arXiv subjects

Lars Hernquist

Publications and source records attributed to Lars Hernquist.

At least 595 records · Page 33Linked to original sources

arXiv2004

The Growth of HII Regions During Reionization

Recently, there has been a great deal of interest in understanding the reionization of hydrogen in the intergalactic medium (IGM). One of the major outstanding questions is how this event proceeds on large scales. Motivated by numerical simulations, we develop a model for the growth of HII regions during the reionization era. We associate ionized regions with large-scale density fluctuations and use the excursion set formalism to model the resulting size distribution. We then consider ways in which to characterize the morphology of ionized regions. We show how to construct the power spectrum of fluctuations in the neutral hydrogen field. The power spectrum contains definite features from the HII regions that should be observable with the next generation of low-frequency radio telescopes through surveys of redshifted 21 cm emission from the reionization era. Finally, we also consider statistical descriptions beyond the power spectrum and show that our model of reionization qualitatively changes the distribution of neutral gas in the IGM.

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arXiv2004

Was the Universe Reionized at Redshift 10?

Recently, Pello et al. (2004) claimed to have discovered a galaxy at a redshift z=10, and identified a feature in its spectrum with a hydrogen Lyman-alpha emission line. If this identification is correct, we show that the intergalactic medium (IGM) around the galaxy must be significantly ionized; otherwise, the damping wing of Lyman-alpha absorption by the neutral IGM would have greatly suppressed the emission line. We find either that the large-scale region surrounding this galaxy must have been largely reionized by z=10 (with a neutral fraction <0.4) or that the stars within the galaxy must be massive (>100 solar masses), and hence capable of producing a sufficiently large HII region around it. We generalize these conclusions and derive the maximum Lyman-alpha line flux for a given UV continuum flux of galaxies prior to the epoch of reionization.

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arXiv2004

Ultraviolet Line Emission from Metals in the Low-Redshift Intergalactic Medium

We use a high-resolution cosmological simulation that includes hydrodynamics, multiphase star formation, and galactic winds to predict the distribution of metal line emission at z~0 from the intergalactic medium (IGM). We focus on two ultraviolet doublet transitions, OVI 1032,1038 and CIV 1548,1551. Emission from filaments with moderate overdensities is orders of magnitude smaller than the background, but isolated emission from enriched, dense regions with T~10^5-10^5.5 K and characteristic sizes of 50-100 kpc can be detected above the background. We show that the emission from these regions is substantially greater when we use the metallicities predicted by the simulation (which includes enrichment through galactic winds) than when we assume a uniform IGM metallicity. Luminous regions correspond to volumes that have recently been influenced by galactic winds. We also show that the line emission is clustered on scales ~1 h^-1 Mpc. We argue that although these transitions are not effective tracers of the warm-hot intergalactic medium, they do provide a route to study the chemical enrichment of the IGM and the physics of galactic winds.

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arXiv2004

Cosmic Reionisation by Stellar Sources: Population III Stars

We combine fast radiative transfer calculations with high resolution hydrodynamical simulations to study an epoch of early hydrogen reionisation by primordial stellar sources at redshifts 15 10^4K at redshifts 5 ~0.13 would bolster the claim for the existence of population III stars similar to the ones studied here.

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arXiv2004

The Era of Massive Population III Stars: Cosmological Implications and Self-Termination

The birth and death of the first generation of stars have important implications for the thermal state and chemical properties of the intergalactic medium (IGM) in the early universe. Sometime after recombination, the neutral, chemically pristine gas was reionized by ultraviolet photons emitted from the first stars, but also enriched with heavy elements when these stars ended their lives as energetic supernovae. Using the results from previous high-resolution cosmological simulations of early structure formation that include radiative transfer, we show that a significant volume fraction of the IGM can be metal-polluted, as well as ionized, by massive Population III stars formed in small-mass (10^6-10^7 Msun) halos early on. If most of the early generation stars die as pair-instability supernovae with energies up to 10^{53} ergs, the volume-averaged mean metallicity will quickly reach Z ~ 10^{-4}Zsun by a redshift of 15-20, possibly causing a prompt transition to the formation of a stellar population that is dominated by low-mass stars. In this scenario, the early chemical enrichment history should closely trace the reionization history of the IGM, and the end of the Population III era is marked by the completion of reionization and pre-enrichment by z=15. We conclude that, while the pre-enrichment may partially account for the ``metallicity-floor'' in high-redshift Lyman-alpha clouds, it does not significantly affect the elemental abundance in the intracluster medium.

