arXiv subjects

Lars Hernquist

Publications and source records attributed to Lars Hernquist.

At least 577 records · Page 32Linked to original sources

arXiv2005

A QED Model for Non-thermal Emission from SGRs and AXPs

Previously, we showed that, owing to effects arising from quantum electrodynamics (QED), magnetohydrodynamic fast modes of sufficient strength will break down to form electron-positron pairs while traversing the magnetospheres of strongly magnetised neutron stars. The bulk of the energy of the fast mode fuels the development of an electron-positron fireball. However, a small, but potentially observable, fraction of the energy ($\sim 10^{33}$ ergs) can generate a non-thermal distribution of electrons and positrons far from the star. In this paper, we examine the cooling and radiative output of these particles. We also investigate the properties of non-thermal emission in the absence of a fireball to understand the breakdown of fast modes that do not yield an optically thick pair plasma. This quiescent, non-thermal radiation associated with fast mode breakdown may account for the recently observed non-thermal emission from several anomalous X-ray pulsars and soft-gamma repeaters.

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arXiv2005

A Physical Model for the Origin of Quasar Lifetimes

We propose a model of quasar lifetimes in which observational quasar lifetimes and an intrinsic lifetime of rapid accretion are strongly distinguished by the physics of obscuration by surrounding gas and dust. Quasars are powered by gas funneled to galaxy centers, but for a large part of the accretion lifetime are heavily obscured by the large gas densities powering accretion. In this phase, starbursts and black hole growth are fueled but the quasar is buried. Eventually, feedback from accretion energy disperses surrounding gas, creating a window in which the black hole is observable optically as a quasar, until accretion rates drop below those required to maintain a quasar luminosity. We model this process and measure the unobscured and intrinsic quasar lifetimes in a hydrodynamical simulation of a major galaxy merger. The source luminosity is determined from the black hole accretion rate, calculated from local gas properties. We calculate the column density of hydrogen to the source along multiple lines of sight and use these column densities and gas metallicities to determine B-band attenuation of the source. Defining the observable quasar lifetime as the total time with an observed B-band luminosity above some limit L_B,min, we find lifetimes ~10-20 Myr for L_B,min=10^11 L_sun (M_B=-23), in good agreement with observationally determined quasar lifetimes. This is significantly smaller than the intrinsic lifetime ~100 Myr obtained if attenuation is neglected. The ratio of observed to intrinsic lifetime is also strong function of both the limiting luminosity and the observed frequency.

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arXiv2005

Energy input from quasars regulates the growth and activity of black holes and their host galaxies

In the early Universe, while galaxies were still forming, black holes as massive as a billion solar masses powered quasars. Supermassive black holes are found at the centers of most galaxies today, where their masses are related to the velocity dispersions of stars in their host galaxies and hence to the mass of the central bulge of the galaxy. This suggests a link between the growth of the black holes and the host galaxies, which has indeed been assumed for a number of years. But the origin of the observed relation between black hole mass and stellar velocity dispersion, and its connection with the evolution of galaxies have remained unclear. Here we report simulations that simultaneously follow star formation and the growth of black holes during galaxy-galaxy collisions. We find that in addition to generating a burst of star formation, a merger leads to strong inflows that feed gas to the supermassive black hole and thereby power the quasar. The energy released by the quasar expels enough gas to quench both star formation and further black hole growth. This determines the lifetime of the quasar phase (approaching 100 million years) and explains the relationship between the black hole mass and the stellar velocity dispersion.

