-redshift (z similar to 0.5) clusters observed with WFPC2 on the Hubble Space Telescope, we derive relations between morphology and local galaxy density similar to that found by Dressier for low-redshift clusters. Taken collectively, the ''morphology-density'' relationship, T-Sigma, for these more distant, presumably younger clusters is qualitatively similar to that found for the local sample, but a detailed comparison shows two substantial differences: (1) For the clusters in our sample, the T-Sigma relation is strong in centrally concentrated ''regular'' clusters, those with a strong correlation of radius and surface density, but nearly absent for clusters that are less concentrated and irregular, in contrast to the situation for low-redshift clusters, where a strong relation has been found for both. (2) In every cluster the fraction of elliptical galaxies is as large or larger than in low-redshift clusters, but the SO fraction is 2-3 times smaller, with a proportional increase of the spiral fraction. Straightforward, though probably not unique, interpretations of these observations are (1) morphological segregation proceeds hierarchically, affecting richer, denser groups of galaxies earlier, and (2) the formation of elliptical galaxies predates the formation of rich clusters and occurs instead in the loose-group phase or even earlier, but SO's are generated in large numbers only after cluster virialization.
1,070 citations
TL;DR: In this paper, it was shown that perturbations arising from discretization of the equations of self-gravitational hydrodynamics can grow into fragments in multiple-grid simulations, a process referred to as artificial fragmentation.
Abstract: We demonstrate with a new three-dimensional adaptive mesh refinement code that perturbations arising from discretization of the equations of self-gravitational hydrodynamics can grow into fragments in multiple-grid simulations, a process we term "artificial fragmentation." We present star formation calculations of isothermal collapse of dense molecular cloud cores. In simulation of a Gaussian-profile cloud free of applied perturbations, we find artificial fragmentation can be avoided across the isothermal density regime by ensuring the ratio of cell size to Jeans length, which we call the Jeans number, J ≡ Δx/λJ, is kept below 0.25. We refer to the constraint that λJ be resolved as the Jeans condition. When an m=2 perturbation is included, we again find it necessary to keep J≤0.25 to achieve a converged morphology. Collapse to a filamentary singularity occurs without fragmentation of the filament, in agreement with the predictions of Inutsuka & Miyama. Simulation beyond the time of this singularity requires an arresting agent to slow the runaway density growth. Physically, the breakdown of isothermality due to the buildup of opacity acts as this agent, but many published calculations have instead used artificial viscosity for this purpose. Because artificial viscosity is resolution dependent, such calculations produce resolution-dependent results. In the context of the perturbed Gaussian cloud, we show that use of artificial viscosity to arrest collapse results in significant violation of the Jeans condition. We also show that if the applied perturbation is removed from such a calculation, numerical fluctuations grow to produce substantial fragments not unlike those found when the perturbation is included. These findings indicate that calculations that employ artificial viscosity to halt collapse are susceptible to contamination by artificial fragmentation. The Jeans condition has important implications for numerical studies of isothermal self-gravitational hydrodynamics problems insofar as it is a necessary but not, in general, sufficient condition for convergence.
1,004 citations
TL;DR: In this article, the authors used the IRAM 30 m telescope of CO in a large sample of ultraluminous IR galaxies out to redshift z = 0.3 to derive the sizes of the FIR- and CO-emitting regions and the enclosed dynamical masses.
Abstract: We present observations with the IRAM 30 m telescope of CO in a large sample of ultraluminous IR galaxies out to redshift z = 0.3. Most of the ultraluminous galaxies in this sample are interacting, but not completed, mergers. The CO(1-0) luminosity of all but one of the ultraluminous galaxies is high, with values of log (L${′}{r CO}$ -->/K km s-1 pc2) = 9.92 ? 0.12. The extremely small dispersion of only 30% is less than that of the far-infrared luminosity. The integrated CO line intensity is strongly correlated with the 100 ?m flux density, as expected for a blackbody model in which the mid- and far-IR radiation is optically thick. We use this model to derive sizes of the FIR- and CO-emitting regions and the enclosed dynamical masses. Both the IR and CO emission originate in regions a few hundred parsecs in radius. The median value of LFIR${r FIR}$ -->/L${′}{r CO}$ -->=160 L?/K km s-1 pc2, within a factor of 2 or 3 of the blackbody limit for the observed far-IR temperatures. The entire ISM is a scaled-up version of a normal galactic disk with the ambient densities a factor of 100 higher, making even the intercloud medium a molecular region. We compare three different techniques of H2 mass estimation and conclude that the ratio of gas mass to CO luminosity is about a factor of 4 times lower than for giant molecular clouds (GMCs) but that the gas mass is a large fraction of the dynamical mass. Our analysis of CO emission from ultraluminous galaxies reduces the H2 mass from previous estimates of 2-5 ? 1010 M? to 0.4-1.5 ? 1010 M?, which is in the range found for molecular gas-rich spiral galaxies. A collision involving a molecular gas-rich spiral could lead to an ultraluminous galaxy powered by central starbursts triggered by the compression of infalling preexisting GMCs. The extremely dense molecular gas in the center of an ultraluminous galaxy is an ideal stellar nursery for a huge starburst.
