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Robert J. Thacker

Bio: Robert J. Thacker is an academic researcher from Saint Mary's University. The author has contributed to research in topics: Galaxy & Star formation. The author has an hindex of 26, co-authored 65 publications receiving 6728 citations. Previous affiliations of Robert J. Thacker include University of Saint Mary & University of California, Berkeley.


Papers
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Journal ArticleDOI
02 Jun 2005-Nature
TL;DR: It is shown that baryon-induced features in the initial conditions of the Universe are reflected in distorted form in the low-redshift galaxy distribution, an effect that can be used to constrain the nature of dark energy with future generations of observational surveys of galaxies.
Abstract: The cold dark matter model has become the leading theoretical picture for the formation of structure in the Universe. This model, together with the theory of cosmic inflation, makes a clear prediction for the initial conditions for structure formation and predicts that structures grow hierarchically through gravitational instability. Testing this model requires that the precise measurements delivered by galaxy surveys can be compared to robust and equally precise theoretical calculations. Here we present a simulation of the growth of dark matter structure using 2,1603 particles, following them from redshift z = 127 to the present in a cube-shaped region 2.230 billion lightyears on a side. In postprocessing, we also follow the formation and evolution of the galaxies and quasars. We show that baryon-induced features in the initial conditions of the Universe are reflected in distorted form in the low-redshift galaxy distribution, an effect that can be used to constrain the nature of dark energy with future generations of observational surveys of galaxies.

4,814 citations

Journal ArticleDOI
TL;DR: In this article, the authors present a detailed description and examine the performance of different approaches to modeling feedback in simulations of galaxy formation using smoothed particle hydrodynamics (SPH).
Abstract: We present a detailed description, and examine the performance of, a number of different approaches to modeling feedback in simulations of galaxy formation. Gasdynamic forces are evaluated using smoothed particle hydrodynamics (SPH). Star formation and supernova feedback are included using a three-parameter model which determines the star formation rate (SFR) normalization, feedback energy, and lifetime of feedback regions. The star formation rate is calculated for all gas particles which fall within prescribed temperature, density, and convergent flow criteria, and for cosmological simulations we also include a self-gravity criterion for gas particles to prevent star formation at high redshifts. A Lagrangian Schmidt law is used to calculate the star formation rate from the SPH density. Conversion of gas to stars is performed when the star mass for a gas particle exceeds a certain limit, typically half that of the gas particle. Feedback is incorporated by returning a precalculated amount of energy to the ISM as thermal heating. We compare the effects of distributing this energy over the smoothing scale or depositing it on a single particle. Radiative losses are prevented from heated particles by adjusting the density used in radiative cooling so that the decay of energy occurs over a set half-life, or by turning off cooling completely and allowing feedback regions a brief period of adiabatic expansion. We test the models on the formation of galaxies from cosmological initial conditions and also on isolated disk galaxies. For isolated prototypes of the Milky Way and the dwarf galaxy NGC 6503 we find feedback has a significant effect, with some algorithms being capable of unbinding gas from the dark matter halo (blow-away). As expected feedback has a stronger effect on the dwarf galaxy, producing significant disk evaporation and also larger feedback bubbles for the same parameters. In the critical-density CDM cosmological simulations, evolved to a redshift z = 1, we find that, barring extreme models, feedback has little effect. Further, feedback only manages to produce a disk with a specific angular momentum value approximately twice that of the run with no feedback, the disk thus has an specific angular momentum value that is characteristic of observed elliptical galaxies. We argue that this is a result of the extreme central concentration of the dark halos in the standard CDM model and the pervasiveness of the core-halo angular momentum transport mechanism (even in light of feedback). A simulation with extremely violent feedback, relative to our fiducial models, leads to a disk that resembles the other simulations at z = 1 and has a specific angular momentum value that is more typical of observed disk galaxies. At later times, z = 0.5, a large amount of halo gas which does not suffer an angular momentum deficit is present; however, the cooling time is too long to accrete on to the disk. We further point out that the disks formed in hierarchical simulations are partially a numerical artifact produced by the minimum mass scale of the simulation acting as a highly efficient support mechanism. Disk formation is strongly affected by the treatment of dense regions in the SPH method. The problems inherent in the treatment of high-density regions in SPH, in concert with the difficulty of representing the hierarchical formation process, means that realistic simulations of galaxy formation require far higher particle resolution than currently used.