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arXiv2005

Confronting cosmological simulations with observations of intergalactic metals

Using the statistics of pixel optical depths, we compare HI, CIV and CIII absorption in a set of six high quality z ~ 3-4 quasar absorption spectra to that in spectra drawn from two different state-of-the-art cosmological simulations that include galactic outflows. We find that the simulations predict far too little CIV absorption unless the UVB is extremely soft, and always predict far too small CIII/CIV ratios. We note, however, that much of the enriched gas is in a phase (T ~ 10^5-10^7 K, overdensity ~ 0.1-10, Z >~0.1 Z_sol) that should cool by metal line emission -- which was not included in our simulations. When the effect of cooling is modeled, the predicted CIV absorption increases substantially, but the CIII/CIV ratios are still far too small because the density of the enriched gas is too low. Finally, we find that the predicted metal distribution is much too inhomogeneous to reproduce the observed probability distribution of CIV absorption. These findings suggest that strong z <~ 6 winds cannot fully explain the observed enrichment, and that an additional (perhaps higher-z) contribution is required.

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arXiv2004

The Temperature Structure of the Warm-Hot Intergalactic Medium

We study the temperature structure of the intergalactic medium (IGM) using a large cosmological N-body/SPH simulation. We employ a two-temperature model for the thermal evolution of the ionized gas, in which the relaxation process between electrons and ions is explicitly included. In the diffuse, hot IGM, the relaxation time is comparable to the age of the Universe and hence the electron temperature in post-shock regions remains significantly smaller than the ion temperature. We show that, at the present epoch, a large fraction of the warm/hot intergalactic medium (WHIM) has a well-developed two temperature structure, with typical temperature differences of order a factor of a few. Consequently, the fraction of metals in various ionization states such as OVI, OVII, and OVIII, as well as their line emissivities, can differ locally by more than an order of magnitude from those computed with a single temperature model. It is thus necessary to follow the evolution of the electron temperature explicitly to determine absorption and emission by the WHIM. Although equipartition is nearly achieved in the denser intracluster medium (ICM), we find an appreciable systematic deviation between the gas-mass weighted electron temperature and the mean temperature even at half the virial radii of clusters. There is thus a reservoir of warm (Te < 1keV) gas in and around massive clusters. Our results imply that relaxation processes need to be considered in describing and interpreting observational data from existing X-ray telescopes as well as from future missions designed to detect the WHIM, such as the Diffuse Intergalactic Oxygen Surveyor and the Missing Baryon Explorer.

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arXiv2004

Formation of a spiral galaxy in a major merger

We use numerical simulations to examine the structure of merger remnants resulting from collisions of gas-rich spiral galaxies. When the gas fraction of the progenitors is small, the remnants structurally and kinematically resemble elliptical galaxies, in agreement with earlier work. However, if the progenitor disks are gas-dominated, new types of outcomes are possible. In fact, we show that a prominent disk may survive in certain cases. To illustrate this scenario, we analyze an extreme example with progenitor galaxies consisting of dark matter halos, pure gas disks, and no bulges, as might be appropriate for mergers at high redshifts. While rapid star formation triggered by tidal torques during the merger forms a central, rotating bulge in the remnant, not all the gas is consumed in the burst. The remaining gas cools very quickly and settles into an extended star-forming disk, yielding an object similar to a spiral galaxy, and not an early type galaxy. This is contrary to the usual view that major mergers invariably destroy disks. The morphological evolution of galaxies can therefore be more complicated than often assumed, and in particular, theoretical constraints based on the fragility of spiral disks need to be reevaluated.

