arXiv subjects

Tom Theuns

Publications and source records attributed to Tom Theuns.

At least 217 records · Page 12Linked to original sources

arXiv2004

Quenching Cluster Cooling Flows with Recurrent Hot Plasma Bubbles

The observed cooling rate of hot gas in clusters is much lower than that inferred from the gas density profiles. This suggests that the gas is being heated by some source. We use an adaptive-mesh refinement code (FLASH) to simulate the effect of multiple, randomly positioned, injections of thermal energy within 50 kpc of the centre of an initially isothermal cluster with mass M_200=3x10^(14) Msol and kT=3.1 keV. We have performed eight simulations with spherical bubbles of energy generated every 10^8 years, over a total of 1.5 Gyr. Each bubble is created by injecting thermal energy steadily for 10^7 years; the total energy of each bubble ranges from 0.1--3x10^(60) erg, depending on the simulation. We find that 2x10^(60) erg per bubble (corresponding to a average power of 6.3x10^(44) erg/s) effectively balances energy loss in the cluster and prevents the accumulation of gas below kT=1 keV from exceeding the observational limits of 30 Msol/yr. This injection rate is comparable to the radiated luminosity of the cluster, and the required energy and periodic timescale of events are consistent with observations of bubbles produced by central AGN in clusters. The effectiveness of this process depends primarily on the total amount of injected energy and the initial location of the bubbles, but is relatively insensitive to the exact duty cycle of events.

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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.

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arXiv2003

Metallicity of the intergalactic medium using pixel statistics. II. The distribution of metals as traced by C IV

(Modified) We measure the distribution of carbon in the intergalactic medium as a function of redshift z and overdensity delta. Using a hydrodynamical simulation to link the H I absorption to the density and temperature of the absorbing gas, and a model for the UV background radiation, we convert ratios of C IV to H I pixel optical depths into carbon abundances. A careful analysis of 19 high-quality quasar absorption spectra reveals that the carbon abundance is spatially highly inhomogeneous and is well-described by a lognormal distribution for fixed delta and z. Using data in the range log(delta) = -0.5 - 1.8 and z = 1.8 - 4.1, and a renormalized version of the Haardt & Madau (2001) model for the UV background radiation from galaxies and quasars, we measure a median metallicity of [C/H] = -3.47 + 0.08(z-3) + 0.65[log(delta)-0.5] and a lognormal scatter of sigma([C/H]) = 0.76 + 0.02(z-3) - 0.23[log(delta)-0.5]. Thus, we find significant trends with overdensity, but no evidence for evolution. These measurements imply that gas in this density range accounts for a cosmic carbon abundance of [C/H] = -2.80 +/- 0.13 (Omega_C = 2e-7), with no evidence for evolution. The dominant source of systematic error is the spectral shape of the UV background, with harder spectra yielding higher carbon abundances. While the systematic errors due to uncertainties in the spectral hardness may exceed the quoted statistical errors for delta < 10, we stress that UV backgrounds that differ significantly from our fiducial model give unphysical results. The measured lognormal scatter is strictly independent of the spectral shape, provided the background radiation is uniform. We also present measurements of the C III/C IV ratio (which rule out collisional ionization) and of the evolution of the effective Lyman-alpha optical depth.

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arXiv2003

The impact of galaxy formation on X-ray groups

Using hydrodynamical simulations of the LCDM cosmology, that include both radiative cooling and a phenomenological model for star formation and supernovae feedback, we investigate the impact of galaxy formation on the X-ray properties of groups at redshift zero. Motivated by the observed ``break'' in the L_x-T_x relation at kT~1-2keV, our feedback model is based on the assumption that supernovae imprint a temperature scale on the hot gas, with the star formation rate and corresponding reheated gas mass then depending only on the available energy budget. We demonstrate that a strong feedback model with a heating temperature comparable to this break (kT_=2keV), and an energy budget twice that available from supernovae (epsilon=2), raises the core entropy of groups sufficiently to produce an adequate match to their observed X-ray properties. A lower value of epsilon increases the star formation rate without significantly affecting the X-ray properties of groups, and a model with epsilon~0.1 reproduces the observed fraction of baryons in stars. However, a heating temperature which is lower than the virial temperatures of the groups leads to an excess of cooling gas that boosts their X-ray luminosities, due to the failure of the reheated material to escape from the gravitational potential. A limited study of numerical resolution effects reveals that the temperature of poorly-resolved objects is underestimated, therefore (in our case) a fully-resolved group population would lead to a steeper L_x-T_x relation, bringing our results into even better agreement with the observations.