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arXiv2003

Star formation rate and metallicity of damped Lyman-alpha absorbers in cosmological SPH simulations

We study the distribution of the star formation rate and metallicity of damped Lyman-alpha absorbers using cosmological SPH simulations of the Lambda cold dark matter model in the redshift range z=0-4.5. Our approach includes a phenomenological model of galactic wind. We find that there is a positive correlation between the projected stellar mass density and the neutral hydrogen column density (NHI) of DLAs for high NHI systems, and that there is a good correspondence in the spatial distribution of stars and DLAs in the simulations. The evolution of typical star-to-gas mass ratios in DLAs can be characterised by an increase from about 2 at z=4.5 to 3 at z=3, to 10 at z=1, and finally to 20 at z=0. We also find that the projected SFR density in DLAs follows the Kennicutt law well at all redshifts, and the simulated values are consistent with the recent observational estimates of this quantity by Wolfe et al. (2003a,b). The rate of evolution in the mean metallicity of simulated DLAs as a function of redshift is mild, and is consistent with the rate estimated from observations. The predicted metallicity of DLAs is generally sub-solar in our simulations, and there is a significant scatter in the distribution of DLA metallicity for a given NHI. However, we find that the median metallicity of simulated DLAs is close to that of Lyman-break galaxies, which is higher than the values typically observed for DLAs by nearly an order of magnitude. This discrepancy with observations could be due to an inadequate treatment of SN feedback in our current simulations, perhaps indicating that metals are not expelled efficiently enough from DLAs by outflows. Alternatively, the current observations might be missing the majority of the high metallicity DLAs due to selection effects. (abridged)

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arXiv2003

Abundance of damped Lyman-alpha absorbers in cosmological SPH simulations

We use cosmological smoothed-particle hydrodynamics (SPH) simulations of the Lambda cold dark matter (CDM) model to study the abundance of damped Lyman-alpha absorbers (DLAs) in the redshift range z=0-4.5. We compute the cumulative DLA abundance by using the relation between DLA cross-section and the total halo mass inferred from the simulations. Our approach includes a phenomenological model of galactic wind. We employ the ``conservative entropy'' formulation of SPH, and utilise a series of simulations of varying box-size and particle number to isolate the impact of numerical resolution on our results. We show that the DLA abundance was overestimated in previous studies for three reasons: (1) the overcooling of gas occurring with non-conservative formulations of SPH, (2) a lack of numerical resolution, and (3) an inadequate treatment of feedback. Our new results for the total neutral hydrogen mass density, DLA abundance, and column density distribution function all agree reasonably well with observational estimates at redshift z=3, indicating that DLAs arise naturally from radiatively cooled gas in dark matter haloes that form in a Lambda CDM universe. Our simulations suggest a moderate decrease in DLA abundance by roughly a factor of two from z=4.5 to 3, consistent with observations. A significant decline in abundance from z=3 to z=1, followed by weak evolution from z=1 to z=0, is also indicated, but our low-redshift results need to be interpreted with caution because they are based on coarser simulations than the ones employed at high redshift. Our highest resolution simulation also suggests that the halo mass-scale below which DLAs do not exist is slightly above 10^8 Msun/h at z=3-4, somewhat lower than previously estimated. (abridged)