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arXiv2004

Massive galaxies at redshift 2 in cosmological hydrodynamic simulations

We study the properties of galaxies at z=2 in a Lambda cold dark matter universe, using two different types of hydrodynamic simulation methods -- Eulerian TVD and smoothed particle hydrodynamics (SPH) -- and a spectrophotometric analysis in the U_n, G, R filter set. The simulated galaxies at z=2 satisfy the color-selection criteria proposed by Adelberger et al. (2004) and Steidel et al. (2004) when we assume Calzetti extinction with E(B-V)=0.15. We find that the number density of simulated galaxies brighter than R<25.5 at z=2 is about 1e-2 h^3 Mpc^-3 for E(B-V)=0.15, which is roughly twice that of the number density found by Erb et al. (2004) for the UV bright sample. This suggests that roughly half of the massive galaxies with M*>10^{10} Msun/h at z=2 are UV bright population, and the other half is bright in the infra-red wavelengths. The most massive galaxies at z=2 have stellar masses >= 10^{11-12} Msun. They typically have been continuously forming stars with a rate exceeding 30 Msun/yr over a few Gyrs from z=10 to z=2, together with significant contribution by starbursts reaching up to 1000 Msun/yr which lie on top of the continuous component. TVD simulations indicate a more sporadic star formation history than the SPH simulations. Our results do not imply that hierarchical galaxy formation fails to account for the observed massive galaxies at z>=1. The global star formation rate density in our simulations peaks at z>=5, a much higher redshift than predicted by the semianalytic models. This star formation history suggests early build-up of the stellar mass density, and predicts that 70 (50, 30)% of the total stellar mass at z=0 had already been formed by z=1 (2, 3). Upcoming observations by Spitzer and Swift might help to better constrain the star formation history at high redshift.

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arXiv2004

Modeling feedback from stars and black holes in galaxy mergers

We describe techniques for incorporating feedback from star formation and black hole accretion into simulations of isolated and merging galaxies. At present, the details of these processes cannot be resolved in simulations on galactic scales. Our basic approach therefore involves forming coarse-grained representations of the properties of the interstellar medium and black hole accretion starting from basic physical assumptions, so that the impact of these effects can be included on resolved scales. We illustrate our method using a multiphase description of star-forming gas. Feedback from star formation pressurises highly overdense gas, altering its effective equation of state. We show that this allows the construction of stable galaxy models with much larger gas fractions than possible in earlier numerical work. We extend the model by including a treatment of gas accretion onto central supermassive black holes in galaxies. Assuming thermal coupling of a small fraction of the bolometric luminosity of accreting black holes to the surrounding gas, we show how this feedback regulates the growth of black holes. In gas-rich mergers of galaxies, we observe a complex interplay between starbursts and central AGN activity when the tidal interaction triggers intense nuclear inflows of gas. Once an accreting supermassive black hole has grown to a critical size, feedback terminates its further growth, and expels gas from the central region in a powerful quasar-driven wind. Our simulation methodology is therefore able to address the coupled processes of gas dynamics, star formation, and black hole accretion during the formation of galaxies.

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arXiv2004

Lyman-alpha Emission from Structure Formation

The nature of the interaction between galaxies and the intergalactic medium (IGM) is one of the most fundamental problems in astrophysics. The accretion of gas onto galaxies provides fuel for star formation, while galactic winds transform the nearby IGM in a number of ways. One exciting technique to study this gas is through the imaging of hydrogen Lyman-alpha emission. We use cosmological simulations to study the Lyman-alpha signals expected from the growth of cosmic structure from z=0-5. We show that if dust absorption is negligible, recombinations following the absorption of stellar ionizing photons dominate the total Lyman-alpha photon production rate. However, galaxies are also surrounded by "Lyman-alpha coronae" of diffuse IGM gas. These coronae are composed of a combination of accreting gas and material ejected from the central galaxy by winds. The Lyman-alpha emission from this phase is powered by a combination of gravitational processes and the photoionizing background. While the former dominates at z~0, collisional excitation following photo-heating may well dominate the total emission at higher redshifts. The central regions of these systems are dense enough to shield themselves from the metagalactic ionizing background; unfortunately, in this regime our simulations are no longer reliable. We therefore consider several scenarios for the emission from the central cores, including one in which self-shielded gas does not emit at all. We show that the combination of star formation and cooling IGM gas can explain most of the observed "Lyman-alpha blobs" at z~3, with the important exception of the largest sources. On the other hand, except under the most optimistic assumptions, cooling IGM gas cannot explain the observations on its own.

