arXiv subjects

Lars Hernquist

Publications and source records attributed to Lars Hernquist.

At least 541 records · Page 30Linked to original sources

arXiv2007

Distribution of Damped Lyman-alpha Absorbers in a Lambda Cold Dark Matter Universe

We present the results of a numerical study of a galactic wind model and its implications on the properties of damped Lyman-alpha absorbers (DLAs) using cosmological hydrodynamic simulations. We vary both the wind strength and the internal parameters of the the wind model in a series of cosmological SPH simulations that include radiative cooling and heating by a UV background, star formation, and feedback from supernovae and galactic winds. To test our simulations, we examine the DLA `rate-of-incidence' as a function of halo mass, galaxy apparent magnitude, and impact parameter. We find that the statistical distribution of DLAs does not depend on the exact values of internal numerical parameters that control the decoupling of hydrodynamic forces when the gas is ejected from starforming regions. The DLA rate-of-incidence in our simulations at z=3 is dominated by the faint galaxies with apparent magnitude R_AB < 25.5. However, interestingly in a `strong wind' run, the differential distribution of DLA sight-lines is peaked at Mhalo = 10^{12} Msun/h (R_AB~27), and the mean DLA halo mass is Mmean=10^{12.4} Msun/h (R_AB ~ 26). These mass-scales are much larger than those if we ignore winds, because galactic wind feedback suppresses the DLA cross section in low-mass halos and increases the relative contribution to the DLA incidence from more massive halos. The DLAs in our simulations are more compact than the present-day disk galaxies, and the impact parameter distribution is very narrow unless we limit the search for the host galaxy to only bright LBGs. The comoving number density of DLAs is higher than that of LBGs down to R_AB=30 mag if the physical radius of each DLA is smaller than 5 kpc/h_70. We discuss conflicts between current simulations and observations, and potential problems with simulations based on the CDM model.

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arXiv2006

Cosmological Parameter Estimation Using 21 cm Radiation from the Epoch of Reionization

A number of radio interferometers are currently being planned or constructed to observe 21 cm emission from reionization. Not only will such measurements provide a detailed view of that epoch, but, since the 21 cm emission also traces the distribution of matter in the Universe, this signal can be used to constrain cosmological parameters at 6 < z < 20. The sensitivity of an interferometer to the cosmological information in the signal may depend on how precisely the angular dependence of the 21 cm 3-D power spectrum can be measured. Utilizing an analytic model for reionization, we quantify all the effects that break the spherical symmetry of the 3-D 21 cm power spectrum and produce physically motivated predictions for this power spectrum. We find that upcoming observatories will be sensitive to the 21 cm signal over a wide range of scales, from larger than 100 to as small as 1 comoving Mpc. We consider three methods to measure cosmological parameters from the signal: (1) direct fitting of the density power spectrum to the signal, (2) using only the velocity field fluctuations in the signal, (3) looking at the signal at large enough scales such that all fluctuations trace the density field. With the foremost method, the first generation of 21 cm observations should moderately improve existing constraints on cosmological parameters for certain low-redshift reionization scenarios, and a two year observation with the second generation interferometer MWA5000 can improve constraints on Omega_w, Omega_m h^2, Omega_b h^2, Omega_nu, n_s, and alpha_s. If the Universe is substantially ionized by z = 12 or if spin temperature fluctuations are important, we show that it will be difficult to place competitive constraints on cosmological parameters with any of the considered methods.

