arXiv subjects

Joop Schaye

Publications and source records attributed to Joop Schaye.

At least 505 records · Page 28Linked to original sources

arXiv2008

Foreground simulations for the LOFAR - Epoch of Reionization Experiment

Future high redshift 21-cm experiments will suffer from a high degree of contamination, due both to astrophysical foregrounds and to non-astrophysical and instrumental effects. In order to reliably extract the cosmological signal from the observed data, it is essential to understand very well all data components and their influence on the extracted signal. Here we present simulated astrophysical foregrounds datacubes and discuss their possible statistical effects on the data. The foreground maps are produced assuming 5 deg x 5 deg windows that match those expected to be observed by the LOFAR Epoch-of-Reionization (EoR) key science project. We show that with the expected LOFAR-EoR sky and receiver noise levels, which amount to ~52 mK at 150 MHz after 300 hours of total observing time, a simple polynomial fit allows a statistical reconstruction of the signal. We also show that the polynomial fitting will work for maps with realistic yet idealised instrument response, i.e., a response that includes only a uniform uv coverage as a function of frequency and ignores many other uncertainties. Polarized galactic synchrotron maps that include internal polarization and a number of Faraday screens along the line of sight are also simulated. The importance of these stems from the fact that the LOFAR instrument, in common with all current interferometric EoR experiments has an instrumentally polarized response.

astro-ph↗

arXiv2008

TRAPHIC - Radiative Transfer for Smoothed Particle Hydrodynamics Simulations

We present TRAPHIC, a novel radiative transfer scheme for Smoothed Particle Hydrodynamics (SPH) simulations. TRAPHIC is designed for use in simulations exhibiting a wide dynamic range in physical length scales and containing a large number of light sources. It is adaptive both in space and in angle and can be employed for application on distributed memory machines. The commonly encountered computationally expensive scaling with the number of light sources in the simulation is avoided by introducing a source merging procedure. The (time-dependent) radiative transfer equation is solved by tracing individual photon packets in an explicitly photon-conserving manner directly on the unstructured grid traced out by the set of SPH particles. To accomplish directed transport of radiation despite the irregular spatial distribution of the SPH particles, photons are guided inside cones. We present and test a parallel numerical implementation of TRAPHIC in the SPH code GADGET-2, specified for the transport of mono-chromatic hydrogen-ionizing radiation. The results of the tests are in excellent agreement with both analytic solutions and results obtained with other state-of-the-art radiative transfer codes.

astro-ph↗

arXiv2008

Simulating galactic outflows with kinetic supernova feedback

Feedback from star formation is thought to play a key role in the formation and evolution of galaxies, but its implementation in cosmological simulations is currently hampered by a lack of numerical resolution. We present and test a sub-grid recipe to model feedback from massive stars in cosmological smoothed particle hydrodynamics simulations. The energy is distributed in kinetic form among the gas particles surrounding recently formed stars. The impact of the feedback is studied using a suite of high-resolution simulations of isolated disc galaxies embedded in dark halos with total mass 10^{10} and 10^{12} Msol/h. We focus in particular on the effect of pressure forces on wind particles within the disc, which we turn off temporarily in some of our runs to mimic a recipe that has been widely used in the literature. We find that this popular recipe gives dramatically different results because (ram) pressure forces on expanding superbubbles determine both the structure of the disc and the development of large-scale outflows. Pressure forces exerted by expanding superbubbles puff up the disc, giving the dwarf galaxy an irregular morphology and creating a galactic fountain in the massive galaxy. Hydrodynamic drag within the disc results in a strong increase of the effective mass loading of the wind for the dwarf galaxy, but quenches much of the outflow in the case of the high-mass galaxy.

astro-ph↗

arXiv2007

On the relation between the Schmidt and Kennicutt-Schmidt star formation laws and its implications for numerical simulations

When averaged over large scales, star formation in galaxies is observed to follow the empirical Kennicutt-Schmidt (KS) law for surface densities above a constant threshold. While the observed law involves surface densities, theoretical models and simulations generally work with volume density laws (i.e. Schmidt laws). We derive analytic relations between star formation laws expressed in terms of surface densities, volume densities, and pressures and we show how these relations depend on parameters such as the effective equation of state of the multiphase interstellar medium. Our analytic relations enable us to implement observed surface density laws into simulations. Because the parameters of our prescription for star formation are observables, we are not free to tune them to match the observations. We test our theoretical framework using high-resolution simulations of isolated disc galaxies that assume an effective equation of state for the multiphase interstellar medium. We are able to reproduce the star formation threshold and both the slope and the normalisation of arbitrary input KS laws without tuning any parameters and with very little scatter, even for unstable galaxies and even if we use poor numerical resolution. Moreover, we can do so for arbitrary effective equations of state. Our prescription therefore enables simulations of galaxies to bypass our current inability to simulate the formation of stars. On the other hand, the fact that we can reproduce arbitrary input thresholds and KS laws, rather than just the particular ones picked out by nature, indicates that simulations that lack the physics and/or resolution to simulate the multiphase interstellar medium can only provide limited insight into the origin of the observed star formation laws.