TL;DR: In this article, a comprehensive discussion of methods for estimating cosmological parameters from large data sets, including the previously published techniques as special cases, is presented, and it is shown that both the problem of estimating several parameters jointly and the problem that not knowing the parameters a priori can be readily solved by adding an extra singular value decomposition step.
Abstract: Since cosmology is no longer "the data-starved science," the problem of how to analyze large data sets best has recently received considerable attention, and Karhunen-Loeve eigenvalue methods have been applied to both galaxy redshift surveys and cosmic microwave background (CMB) maps. We present a comprehensive discussion of methods for estimating cosmological parameters from large data sets, which includes the previously published techniques as special cases. We show that both the problem of estimating several parameters jointly and the problem of not knowing the parameters a priori can be readily solved by adding an extra singular value decomposition step. It has recently been argued that the information content in a sky map from a next-generation CMB satellite is sufficient to measure key cosmological parameters (h, Ω, Λ, etc.) to an accuracy of a few percent or better—in principle. In practice, the data set is so large that both a brute force likelihood analysis and a direct expansion in signal-to-noise eigenmodes will be computationally unfeasible. We argue that it is likely that a Karhunen-Loeve approach can nonetheless measure the parameters with close to maximal accuracy, if preceded by an appropriate form of quadratic "precompression." We also discuss practical issues regarding parameter estimation from present and future galaxy redshift surveys and illustrate this with a generalized eigenmode analysis of the IRAS 1.2 Jy survey optimized for measuring β ≡ Ω0.6/b using redshift space distortions.
TL;DR: In this article, a new chemical evolution model for the Galaxy that assumes two main infall episodes, for the formation of the halo-thick disk and thin disk, respectively, is presented.
Abstract: We present a new chemical evolution model for the Galaxy that assumes two main infall episodes, for the formation of the halo-thick disk and thin disk, respectively. We do not try to take into account explicitly the evolution of the halo since our model is better suited for computing the evolution of the disk (thick plus thin), but we implicitly assume that the timescale for the formation of the halo was of the same order as the timescale for the formation of the thick disk. The formation of the thin disk is much longer than that of the thick disk, implying that the infalling gas forming the thin disk comes not only from the thick disk but mainly from the intergalactic medium. The timescale for the formation of the thin disk is assumed to be a function of Galactocentric distance, leading to an inside-out picture for the Galaxy's building. The model takes into account the most up-to-date nucleosynthesis prescriptions and adopts a threshold in the star formation process, which naturally produces a hiatus in the star formation rate at the end of the thick-disk phase, as suggested by recent observations. The model results are compared with an extended set of observational constraints both for the solar neighborhood and for the whole disk. Among these constraints, the tightest is the metallicity distribution of the G-dwarf stars, for which new data are now available. Our model fits these new data very well. The model also predicts the evolution of the gas mass, the star formation rate, the supernova rates, and the abundances of 16 chemical elements as functions of time and Galactocentric distance. We show that, in order to reproduce most of these constraints, a timescale of ≤1 Gyr for the (halo) thick disk and of 8 Gyr for the thin disk's formation in the solar vicinity are required. We predict that the radial abundance gradients in the inner regions of the disk (R < 1 R☉) are steeper than in the outer regions, a result confirmed by recent abundance determinations, and that the inner gradients steepen during the Galactic lifetime. The importance and the advantages of assuming a threshold gas density for the onset of the star formation process are discussed.
TL;DR: In this article, the minimum mass that a virialized gas cloud must have in order to be able to cool in a Hubble time is computed, using a detailed treatment of the chemistry of molecular hydrogen.
Abstract: The minimum mass that a virialized gas cloud must have in order to be able to cool in a Hubble time is computed, using a detailed treatment of the chemistry of molecular hydrogen. With a simple model for halo profiles, we reduce the problem to that of numerically integrating a system of chemical equations. The results agree well with numerically expensive three-dimensional simulations, and our approach has the advantage of being able to explore large regions of parameter space rapidly. The minimum baryonic mass Mb is found to be strongly redshift dependent, dropping from 106 M☉ at z ~ 15 to 5 × 103 M☉ at z ~ 100 as molecular cooling becomes effective. For z 100, Mb rises again, as cosmic microwave background photons inhibit H2 formation through the H- channel. Finally, for z 200, the H -->+2 channel for H2 formation becomes effective, driving Mb down toward Mb ~ 103 M☉. With a standard cold dark matter power spectrum with σ8 = 0.7, this implies that a fraction 10-3 of all baryons may have formed luminous objects by z = 30, which could be sufficient to reheat the universe.
TL;DR: In this article, the authors investigate the 21 cm signature that may arise from the intergalactic medium (IGM) prior to the epoch of full reionization (z > 5) in scenarios in which the neutral gas that has not yet been engulfed by an H II region may easily be preheated to temperatures well above that of the cosmic background radiation (CBR).
