Lars Hernquist - Academia.edu (original) (raw)

Papers by Lars Hernquist

Recent high precision proper motions from the Hubble Space Telescope (HST) suggest that the Large... more Recent high precision proper motions from the Hubble Space Telescope (HST) suggest that the Large and Small Magellanic Clouds (LMC and SMC, respectively) are either on their first passage or on an eccentric long period (>=6 Gyr) orbit about the Milky Way (MW). This differs markedly from the canonical picture in which the Clouds travel on a quasi-periodic orbit about

We compare the predictions of four different algorithms for the distribution of ionized gas durin... more We compare the predictions of four different algorithms for the distribution of ionized gas during the Epoch of Reionization. These algorithms are all used to run a 100 Mpc/h simulation of reionization with the same initial conditions. Two of the algorithms are state-of-the-art ray-tracing radiative transfer codes that use disparate methods to calculate the ionization history. The other two algorithms are fast but more approximate schemes based on iterative application of a smoothing filter to the underlying source and density fields. We compare these algorithms' resulting ionization and 21 cm fields using several different statistical measures. The two radiative transfer schemes are in excellent agreement with each other (with the cross-correlation coefficient of the ionization fields >0.8 for k < 10 h/Mpc and in good agreement with the analytic schemes (>0.6 for k < 1 h/Mpc). When used to predict the 21cm power spectrum at different times during reionization, all i...

We study the distribution of the star formation rate (SFR) and metallicity of damped Lyman-α abso... more We study the distribution of the star formation rate (SFR) and metallicity of damped Lyman-α absorbers (DLAs) in the redshift range z = 0 4.5 using cosmological smoothed particle hydrodynamics (SPH) simulations of thecold dark matter model. Our simulations include standard radiative cooling and heating with a uniform UV background, star formation, supernova (SN) feedback, as well as a phenomenological model for feedback by galactic winds. The latter allows us to examine, in particular, the effect of galactic outflows on the distribution of the SFR and metallicity of DLAs. We employ a "conservative entropy" formulation of SPH which alleviates numerical overcooling effects that affected earlier simulations. In addition, we utilise a series of simulations of varying boxsize and particle number to investigate the impact of numerical resolution on our results. We find that there is a positive correlation between the projected stellar mass density and the neutral hydrogen column...

We use cosmological smoothed-particle hydrodynamics (SPH) simulations of the Lambda cold dark mat... more 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 re...

We study the properties of galaxies at z=2 in a Lambda cold dark matter universe, using two diffe... more 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 exc...

We perform simulated observations on the outputs of up-to-date SPH simulations in a Lambda cold d... more We perform simulated observations on the outputs of up-to-date SPH simulations in a Lambda cold dark matter cosmology, at redshifts of 3-6. We determine the luminosity function over the rest-frame UV magnitude range MAB(1700Å) ˜ -24 - -16 in simulations with a variety of box sizes. We highlight the evolution of the luminosity function for z=3-6, and compare the simulation results with those of recent surveys of Lyman-break galaxies. We use a variety of values for galactic dust extinction covering the range E(B-V) = 0.0-0.3 for use in comparison with observational data. Overall, our simulated UV luminosity functions show good agreement with observational data when moderate amount of dust extinction E(B-V) ˜ 0.15 is assumed.

We study the optical and near infrared properties of galaxies at z=1-3 in a Lambda cold dark matt... more We study the optical and near infrared properties of galaxies at z=1-3 in a Lambda cold dark matter universe using two types of cosmological hydrodynamic simulations. We present a unified global picture of massive galaxies at these redshifts when compared with UV-selected and near-IR selected galaxy surveys; namely, roughly a half of the massive galaxies with M* >1010h-1M&sun; at z=2 can be observed with the UV color selection criteria, and the other half observed in near-IR wavelengths, with some overlap between the two populations. Furthermore, the near-IR selected sample can be divided into two categories: dusty starbursts and very red, dead & old galaxies. Our simulations are able to account for the number density of these objects depending on the amount of dust extinction. Contrary to existing claims, we find that models of hierarchical galaxy formation model can account for the massive galaxies observed at z>= 1.

Proceedings of the International Astronomical Union

We study the distribution of star formation rate (SFR) and metallicity of damped Lyman-alpha abso... more 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 a...