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arXiv2003

Dark energy effects on the Lyman-alpha forest

In quintessence models, the dark energy content of the universe is described by a slowly rolling scalar field whose pressure and energy density obey an equation of state of the form p=w $ρ$; w is in general a function of time such that w<-1/3, in order to drive the observed acceleration of the Universe today. The cosmological constant model LCDM corresponds to the limiting case w=-1. In this paper, we explore the prospects of using the Lyman-alpha forest to constrain w, using semi-analytical techniques to model the intergalactic medium (IGM). A different value of w changes both the growth factor and the Hubble parameter as a function of time. The resulting change in the optical depth distribution affects the optical depth power spectrum, the number of regions of high transmission per unit redshift and the cross-correlation coefficient of spectra of quasar pairs. These can be detected in current data, provided we have independent estimates of the thermal state of the IGM, its ionization parameter and the baryon density.

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arXiv2002

Galactic winds in the intergalactic medium

We have performed hydrodynamical simulations to investigate the effects of galactic winds on the high-redshift (z=3) universe. Strong winds suppress the formation of low-mass galaxies significantly, and the metals carried by them produce C IV absorption lines with properties in reasonable agreement with observations. The winds have little effect on the statistics of the H I absorption lines, because the hot gas bubbles blown by the winds fill only a small fraction of the volume and because they tend to escape into the voids, thereby leaving the filaments that produce these lines intact.

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arXiv2002

Numerical simulations of the intergalactic medium

The intergalactic medium at redshifts 2--6 can be studied observationally through the absorption features it produces in the spectra of background quasars. Most of the UV-absorption lines arise in mildly overdense regions, which can be simulated reliably with current hydrodynamical simulations. Comparison of observed and simulated spectra allows one to put contraints on the model's parameters.

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arXiv2002

Metallicity of the Intergalactic Medium Using Pixel Statistics: I. Method

Studies of absorption spectra of high-z QSOs have revealed that the intergalactic medium at z ~ 2-3 is enriched to ~ 0.1%-1% solar for gas densities more than a few times the mean cosmic density, but have not yet produced an accurate metallicity estimate, nor constrained variations in the metallicity with density, redshift, or spatial location. This paper discusses the ``pixel optical depth'' (POD) method of QSO spectrum analysis, using realistic simulated spectra from cosmological simulations. In this method, absorption in Ly-alpha is compared to corresponding metal absorption on a pixel-by-pixel basis, yielding for each analyzed spectrum a single statistical correlation encoding metal enrichment information. Our simulations allow testing and optimization of each step of the technique's implementation. Tests show that previous studies have probably been limited by C IV self-contamination and O VI contamination by HI lines; we have developed and tested an effective method of correcting for both contaminants. We summarize these and other findings, and provide a useful recipe for the POD technique's application to observed spectra. Our tests reveal that the POD technique applied to spectra of presently available quality is effective in recovering useful metallicity information even in underdense gas. We present an extension of the POD technique to directly recover the intergalactic metallicity as a function of gas density. For a given ionizing background, both the oxygen and carbon abundance can be measured with errors of at most a factor of a few over at least an order of magnitude in density, using a single high-quality spectrum.

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arXiv2002

Modelling the IGM and the Lyalpha forest at high redshift from the dark matter distribution

A variety of approximate schemes for modelling the low-density Intergalactic Medium (IGM) in the high-redshift Universe is compared to the results of a large high-resolution hydro-dynamical simulation. These schemes use either an analytical description of the dark matter distribution and the IGM or numerical simulations of the DM distributions combined with different approximate relations between dark matter field and the gas distribution. Schemes based on a filtering of the dark matter distribution with a global Jeans scale result in a rather poor description of the gas distribution. An adaptive filtering which takes into account the density/temperature dependence of the Jeans scale is required. A reasonable description of the gas distribution can be achieved using a fit of the mean relation between the dark matter and gas densities in the hydro-dynamical simulation to relate dark matter and gas distribution. In the hydro-dynamical simulations deviations from this mean relation are correlated with gradients in the dark matter peculiar velocity field indicative of shocks in the gas component. A scheme which takes into account this correlation results in a further improved gas distribution. Such adaptive filtering schemes applied to dark matter simulations will be very well suited for studies of statistical properties of the Lyalpha forest which investigate the IGM and the underlying dark matter distribution and require a large dynamic range and/or an extensive parameter study.