astro-ph↗

arXiv2007

Star Formation Thresholds

To make predictions for the existence of ``dark galaxies'', it is necessary to understand what determines whether a gas cloud will form stars. Star formation thresholds are generally explained in terms of the Toomre criterion for gravitational instability. I contrast this theory with the thermo-gravitational instability hypothesis of Schaye (2004), in which star formation is triggered by the formation of a cold gas phase and which predicts a nearly constant surface density threshold. I argue that although the Toomre analysis is useful for the global stability of disc galaxies, it relies on assumptions that break down in the outer regions, where star formation thresholds are observed. The thermo-gravitational instability hypothesis can account for a number of observed phenomena, some of which were thought to be unrelated to star formation thresholds.

astro-ph↗

arXiv2007

A large population of metal-rich, compact, intergalactic C IV absorbers - Evidence for poor small-scale metal mixing

We carried out a survey for high-metallicity C IV absorbers at redshift z ~ 2.3 in the spectra of 9 high-quality quasar spectra. Using a novel analysis technique, based on detections of C IV lines and automatically determined upper limits on the column densities of H I, C III, N V, and O VI, we find a large (dN/dz > 7) population of photo-ionized, compact (R ~ 10^2 pc), metal-rich (Z >~ Z_solar) C IV clouds with moderate densities (n_H ~ 10^{-3.5} cm^{-3}), properties that we show are robust with respect to uncertainties in the ionization model. In particular, local sources of ionizing radiation, overabundance of oxygen, departures from ionization equilibrium, and collisional ionization would all imply more compact clouds. The clouds are too small to be self-gravitating and pressure confinement is only consistent under special conditions. We argue that the clouds are, in any case, likely to be short-lived and we demonstrate that this implies that the clouds could easily have been responsible for the transport of all metals that end up in the intergalactic medium (IGM). When the clouds reach pressure equilibrium with the general, photo-ionized IGM, the metals will still be concentrated in small high-metallicity patches, but they will look like ordinary, low-metallicity absorbers. We conclude that intergalactic metals are poorly mixed on small scales and that nearly all of the IGM, and thus the universe, may therefore be of primordial composition.

astro-ph↗

arXiv2006

How Did the IGM Become Enriched?

The enrichment of the intergalactic medium with heavy elements is a process that lies at the nexus of poorly-understood aspects of physical cosmology. We review current understanding of the processes that may remove metals from galaxies, the basic predictions of these models, the key observational constraints on enrichment, and how intergalactic enrichment may be used to test cosmological simulations.

astro-ph↗

arXiv2006

On the importance of local sources of radiation for quasar absorption line systems

A generic assumption of ionization models of quasar absorption systems is that radiation from local sources is negligible compared with the cosmological background. We test this assumption and find that it is unlikely to hold for absorbers as rare as H I Lyman limit systems. Assuming that the absorption systems are gas clouds centered on sources of radiation, we derive analytic estimates for the cross-section weighted moments of the flux seen by the absorbers, of the impact parameter, and of the luminosity of the central source. In addition, we compute the corresponding medians numerically. For the one class of absorbers for which the flux has been measured: damped Ly-alpha systems at z~3, our prediction is in excellent agreement with the observations if we assume that the absorption arises in clouds centered on Lyman-break galaxies. Finally, we show that if Lyman-break galaxies dominate the UV background at redshift 3, then consistency between observations of the UV background, the UV luminosity density from galaxies, and the number density of Lyman limit systems requires escape fractions of order 10 percent.

astro-ph↗

arXiv2005

The Spatial Distribution of Metals in the Intergalactic Medium

We investigate the impact of environment on the metallicity of the diffuse intergalactic medium. We use pixel correlation techniques to search for weak CIV and OVI absorption in the spectrum of quasar Q1422+231 in regions of the spectrum close to and far from galaxies at z ~ 3. This is achieved both by using the positions of observed Lyman break galaxies and by using strong CIV absorption as a proxy for the presence of galaxies near the line of sight. We find that the metal line absorption is a strong function of not only the HI optical depth (and thus gas density) but also proximity to highly enriched regions (and so proximity to galaxies). The parameter ``proximity to galaxies'' can account for some, but not all, of the scatter in the strength of CIV absorption for fixed HI. Finally, we find that even if we limit our analysis to the two thirds of the pixels that are at least 600 km/s from any CIV line that is strong enough to detect unambiguously (tau_CIV > 0.1), our statistical analysis reveals only slightly less CIV for fixed HI than when we analyze the whole spectrum. We conclude that while the metallicity is enhanced in regions close to (Lyman-break) galaxies, the enrichment is likely to be much more widespread than their immediate surroundings.