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arXiv2006

An Evolutionary Model For Submillimeter Galaxies

[abridged]We calculate multi-wavelength spectral energy distributions (SEDs) (spanning optical to millimeter wavelengths) from simulations of major galaxy mergers with black hole feedback which produce submillimeter bright galaxies (SMGs), using a self-consistent three-dimensional radiative transfer code. We reproduce correlations for local AGN observed in Spitzer Space Telescope's IRAC bands, and make definitive predictions for infrared X-ray correlations that should be testable by combining observations by Spitzer and the upcoming Herschel mission with X-ray surveys. Our dynamical approach allows us to directly correlate observed clustering in the data as seen in IRAC color-color plots with the relative amount of time the system spends in a region of color-color space. We also find that this clustering is positively correlated with the stars dominating in their contribution to the total bolometric luminosity. We present photo albums spanning the lifetime of SMGs, from their infancy in the pre-merger phase to the final stage as an elliptical galaxy, as seen in the observed $3.6 \micron$ and $450 \micron$ band to visually illustrate some of the morphological differences between mergers of differing orbital inclination and progenitor redshift. We find that SMGs are a broader class of systems than starbursts or quasars. We introduce a simple, heuristic classification scheme on the basis of the $L_{\rm IR}/L_{\rm x}$ ratios of these galaxies, which may be interpreted qualitatively as an evolutionary scheme, as these galaxies evolve in $L_{\rm IR}/L_{\rm x}$ while transiting from the pre-merger stage, through the quasar phase, to a merger remnant.

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arXiv2006

Determining the Properties and Evolution of Red Galaxies from the Quasar Luminosity Function

(Abridged) We study the link between quasars and the red galaxy population using a model for the self-regulated growth of supermassive black holes in mergers involving gas-rich galaxies. Using a model for quasar lifetimes and evolution motivated by hydrodynamical simulations of galaxy mergers, we de-convolve the observed quasar luminosity function at various redshifts to determine the rate of formation of black holes of a given final mass. Identifying quasar activity with the formation of spheroids in the framework of the merger hypothesis, this enables us to deduce the corresponding rate of formation of spheroids with given properties as a function of redshift. This allows us to predict, for the red galaxy population, the distribution of galaxy velocity dispersions, the mass function, mass density, star formation rates, the luminosity function in many observed wavebands (NUV, U, B, V, R, I, J, H, K), the total red galaxy number density and luminosity density, the distribution of colors as a function of magnitude and velocity dispersion for several different wavebands, the distribution of mass to light ratios vs. mass, the luminosity-size relations, and the typical ages and distribution of ages (formation redshifts) as a function of both mass and luminosity. For each of these quantities, we predict the evolution from redshift z=0-6. Each of our predictions agrees well with existing observations, without the addition of tunable parameters; the essential observational inputs come from the observed quasar luminosity function. These predictions are skewed by several orders of magnitude if we adopt simpler, traditional models of quasar lifetimes in which quasars turn on/off or follow simple exponential light curves, instead of the more complicated evolution implied by our simulations.

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arXiv2006

How Much Mass do Supermassive Black Holes Eat in their Old Age?

We consider the distribution of local supermassive black hole Eddington ratios and accretion rates, accounting for the dependence of radiative efficiency and bolometric corrections on the accretion rate. We find that black hole mass growth, both of the integrated mass density and the masses of most individual objects, must be dominated by an earlier, radiatively efficient, high accretion rate stage, and not by the radiatively inefficient low accretion rate phase in which most local supermassive black holes are currently observed. This conclusion is particularly true of supermassive black holes in elliptical host galaxies, as expected if they have undergone merger activity in the past which would fuel quasar activity and rapid growth. We discuss models of the time evolution of accretion rates and show that they all predict significant mass growth in a prior radiatively efficient state. The only way to avoid this conclusion is through careful fine-tuning of the accretion/quasar timescale to a value that is inconsistent with observations. Our results agree with a wide range of observational inferences drawn from the quasar luminosity function and X-ray background synthesis models, but our approach has the virtue of being independent of the modeling of source populations. Models in which black holes spend the great majority of their time in low accretion rate phases are thus completely consistent both with observations implying mass gain in relatively short, high accretion rate phases and with the local distribution of accretion rates.

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arXiv2006

The Relation Between Quasar and Merging Galaxy Luminosity Functions and the Merger-Induced Star Formation Rate of the Universe