232 citations

Journal ArticleDOI
TL;DR: In this article, the authors examined radial and vertical metallicity gradients using a suite of disk galaxy hydrodynamical simulations, supplemented with two classic chemical evolution approaches to reconcile the differences existing between extant models and observations within the canonical “inside-out” disk growth paradigm.
Abstract: Aims. We examine radial and vertical metallicity gradients using a suite of disk galaxy hydrodynamical simulations, supplemented with two classic chemical evolution approaches. We determine the rate of change of gradient slope and reconcile the differences existing between extant models and observations within the canonical “inside-out” disk growth paradigm. Methods. A suite of 25 cosmological disks is used to examine the evolution of metallicity gradients; this consists of 19 galaxies selected from the RaDES (Ramses Disk Environment Study) sample, realised with the adaptive mesh refinement code ramses, including eight drawn from the “field” and six from “loose group” environments. Four disks are selected from the MUGS (McMaster Unbiased Galaxy Simulations) sample, generated with the smoothed particle hydrodynamics (SPH) code gasoline. Two chemical evolution models of inside-out disk growth were employed to contrast the temporal evolution of their radial gradients with those of the simulations. Results. We first show that generically flatter gradients are observed at redshift zero when comparing older stars with those forming today, consistent with expectations of kinematically hot simulations, but counter to that observed in the Milky Way. The vertical abundance gradients at ~1−3 disk scalelengths are comparable to those observed in the thick disk of the Milky Way, but significantly shallower than those seen in the thin disk. Most importantly, we find that systematic differences exist between the predicted evolution of radial abundance gradients in the RaDES and chemical evolution models, compared with the MUGS sample; specifically, the MUGS simulations are systematically steeper at high-redshift, and present much more rapid evolution in their gradients. Conclusions. We find that the majority of the models predict radial gradients today which are consistent with those observed in late-type disks, but they evolve to this self-similarity in different fashions, despite each adhering to classical “inside-out” growth. We find that radial dependence of the efficiency with which stars form as a function of time drives the differences seen in the gradients; systematic differences in the sub-grid physics between the various codes are responsible for setting these gradients. Recent, albeit limited, data at redshift z ~ 1.5 are consistent with the steeper gradients seen in our SPH sample, suggesting a modest revision of the classical chemical evolution models may be required.

191 citations

Journal ArticleDOI
TL;DR: In this paper, a supernova feedback algorithm is used to allow energy to persist in the model interstellar medium for a period corresponding to the lifetime of stellar associations, leading to a disk at a redshift z = 0.52, with a specific angular momentum content within 10% of the value required to fit observations.
Abstract: We present a smoothed particle hydrodynamic simulation that reproduces a galaxy that is a moderate facsimile of those observed. The primary failing point of previous simulations of disk formation, namely, excessive transport of angular momentum from gas to dark matter, is ameliorated by the inclusion of a supernova feedback algorithm that allows energy to persist in the model interstellar medium for a period corresponding to the lifetime of stellar associations. The inclusion of feedback leads to a disk at a redshift z = 0.52, with a specific angular momentum content within 10% of the value required to fit observations. An exponential fit to the disk baryon surface density gives a scale length within 17% of the theoretical value. Runs without feedback, with or without star formation, exhibit the drastic angular momentum transport observed elsewhere.

