This Thesis concerns various aspects of galaxy clusters and their X-ray properties drawn from cosmological simulations. Such simulations allow structures to evolve on a cosmological time-scale hence ...providing promising prospects as an independent means of comparing and reconciling the differences between our current understanding of intracluster medium (ICM) properties and structure formation from theoretical predictions and observations. The main emphasis of this Thesis is to study whether additional physics, i.e. radiative cooling and preheating, can assist us to understand such differences. Following a general overview of the Thesis in Chapter 1, we will review observational aspects of clusters in Chapter 2, in particular the observed ICM properties and the use of clusters as cosmological tools. Then in Chapter 3, we will fully describe our numerical models used in the studies and how we identify clusters in our simulations. In Chapters 4 and 5, we will study the simulated clusters at the present day redshift both of their ICM properties and scaling relations, namely the temperature - mass and luminosity - temperature relations, respectively. In Chapter 6, we will follow the evolution of the scaling relations from the present day to redshift 2. The implications of the simulated scalings as well as their evolution for observational cosmology are discussed in Chapter 7, where we draw our final conclusions.
We have extracted over 400 clusters, covering more than two decades in mass, from three simulations of the tauCDM cosmology. This represents the largest uniform catalogue of simulated clusters ever ...produced. The clusters exhibit a wide variety of density profiles. Only a minority are well-fitted in their outer regions by the widely used density profile of Navarro, Frenk, and White (NFW, 1995-1997), which is applicable to relaxed haloes. Others have steeper outer density profiles, show sharp breaks in their density profiles, or have significant substructure. If we force a fit to the NFW profile, then the best-fitting concentrations decline with increasing mass, but this is driven primarily by an increase in substructure as one moves to higher masses. The temperature-mass relations for properties measured within a sphere enclosing a fixed overdensity all follow the self-similar form, T varies as M exp 2/3; however, the normalization is lower than the value inferred for observed clusters. The temperature-mass relations for properties measured within a fixed physical radius are significantly steeper then this. Both can be accurately predicted using the NFW model. (Author)
We present results of including heating and cooling in cosmological
simulations of the $\Lambda$CDM cosmology and demonstrate their effects on
scaling laws of galaxy clusters. The scaling relations ...when radiative cooling
is included are in good agreement with observations but the fraction of cooled
gas is on the upper limit allowed by observations. On the contrary, the
preheating model has a more realistic cooled fraction but the scaling relations
are less well produced.
We present results of including heating and cooling in cosmological simulations of the \(\Lambda\)CDM cosmology and demonstrate their effects on scaling laws of galaxy clusters. The scaling relations ...when radiative cooling is included are in good agreement with observations but the fraction of cooled gas is on the upper limit allowed by observations. On the contrary, the preheating model has a more realistic cooled fraction but the scaling relations are less well produced.
Astrophys.J.649:640-648,2006 We investigate the redshift dependence of X-ray cluster scaling relations
drawn from three hydrodynamic simulations of the LCDM cosmology: a Radiative
model that ...incorporates radiative cooling of the gas, a Preheating model that
additionally heats the gas uniformly at high redshift, and a Feedback model
that self-consistently heats cold gas in proportion to its local star-formation
rate. While all three models are capable of reproducing the observed local
Lx-Tx relation, they predict substantially different results at high redshift
(to z=1.5), with the Radiative, Preheating and Feedback models predicting
strongly positive, mildly positive and mildly negative evolution, respectively.
The physical explanation for these differences lies in the structure of the
intracluster medium. All three models predict significant temperature
fluctuations at any given radius due to the presence of cool subclumps and, in
the case of the Feedback simulation, reheated gas. The mean gas temperature
lies above the dynamical temperature of the halo for all models at z=0, but
differs between models at higher redshift with the Radiative model having the
lowest mean gas temperature at z=1.5.
We have not attempted to model the scaling relations in a manner that mimics
the observational selection effects, nor has a consistent observational picture
yet emerged. Nevertheless, evolution of the scaling relations promises to be a
powerful probe of the physics of entropy generation in clusters. First
indications are that early, widespread heating is favored over an extended
period of heating that is associated with galaxy formation.
We investigate the redshift dependence of X-ray cluster scaling relations drawn from three hydrodynamic simulations of the LCDM cosmology: a Radiative model that incorporates radiative cooling of the ...gas, a Preheating model that additionally heats the gas uniformly at high redshift, and a Feedback model that self-consistently heats cold gas in proportion to its local star-formation rate. While all three models are capable of reproducing the observed local Lx-Tx relation, they predict substantially different results at high redshift (to z=1.5), with the Radiative, Preheating and Feedback models predicting strongly positive, mildly positive and mildly negative evolution, respectively. The physical explanation for these differences lies in the structure of the intracluster medium. All three models predict significant temperature fluctuations at any given radius due to the presence of cool subclumps and, in the case of the Feedback simulation, reheated gas. The mean gas temperature lies above the dynamical temperature of the halo for all models at z=0, but differs between models at higher redshift with the Radiative model having the lowest mean gas temperature at z=1.5. We have not attempted to model the scaling relations in a manner that mimics the observational selection effects, nor has a consistent observational picture yet emerged. Nevertheless, evolution of the scaling relations promises to be a powerful probe of the physics of entropy generation in clusters. First indications are that early, widespread heating is favored over an extended period of heating that is associated with galaxy formation.
Mon.Not.Roy.Astron.Soc. 346 (2003) 319 The amplitude of density perturbations, for the currently-favoured LambdaCDM
cosmology, is constrained using the observed properties of galaxy clusters. The
...catalogue used is that of Ikebe et al. (2002). The cluster temperature to mass
relation is obtained via N-body/hydrodynamical simulations including radiative
cooling and preheating of cluster gas, which we have previously shown to
reproduce well the observed temperature--mass relation in the innermost parts
of clusters (Thomas et al. 2002). We generate and compare mock catalogues via a
Monte Carlo method, which allows us to constrain the relation between X-ray
temperature and luminosity, including its scatter, simultaneously with
cosmological parameters. We find a luminosity-temperature relation in good
agreement with the results of Ikebe et al. (2002), while for the matter power
spectrum normalization, we find $\sigma_8 = 0.78_{-0.06}^{+0.30}$ at 95 per
cent confidence for $\Omega_0 = 0.35$. Scaling to WMAP's central value of
$\Omega_0 = 0.27$ would give a best-fit value of $\sigma_8 \simeq 0.9$.
Mon.Not.Roy.Astron.Soc. 336 (2002) 527 We calculate X-ray properties of present-day galaxy clusters from
hydrodynamical cosmological simulations of the LCDM cosmology and compare these
with recent ...X-ray observations. Results from three simulations are presented,
each of which uses the same initial conditions: a standard adiabatic,
Non-radiative model, a Radiative model that includes radiative cooling of the
gas, and a Preheating model that also includes cooling but in addition
impulsively heats the gas prior to cluster formation. At the end of the
simulations, the global cooled baryon fractions in the latter two runs are 15
per cent and 0.4 per cent respectively which bracket the recent result from the
K-band luminosity function. We construct cluster catalogues which consist of
over 500 clusters and are complete in mass down to 1.18*10^{13} Msun/h. While
clusters in the Non-radiative model behave in accord with the self-similar
picture, those of the other two models reproduce key aspects of the observed
X-ray properties: the core entropy, temperature-mass and luminosity-temperature
relations are all in good agreement with recent observations. This agreement
stems primarily from an increase in entropy with respect to the Non-radiative
clusters. Although the physics affecting the intra-cluster medium is very
different in the two models, the resulting cluster entropy profiles are very
similar.
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