Using a model for self-regulated growth of black holes (BHs) in mergers involving gas-rich galaxies, we study the relationship between quasars and the population of merging galaxies and predict the merger-induced star formation rate density of the Universe. Mergers drive nuclear gas inflows, fueling starbursts and 'buried quasars' until accretion feedback expels the gas, rendering a briefly visible optical quasar. Star formation is shut down and accretion declines, leaving a passively evolving remnant with properties typical of red, elliptical galaxies. Based on evolution of these events in our simulations, we demonstrate that the observed statistics of merger rates, luminosity functions (LFs) and mass functions, SFR distributions, specific SFRs, quasar and quasar host galaxy LFs, and elliptical/red galaxy LFs are self-consistent and follow from one another as predicted by the merger hypothesis. We use our simulations to de-convolve both quasar and merging galaxy LFs to determine the birthrate of black holes of a given final mass and merger rates as a function of stellar mass. We use this to predict the merging galaxy LF in several observed wavebands, color-magnitude relations, mass functions, absolute and specific SFR distributions and SFR density, and quasar host galaxy LFs, as a function of redshift from z=0-6. We invert this and predict e.g. quasar LFs from observed merger LFs or SFR distributions. Our results agree well with observations, but idealized models of quasar lightcurves are ruled out by comparison of merger and quasar observations at >99.9% confidence. Using only observations of quasars, we estimate the contribution of mergers to the SFR density of the Universe even to high redshifts z~4.

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arXiv2006

Higher Order Contributions to the 21 cm Power Spectrum

We consider the contribution of 3rd and 4th order terms to the power spectrum of 21 cm brightness temperature fluctuations during the epoch of reionization, which arise because the 21 cm brightness temperature involves a product of the hydrogenic neutral fraction and the gas density. The 3rd order terms vanish for Gaussian random fields, and have been previously neglected or ignored. We measure these higher order terms from radiative transfer simulations and estimate them using cosmological perturbation theory. In our simulated models, the higher order terms are significant: neglecting them leads to a >~100% error in 21 cm power spectrum predictions on scales of k >~ 1 Mpc^{-1} when the neutral fraction is ~0.5. The higher order terms have a simple physical interpretation. On small scales they are produced by gravitational mode coupling. Small scale structure grows more readily in large-scale overdense regions, but the same regions tend to be ionized and hence do not contribute to the 21 cm signal. This acts to suppress the influence of non-linear density fluctuations and the small-scale amplitude of the 21 cm power spectrum. These results modify earlier intuition that the 21 cm power spectrum simply traces the density power spectrum on scales smaller than that of a typical bubble, and imply that small scale measurements contain more information about the nature of the ionizing sources than previously believed. On large scales, higher order moments are not directly related to gravity. They are non-zero because over-dense regions tend to ionize first and are important in magnitude at late times owing to the large fluctuations in the neutral fraction. (Abridged)

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arXiv2006

An Observational Determination of the Bolometric Quasar Luminosity Function

We combine a large set of quasar luminosity function (QLF) measurements from the rest-frame optical, soft and hard X-ray, and near- and mid-infrared bands to determine the bolometric QLF in the redshift interval z=0-6. Accounting for the observed distributions of quasar column densities and variation of spectral energy distribution (SED) shapes, and their dependence on luminosity, makes it possible to integrate the observations in a reliable manner and provides a baseline in redshift and luminosity larger than that of any individual survey. We infer the QLF break luminosity and faint-end slope out to z~4.5 and confirm at high significance (>10sigma) previous claims of a flattening in both the faint- and bright-end slopes with redshift. With the best-fit estimates of the column density distribution and quasar SED, which both depend on luminosity, a single bolometric QLF self-consistently reproduces the observed QLFs in all bands and at all redshifts for which we compile measurements. Ignoring this luminosity dependence does not yield a self-consistent bolometric QLF and there is no evidence for any additional dependence on redshift. We calculate the expected relic black hole mass function and mass density, cosmic X-ray background, and ionization rate as a function of redshift and find they are consistent with existing measurements. The peak in the total quasar luminosity density is well-constrained at z=2.15+/-0.05. We provide a number of fitting functions to the bolometric QLF and its manifestations in various bands, and a script to return the QLF at arbitrary frequency and redshift from these fits, as the most simple inferences from the QLF measured in a single band can be misleading.

