Spotting clusters, hunting for planets: a combined study of galaxy clusters in the X-ray and optical regime

Observational studies of the distribution of galaxies in the Universe reveal inhomogeneity and structure on Mpc and larger scales. Galaxy clusters are the largest gravitationally bound structures containing a virialized congregation of galaxies; therefore, studying them is essential for understanding the constitution and assembly history of these systems and probing the large-scale structure of the Universe. The Swift AGN and Cluster survey is a serendipitous X-ray survey aimed at building a large X-ray-selected cluster catalog with $\sim 1000$ cluster detections expected by its final release. In this thesis, I perform an optical analysis of 348 (out of 442) X-ray selected cluster candidates from the Swift cluster catalog using multi-band imaging from MDM 2.4m and the Pan-STARRS survey for the northern sky, and CTIO 4m and DES for the southern sky. I report the optical confirmation of 109 clusters with $> 3 \sigma$ galaxy over-density with photometric redshift estimates extending up to $z\sim1$. The Swift survey is nearly complete for $z\leq 0.3$ and $85\%$ complete for $z \leq 0.5$. The undetected clusters are possibly high redshift clusters with $z >0.8$ that warrant follow-up observations in the near-infrared. Furthermore, I also study the scaling relations between the X-ray and optical cluster mass observables and the offset distribution for all the optically verified SACS clusters and find them to be in agreement with other studies in literature. Another facet of my dissertation involves using quasar microlensing to probe the intracluster region of a galaxy cluster. I employ this novel technique to exert effective constraints on planet-mass objects in two extragalactic systems, Q J0158-4325 and SDSS J1004+4112, by studying their induced microlensing signatures. Chandra observations for these two gravitationally-lensed quasars reveal variations of the emission line peak energy, which can be explained as microlensing of the FeK$\alpha$ emission region surrounding the supermassive blackhole induced by planet-mass microlenses. To corroborate this, I have performed microlensing simulations and developed an edge detection algorithm to determine the probability of caustic transiting events. Comparison with the observed rates has yielded constraints on the substellar population, with masses ranging from Lunar to Jovian mass bodies within these galaxy or cluster scale structures. These results suggest that unbound planet-mass objects are common in galaxies, and these are surmised to be either free-floating planets or primordial black holes. These are the first-ever constraints on the substellar mass distribution in the intracluster light of a galaxy cluster. This analysis yields the most stringent limit for primordial black holes at the mass range

The Most Powerful Lenses in the Universe: Quasar Microlensing as a Probe of the Lensing Galaxy

Optical and X-ray observations of strongly gravitationally lensed quasars (especially when four separate images of the quasar are produced) determine not only the amount of matter in the lensing galaxy but also how much is in a smooth component and how much is composed of compact masses (e.g., stars, stellar remnants, primordial black holes, CDM sub-halos, and planets). Future optical surveys will discover hundreds to thousands of quadruply lensed quasars, and sensitive X-ray observations will unambiguously determine the ratio of smooth to clumpy matter at specific locations in the lensing galaxies and calibrate the stellar mass fundamental plane, providing a determination of the stellar $M/L$. A modest observing program with a sensitive, sub-arcsecond X-ray imager, combined with the planned optical observations, can make those determinations for a large number (hundreds) of the lensing galaxies, which will span a redshift range of $\sim$$0.25<z<1.5$Comment: Astro2020 Science White Pape

The Most Powerful Lenses in the Universe: Quasar Microlensing as a Probe of the Lensing Galaxy

We describe how quasar microlensing is the only way to determine the dark/stellar ratio at the interesting sub-galactic scales of several kpc and how it is the only way to measure the stellar M/L outside the solar neighborhood. Both can be done as a function of redshift, and this requires sub-arcsecond X-ray imaging of LSST-discovered lenses