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arXiv2002

Temperature fluctuations in the intergalactic medium

The temperature of the low-density intergalactic medium is set by the balance between adiabatic cooling resulting from the expansion of the universe, and photo-heating by the UV-background. We have analysed the Lyman-alpha forest of eleven high-resolution quasar spectra using wavelets, and find strong evidence of a marked jump in the temperature at the mean density, T_0, of 60 per cent around a redshift z=3.3, which we attribute to reionization of HeliumII. The jump can be seen in all three of our spectra that straddle redshift 3.3, at a significance of 99 per cent. Below z=3.1, our results are consistent with a smooth cooling down of the universe, as expected when adiabatic expansion dominates over photo-heating by a UV-background from QSOs and galaxies. We find no evidence of thermal fluctuations on scales greater than 5000 km/s larger than 50 per cent, which could be detected by our method, suggesting that the IGM follows a reasonably well-defined temperature-density relation. We demonstrate that the mean wavelet amplitude scales inversely with T_0 and calibrate the relation with hydrodynamical simulations. We find T_0= 12000K at z>3.6. Such high temperature suggest that Hydrogen reionization occured relatively recent.

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arXiv2002

Constraints on reionization from the thermal history of the intergalactic medium

The temperature of the diffuse, photo-heated intergalactic medium (IGM) depends on its reionization history because the thermal time scales are long. The widths of the hydrogen Lyman-alpha absorption lines seen in the spectra of distant quasars that arise in the IGM can be used to determine its temperature. We use a wavelet analysis of the Lyman-alpha forest region of quasar spectra to demonstrate that there is a relatively sudden increase in the line widths between redshifts z=3.5 and 3.0, which we associate with entropy injection resulting from the reionization of HeII. The subsequent fall-off in temperature after z~3.5, is consistent with a thermal evolution dominated by adiabatic expansion. If, as expected, the temperature also drops rapidly after hydrogen reionization, then the high temperatures inferred from the line widths before HeII reionization imply that hydrogen reionization occurred below redshift z=9.

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arXiv2001

Detecting HeliumII reionization from a sudden injection of entropy in the intergalactic medium

The temperature of the low-density intergalactic medium is set by the balance between adiabatic cooling resulting from the expansion of the universe, and photo-heating by the UV-background. A sudden injection of entropy from the reionization will increase the temperature of the gas, leading to a broadening of the hydrogen Lyman-alpha absorption lines produced in the IGM, and observed in the spectra of background quasars. We present a method based on wavelets to characterise objectively the line widths of such absorption lines. We use high resolution hydrodynamical simulations to demonstrate that the algorithm can detect changes in temperature of order of 50 per cent on scales > 5000 km/s. We apply the method to a UVES/VLT spectrum of quasar 0055--269 (emission redshift=3.7) and detect at the 99 per cent confidence level a sudden increase in temperature below redshift z=3.3, which we interpret as evidence for HeliumII reionization.

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arXiv2001

Probing the Intergalactic Medium with the Lyman alpha forest along multiple lines of sight to distant QSOs

(abridged) We present an effective implementation of analytical calculations of the Lyalpha opacity distribution of the Intergalactic Medium (IGM) along multiple lines of sight (LOS) to distant quasars in a cosmological setting. This method fully accounts for the expected correlations between LOS and the cosmic variance in the large-scale modes of the dark matter distribution. Strong correlations extending up to ~300 kpc (proper) and more are found at redshift z~2->3, in agreement with observations. These correlations are investigated using the cross-correlation coefficient and the cross-power spectrum of the flux distribution along different LOS and by identifying coincident absorption features as fitted with a Voigt profile fitting routine. The cross-correlation coefficient between the LOS can be used to constrain the shape-parameter Gamma of the power spectrum if the temperature and the temperature density relation of the IGM can be determined indepedently. We also propose a new technique to recover the 3D linear dark matter power spectrum by integrating over 1D flux cross-spectra which is complementary to the usual `differentiation' of 1D auto-spectra. The cross-power spectrum suffers much less from errors uncorrelated in different LOS, like those introduced by continuum fitting. Investigations of the flux correlations in adjacent LOS should thus allow to extend studies of the dark matter power spectrum with the Lyalpha forest to significantly larger scales than is possible with flux auto-power spectra. 30 pairs with separation of 1-2 arcmin should be sufficient to determine the 1D cross-spectrum at scales of 60 Mpc/h with an accuracy of about 30% if the error is dominated by cosmic variance.

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arXiv2001

Observational signatures of feedback in QSO absorption spectra

Models for the formation of galaxies and clusters of galaxies require strong feedback in order to explain the observed properties of these systems. We investigate whether such feedback has observational consequences for the intergalactic medium, as probed in absorption towards background quasars. A typical quasar sight-line intersects one proto-cluster per unit redshift, and significant feedback from forming galaxies or AGN, heating the proto-cluster gas, will result in a large clearing of reduced absorption in the Lyman alpha-forest. Such a gap could be detected at redshift > 3 when the mean opacity is high. Feedback from Lyman-break galaxies in proto-clusters can be probed by the absorption lines produced in their winds. Strong feedback from galaxies has a major impact on the number and properties of absorption lines with neutral hydrogen column densities of 10^16/ cm^2. This feedback can be probed with multiple sight-lines and by studying the unsaturated higher-order lines of the Lyman series. Galactic winds from dwarf galaxies should break-up into clouds, in order not to over produce the number of absorption lines. These clouds can then coast to large distances.