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arXiv2006

An Upper Limit to the Degree of Evolution Between Supermassive Black Holes and their Host Galaxies

We describe a model-independent integral constraint which defines an upper limit to the allowed degree of evolution in the ratio of black hole (BH) mass to host galaxy luminosity or mass, as a function of redshift. Essentially, if the BH/host ratio is excessive at redshift z, then it would imply that the total mass density in BHs above some M_BH(min) is larger at that redshift than at z=0, which is impossible. This argument requires no knowledge of host or BH properties, only a lower limit to the observed luminosity density in the brightest galaxies at some z. We calculate this upper limit from a variety of luminosity and mass functions in different bands from redshifts z=0-2. We show that it is consistent with passive evolution of spheroid populations (with a fixed M_BH/M_host relation) in all cases, and provides tighter constraints than have generally been obtained previously, ruling out at >6 sigma observational and theoretical estimates suggesting that M_BH/M_host was significantly larger at high redshifts than locally, although relatively weak (factor ~2 by z=2) evolution is still allowed. We discuss a variety of possible 'loopholes' or changes in the BH/host populations and correlations, and show that they typically lower the upper limits and strengthen our conclusions.

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arXiv2006

Formation of Primordial Stars in a LCDM Universe

We study the formation of the first generation of stars in the standard cold dark matter model, using a very high-resolution hydordynamic simulations. Our simulation achieves a dynamic range of 10^{10} in length scale. With accurate treatment of atomic and molecular physics, it allows us to study the chemo-thermal evolution of primordial gas clouds to densities up to n = 10^{16}/cc without assuming any a priori equation of state; a six orders of magnitudes improvement over previous three-dimensional calculations. All the relevant atomic and molecular cooling and heating processes, including cooling by collision-induced continuum emission, are implemented. For calculating optically thick H2 cooling at high densities, we use the Sobolev method. To examine possible gas fragmentation owing to thermal instability, we compute explicitly the growth rate of isobaric perturbations. We show that the cloud core does not fragment in either the low-density or high-density regimes. We also show that the core remains stable against gravitational deformation and fragmentation. We obtain an accurate gas mass accretion rate within a 10 Msun innermost region around the protostar. The protostar is accreting the surrounding hot gas at a rate of 0.001-0.01 Msun/yr. From these findings we conclude that primordial stars formed in early minihalos are massive. We carry out proto-stellar evolution calculations using the obtained accretion rate. The resulting mass of the first star is M_ZAMS = 60-100 Msun, with the exact mass dependent on the actual accretion rate.

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arXiv2006

Kinematic Structure of Merger Remnants

We use numerical simulations to study the kinematic structure of remnants formed from mergers of equal-mass disk galaxies. In particular, we show that remnants of dissipational mergers, which include the radiative cooling of gas, star formation, feedback from supernovae, and the growth of supermassive black holes, are smaller, rounder, have, on average, a larger central velocity dispersion, and show significant rotation compared to remnants of dissipationless mergers. The increased rotation speed of dissipational remnants owes its origin to star formation that occurs in the central regions during the galaxy merger. We have further quantified the anisotropy, three-dimensional shape, minor axis rotation, and isophotal shape of each merger remnant, finding that dissipational remnants are more isotropic, closer to oblate, have the majority of their rotation along their major axis, and are more disky than dissipationless remnants. Individual remnants display a wide variety of kinematic properties. A large fraction of the dissipational remnants are oblate isotropic rotators. Many dissipational, and all of the dissipationless, are slowly rotating and anisotropic. The remnants of gas-rich major mergers can well-reproduce the observed distribution of projected ellipticities, rotation parameter (V/σ)*, kinematic misalignments, Psi, and isophotal shapes. The dissipationless remnants are a poor match to this data. Our results support the merger hypothesis for the origin of low-luminosity elliptical galaxies provided that the progenitor disks are sufficiently gas-rich, however our remnants are a poor match to the bright ellipticals that are slowly rotating and uniformly boxy.

