Tom Theuns

The thermal history of the intergalactic medium

At redshifts z≳2, most of the baryons reside in the smooth intergalactic medium which is responsible for the low column density Lyα forest. This photoheated gas follows a tight temperature–density relation which introduces a cut-off in the distribution of widths of the Lyα absorption lines (b-parameters) as a function of column density. We have measured this cut-off in a sample of nine high-resolution, high signal-to-noise ratio quasar spectra and determined the thermal evolution of the intergalactic medium in the redshift range 2.0–4.5. At a redshift z∼3, the temperature at the mean density shows a peak and the gas becomes nearly isothermal. We interpret this as evidence for the reionization of Hexa0ii.

Measuring the equation of state of the intergalactic medium

Numerical simulations indicate that the smooth, photoionized intergalactic medium (IGM) responsible for the low column density Lyα forest follows a well-defined temperature–density relation, which is well described by a power law . We demonstrate that such an equation of state results in a power-law cut-off in the distribution of linewidths (b-parameters) as a function of column density (N) for the low column density (N≲1014.5xa0cm−2) absorption lines. This explains the existence of the lower envelope that is clearly seen in scatter plots of the b(N) distribution in observed QSO spectra. Even a strict power-law equation of state will not result in an absolute cut-off because of line blending and contamination by unidentified metal lines. We develop an algorithm to determine the cut-off, which is insensitive to these narrow lines. We show that the parameters of the cut-off in the b(N) distribution are strongly correlated with the parameters of the underlying equation of state. We use simulations to determine these relations, which can then be applied to the observed cut-off in the b(N) distribution to measure the equation of state of the IGM. We show that systematics that change the b(N) distribution, such as cosmology (for a fixed equation of state), peculiar velocities, the intensity of the ionizing background radiation and variations in the signal-to-noise ratio, do not affect the measured cut-off. We argue that physical processes that have not been incorporated in the simulations, e.g. feedback from star formation, are unlikely to affect the results. Using Monte Carlo simulations of Keck spectra at z=3, we show that determining the slope of the equation of state will be difficult, but that the amplitude can be determined to within 10xa0per cent, even from a single QSO spectrum. Measuring the evolution of the equation of state with redshift will allow us to put tight constraints on the reionization history of the Universe.

Broadening of QSO Lyα forest absorbers

We investigate the dependence of QSO Lyα 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 the values 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, T0≳15xa0000xa0K. The flux decrement distribution of the observed spectrum is fitted well by that of a ΛCDM model with that temperature.

Observational signatures of feedback in QSO absorption spectra

ABSTRA C T 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 protocluster per unit redshift, and significant feedback from forming galaxies or active galactic nuclei, heating the protocluster gas, will result in a large clearing of reduced absorption in the Lya forest. Such a gap could be detected at redshift *3 when the mean opacity is high. Feedback from Lyman-break galaxies in protoclusters 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 column densities NH i , 10 16 cm 22 : This feedback

A wavelet analysis of the spectra of quasi-stellar objects

The temperature of the intergalactic medium (IGM) is an important factor in determining the line-widths of the absorption lines in the Lyman-alpha forest. We present a method to characterise the line-widths distribution using a decomposition of a Lyman-alpha spectrum in terms of discrete wavelets. Such wavelets form an orthogonal basis so the decomposition is unique. We demonstrate using hydrodynamic simulations that the mean and dispersion of the wavelet amplitudes is strongly correlated with both the temperature of the absorbing gas and its dependence on the gas density. Since wavelets are also localised in space, we are able to analyse the temperature distribution as a function of position along the spectrum. We illustrate how this method could be used to identify fluctuations in the IGM temperature which might result from late reionization or local effects.