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arXiv2004

21 Centimeter Fluctuations from Cosmic Gas at High Redshifts

The relatively large Thomson optical depth, tau_e, inferred recently from the WMAP observations suggests that the Universe was reionized in a more complex manner than previously believed. However, the value of tau_e provides only an integral constraint on the history of reionization and, by itself, cannot be used to determine the nature of the sources responsible for this transition. Here, we show that the evolution of the ionization state of the intergalactic medium at high redshifts can be measured statistically using fluctuations in 21 centimeter radiation from neutral hydrogen. By analogy with the mathematical description of anisotropies in the cosmic microwave background, we develop a formalism to quantify the variations in 21 cm emission as a function of both frequency and angular scale. Prior to and following reionization, fluctuations in the 21 cm signal are mediated by density perturbations in the distribution of matter. Between these epochs, pockets of gas surrounding luminous objects become ionized, producing large HII regions. These ``bubbles'' of ionized material imprint features into the 21 cm power spectrum that make it possible to distinguish them from fluctuations produced by the density perturbations. The variation of the power spectrum with frequency can be used to infer the evolution of this process. As has been emphasized previously by others, the absolute 21 cm signal from neutral gas at high redshifts is undetectable owing to contamination by foreground sources. However, we argue that this source of noise can be suppressed by comparing maps closely spaced in frequency, i.e. redshift, so that 21 cm fluctuations from the IGM can be measured against a much brighter, but smoothly varying (in frequency) background.

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arXiv2004

Black holes in galaxy mergers: The formation of red elliptical galaxies

We use hydrodynamical simulations to study the color transformations induced by star formation and active galactic nuclei (AGN) during major mergers of spiral galaxies. Our modeling accounts for radiative cooling, star formation, and supernova feedback. Moreover, we include a treatment of accretion onto supermassive black holes embedded in the nuclei of the merging galaxies. We assume that a small fraction of the bolometric luminosity of an accreting black hole couples thermally to surrounding gas, providing a feedback mechanism that regulates its growth. The encounter and coalescence of the galaxies triggers nuclear gas inflow which fuels both a powerful starburst and strong black hole accretion. Comparing simulations with and without black holes, we show that AGN feedback can quench star formation and accretion on a short timescale,particularly in large galaxies where the black holes can drive powerful winds once they become sufficiently massive. The color evolution of the remnant differs markedly between mergers with and without central black holes. Without AGN, gas-rich mergers lead to ellipticals which remain blue owing to residual star formation, even after more than 7 Gyrs have elapsed. In contrast, mergers with black holes produce ellipticals that redden much faster, an effect that is more pronounced in massive remnants where a nearly complete termination of star formation occurs, allowing them to redden to u-r ~ 2.3 in less than one Gyr. AGN feedback may thus be required to explain the population of extremely red massive early type-galaxies, and it appears to be an important driver in generating the observed bimodal color distribution of galaxies in the Local Universe.

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arXiv2004

Photometric Properties of Lyman-break Galaxies at z=3 in Cosmological SPH Simulations

We study the photometric properties of Lyman-break galaxies (LBGs) formed by redshift z=3 in a set of large cosmological smoothed-particle hydrodynamics simulations of the Lambda cold dark matter (CDM) model. Our numerical simulations include radiative cooling and heating with a uniform UV background, star formation, supernova feedback, and a phenomenological model for galactic winds. Analysing a series of simulations of varying boxsize and particle number allows us to isolate the impact of numerical resolution on our results. We compute spectra of simulated galaxies using a population synthesis model, and derive colours and luminosity functions of galaxies at z=3 after applying local dust extinction and absorption by the intergalactic medium (IGM). We find that the simulated galaxies have U-G and G-R colours consistent with observations, provided that intervening absorption by the IGM is applied. The observed properties of LBGs, including their number density, colours, and luminosity functions, can be explained if LBGs are identified with the most massive galaxies at z=3, having typical stellar mass of M_{star} ~ 1e10 Msun/h, a conclusion broadly consistent with earlier studies based on hydrodynamic simulations of the Lamda CDM model. We also find that most simulated LBGs were continuously forming stars at a high rate for more than one Gyr up until z=3, but with numerous starbursts lying on top of the continuous component. Interestingly, our simulations suggest that more than 50% of the total stellar mass and star formation rate in the Universe are accounted for by galaxies that are not detected in the current generation of LBG surveys.