Abstract: We investigate the 21 cm signature that may arise from the intergalactic medium (IGM) prior to the epoch of full reionization (z > 5) In scenarios in which the IGM is reionized by discrete sources of photoionizing radiation, the neutral gas that has not yet been engulfed by an H II region may easily be preheated to temperatures well above that of the cosmic background radiation (CBR), rendering the IGM invisible in absorption against the CBR We identify three possible preheating mechanisms: (1) photoelectric heating by soft X-rays from QSOs, (2) photoelectric heating by soft X-rays from early galactic halos, and (3) resonant scattering of the continuum UV radiation from an early generation of stars We find that bright quasars with only a small fraction of the observed comoving density at z ~ 4 will suffice to preheat the entire universe at z 6 We also show that, in a cold dark matter dominated cosmology, the thermal bremsstrahlung radiation associated with collapsing galactic mass halos (1010-1011 M?) may warm the IGM to ~100 K by z ~ 7 Alternatively, the equivalent of ~10% of the star formation rate density in the local universe, whether in isolated pregalactic stars, dwarf, or normal galaxies, would be capable of heating the entire IGM to a temperature above that of the CBR by Ly? scattering in a small fraction of the Hubble time at z ~ 6 In the presence of a sufficiently strong ambient flux of Ly? photons, the hyperfine transition in the warmed H I will be excited A beam differencing experiment would detect a patchwork of emission, both in frequency and in angle across the sky This patchwork could serve as a valuable tool for understanding the epoch, nature, and sources of the reionization of the universe, and their implications for cosmology We demonstrate that isolated QSOs will produce detectable signals at meter wavelengths within their spheres of influence over which they warm the IGM As a result of the redshifted 21 cm radiation emitted by warm H I bubbles, the spectrum of the radio extragalactic background will display frequency structure with velocity widths up to 10,000 km s-1 Broad beam observations would reveal corresponding angular fluctuations in the sky intensity with ?T/T 10-3 on scales ? ~ 1? This scale is set either by the thermalization distance from a QSO within which Ly? pumping determines the spin temperature of the IGM or by the quasar lifetime Radio measurements near 235 and 150 MHz, as will be possible in the near future using the Giant Metrewave Radio Telescope, may provide the first detection of a neutral IGM at 5 z 10 A next generation facility like the Square Kilometer Array Interferometer could effectively open much of the universe to a direct study of the reheating epoch and possibly probe the transition from a neutral universe to one that is fully ionized
TL;DR: In this article, the effects of tangential forces on the contact in some detail are investigated. But the results of these studies are limited to the case of collisions of aggregates with each other or with large grains.
Abstract: Even though dust coagulation is a very important dust-processing mechanism in interstellar space and protoplanetary disks, there are still important parts of the physics involved that are poorly understood. This imposes a serious problem for model calculations of any kind. In this paper, we attempt to improve the situation by including the effects of tangential forces on the contact in some detail. These have been studied in recent papers. We summarize the main results from these papers and apply them to detailed simulations of the coagulation process and of collisions between dust aggregates. Our results show the following: (1) the growth of aggregates by monomers will normally not involve major restructuring of the aggregates, (2) the classical hit-and-stick assumption is reasonably valid for this case, (3) collisions of aggregates with each other or with large grains can lead to significant compaction, and (4) the results can be easily understood in terms of critical energies for different restructuring processes. We also derive a short summary that may be used as a recipe for determining the outcome of collisions in coagulation calculations. It is shown that turbulent velocity fields in interstellar clouds are capable of producing considerably compressed aggregates, while the small aggregates forming early on in the solar nebula will not be compacted by collisions. However, compaction provides an important energy sink in collisions of larger aggregates in the solar nebula.
TL;DR: In this paper, the high-energy diffuse gamma-ray emission from the Galactic plane, |b| ≤ 10°, was studied using observations from the Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma-ray Observatory, and the spatial distribution of the diffuse emission was determined for four broad energy ranges after removing the contribution from point sources detected with greater than 5 σ significance.
