Cataloging astrophysical sources is a fundamental operation in astronomy. While simple in principle, cataloging becomes more complicated for images with low signal to noise and degeneracies across different emission components. We analyze Chandra Deep Field - South (CDF-S) data using a novel method called probabilistic cataloging, which extracts information by sampling from the catalog space, i.e. the space of different point source configurations consistent with a given image. By employing a reversible-jump Markov Chain Monte Carlo (RJMCMC) sampling method, we are able to infer the flux and color distribution for Active Galactic Nuclei (AGN) in the region. We are also able to infer the number of AGN by marginalizing over faint members below the detection threshold. To validate our method, we use simulated deep Chandra exposures and show that the isotropic background emission can be constrained at the 10% level. This result takes into account its covariance with unresolved AGN and the particle background of Chandra. We then present results of probabilistic cataloging applied to the CDF-S 2Ms, 4Ms, and 7Ms datasets in order to evaluate the fidelity of our method. Furthermore, by incorporating auxiliary redshift information from the COMBO Survey within our framework, we present the first three-dimensional probabilistic catalog of AGN and discuss possible implications for AGN synthesis models.
The Giant Magellan Telescope (GMT) will be the world\rsquos largest telescope upon completion. The GMT employs seven 8 m primary mirror segments and seven 1 m secondary mirror segments. One challenge of the GMT is keeping the seven pairs of mirror segments on the GMT in phase. In this project, we developed and began assembly on a design for a dispersed fringe sensor prototype consisting of an optical and basic mechanical layout. The prototype design will be tested on the Magellan Clay Telescope as an experiment for future phasing methods to be used on the GMT.
Multiple groups have been working on modeling the mass distributions of the six lensing galaxy clusters in the Hubble Space Telescope Frontier Fields data set. The magnification maps produced from these mass models will be important for the future study of the lensed background galaxies, but there exists significant variation in the different groups\rsquo models and magnification maps. We explore the use of two-dimensional histograms as a tool for visualizing these magnification map variations. Using a number of simple, one- or two-halo singular isothermal sphere models, we explore the features that are produced in 2D histogram model comparisons when parameters such as halo mass, ellipticity, and location are allowed to vary. Our analysis demonstrates the potential of 2D histograms as a means of observing the full range of differences between the Frontier Fields groups\rsquo models.This work has been supported by funding from National Science Foundation grants PHY-1560077 and AST-1211385, and from the Space Telescope Science Institute.
The Event Horizon Telescope (EHT) aims to resolve the general relativistic shadow of Sgr A*, the supermassive black hole at the center of our galaxy, via Very Long Baseline Interferometry (VLBI) measurements with a multinational array of radio observatories. In order to optimize the scheduling of future observations, we have developed tools to model the atmospheric opacity at each EHT site using the past 10 years of Global Forecast System (GFS) data describing the atmospheric state. These tools allow us to determine the ideal observing windows for EHT observations and to assess the suitability and impact of new EHT sites. We describe our modeling framework, compare our models to in-situ measurements at EHT sites, and discuss the implications of weather limitations for planned extensions of the EHT to higher frequencies, as well as additional sites and observation windows.
We present results from an analysis comparing simulations of isolated spiral galaxies with recent observations of the circumgalactic medium (CGM). As the interface containing inflows and outflows between the interstellar and intergalactic media, the CGM plays an important role in the composition and evolution of galaxies. Using a set of isolated galaxy simulations over different initial conditions and star formation and feedback parameters, we investigate the evolution of CGM gas. Specifically, in light of recent observational studies, we compute the radial column density profiles and covering fractions of various observable ion species (H I, C IV, O VI, Mg II, Si III) for each simulated galaxy. Taking uniformly random sightlines through the CGM of each simulated galaxy, we find the abundance of gas absorbers and analyze their contribution to the overall column density along each sightline. By identifying the prevalence of high column density absorbers, we seek to characterize the distribution and evolution of observable ion species in the CGM. We also highlight a subset of our isolated galaxy simulations that produce and maintain a stable precipitating CGM that fuels high rates of sustained star formation. This project was supported in part by the NSF REU grant AST-1358980 and by the Nantucket Maria Mitchell Association.
Bureau of Study Counsel The Bureau of Study Counsel (BSC) supports students in their efforts to learn and develop at Harvard. Among the Bureau's many services are individual counseling for academic and personal development, workshop and discussion groups, peer tutoring, consultation to student organizations, and a course in reading and study strategies.