An X-Ray Study Of Intergalactic Medium In Galaxy Clusters And Groups And Its Application To Low-Frequency Radio Observations

Galaxy clusters and groups are the largest celestial sources bound by the gravity in our u-niverse, and they are also the places where the dark matter was discovered via the study of themotions of member galaxies in early1930s. In clusters and groups, the dominating baryonic mat-ter component is distributed between galaxies in the form of extremely hot (1078K) plasma, andit accounts for about7085%of the total gravitating mass, meanwhile emits thermal radiationmostly in the X-ray band. This baryonic matter component, i.e., the intergalactic medium (IGM),has played an important role in the space-borne X-ray observations performed in the past43yearsby providing us with valuable information about the structure and evolution of not only the clustersand groups themselves, but also our universe.One of the unsolved mysteries in the X-ray study of galaxy clusters and groups is the observeddeviations of the scaling relations from the theoretical predictions. These scaling laws are expectedto be intrinsically determined by the essential physics that governs the formation and evolution ofthe Mpc-scale structures, so that there should exist tight correlations between, e.g., total gravitatingmass, gas temperature, X-ray luminosity, and some other fundamental physical quantities. In thepast decade a tremendous amount of high quality imaging and spectroscopic data have been ac-quired with the new generation X-ray satellites in more than3000pointing observations of galaxyclusters and groups. And this makes it feasible to construct cluster-group samples that are largeenough to re-examine the scaling laws with sufficiently high accuracy and precision that has notbeen achieved by the previous small-and medium-sized samples, and to explain why the scalinglaws are biased as compared with the theoretical predictions.To help this, it is equally important to prepare for the search in the near future in the un-explored low-frequency radio band for any possible evidence for AGN feedback and/or mergerevents, which are believed to be responsible for the biases of the scaling laws and to be moreprominent in the low-frequency radio band. New techniques should be developed based on theexisting multi-band information to simulate the low-frequency radio sky with sufficiently accu-rate details, and to identify and separate the cluster/group signals correctly. Apparently, once the cluster/group signal is successfully separated, it will also greatly benefit the research of the cos-mic reionization epoch and dark age, since the radio radiations of clusters and groups stronglycontaminate the extremely weak redshifted21cm signals to be detected.The research results of the thesis work is summarized as follows.(1) By constructing a largesample of342galaxy clusters and groups (z=01.4, TX=0.516keV), which are drawn fromthe Chandra archive database by filtering and analyzing all the corresponding data acquired in thepast decade, we calculate the total gravitating mass-X-ray gas temperature relation and gas fractionwithin r500, and find that the mass-temperature relation of the high-temperature clusters followsthe theoretical prediction (Mαtot∝TX，α=1.5), while the low-temperature clusters and groupsexhibit an apparently steeper distribution (α=1.80±0.02), with a break at Tbreak=2.58±0.17keV. We also find that the TX> Tbreakclusters show an average gas fraction of12.9±3.3%thatsatisfy the standard cosmological model, and the gas fraction decreases significantly towards thelow-temperature end. We argue that both the phenomena are caused by the fact that the effects ofextra heating is more prominent in the low-temperature systems.(2) Based on the results of work1,we simulate the50-200MHz radio sky with high accuracy by carrying out Monte Carlo simulationsto model the strong contaminating foreground of the redshifted cosmological reionization signals,including emissions from our Galaxy, galaxy clusters, and extragalactic discrete sources (i.e., star-forming galaxies, radio-quiet active galactic nuclei (AGNs), and radio-loud AGNs). We considerin detail not only random variations of morphological and spectroscopic parameters within theranges allowed by multi-band observations, but also the evolution of radio halos in galaxy clusters,assuming that relativistic electrons are re-accelerated in the IGM in merger events and lose energyvia both synchrotron emission and inverse Compton scattering with cosmic microwave backgroundphotons.(3) By adopting the complex foreground model built in work2and introducing a newapproach designed on the basis of independent component analysis and wavelet detection algo-rithm, we prove that, with a cumulative observation of one month with the21CMA array built atXinjiang, about80%of galaxy clusters with central brightness temperatures of>10K at65MHzcan be safely identified and separated from the overwhelmingly bright foreground.(4) By adoptingthe complex foreground model built in work2, we re-examine the separation approaches based onthe quadratic polynomial fitting technique in frequency space in order to investigate whether theywork satisfactorily by quantitatively evaluating the quality of restored21cm signals (both spectraand power spectra) in terms of sample statistics. We find that a significant part of the Mpc-scalecomponents of the21cm signals (75%for6h1Mpc scales and34%for1h1Mpc scales)is lost using the traditional separation algorithm, because it tends to be misidentified as part of theforeground when the single-narrow-segment separation approach is applied. The best restorationof the21cm signals and the tightest determination of the mean halo b and average ionization frac-tion xecan be obtained with the three-narrow-segment fitting technique as proposed in this paper. Similar results can be obtained at other redshifts, showing its potential use in the research of theBAO signals.