Abstract: The high-energy diffuse gamma-ray emission from the Galactic plane, |b| ≤ 10°, is studied using observations from the Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory. The spatial distribution of the diffuse emission has been determined for four broad energy ranges after removing the contribution from point sources detected with greater than 5 σ significance. The longitude and latitude distributions of the intensity, averaged over 4° latitude ranges and 10° longitude ranges, respectively, are shown for the four energy ranges. Spectra of the diffuse emission in 11 energy bands, covering the energy range 30 MeV to 30 GeV, were determined for 10° × 4° (l × b) bins after correcting for the finite EGRET angular resolution. The average spectrum from the direction of the inner Galaxy is shown for 29 energy bands, covering the energy range 30 MeV to 50 GeV. At latitudes |b| > 2°, corresponding to gamma rays emitted within about 3 kpc of the Sun, there is no significant variation in the spectra with Galactic longitude. Comparison of the spectra from the Galactic plane (|b| < 2°) reveals no significant variation with Galactic longitude below about 4 GeV, which suggests that the cosmic-ray electron to proton ratio does not vary significantly throughout the Galaxy. Above 4 GeV, however, there is weak (about 3 σ) evidence for variation of the Galactic plane (|b| < 2°) spectrum with longitude. The spectrum is softer in the direction of the outer Galaxy by about E compared to the spectrum from the inner Galaxy. This variation of the diffuse gamma-ray emission hints at a variation of the cosmic-ray proton spectrum with Galactic radius, which might be expected if cosmic rays are accelerated primarily in the inner Galaxy and then propagate to the outer Galaxy or if the high-energy cosmic rays are confined less well in the outer Galaxy. The spatial and spectral distributions of the diffuse emission are compared with a model calculation of this emission based on dynamic balance and realistic interstellar matter and photon distributions. The spatial comparison is used to establish the value of the molecular mass calibrating ratio N(H2)/WCO and the cosmic-ray/matter coupling scale r0, which are the only adjustable parameters of the model. Comparisons with the observations indicates N(H2)/WCO = (1.56 ± 0.05) × 1020 mol cm-2 (K km s-1)-1 and r0 = (1.76 ± 0.2) kpc. The spatial agreement between this model and the observation is very good. However, above about 1 GeV the integral intensity predicted by the model is about 60% less than the observed intensity. Although the explanation of this excess is unclear, uncertainties in the neutral pion production function or variations in the cosmic-ray spectrum with Galactic radius may partially account for the underprediction. A small medium-latitude (2° < |b| < 10°) excess in the direction of the inner Galaxy exists and may indicate that the low-energy photon density used in the model is too low.
TL;DR: In this paper, the authors presented new maps of the soft X-ray background from the ROSAT all-sky survey, which represent a significant improvement over the previous version in that position resolution of the PSPC has been used to improve the angular resolution from 2? to 12' and the contributions of point sources have been removed to a uniform source flux level over most of the sky.
Abstract: This paper presents new maps of the soft X-ray background from the ROSAT all-sky survey. These maps represent a significant improvement over the previous version in that (1) the position resolution of the PSPC has been used to improve the angular resolution from ~2? to 12', (2) there are six energy bands that divide each of the previous three into two parts, and (3) the contributions of point sources have been removed to a uniform source flux level over most of the sky. These new maps will be available in electronic format later in 1997. In this paper we also consider the bright emission in the general direction of the Galactic center in the 0.5-2.0 keV band, and the apparent absorption trough that runs through it along the Galactic plane. We find that while the northern hemisphere data are confused by emission from Loop I, the emission seen south of the plane is consistent with a bulge of hot gas surrounding the Galactic center (in our simple model, a cylinder with an exponential fall-off of density with height above the plane). The cylinder has a radial extent of ~5.6 kpc. The X-ray emitting gas has a scale height of 1.9 kpc, an in-plane electron density of ~0.0035 cm-3, a temperature of ~106.6 K, a thermal pressure of ~28,000 cm-3 K, and a total luminosity of ~2 ? 1039 ergs s-1 using a collisional ionization equilibrium (CIE) plasma emission model.
TL;DR: In this article, the authors investigate the dynamical evolution of the Galactic globular cluster system in considerably greater detail than has been done hitherto, finding that destruction rates are significantly larger than given by previous estimates.
Abstract: We investigate the dynamical evolution of the Galactic globular cluster system in considerably greater detail than has been done hitherto, finding that destruction rates are significantly larger than given by previous estimates. The general scheme (but not the detailed implementation) follows Aguilar, Hut, & Ostriker. For the evolution of individual clusters, we use a Fokker-Planck code including the most important physical processes governing the evolution: two-body relaxation, tidal truncation of clusters, compressive gravitational shocks while clusters pass through the Galactic disk, and tidal shocks due to passage close to the bulge. Gravitational shocks are treated comprehensively, using a recent result by Kundi? & Ostriker that the ?E2 shock-induced relaxation term, driving an additional dispersion of energies, is generally more important than the usual energy shift term ?E. Various functional forms of the correction factor are adopted to allow for the adiabatic conservation of stellar actions in a presence of transient gravitational perturbation. We use a recent compilation of the globular cluster positional and structural parameters, and a collection of radial velocity measurements. Two transverse to the line-of-sight velocity components were assigned randomly according to the two kinematic models for the cluster system (following the method of Aguilar, Hut, & Ostriker): one with an isotropic peculiar velocity distribution, corresponding to the present-day cluster population, and the other with the radially preferred peculiar velocities, similar to those of the stellar halo. We use the Ostriker & Caldwell and the Bahcall, Schmidt, & Soneira models for our Galaxy. For each cluster in our sample, we calculated its orbits over a Hubble time, starting from the present observed positions and assumed velocities. Medians of the resulting set of peri- and apogalactic distances and velocities are used then as an input for the Fokker-Planck code. Evolution of the cluster is followed up to its total dissolution due to a coherent action of all of the destruction mechanisms. The rate of destruction is then obtained as a median over all the cluster sample, in accord with Aguilar, Hut, & Ostriker. We find that the total destruction rate is much larger than that given by Aguilar, Hut, & Ostriker with more than half of the present clusters (52%-58% for the Ostriker & Caldwell model, and 75%-86% for the Bahcall, Schmidt, & Soneira model) destroyed in the next Hubble time. Alternatively put, the typical time to destruction is comparable to the typical age, a result that would follow from (but is not required by) an initially power law distribution of destruction times. We discuss some implications for a past history of the globular cluster system and the initial distribution of the destruction times, raising the possibility that the current population is but a very small fraction of the initial population with the remnants of the destroyed clusters constituting presently a large fraction of the spheroid (bulge + halo) stellar population.