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arXiv2003

Mapping the Cosmic Web with Ly-alpha Emission

We use a high-resolution cosmological simulation to predict the distribution of HI Ly-alpha emission from the low-redshift (z<0.5) intergalactic medium (IGM). Our simulation can be used to reliably compute the emission from optically thin regions of the IGM but not that of self-shielded gas. We therefore consider several models that bracket the expected emission from self-shielded regions. Most galaxies are surrounded by extended (>10^2 kpc) ``coronae'' of optically thin gas with Ly-alpha surface brightness close to the expected background. Most of these regions contain smaller cores of dense, cool gas. Unless self-shielded gas is able to cool to T<10^4.1 K, these cores are much brighter than the background. The Ly-alpha coronae represent ``cooling flows'' of IGM gas accreting onto galaxies. We also estimate the number of Ly-alpha photons produced through the reprocessing of stellar ionizing radiation in the interstellar medium of galaxies; while this mechanism is responsible for the brightest Ly-alpha emission, it occurs on small physical scales and can be separated using high-resolution observations. In all cases, we find that Ly-alpha emitters are numerous (with a space density ~0.1 h^3 Mpc^-3) and closely trace the filamentary structure of the IGM, providing a new way to map gas inside the cosmic web.

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arXiv2003

Damped Lyman-alpha Absorbers in Cosmological SPH Simulations: the "metallicity problem"

We study the distribution of star formation rate (SFR) and metallicity of damped Lyman-alpha absorbers (DLAs) using cosmological smoothed particle hydrodynamics (SPH) simulations of the Lambda cold dark matter model. Our simulations include a phenomenological model for feedback by galactic winds which allows us to examine the effect of galactic outflows on the distribution of SFR and metallicity of DLAs. For models with strong galactic winds, we obtain good agreement with recent observations with respect to total neutral hydrogen mass density, N_HI column-density distribution, abundance of DLAs, and for the distribution of SFR in DLAs. However, we also find that the median metallicity of simulated DLAs is higher than the values typically observed by nearly an order of magnitude. This discrepancy with observations could be due to shortcomings in the treatment of the supernova feedback or the multiphase structure of the gas in our current simulations. Recent observations by Wolfe et al. (2003a,b) seem to point to the same problem; i.e. the observed DLA metallicities are much lower than those expected from the (either observed or simulated) DLA star formation rates, a puzzle that has been known as the ``missing metals'' problem for the globally averaged quantities.

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arXiv2003

The First Supernova Explosions in the Universe

We investigate the supernova explosions that end the lives of massive Population III stars in low-mass minihalos (M~10^6 M_sun) at redshifts z~20. Employing the smoothed particle hydrodynamics method, we carry out numerical simulations in a cosmological set-up of pair-instability supernovae with explosion energies of E_SN=10^51 and 10^53 ergs. We find that the more energetic explosion leads to the complete disruption of the gas in the minihalo, whereas the lower explosion energy leaves much of the halo intact. The higher energy supernova expels > 90% of the stellar metals into a region ~1 kpc across over a timescale of 3-5 Myr. Due to this burst-like initial star formation episode, a large fraction of the universe could have been endowed with a metallicity floor, Z_min>10^-4 Z_sun, already at z>15.

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arXiv2003

Searching for Star Formation Beyond Reionization

The goal of searching back in cosmic time to find star formation during the epoch of reionization will soon be within reach. We assess the detectability of high-redshift galaxies by combining cosmological hydrodynamic simulations of galaxy formation, stellar evolution models appropriate for the first generations of stars, and estimates of the efficiency for Lyman alpha to escape from forming galaxies into the intergalactic medium. Our simulated observations show that Lyman alpha emission at z ~ 8 may be observable in the near-infrared with 8-meter class telescopes and present-day technology. Not only is the detection of early star-forming objects vital to understanding the underlying cause of the reionization of the universe, but the timely discovery of a z > 7 star-forming population -- or even an interesting upper limit on the emergent flux from these objects -- will have implications for the design of the next generation of ground- and space-based facilities.

