With optical and X-ray data, numerical simulations and analytical modeling, we study two classes of extreme celestial objects—galaxy clusters (the largest gravitationally-bound objects) and supermassive black holes (SMBHs, objects with the deepest gravitational potential)—and their use as probes of cosmology.
To explain certain observational results of galaxy clusters, it is suggested that the intra-cluster gas might have been pre-heated by feedback processes before the formation of the cluster. Such "preheating" could produce voids with little or no absorption in quasar spectra by ionizing hydrogen in the proto-cluster region. We examine the spectra of 137 quasars in the Sloan Digital Sky Survey (SDSS) to search for such voids, and find no clear evidence of their existence. Employing a bubble growth model adapted from cosmic reionization, we find that preheating models in which the volume filling factor of ionized bubbles exceed 20% at z ∼ 3 can be ruled out.
The abundance evolution of galaxy clusters offers a probe of cosmology, that is independent and complementary to those of the cosmic microwave background or supernovae. We present the results of a survey by the Suzaku telescope of 14 low-redshift galaxy clusters, which had otherwise never been observed in direct, pointed X-ray observations with earlier, spectrally sensitive instruments. Together with 47 other systems, they form a flux- limited sample extending to redshift z ≤ 0.1 in the northern celestial hemisphere. Using this sample, we fit the X-ray luminosity-temperature relation, and determine the low-redshift temperature function. In general, the low-redshift cluster temperature function from our sample is in agreement with other published estimates; however, the sample used by us exhibits slightly lower space densities at gas temperatures below 4–5 keV.
We also explore a new way of extracting cosmological information from galaxy clusters, namely, by measuring the scaling relations among cluster observables. Employing the Fisher matrix technique and a physically motivated parametric model to describe cluster structure, we studied the utility of the scaling relation between Sunyaev-Zel'dovich (SZ) decrement and X-ray temperature in probing cosmology. In general, we find that the cosmology constraints from the scaling relation are comparable to those expected from the number counts of the same clusters, and combining these two approaches help break degeneracies and disentangle cluster physics from cosmology.
A compelling explanation of the existence of SMBHs at z ∼ 6 is that the primordial gas in early dark matter halos might be able to avoid fragmentation and collapse directly into a massive black hole if H2 formation and cooling are strongly suppressed by a sufficiently intense ultraviolet (UV) flux. We perform a suite of adaptive mesh refinement (AMR) simulations of gas collapse in protogalactic halos with virial temperature above ∼ 104K, irradiated by a UV flux with various intensities and spectra. We determine the critical specific intensity required to suppress H2 formation. The values found are a factor of 3–10 lower than previous estimates, which means an exponential increase in the number of rare halos exposed to super-critical radiation, making these halos possible sites of SMBH formation.
Observations of gravitational waves (GWs) emitted by inspiraling SMBH binaries (so-called standard sirens) could yield accurate measurements of the luminosity distance. We study cosmological parameter constraints from such systems, in the presence of gravitational lensing by large-scale structure. In particular, we study how the non-Gaussian nature of the lensing magnification distribution affects the cosmological constraints. With Monte-Carlo simulations, we find that the constraints on the matter density and the dark energy equation of state can be improved by 50%–80% by exploiting the non-Gaussianity, while the constraints on the amplitude of the matter power spectrum is ∼ 20% less tight than in the case of a Gaussian distribution.