TL;DR: In this article, the authors simulated the generation of Ca II H2V bright grains by acoustic shocks using a one-dimensional, non-LTE radiation-hydrodynamic code with six-level model atoms for hydrogen and singly ionized calcium.
Abstract: We have simulated the generation of Ca II H2V bright grains by acoustic shocks. We employ a one-dimensional, non-LTE radiation-hydrodynamic code, with six-level model atoms for hydrogen and singly ionized calcium. We drive acoustic waves through a stratified radiative equilibrium atmosphere by a piston, whose velocity is chosen to match the Doppler shift observed in the Fe I 396.68 nm line in the H line wing, formed at about 260 km above τ500 = 1. The simulations closely match the observed behavior of Ca II H2V bright grains down to the level of individual grains. The bright grains are produced by shocks near 1 Mm above τ500 = 1. Shocks in the mid-chromosphere produce a large source function (and therefore high emissivity) because the density is high enough for collisions to couple the Ca II populations to the local conditions. The asymmetry of the line profile is due to velocity gradients near 1 Mm. Material motion Doppler-shifts the frequency at which atoms emit and absorb photons, so the maximum opacity is located at—and the absorption profile is symmetric about—the local fluid velocity, which is shifted to the blue behind shocks. The optical depth depends upon the velocity structure higher up. Shocks propagate generally into downflowing material, so there is little matter above to absorb the Doppler-shifted radiation. The corresponding red peak is absent because of small opacity at the source function maximum and large optical depth due to overlying material. The bright grains are produced primarily by waves from the photosphere that are slightly above the acoustic cutoff frequency. The precise time and strength of a grain depend upon the interference between these waves near the acoustic cutoff frequency and higher frequency waves. When waves near the acoustic cutoff frequency are weak, then higher frequency waves may produce grains. The "5 minute" trapped p-mode oscillations are not the source of the grains, although they can slightly modify the behavior of higher frequency waves.
TL;DR: In this paper, the authors present evidence for widespread relativistic effects in the central regions of active galactic nuclei, showing that a strong red wing is indicative of gravitational redshifts close to a central black hole, and accretion disk models provide an excellent description of the data.
Abstract: We present evidence for widespread relativistic effects in the central regions of active galactic nuclei. In a sample of 18 Seyfert 1 galaxies observed by ASCA, 14 show an iron K? line that is resolved, with mean width ?K? = 0.43 ? 0.12 keV for a Gaussian profile (full width at half-maximum, FWHM ~ 50,000 km s-1). However, many of the line profiles are asymmetric. A strong red wing is indicative of gravitational redshifts close to a central black hole, and accretion disk models provide an excellent description of the data. The peak energy of the line is 6.4 keV, which indicates that it arises by fluorescence in near-neutral material. Our fits imply a low inclination for the disk in these Seyfert 1 galaxies, with a mean of 30?, consistent with orientation-dependent unification schemes. Differences in the line profiles from source to source imply slight variations in geometry, which cannot be accounted for solely by inclination. In most cases, we require that the line emission arises from a range of radii. Although a small contribution to the emission from a region other than the disk is not ruled out, it is not generally required and has little effect on our conclusions regarding the disk line. Our data are fit equally well with rotating (Kerr) and nonrotating (Schwarzschild) black hole models. We find a mean spectral index in the 3-10 keV range of ?3-10 = 1.91 ? 0.07 after accounting for the effects of reflection. Such observations probe the innermost regions of active galactic nuclei and arguably provide the best evidence yet obtained for the existence of supermassive black holes in the centers of active galaxies.
TL;DR: In this article, a universal mechanism for cluster formation in all epochs and environments is found to be consistent with the properties and locations of young and old globular clusters, open clusters and unbound associations, and interstellar clouds.