astro-ph↗

arXiv2005

Abundances in the high-redshift Intergalactic Medium

The enrichment of the intergalactic medium (IGM) with heavy elements provides us with a record of past star formation and with an opportunity to study the interactions between galaxies and their environments. We summarize current data analysis methods and observational constraints on abundances in the diffuse, high-redshift (z > 2) IGM. This review is targeted at interested outsiders and attempts to answer the following questions: Why should you care? What do we want to measure? How do we do it? What do we know? What are the common misconceptions?

astro-ph↗

arXiv2005

Observational Tests of Intergalactic Enrichment Models

We summarize recent results assessing the carbon and silicon abundances of the intergalactic medium (IGM) using the `pixel optical depth' technique. We briefly discuss the implications of these results for models of intergalactic enrichment, focusing on distinguishing `early' z >> 4 enrichment by the first generations of stars and objects from `late' enrichment by 2 < z < 5 Ly-break galaxies. We then discuss the comparison of observed QSO spectra to simulated spectra generated from cosmological simulations that self-consistently include enrichment, and draw qualitative implications for the general picture of intergalactic enrichment at z > 2.

astro-ph↗

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.

astro-ph↗

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.

astro-ph↗

arXiv2004

Star formation thresholds and galaxy edges: why and where

We study global star formation thresholds in the outer parts of galaxies by investigating the stability of disk galaxies embedded in dark halos. The disks are self-gravitating, contain metals and dust, and are exposed to UV radiation. We find that the critical surface density for the existence of a cold interstellar phase depends only weakly on the parameters of the model and coincides with the empirically derived surface density threshold for star formation. Furthermore, it is shown that the drop in the thermal velocity dispersion associated with the transition from the warm to the cold gas phase triggers gravitational instability on a wide range of scales. The presence of strong turbulence does not undermine this conclusion if the disk is self-gravitating. Models based on the hypothesis that the onset of thermal instability determines the star formation threshold in the outer parts of galaxies can reproduce many observations, including the threshold radii, column densities, and the sizes of stellar disks as a function of disk scale length and mass. Finally, prescriptions are given for implementing star formation thresholds in (semi-)analytic models and three-dimensional hydrodynamical simulations of galaxy formation.

astro-ph↗

arXiv2004

The Lyman-alpha Forest Power Spectrum from the Sloan Digital Sky Survey

We measure the power spectrum, P_F(k,z), of the transmitted flux in the Ly-alpha forest using 3035 high redshift quasar spectra from the Sloan Digital Sky Survey. This sample is almost two orders of magnitude larger than any previously available data set, yielding statistical errors of ~0.6% and ~0.005 on, respectively, the overall amplitude and logarithmic slope of P_F(k,z). This unprecedented statistical power requires a correspondingly careful analysis of the data and of possible systematic contaminations in it. For this purpose we reanalyze the raw spectra to make use of information not preserved by the standard pipeline. We investigate the details of the noise in the data, resolution of the spectrograph, sky subtraction, quasar continuum, and metal absorption. We find that background sources such as metals contribute significantly to the total power and have to be subtracted properly. We also find clear evidence for SiIII correlations with the Ly-alpha forest and suggest a simple model to account for this contribution to the power. While it is likely that our newly developed analysis technique does not eliminate all systematic errors in the P_F(k,z) measurement below the level of the statistical errors, our tests indicate that any residual systematics in the analysis are unlikely to affect the inference of cosmological parameters from P_F(k,z). These results should provide an essential ingredient for all future attempts to constrain modeling of structure formation, cosmological parameters, and theories for the origin of primordial fluctuations.

astro-ph↗

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.

astro-ph↗

arXiv2004

Metallicity of the intergalactic medium using pixel statistics: III. Silicon

(Modified) We study the abundance of silicon in the intergalactic medium by analyzing the statistics of SiIV, CIV, and HI pixel optical depths in a sample of 19 high-quality quasar absorption spectra spanning redshifts z ~ 2 - 4, which we compare to realistic spectra drawn from a hydrodynamical simulation. We find that silicon is highly overabundant relative to carbon and that the scatter in Si/C is much smaller than that in C/H, implying a common origin for Si and C. The inferred [Si/C] depends upon the shape of the UV background (UVB) (harder backgrounds leading to higher [Si/C]), ranging from [Si/C] ~ 1.5 for a quasar-only UVB, to [Si/C] ~ 0.25 for a UVB including both galaxies and an artificial softening. For our fiducial UVB, which includes both quasars and galaxies, we find [Si/C]=0.77 +/- 0.05, with a possible systematic error of ~ 0.1 dex. There is no evidence for evolution in [Si/C] and the data are inconsistent with previous claims of a sharp change in the SiIV/CIV ratio (or the UVB) at z ~ 3. Comparisons with low-metallicity halo stars and nucleosynthetic yields suggest that either our fiducial UVB is too hard or that supermassive Pop III stars might have to be included. The inferred [Si/C] corresponds to a contribution to the cosmic Si abundance of [Si/H] = -2.0, a significant fraction of all Si production expected by z ~ 3.

astro-ph↗

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.

astro-ph↗