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arXiv2001

Do angular momentum induced ellipticity correlations contaminate weak lensing measurements?

Alignments in the angular momentum vectors of galaxies can induce large scale correlations in their projected orientations. Such alignments arise from the tidal torques exerted on neighboring proto-galaxies by the smoothly varying shear field. Weak gravitational lensing can also induce ellipticity correlations since the images of neighboring galaxies will be distorted coherently by the intervening mass distribution. Comparing these two sources of shape correlations, it is found that for current weak lensing surveys with a median redshift of 1, the intrinsic signal is a contaminant on the order of 1-10% of the measured signal. However, for shallower surveys, the intrinsic correlations dominate over the lensing signal. The distortions induced by lensing are curl-free, whereas those resulting from intrinsic alignments are not. This difference can be used to disentangle these two sources of ellipticity correlations. When the distortions are dominated by lensing, as occurs at high redshifts, the decomposition provides a valuable tool for understanding properties of the noise and systematic errors.

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arXiv2000

Discriminating weak lensing from intrinsic spin correlations using the curl-gradient decomposition

The distortion field defined by the ellipticities of galaxy shapes projected on the sky can be uniquely decomposed into a gradient and a curl component. If the observed ellipticities are induced by weak gravitational lensing, then the distortion field is curl free. Here we show that, in contrast, the distortion field resulting from intrinsic spin alignments is not curl free. This provides a powerful discriminant between lensing and intrinsic contributions to observed ellipticity correlations. We also show how these contributions can be disentangled statistically from the ellipticity correlations or computed locally from circular integrals of the ellipticity field. This allows for an unambiguous detection of intrinsic galaxy alignments in the data. When the distortions are dominated by lensing, as occurs at high redshifts, the decomposition provides a valuable tool for understanding properties of the noise and systematic errors. These techniques can be applied equally well to the polarization of the microwave background, where it can be used to separate curl-free scalar perturbations from those produced by gravity waves or defects.

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arXiv2000

Spin induced galaxy alignments and their implications for weak lensing measurements

Large scale correlations in the orientations of galaxies can result from alignments in their angular momentum vectors. These alignments arise from the tidal torques exerted on neighboring proto-galaxies by the smoothly varying shear field. We compute the predicted amplitude of such ellipticity correlations using the Zel'dovich approximation for a realistic distribution of galaxy shapes. Weak gravitational lensing can also induce ellipticity correlations since the images of neighboring galaxies will be distorted coherently. On comparing these two effects that induce shape correlations, we find that for current weak lensing surveys with a median redshift of z_m = 1, the intrinsic signal is of order 1 - 10 percent of the measured signal. However, for shallower surveys with z_m < 0.3, the intrinsic correlations dominate over the lensing signal. The distortions induced by lensing are curl-free, whereas those resulting from intrinsic alignments are not. This difference can be used to disentangle these two sources of ellipticity correlations.

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arXiv2000

Broadening of QSO Lyman-alpha forest absorbers

We investigate the dependence of QSO Lyman-alpha absorption features on the temperature of the absorbing gas and on the amplitude of the underlying dark matter fluctuations. We use high-resolution hydrodynamic simulations in cold dark matter dominated cosmological models. In models with a hotter intergalactic medium (IGM), the increased temperature enhances the pressure gradients between low and high density regions and this changes the spatial distribution and the velocity field of the gas. Combined with more thermal broadening, this leads to significantly wider absorption features in hotter models. Cosmological models with little small scale power also have broader absorption features, because fluctuations on the scale of the Jeans length are still in the linear regime . Consequently, both the amplitude of dark matter fluctuations on small scales and thermal smoothing affect the flux decrement distribution in a similar way. However, the b-parameter distribution of Voigt profile fits, obtained by deblending the absorption features into a sum of thermally broadened lines, is largely independent of the amount of small scale power, but does depend strongly on the IGM temperature. The same is true for the two-point function of the flux and for the flux power spectrum on small scales. These three flux statistics are thus sensitive probes of the temperature of the IGM. We compare them computed for our models and obtained from a HIRES spectrum of the quasar Q1422+231 and conclude that the IGM temperature at z=3.25 is fairly high, T_0>= 15000K.

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