Proceedings of the International Astronomical Union

We study the distribution of star formation rate (SFR) and metallicity of damped Lyman-alpha abso... more 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 (CDM) 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 Wo...

Publications- Astronomical Society of Japan

We study the properties of Lyman-alpha emitters (LAEs) and Lyman-break galaxies (LBGs) at z=3-6 u... more We study the properties of Lyman-alpha emitters (LAEs) and Lyman-break galaxies (LBGs) at z=3-6 using cosmological SPH simulations. We investigate two simple scenarios for explaining the observed Ly-a and rest-frame UV luminosity functions (LFs) of LAEs: (i) the "escape fraction" scenario, in which the "effective" escape fraction (including the IGM attenuation) of Ly-a photons is f_Lya ~0.1 (0.15) at z=3 (6), and (ii) the "stochastic" scenario, in which the fraction of LAEs that are turned on at z=3 (6) is \Cstoc ~0.07 (0.2) after correcting for the IGM attenuation. Our comparisons with a number of different observations suggest that the stochastic scenario is preferred over the escape fraction scenario. We find that the mean values of stellar mass, metallicity and black hole mass hosted by LAEs are all smaller in the stochastic scenario than in the escape fraction scenario. In our simulations, the galaxy stellar mass function evolves rapidly, as expect...

We present cosmological hydrodynamic simulations of galaxy formation with the new moving-mesh cod... more We present cosmological hydrodynamic simulations of galaxy formation with the new moving-mesh code AREPO, which promises higher accuracy compared with the traditional SPH technique that has been widely employed for this problem. We use the same initial conditions, set of physics and gravity solver to compare these results to the ones with well-tested SPH code GADGET-3 which enables us to cleanly test the differences in hydrodynamics. We find that AREPO leads to significantly higher star formation rates for galaxies in massive halos enabled by a more efficient cooling of the hot halo gas. Furthermore, galaxies in AREPO show more extended gaseous disks, which also feature a thinner and smoother morphology than their GADGET counterparts. Consequently, galaxies formed in AREPO have higher specific angular momentum than their SPH correspondents. We discuss the causes of these differences which can be connected to shortcomings of the standard SPH implementation. We point out that AREPO ca...

We present new type of cosmological hydrodynamic simulations to study formation of galaxies. Simu... more We present new type of cosmological hydrodynamic simulations to study formation of galaxies. Simulations were performed using newly developed moving mesh code AREPO in a cosmological box evolved to z=0. We compare the results from this new technique to the results from well-tested SPH code GADGET-3 which uses identical gravity solver, which enables to cleanly test the differences in hydrodynamics. We found several important, systematic differences in properties of galaxies and gaseous components of halos. These include significantly more extended disks and faster cooling of the hot halo gas in massive halos in AREPO. Owing to hierarchical nature of halo and galaxy buildup these differences than propagate to many other properties of simulated halos and galaxies. Given the advantages of hydrodynamic scheme in AREPO in idealized test problems, as well as tests we did in cosmological environment, our new findings are questioning reliability of some the previous results based on SPH tech...

Entropy profiles of galaxy clusters are systematically different when simulated with SPH versus E... more Entropy profiles of galaxy clusters are systematically different when simulated with SPH versus Eulerian mesh codes. This systematic difference has persisted until now, casting doubt hydrodynamical schemes can follow complex hydrodynamical flows in full cosmological settings reliably. We present results from a novel set of simulations performed both with a Lagrangian SPH code GADGET and with a recently developed moving mesh code AREPO. This suite of simulations sheds light on the physical and numerical mechanisms which are responsible for the systematic discrepancies in the entropy profiles.

We employ a set of high-resolution simulations of disk galaxies to argue that dark matter substru... more We employ a set of high-resolution simulations of disk galaxies to argue that dark matter substructures passing through the disk hardly produce transient features in disk, like multi-armed spiral patterns. Indeed, our studies indicate that stochastic spiral arms in galaxies have different origin.

Spiral structure in galaxies was recognized as likely originating from the action of density wave... more Spiral structure in galaxies was recognized as likely originating from the action of density waves propagating through a differentially rotating disk. However, the origin of these features remains controversial. Using very high resolution numerical simulations we show that spiral arms might be seeded by density inhomogeneities orbiting in the disk itself. These perturbations can be identified with fluctuations in the distribution of gas in the interstellar medium of galaxies, such as giant molecular clouds. Our simulations show that when sufficient numbers of these perturbers are present, they collectively amplify to yield large-scale spiral patterns that resemble the spiral arms in flocculent and intermediate late type spiral galaxies.