Abstract: A universal mechanism for cluster formation in all epochs and environments is found to be consistent with the properties and locations of young and old globular clusters, open clusters and unbound associations, and interstellar clouds. The primary structural differences between various cluster types result from differences in pressure at the time of formation, combined with different ages for subsequent evolution. All clusters begin with a mass distribution similar to that for interstellar clouds, which is approximately n(M)dM M-2 dM. Old halo globulars have a current mass distribution that falls off at low mass because of a Hubble time of cluster destruction. Young globulars have not yet had time for a similar loss, and some old open clusters have survived because of their low densities. The peak in the halo cluster luminosity function depends only on age, and is independent of the host galaxy luminosity, as observed. The peak globular cluster mass is not a characteristic or Jeans mass in the primordial galaxy, as previously suggested. The initial mass distribution functions for young and old globular clusters, open clusters and associations, and interstellar clouds are all power laws with a slope of ~ -2. This distribution could be the result of fractal structure in turbulent gas. New data on clusters in the LMC also follow this power law. The slope is so steep that it implies a significant fraction of star formation occurs in small clusters. Numerous halo field stars should come from the evaporation of small halo clusters, and a high fraction of disk field stars should arise in small unbound disk clusters. This differs significantly from previous suggestions that most disk stars form in large OB associations. Globular clusters of all ages preferentially form in high-pressure regions. This is directly evident today in the form of large kinematic pressures from the densities and relative velocities of member stars. High pressures at the time of globular cluster formation are either the result of a high background virial density in that part of the galaxy (as in dwarf galaxies or galactic nuclei and nuclear rings), turbulence compression (in halo globulars), or large-scale shocks (in interacting galaxies). Massive clusters that form in such high-pressure environments are more likely to be bound than low-mass clusters or clusters of equal mass in low-pressure regions. This is because virialized clouds are more tightly bound at high pressure. A simple model illustrates this effect. One implication of this result is that starburst regions preferentially make globular clusters, in which case some elliptical galaxies could have formed by the violent merger of spiral galaxies.
TL;DR: In this paper, the systematic errors in the virial mass-to-light ratio, Mv/L, of galaxy clusters as an estimator of the field M/L value are assessed.
Abstract: The systematic errors in the virial mass-to-light ratio, Mv/L, of galaxy clusters as an estimator of the field M/L value are assessed. We overlay 14 clusters in redshift space to create an ensemble cluster that averages over substructure and asymmetries. The combined sample, including background, contains about 1150 galaxies, extending to a projected radius of about twice r200. The radius r200, defined as where the mean interior density is 200 times the critical density, is expected to contain the bulk of the virialized cluster mass. The dynamically derived M(r200)/L(r200) of the ensemble is (0.82 ? 0.14)Mv/L. The Mv/L overestimate is attributed to not taking into account the surface pressure term in the virial equation. Under the assumption that the velocity anisotropy parameter is in the range 0 ? ? ? 2/3, the galaxy distribution accurately traces the mass profile beyond about the central 0.3r200. There are no color or luminosity gradients in the galaxy population beyond 2r200, but there is 0.11 ? 0.05 mag fading in the r-band luminosities between the field and cluster galaxies. We correct the cluster virial mass-to-light ratio, Mv/L = 289 ? 50 h M?/L? (calculated assuming q0 = 0.1), for the biases in Mv and mean luminosity to estimate the field M/L = 213 ? 59 h M?/L?. With our self-consistently derived field luminosity density, j/?c = 1136 ? 138 h M?/L? (at z 1/3), the corrected M/L indicates ?0 = 0.19 ? 0.06 ? 0.04 (formal 1 ? random error and estimated potential systematic errors) for those components of the mass field in rich clusters.
TL;DR: In this article, the authors discussed the possibility that gamma-ray bursts result from internal shocks in ultrarelativistic matter and used a simple model to calculate the temporal structure and estimate the efficiency of this process.
Abstract: We discuss the possibility that gamma-ray bursts result from internal shocks in ultrarelativistic matter. Using a simple model, we calculate the temporal structure and estimate the efficiency of this process. In this model the flow of ultrarelativistic matter is represented by a succession of shells with random values of the Lorentz factor. We calculate the shocks that take place between those shells, and we estimate the resulting emission. Internal shocks can produce the highly variable temporal structure observed in most of the bursts, provided that the source emitting the relativistic flow is highly variable. The observed peaks are in almost one-to-one correlation with the activity of the emitting source. A large fraction of the kinetic energy is converted to radiation. The most efficient case is when an inner engine produces shells with comparable energy but very different Lorentz factors. It also gives the most desirable temporal structure.
TL;DR: In this article, the authors simulate a plausible cosmological model in considerable physical and numerical detail through the successive phases of reheating (at 10 z 20) and reionization at z ≈ 7.4.
Abstract: We simulate a plausible cosmological model in considerable physical and numerical detail through the successive phases of reheating (at 10 z 20) and reionization at z ≈ 7. We assume an efficiency of high-mass star formation appropriate to leave the universe, after it becomes transparent, with an ionizing background J21 ≈ 0.4 (at z = 4), near (and perhaps slightly below) the observed value. Since the same stars produce the ionizing radiation and the first generation of heavy elements, a mean metallicity of Z/Z☉ ~ 1/200 is produced in this early phase, but there is a large variation about this mean, with the high density regions having Z/Z☉ ≈ 1/30 and the low density regions (or the Lyα forest with NH I 1013.5 cm2) having essentially no metals. When it occurs, reionization is very rapid (phase change-like), which will leave a signature that may be detectable by very large area meter-wavelength radio instruments. Also, the background UV radiation field will show a sharp drop of ~10-3 from 1 to 4 ryd because of absorption edges. The simulated volume is too small to form L* galaxies, but the smaller objects that are found in the simulation obey the Faber-Jackson relation. In order to explore theoretically this domain of "the end of the dark ages" quantitatively, numerical simulations must have a mass resolution of the order of 104.5 M☉ in baryons, have high spatial resolution (1 kpc) to resolve strong clumping, and allow for detailed and accurate treatment of both the radiation field and atomic/molecular physics.