132 citations

Journal ArticleDOI
TL;DR: In this paper, the clustering properties of metals in the intergalactic medium (IGM) were studied by 619 Civ and 81 Siiv absorption components with N ≥ 10 12 cm -2 and 316 Mg II and 82 Fell absorption component with n ≥ 10 11.5 cm-2 in 19 high signal-to-noise ratio (60-100 pixel -1 ), high-resolution (R = 45 000) quasar spectra.
Abstract: We study the clustering properties of metals in the intergalactic medium (IGM) as traced by 619 Civ and 81 Siiv absorption components with N ≥ 10 12 cm -2 and 316 Mg II and 82 Fell absorption components with N ≥ 10 11.5 cm -2 in 19 high signal-to-noise ratio (60-100 pixel -1 ), high-resolution (R = 45 000) quasar spectra. C IV and Si iv trace each other closely and their line-of-sight correlation functions ξ(v) exhibit a steep decline at large separations and a flatter profile below 150 km s -1 , with a large overall bias. These features do not depend on absorber column densities, although there are hints that the overall amplitude of ξ CIV (v) increases with time over the redshift range detected (1.5-3). Carrying out a detailed smoothed particle hydrodynamic simulation (2 x 320 3 , 57 Mpc 3 comoving), we show that the C iv correlation function cannot be reproduced by models in which the IGM metallicity is constant or a local function of overdensity (Z Δ 2/3 ). However, the properties of ξ CIV (v) are generally consistent with a model in which metals are confined within bubbles with a typical radius R s about sources of mass ≥M s . We derive best-fitting values of R s 2 comoving Mpc and M s 10 12 M ○. at z = 3. Our lower-redshift (0.5-2) measurements of the Mg II and Fe II correlation functions also uncover a steep decline at large separations and a flatter profile at small separations, but the clustering is even higher than in the z = 1.5-3 measurements, and the turnover is shifted to somewhat smaller distances, 75 km s -1 . Again, these features do not change with column density, but there are hints that the amplitudes of ξ Mg II (v) and ξ Fe II (v) increase with time. We describe an analytic 'bubble' model for these species, which come from regions that are too compact to be accurately simulated numerically, deriving best-fitting values of R s 2.4 Mpc and M s 10 12 M ○. . Equally good analytic fits to all four species are found in a similarly biased high-redshift enrichment model in which metals are placed within 2.4 comoving Mpc of M s 3 x 10 9 sources at z = 7.5.

99 citations


Cited by
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Journal ArticleDOI
TL;DR: GADGET-2 as mentioned in this paper is a massively parallel tree-SPH code, capable of following a collisionless fluid with the N-body method, and an ideal gas by means of smoothed particle hydrodynamics.
Abstract: We discuss the cosmological simulation code GADGET-2, a new massively parallel TreeSPH code, capable of following a collisionless fluid with the N-body method, and an ideal gas by means of smoothed particle hydrodynamics (SPH). Our implementation of SPH manifestly conserves energy and entropy in regions free of dissipation, while allowing for fully adaptive smoothing lengths. Gravitational forces are computed with a hierarchical multipole expansion, which can optionally be applied in the form of a TreePM algorithm, where only short-range forces are computed with the ‘tree’ method while long-range forces are determined with Fourier techniques. Time integration is based on a quasi-symplectic scheme where long-range and short-range forces can be integrated with different time-steps. Individual and adaptive short-range time-steps may also be employed. The domain decomposition used in the parallelization algorithm is based on a space-filling curve, resulting in high flexibility and tree force errors that do not depend on the way the domains are cut. The code is efficient in terms of memory consumption and required communication bandwidth. It has been used to compute the first cosmological N-body simulation with more than 10 10 dark matter particles, reaching a homogeneous spatial dynamic range of 10 5 per dimension in a three-dimensional box. It has also been used to carry out very large cosmological SPH simulations that account for radiative cooling and star formation, reaching total particle numbers of more than 250 million. We present the algorithms used by the code and discuss their accuracy and performance using a number of test problems. GADGET-2 is publicly released to the research community. Ke yw ords: methods: numerical ‐ galaxies: interactions ‐ dark matter.