We reanalyze the artificial spectra generated in the Hernquist, et al (1996) CDM hydro simulation... more We reanalyze the artificial spectra generated in the Hernquist, et al (1996) CDM hydro simulations of the Lyman alpha forest using Voigt profiles fitting, and find substantial differences in the resulting statistical properties of Lyalpha forest absorbers. The semi-automated analysis algorithm adds noise to artificial spectra to emulate HIRES data, fits a continuum in IRAF, identifies lines as > 5sigma fluctuations, deconvolves blended lines into individual Voigt profiles, and uses a chi (2) -minimization routine to find the best-fit values for N_HI and b. This algorithm is intended to be applicable to both real and artifical data, and is intended to emulate the manner in which QSO spectra are generally analyzed. In contrast, the algorithm in Hernquist, et al (1996) used a flux threshold algorithm to identify lines, and performed no deblending. The resulting distributions of N_HI and b-parameters are in significantly better agreement with Keck HIRES results. The sensitivity of th...

Using a high-resolution cosmological hydrodynamic simulation, we present a method to constrain ex... more Using a high-resolution cosmological hydrodynamic simulation, we present a method to constrain extinction due to intergalactic grey dust based on the observed magnitudes of distant Type IA supernovae. We apply several simple prescriptions to relate the intergalactic dust density to the gas density in the simulation, thereby obtaining dust extinctions that may be directly compared to the observed distribution of

Cold fronts (CFs) are found in most galaxy clusters, as well as in some galaxies and groups of ga... more Cold fronts (CFs) are found in most galaxy clusters, as well as in some galaxies and groups of galaxies. We propose that some CFs are relics of collisions between trailing shocks. Such a collision typically results in a spherical, factor ~1.4-2.7 density/temperature discontinuity. These CFs may be found as far as the virial shock, unlike in other CF formation models. As a demonstration of this effect, we use one dimensional simulations where halo reverberations involving periodic collisions between the virial shock and outgoing secondary shocks exist. These collisions yield a distinctive, concentric geometric sequence of CFs which trace the expansion of the virial shock. Comment: 5 pages, 4 figures, this letter is superseded by a more detailed version in arXiv:1006.1892

Monthly Notices of the Royal Astronomical Society

Cold fronts (CFs) are found in most galaxy clusters, as well as in some galaxies and groups of ga... more Cold fronts (CFs) are found in most galaxy clusters, as well as in some galaxies and groups of galaxies. We propose that some CFs are relics of merging between two shocks propagating in the same direction. Such shock mergers typically result in a quasi-spherical, factor ~1.4-2.7 discontinuity in density and in temperature. These CFs may be found as far out as the virial shock, unlike what is expected in other CF formation models. As a demonstration of this effect, we use one dimensional simulations of clusters and show that shock induced cold fronts form when perturbations such as explosions or mergers occur near the cluster's centre. Perturbations at a cluster's core induce periodic merging between the virial shock and outgoing secondary shocks. These collisions yield a distinctive, concentric, geometric sequence of CFs which trace the expansion of the virial shock.

Proceedings of the International Astronomical Union

We study the distribution of star formation rate (SFR) and metallicity of damped Lyman-alpha abso... more 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 (CDM) 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 Wo...

Publications- Astronomical Society of Japan

Monthly Notices of the Royal Astronomical Society

Cold fronts (CFs) are found in most galaxy clusters, as well as in some galaxies and groups of ga... more Cold fronts (CFs) are found in most galaxy clusters, as well as in some galaxies and groups of galaxies. We propose that some CFs are relics of merging between two shocks propagating in the same direction. Such shock mergers typically result in a quasi-spherical, factor ~1.4-2.7 discontinuity in density and in temperature. These CFs may be found as far out as the virial shock, unlike what is expected in other CF formation models. As a demonstration of this effect, we use one dimensional simulations of clusters and show that shock induced cold fronts form when perturbations such as explosions or mergers occur near the cluster's centre. Perturbations at a cluster's core induce periodic merging between the virial shock and outgoing secondary shocks. These collisions yield a distinctive, concentric, geometric sequence of CFs which trace the expansion of the virial shock.