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arXiv2006

X-ray Emission from Hot Gas in Galaxy Mergers

We examine X-ray emission produced from hot gas during collisions and mergers of disk galaxies. To study this process, we employ simulations that incorporate cosmologically motivated disk-galaxy models and include the effects of radiative cooling, star formation, supernova feedback, and accreting supermassive black holes. We find that during a merger, the colliding gas in the disks is shock-heated to X-ray-emitting temperatures. The X-ray luminosity is spatially extended, rises during the initial stages of the merger, and peaks when the galactic centers coalesce. When a physical model for accreting black holes is included, the resulting feedback can drive powerful winds that contribute significantly to the amount and metallicity of hot gas, both of which increase the X-ray luminosity. In terms of their stellar kinematics and structural properties, the merger remnants in our simulations resemble elliptical galaxies. We find that the X-ray luminosities of the remnants with B-band luminosities in the range L_B ~ 10^10 - 10^11 Lsun are consistent with observations, while remnants with smaller or larger masses are underluminous in X-rays. Moreover, because the majority of the merger remnants are broadly consistent with the observed scaling relations between temperature, B-band luminosity and X-ray luminosity we conclude that major mergers are a viable mechanism for producing the X-ray halos of large, luminous elliptical galaxies.

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arXiv2006

The Impact of Temperature Fluctuations on the Lyman-alpha Forest Power Spectrum

We explore the impact of spatial fluctuations in the intergalactic medium temperature on the Lyman-alpha forest flux power spectrum near z ~ 3. We develop a semianalytic model to examine temperature fluctuations resulting from inhomogeneous HI and incomplete HeII reionizations. Detection of these fluctuations might provide insight into the reionization histories of hydrogen and helium. Furthermore, these fluctuations, neglected in previous analyses, could bias constraints on cosmological parameters from the Lyman-alpha forest. We find that the temperature fluctuations resulting from inhomogeneous HI reionization are likely to be very small, with an rms amplitude of < 5%, $σ_{T_0}/ < 0.05$. More important are the temperature fluctuations that arise from incomplete HeII reionization, which might plausibly be as large as 50%, $σ_{T_0}/ ~ 0.5$. In practice, however, these temperature fluctuations have only a small effect on flux power spectrum predictions. The smallness of the effect is possibly due to density fluctuations dominating over temperature fluctuations on the scales probed by current measurements. On the largest scales currently probed, k ~ 0.001 s/km (~0.1 h/Mpc), the effect on the flux power spectrum may be as large as ~10% in extreme models. The effect is larger on small scales, up to ~20% at k = 0.1 s/km, due to thermal broadening. Our results suggest that the omission of temperature fluctuations effects from previous analyses does not significantly bias constraints on cosmological parameters.

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arXiv2006

Detectability of [CII] 158 micron emission from high-redshift galaxies: predictions for ALMA and SPICA

We discuss the detectability of high-redshift galaxies via [CII] 158 micron line emission by coupling an analytic model with cosmological Smoothed Particle Hydrodynamics (SPH) simulations that are based on the concordance Lambda cold dark matter (CDM) model. Our analytic model describes a multiphase interstellar medium irradiated by the far ultra-violet radiation from local star-forming regions, and it calculates thermal and ionization equilibrium between cooling and heating. The model allows us to predict the mass fraction of a cold neutral medium (CNM) embedded in a warm neutral medium (WNM). Our cosmological SPH simulations include a treatment of radiative cooling/heating, star formation, and feedback effects from supernovae and galactic winds. Using our method, we make predictions for the [CII] luminosity from high-redshift galaxies which can be directly compared with upcoming observations by the Atacama Large Millimeter Array (ALMA) and the Space Infrared Telescope for Cosmology and Astrophysics (SPICA). We find that the number density of high-redshift galaxies detectable by ALMA and SPICA via [CII] emission depends significantly on the amount of neutral gas which is highly uncertain. Our calculations suggest that, in a CDM universe, most [CII] sources at z=3 are faint objects with \Snu < 0.01 mJy. Lyman-break galaxies (LBGs) brighter than R_AB=23.5 mag are expected to have flux densities \Snu = 1-3 mJy depending on the strength of galactic wind feedback. The recommended observing strategy for ALMA and SPICA is to aim at very bright LBGs or star-forming DRG/BzK galaxies.