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arXiv2004

A QED Model for the Origin of Bursts from SGRs and AXPs

We propose a model to account for the bursts from soft gamma repeaters (SGRs) and anomalous X-ray pulsars (AXPs) in which quantum electrodynamics (QED) plays a vital role. In our theory, that we term ``fast-mode breakdown,'' magnetohydrodynamic (MHD) waves that are generated near the surface of a neutron star and propagate outward through the magnetosphere will be modified by the polarization of the vacuum. For neutron star magnetic fields $B_\rmscr{NS} \gtrsim B_\rmscr{QED} \approx 4.4 \times 10^{13}$ G, the interaction of the wave fields with the vacuum produces non-linearities in fast MHD waves that can steepen in a manner akin to the growth of hydrodynamic shocks. Under certain conditions, fast modes can develop field discontinuities on scales comparable to an electron Compton wavelength, at which point the wave energy will be dissipated through electron-positron pair production. We show that this process operates if the magnetic field of the neutron star is sufficiently strong and the ratio of the wavelength of the fast mode to its amplitude is sufficiently small, in which case the wave energy will be efficiently converted into an extended pair-plasma fireball. The radiative output from this fireball will consist of hard X-rays and soft $γ$-rays, with a spectrum similar to those seen in bursts from SGRs and AXPs (abridged).

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arXiv2004

Massive galaxies in cosmological simulations: UV-selected sample at redshift z=2

We study the properties of galaxies at z=2 in a Lambda CDM universe, using two different types of hydrodynamic simulation methods (Eulerian TVD and SPH) and a spectrophotometric analysis in the Un, G, R filter set. The simulated galaxies at z=2 satisfy the color-selection criteria proposed by Adelberger et al. (2004) when we assume Calzetti extinction with E(B-V)=0.15. We find that the number density of simulated galaxies brighter than R<25.5 at z=2 is about 2e-2 h^3/Mpc^3, roughly one order of magnitude larger than that of Lyman break galaxies at z=3. The most massive galaxies at z=2 have stellar masses >~1e11 Msun, and their observed-frame G-R colors lie in the range 0.0 = 1.0 and Mstar>1e10 Msun/h finished the build-up of their stellar mass by z~3. Interestingly, our study suggests that the majority of the most massive galaxies at z=2 should be detectable at rest-frame UV wavelengths, contrary to some recent claims made on the basis of near-IR studies of galaxies at the same epoch, provided the median extinction is less than E(B-V)<0.3. However, our results also suggest that the fraction of stellar mass contained in galaxies that pass the color-selection criteria could be as low as 50% of the total stellar mass in the Universe at z=2. Our simulations suggest that the missing stellar mass is contained in fainter (R>25.5) and intrinsically redder galaxies. Our results do not suggest that hierarchical galaxy formation fails to account for the massive galaxies at z>=1. (abridged)

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arXiv2004

Detecting Sunyaev-Zel'dovich clusters with PLANCK: I. Construction of all-sky thermal and kinetic SZ-maps

All-sky thermal and kinetic Sunyaev-Zel'dovich (SZ) maps are presented for assessing how well the PLANCK-mission can find and characterise clusters of galaxies, especially in the presence of primary anisotropies of the cosmic microwave background (CMB) and various galactic and ecliptic foregrounds. The maps have been constructed from numerical simulations of structure formation in a standard LCDM cosmology and contain all clusters out to redshifts of z = 1.46 with masses exceeding 5e13 M_solar/h. By construction, the maps properly account for the evolution of cosmic structure, the halo-halo correlation function, the evolving mass function, halo substructure and adiabatic gas physics. The velocities in the kinetic map correspond to the actual density environment at the cluster positions. We characterise the SZ-cluster sample by measuring the distribution of angular sizes, the integrated thermal and kinetic Comptonisations, the source counts in the three relevant PLANCK-channels, and give the angular power-spectra of the SZ-sky. While our results are broadly consistent with simple estimates based on scaling relations and spherically symmetric cluster models, some significant differences are seen which may affect the number of cluster detectable by PLANCK.