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arXiv2003

The history of star formation in a LCDM universe

Employing hydrodynamic simulations of structure formation in a LCDM cosmology, we study the history of cosmic star formation from the "dark ages" at redshift z~20 to the present. In addition to gravity and ordinary hydrodynamics, our model includes radiative heating and cooling of gas, star formation, supernova feedback, and galactic winds. By making use of a comprehensive set of simulations on interlocking scales and epochs, we demonstrate numerical convergence of our results on all relevant halo mass scales, ranging from 10^8 to 10^15 Msun/h. The predicted density of cosmic star formation is broadly consistent with measurements, given observational uncertainty. From the present epoch, it gradually rises by about a factor of ten to a peak at z~5-6, which is beyond the redshift range where it has been estimated observationally. 50% of the stars are predicted to have formed by redshift z~2.1, and are thus older than 10.4 Gyr, while only 25% form at redshifts lower than z~1. The mean age of all stars at the present is about 9 Gyr. Our model predicts a total stellar density at z=0 of Omega_*=0.004, corresponding to about 10% of all baryons being locked up in long-lived stars, in agreement with recent determinations of the luminosity density of the Universe. We determine the "multiplicity function of cosmic star formation" as a function of redshift; i.e. the distribution of star formation with respect to halo mass. We also briefly examine possible implications of our predicted star formation history for reionisation of hydrogen in the Universe. We find that the star formation rate predicted by the simulations is sufficient to account for hydrogen reionisation by z~6, but only if a high escape fraction close to unity is assumed. (abridged)

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arXiv2003

Hubble Space Telescope Observations of NGC 6240: a Case Study of an Ultra-Luminous Infrared Galaxy with Obscured Activity

We present results from an HST study of the morphology and kinematics of NGC 6240. This merging galaxy with a double nucleus is one of the nearest and best-studied ultraluminous infrared galaxies. HST resolves both nuclei into seperate components. The distance between the northern and southern optical/near-infrared components is greater than that observed in radio and X-ray studies, arguing that even in K-band we may not be seeing all the way through the dust to the true nuclei. The ionized gas does not display rotation around either of the nuclei, or equilibrium motion in general. There is a strong velocity gradient between the nuclei, similar to what is seen in CO data. There is no such gradient in our stellar kinematics. The velocity dispersion of the gas is larger than expected for a cold disk. We also map and model the emission-line velocity field at an off-nuclear position where a steep velocity gradient was previously detected in ground-based data. Overall, the data indicate that line-of-sight projection effects, dust absorption, non-equilibrium merger dynamics, and the possible influence of a wind may be playing an important role in the observed kinematics. Chandra observations of hard X-rays have shown that both of the nuclei contain an Active Galactic Nucleus (AGN). The HST data show no clear sign of the two AGNs: neither continuum nor narrow-band imaging shows evidence for unresolved components in the nuclei, and there are no increased emission line widths or rapid rotation near the nuclei. This underscores the importance of X-ray data for identifying AGNs in highly dust-enshrouded environments.

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arXiv2003

The cosmological evolution of metal enrichment in quasar host galaxies

We study the gas metallicity of quasar hosts using cosmological hydrodynamic simulations of the Lambda-cold dark matter model. Galaxy formation in the simulations is coupled with a prescription for black hole activity enabling us to study the evolution of the metal enrichment in quasar hosts and hence explore the relationship between star/spheroid formation and black hole growth/activity. We find a steep radial metallicity gradient in quasar host galaxies, with gas metallicities close to solar values in the outer parts but becoming supersolar in the center. The hosts of the rare bright quasars at z~5-6 have star formation rates of several hundred solar masses per year and halo masses of order ~10^12 solar masses. Already at these redshifts they have supersolar (Z ~2-3 solar) central metallicities, with a mild dependence of metallicity on luminosity, consistent with observed trends. The mean value of metallicity is sensitive to the assumed quasar lifetime, providing a useful new probe of this parameter. We find that lifetimes from 10^7-4x10^7yr are favored by comparison to observational data. In both the models and observations, the rate of evolution of the mean quasar metallicity as a function of redshift is generally flat out to z ~ 4-5. Beyond the observed redshift range and out to redshift z ~ 6-8, we predict a slow decline of the mean central metallicity towards solar and slightly subsolar values as we approach the epoch of the first significant star formation activity