6,196 citations

Journal ArticleDOI
TL;DR: In this article, the authors review the range of complementary techniques and theoretical tools that allow astronomers to map the cosmic history of star formation, heavy element production, and reionization of the Universe from the cosmic "dark ages" to the present epoch.
Abstract: Over the past two decades, an avalanche of data from multiwavelength imaging and spectroscopic surveys has revolutionized our view of galaxy formation and evolution. Here we review the range of complementary techniques and theoretical tools that allow astronomers to map the cosmic history of star formation, heavy element production, and reionization of the Universe from the cosmic "dark ages" to the present epoch. A consistent picture is emerging, whereby the star-formation rate density peaked approximately 3.5 Gyr after the Big Bang, at z~1.9, and declined exponentially at later times, with an e-folding timescale of 3.9 Gyr. Half of the stellar mass observed today was formed before a redshift z = 1.3. About 25% formed before the peak of the cosmic star-formation rate density, and another 25% formed after z = 0.7. Less than ~1% of today's stars formed during the epoch of reionization. Under the assumption of a universal initial mass function, the global stellar mass density inferred at any epoch matches reasonably well the time integral of all the preceding star-formation activity. The comoving rates of star formation and central black hole accretion follow a similar rise and fall, offering evidence for co-evolution of black holes and their host galaxies. The rise of the mean metallicity of the Universe to about 0.001 solar by z = 6, one Gyr after the Big Bang, appears to have been accompanied by the production of fewer than ten hydrogen Lyman-continuum photons per baryon, a rather tight budget for cosmological reionization.

3,104 citations

Journal ArticleDOI
TL;DR: The Virgo Consortium's EAGLE project as discussed by the authors is a suite of hydrodynamical simulations that follow the formation of galaxies and black holes in representative volumes, where thermal energy is injected into the gas, allowing winds to develop without predetermined speed or mass loading factors.
Abstract: We introduce the Virgo Consortium's EAGLE project, a suite of hydrodynamical simulations that follow the formation of galaxies and black holes in representative volumes. We discuss the limitations of such simulations in light of their finite resolution and poorly constrained subgrid physics, and how these affect their predictive power. One major improvement is our treatment of feedback from massive stars and AGN in which thermal energy is injected into the gas without the need to turn off cooling or hydrodynamical forces, allowing winds to develop without predetermined speed or mass loading factors. Because the feedback efficiencies cannot be predicted from first principles, we calibrate them to the z~0 galaxy stellar mass function and the amplitude of the galaxy-central black hole mass relation, also taking galaxy sizes into account. The observed galaxy mass function is reproduced to ≲0.2 dex over the full mass range, 108

2,828 citations

Journal ArticleDOI
TL;DR: In this paper, supermassive black holes (BHs) have been found in 85 galaxies by dynamical modeling of spatially resolved kinematics, and it has been shown that BHs and bulges coevolve by regulating each other's growth.
Abstract: Supermassive black holes (BHs) have been found in 85 galaxies by dynamical modeling of spatially resolved kinematics. The Hubble Space Telescope revolutionized BH research by advancing the subject from its proof-of-concept phase into quantitative studies of BH demographics. Most influential was the discovery of a tight correlation between BH mass and the velocity dispersion σ of the bulge component of the host galaxy. Together with similar correlations with bulge luminosity and mass, this led to the widespread belief that BHs and bulges coevolve by regulating each other's growth. Conclusions based on one set of correlations from in brightest cluster ellipticals to in the smallest galaxies dominated BH work for more than a decade. New results are now replacing this simple story with a richer and more plausible picture in which BHs correlate differently with different galaxy components. A reasonable aim is to use this progress to refine our understanding of BH-galaxy coevolution. BHs with masses of 105−106M...

2,804 citations