TL;DR: In this article, the authors tracked the birth and outflow of 50-100 of the most prominent moving coronal features and found that they originate about 3-4 R☉ from Sun center as radially elongated structures above the cusps of helmet streamers.
Abstract: Time-lapse sequences of white-light images, obtained during sunspot minimum conditions in 1996 by the Large Angle Spectrometric Coronagraph on the Solar and Heliospheric Observatory, give the impression of a continuous outflow of material in the streamer belt, as if we were observing Thomson scattering from inhomogeneities in the solar wind. Pursuing this idea, we have tracked the birth and outflow of 50-100 of the most prominent moving coronal features and find that: 1. They originate about 3-4 R☉ from Sun center as radially elongated structures above the cusps of helmet streamers. Their initial sizes are about 1 R☉ in the radial direction and 0.1 R☉ in the transverse direction. 2. They move radially outward, maintaining constant angular spans and increasing their lengths in rough accord with their speeds, which typically double from 150 km s-1 near 5 R☉ to 300 km s-1 near 25 R☉. 3. Their individual speed profiles v(r) cluster around a nearly parabolic path characterized by a constant acceleration of about 4 m s-2 through most of the 30 R☉ field of view. This profile is consistent with an isothermal solar wind expansion at a temperature of about 1.1 MK and a sonic point near 5 R☉. Based on their relatively small initial sizes, low intensities, radial motions, slow but increasing speeds, and location in the streamer belt, we conclude that these moving features are passively tracing the outflow of the slow solar wind.
TL;DR: In this article, the authors used a gravitationally self-consistent model for disk collapse to calculate the observable properties of disk galaxies as a function of mass and angular momentum.
Abstract: We present a scenario for the formation of disks that explains not only the properties of normal galaxies but also the properties of the population of low surface brightness galaxies (LSBs) as well. We use a gravitationally self-consistent model for disk collapse to calculate the observable properties of disk galaxies as a function of mass and angular momentum of the initial protogalaxy. The model naturally produces both smooth, asymptotically flat rotation curves and exponential surface brightness profiles over many disk scale lengths. In this scenario, low-mass and/or high angular momentum halos naturally form low baryonic surface density disks, which will tend to be low surface brightness. Theoretical and numerical calculations suggest galaxy halos should form with a wide range of mass and angular momenta, and thus, the disks that form within these halos should have a wide range of surface brightnesses and scale lengths. We use theoretical predictions for the distribution of halo masses and angular momenta to calculate explicitly the expected number density of disk galaxies as a function of central surface brightness and disk scale length. The resulting distribution is compared to the observed properties of galactic disks and is shown to explain the range of observed disk properties, including the cutoff in the maximum disk scale length as a function of surface brightness. We also show that disk instabilities explain the observed lack of high surface density disks. The calculated distribution of disk properties also suggests that there are large numbers of galaxies that remain undetected owing to biases against galaxies with either low surface brightness or small scale length. We quantify this by calculating the difference between the intrinsic luminosity function and the luminosity function that would be measured in a galaxy survey with a given limiting surface brightness. We show that current measurements of the galaxy luminosity function may be missing more than half of all L* galaxies, and an even larger fraction of faint galaxies, given the correlation between mass and surface brightness. The likely underestimate of the luminosity density is also expected to be large. We discuss how this affects observations of the "faint blue galaxy" population. We also investigate the dynamics of galaxies as a function of surface brightness. We show that, in the absence of any systematic change in the ratio of disk mass to disk luminosity, galaxies of all surface brightnesses should lie on the same Tully-Fisher relation. Our models also show systematic changes in the shape of the rotation curve as a function of angular momentum, which leads to LSBs having slowly rising rotation curves. Furthermore, because high angular momentum LSB disks have their baryonic mass spread over a larger area than normal galaxies of comparable mass, LSB disks contribute very little to the observed dynamics of the galaxy. Thus, LSBs provide a very effective tracer of the shape and mass profile of the dark matter halo, out to proportionally larger radii than is possible to observe with normal galaxy rotation curves.
University of California, Berkeley1, Lawrence Livermore National Laboratory2, Australian National University3, University of California, Davis4, University of Washington5, University of California, San Diego6, University of California, Santa Barbara7, European Southern Observatory8, University of Oxford9, McMaster University10
TL;DR: In this paper, the authors estimate the total microlensing optical depth toward the LMC from events with 2 < < 200 days to be τ2002=2.9×10−7 based upon their eight event sample.