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arXiv2006

Molecular Outflows in Galaxy Merger Simulations with Embedded AGN

We study the effects of feedback from active galactic nuclei (AGN) on emission from molecular gas in galaxy mergers by combining hydrodynamic simulations which include black holes with a three-dimensional, non-local thermodynamic equilibrium (LTE) radiative transfer code. We find that molecular clouds entrained in AGN winds produce an extended CO morphology with significant off-nuclear emission, which may be detectable via contour mapping. Furthermore, kinematic signatures of these molecular outflows are visible in emission line profiles when the outflow has a large line of sight velocity. Our results can help interpret current and upcoming observations of luminous infrared galaxies, as well as provide a detailed test of subresolution prescriptions for supermassive black hole growth in galaxy-scale hydrodynamic simulations.

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arXiv2006

A Merger-Driven Scenario for Cosmological Disk Galaxy Formation

(Abridged) The violent hierarchical nature of the LCDM cosmology poses serious difficulties for the formation of disk galaxies. To help resolve these issues, we describe a new, merger-driven scenario for the cosmological formation of disk galaxies at high redshifts that supplements the standard model based on dissipational collapse.In this picture, large gaseous disks may be produced from high-angular momentum mergers of systems that are gas-dominated, i.e. M_gas/(M_gas +M_star > 0.5 at the height of the merger. Pressurization from the multiphase structure of the interstellar medium prevents the complete conversion of gas into stars during the merger, and if enough gas remains to form a disk, the remnant eventually resembles a disk galaxy. We perform numerical simulations of galaxy mergers to study how supernovae feedback strength, supermassive black hole growth and feedback, progenitor gas fraction, merger mass-ratio, and orbital geometry impact the formation of remnant disks. We find that disks can build angular momentum through mergers and the degree of rotational support of the baryons in the merger remnant is primarily related to feedback processes associated with star formation. Disk-dominated remnants are restricted to form in mergers that are gas-dominated at the time of final coalescence and gas-dominated mergers typically require extreme progenitor gas fractions (>80%). We also show that the formation of rotationally-supported stellar systems in mergers is not restricted to idealized orbits, or major or minor mergers. We suggest that the hierarchical nature of the LCDM cosmology and the physics of the interstellar gas may act together to form spiral galaxies by building the angular momentum of disks through early, gas-dominated mergers.

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arXiv2006

Simulated Molecular Gas Emission in Galaxy Mergers with Embedded AGN

We investigate the effect of embedded active galactic nuclei (AGN) in galaxy mergers on the CO molecular line emission by combining non-local thermodynamic equilibrium (LTE) radiative transfer calculations with hydrodynamic simulations. We find that AGN feedback energy in gas rich galaxy mergers can contribute to large molecular outflows which may be detectable via velocity-integrated emission contour maps, as well as through kinematic features in the emission line profiles.

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arXiv2006

Fueling Low-Level AGN Activity Through the Stochastic Accretion of Cold Gas

Using a simple description of feedback from black hole (BH) growth, we develop an analytic model for the fueling of Seyferts (low-luminosity AGN) and their relation to their host galaxies, Eddington ratio distributions, and cosmological evolution. We derive a solution for evolution of accretion rates in a feedback-driven blast wave, applicable to large-scale outflows from bright quasars in galaxy mergers, low-luminosity AGN, and BHs in supernova remnants. Under the assumption that cold gas stochastically accretes onto a central SMBH at a rate set by the dynamics of that gas, our solution determines the Seyfert light curve evolution. We use this to predict the Seyfert luminosity function (LF), duty cycles, lifetimes, and distribution of host morphologies, Eddington ratios, and obscuration as a function of AGN luminosity and mass, and find agreement with observations at z=0. We consider the breakdown of contributions from this and stellar wind accretion and merger-driven activity. We make specific predictions for the weak evolution of the Seyfert LF with redshift (LF of non-merger driven activity), and for changes in both the slope and scatter of the M_BH-sigma relation at low-M_BH. Quiescent fueling dominates over a wide luminosity range (-14 > M_B > -22) at z=0, where most BH growth occurs around M_BH~10^7 M_sun, in S0 and Sa/b galaxies. However, quasar activity from gas-rich mergers evolves more rapidly with redshift, and by z=1 quiescent fueling is important only at luminosities an order of magnitude or more below the LF break. Although non-merger driven fueling is important for BH growth and the M_BH-sigma relation at low M_BH, it does not significantly contribute to the total BH mass density or cosmological backgrounds.

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