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arXiv2003

The Angular Momentum of Gas in Proto-Galaxies: II -- The Impact of Preheating

We examine the effect of preheating of the intergalactic medium on galaxy formation using cosmological hydrodynamical simulations. By performing simulations both with and without a simple model for preheating, we analyse and compare the angular momentum distributions of the dark matter and the baryons. Preheating unbinds baryons from their dark matter haloes, yielding a baryonic mass fraction that declines with decreasing halo mass. In addition, the spin parameter of the gas is reduced with respect to the case without preheating, while the misalignment between the angular momentum directions of the gas and dark matter increases strongly. The angular momentum distributions of individual haloes reveal that preheating decreases (increases) the mass fraction with low (negative) specific angular momentum. We discuss the implications of these findings for the formation of disk galaxies in a preheated intergalactic medium, and compare our results to the predictions of Maller & Dekel (2002), who propose an alternative interpretation for the origin of the angular momentum of (proto)-galaxies.

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arXiv2003

Early Structure Formation and Reionization in a Cosmological Model with a Running Primordial Power Spectrum

(abridged) We study high redshift structure formation and reionization in a LCDM universe under the assumption that the spectral power index of primordial density fluctuations is a function of length scale. We adopt a particular formulation of the running spectral index (RSI) model as suggested by the recent WMAP data. While early structure forms hierarchically in the RSI model, the reduced power on small scales causes a considerable delay in the formation epoch of low mass (~ 10^6 Msun) ``mini-halos'' compared to the LCDM model. The extremely small number of gas clouds in the RSI model indicates that reionization is initiated later than z<15, generally resulting in a smaller total Thomson optical depth than in the LCDM model. By carrying out radiative transfer calculations, we also study reionization by stellar populations formed in galaxies. Even with a top-heavy intial mass function representing an early population of massive stars and/or an extraordinarily high photon emission rate from galaxies, the total optical depth can only be as large as tau ~ 0.1 for reasonable models of early star-formation. The RSI model is thus in conflict with the large Thomson optical depth inferred by the WMAP satellite.

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arXiv2003

The Evolution of Baryon Density Fluctuations in Multi-Component Cosmological Simulations

We critically examine how the evolution of the matter density field in cosmological simulations is affected by details of setting up initial conditions. We show that it is non-trivial to realise an initial distribution of matter in N-body/hydrodynamic simulations so that the baryon and dark matter density fluctuations and their velocities evolve consistently as theoretically predicted. We perform a set of cosmological simulations and use them to distinguish and verify an appropriate method for generating initial conditions. We show that a straightforward way of applying the Zel'dovich approximation to each component using distinct transfer functions results in an incorrect growth of density fluctuations and that it is necessary to correct velocities at the initial epoch. The unperturbed uniform particle distribution must be also generated appropriately to avoid tight coupling of the baryonic and dark matter components. We recommend using independent ``glass'' particle distributions, using distinct transfer functions for baryons and dark matter, and taking into account the difference in the velocity fields at the initialisation epoch. The proposed method will be useful for studies of the evolution of the intergalactic medium and the formation of the first cosmological objects using numerical simulations.

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arXiv2003

Early Structure Formation and Reionization in a Warm Dark Matter Cosmology

We study first structure formation in Lambda-dominated universes using large cosmological N-body/SPH simulations. We consider a standard LCDM model and a LWDM model in which the mass of the dark matter particles is taken to be m_X=10 keV. The linear power spectrum for the LWDM model has a characteristic cut-off at a wavenumber k=200 /Mpc, suppressing the formation of low mass (< 10^6 Msun) nonlinear objects early on. The absence of low mass halos in the WDM model makes the formation of primordial gas clouds with molecular hydrogen very inefficient at high redshifts. The first star-forming gas clouds form at z~21 in the WDM model, considerably later than in the CDM counterpart, and the abundance of these gas clouds differs by an order of magnitude between the two models. We carry out radiative transfer calculations by embedding massive Population III stars in the gas clouds. We show that the volume fraction of ionized gas rises up close to 100% by z=18 in the CDM case, whereas that of the WDM model remains extremely small at a level of a few percent. Thus the WDM model with m_X=10 keV is strongly inconsistent with the observed high optical depth by the WMAP satellite.

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