Abstract: The MACHO Project is a search for dark matter in the form of massive compact halo objects (MACHOs). Photometric monitoring of millions of stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud (SMC), and Galactic bulge is used to search for gravitational microlensing events caused by these otherwise invisible objects. Analysis of the first 2.1 yr of photometry of 8.5 million stars in the LMC reveals eight candidate microlensing events. This is substantially more than the number expected (~1.1) from lensing by known stellar populations. The timescales (t) of the events range from 34 to 145 days. We estimate the total microlensing optical depth toward the LMC from events with 2 < < 200 days to be τ2002=2.9+ 1.4−0.9×10−7 based upon our eight event sample. This exceeds the optical depth, τbackgnd = 0.5 × 10-7, expected from known stars, and the difference is to be compared with the optical depth predicted for a "standard" halo composed entirely of MACHOs: τhalo = 4.7 × 10-7. To compare with Galactic halo models, we perform likelihood analyses on the full eight-event sample and a six-event subsample (which allows for two events to be caused by a nonhalo "background"). This gives a fairly model-independent estimate of the halo mass in MACHOs within 50 kpc of 2.0+ 1.2−0.7×1011 M☉, which is about half of the "standard halo" value. We also find a most probable MACHO mass of 0.5+ 0.3−0.2 M☉, although this value is strongly model dependent. In addition, the absence of short duration events places stringent upper limits on the contribution of low-mass MACHOs: objects from 10-4 M☉ to 0.03 M☉ contribute 20% of the "standard" dark halo.
TL;DR: In this article, the authors used the sample of emission-line nuclei derived from a recently completed optical spectroscopic survey of nearby galaxies to quantify the incidence of local (z ≈ 0) nuclear activity.
Abstract: We use the sample of emission-line nuclei derived from a recently completed optical spectroscopic survey of nearby galaxies to quantify the incidence of local (z ≈ 0) nuclear activity. Particular attention is paid to obtaining accurate measurements of the emission lines and reliable spectral classifications. The resulting database contains the largest collection of star-forming nuclei and active galactic nuclei (AGNs) currently known for nearby galaxies. It consists of 420 emission-line nuclei detected from a nearly complete, magnitude-limited sample of 486 galaxies with BT ≤ 12.5 mag and declination greater than 0°; the equivalent width detection limit of the brightest emission line, usually Hα, is ~0.25 A. As is consistent with previous studies, we find detectable amounts of ionized gas in the central few hundred parsecs of most galaxies (86%); emission lines are present in essentially every spiral galaxy and in a large fraction of ellipticals and lenticulars. Based on their narrow-line spectra, half of the objects can be classified as H II or star-forming nuclei and the other half as some form of AGN, of which we distinguish three classes: Seyfert nuclei, low-ionization nuclear emission-line regions (LINERs), and transition objects that we assume to be composite LINER/H II-nucleus systems. The population of AGNs is consequently very large; approximately 43% of the galaxies in our survey can be regarded as "active," although, for a number of reasons, this fraction is still rather uncertain. Most of the objects have much lower luminosities than commonly studied AGNs; the median luminosity of the narrow Hα line, after correcting for extinction, is only 2 × 1039 ergs s-1. Our sample therefore occupies the extreme faint end of the AGN luminosity function. We detect signatures of a broad-line region, as revealed by visible broad Hα emission, in ~20% of the AGN sample. Seyfert nuclei, both type 1 and type 2, reside in ~10% of all galaxies. LINERs make up the bulk (1/2-3/4) of the AGN population and a significant fraction (1/5-1/3) of all galaxies. A nonnegligible subset of LINERs emit broad Hα emission, furnishing direct evidence that a least some LINERs are indeed physically related to the AGN phenomenon. The dominant ionization mechanism of the nuclear emission depends strongly on the morphological type and luminosity of the host galaxy. AGNs are found predominantly in luminous, early-type (E-Sbc) galaxies, while H II nuclei prefer less luminous, late-type (Sbc and later) systems. The various AGN subclasses have broadly similar host galaxies.
TL;DR: In this paper, a representative sample of 20 intrinsically luminous quasars with redshifts smaller than 0.30 were observed with the Wide-Field Camera of the Hubble Space Telescope (HST).
Abstract: Observations with the Wide-Field Camera of the Hubble Space Telescope (HST) are presented for a representative sample of 20 intrinsically luminous quasars with redshifts smaller than 0.30. These observations show that luminous quasars occur in diverse environments that include ellipticals as bright as the brightest cluster galaxies (two), apparently normal ellipticals (10), apparently normal spirals with H II regions (three), complex systems of gravitationally interacting components (three), and faint surrounding nebulosity (two). The quasar host galaxies are centered on the quasar to the accuracy of our measurements, 400 pc. There are more radio-quiet quasars in galaxies that appear to be ellipticals (seven) than in spiral hosts (three), contrary to expectations. However, three, and possibly five, of the six radio-loud quasars have detectable elliptical hosts, in agreement with expectations. The luminous quasars studied in this paper occur preferentially in luminous galaxies. The average absolute magnitude of the hosts is 2.2 mag brighter than expected for a field galaxy luminosity function. The superb optical characteristics of the repaired HST make possible the detection of close galactic companions; we detect eight companion galaxies within projected distances of 10 kpc from quasar nuclei. The presence of very close companions, the images of current gravitational interactions, and the higher density of galaxies around the quasars suggest that gravitational interactions play an important role in triggering the quasar phenomenon.