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arXiv2004

Constraining the Topology of Reionization Through Lyman-alpha Absorption

The reionization of hydrogen in the intergalactic medium (IGM) is a crucial landmark in the history of the universe, but the processes through which it occurs remain mysterious. In particular, recent numerical and analytic work suggest that reionization by stellar sources is driven by large-scale density fluctuations and must be inhomogeneous on scales of many comoving Mpc. We examine the prospects for constraining the topology of neutral and ionized gas through Lyman-alpha absorption of high-redshift sources. One method is to search for gaps in the Gunn-Peterson absorption troughs of luminous sources. These could occur if the line of sight passes sufficiently close to the center of a large HII region. In contrast to previous work, we find a non-negligible (though still small) probability of observing such a gap before reionization is complete. In our model the transmission spike at z=6.08 in the spectrum of SDSS J1148+5251 does not necessarily require overlap to have been completed at an earlier epoch. We also examine the IGM damping wing absorption of the Lyman-alpha emission lines of star-forming galaxies. Because most galaxies sit inside of large HII regions, we find that the severity of absorption is significantly smaller than previously thought and decoupled from the properties of the observed galaxy. While this limits our ability to constrain the mean neutral fraction of the IGM from observations of individual galaxies, it presents the exciting possibility of measuring the size distribution and evolution of the ionized bubbles by examining the distribution of damping wing optical depths in a large sample of galaxies.

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arXiv2004

Is There a Missing Galaxy Problem at High Redshift?

We study the evolution of the global stellar mass density in a Lambda cold dark matter universe using two different types of hydrodynamical simulations (Eulerian TVD and SPH) and the analytical model of Hernquist & Springel (2003). We find that the theoretical calculations all predict both a higher stellar mass density at z~3 than indicated by current observations, and that the peak of the cosmic star formation rate history should lie at z~5. Such a star formation history implies that as much as (70%, 30%) of the total stellar mass density today must already have formed by z=(1, 3). Our results suggest that current observations at z~3 are missing as much as 50% of the total stellar mass density in the Universe, perhaps owing to an inadequate allowance for dust obscuration in star-forming galaxies, limited sample sizes, or cosmic variance. We also compare our results with some of the updated semi-analytic models of galaxy formation.

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arXiv2004

Statistical Probes of Reionization With 21 cm Tomography

We consider the degree to which "21 cm tomography" of the high-redshift Universe can distinguish different ionization histories. Using a new analytic model for the size distribution of HII regions that associates these ionized bubbles with large-scale galaxy overdensities, we compute the angular power spectrum and other statistical properties of the 21 cm brightness temperature during reionization. We show that the HII regions imprint features on the power spectrum that allow us to separate histories with discrete bubbles from those with partial uniform ionization (by, for example, X-rays). We also show that "double reionization" scenarios will modify the morphology of the bubbles in ways that depend on the mechanism through which the first generation of sources shuts off. If, for example, the transition occurs globally at a fixed redshift, the first generation imprints a persistent feature on the 21 cm power spectrum. Finally, we compare our model to one in which voids are ionized first. While the power spectra of these two models are qualitatively similar, we show that the underlying distributions of neutral hydrogen differ dramatically and suggest that other statistical tests can distinguish them. The next generation of low-frequency radio telescopes will have the sensitivity to distinguish all of these models and strongly constrain the history and morphology of reionization.

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