Dr. Neelima Sehgal is awarded an NSF Astronomy and Astrophysics Postdoctoral Fellowship to carry out a program of research and education at Princeton University. Ground-based millimeter-wavelength surveys are underway and are detecting tens to hundreds of galaxy clusters by their Sunyaev-Zel'dovich (SZ) signal, in which high-energy electrons distort the cosmic microwave background radiation (CMB) through inverse Compton scattering, in which the low energy CMB photons receive energy boost during collision with the high energy cluster electrons. These surveys have presented their first measurements of the cluster number density evolution over cosmic time and can potentially provide precision measurements of structure growth. This, in turn, could discriminate between dark energy as a component of the Universe or as a manifestation of General Relativity's failure on the largest scales. Dr. Sehgal will build on her previous work to lead an analysis of the first cosmological constraints from an SZ-selected sample of galaxy clusters detected with the Atacama Cosmology Telescope. She will play a leading role in the determination of cosmological parameters from larger and more sensitive SZ cluster surveys and in upgrading simulations of the millimeter-wavelength sky by calibrating to the latest multi-wavelength observations. The latter will help reduce sources of systematic uncertainty in the cosmological analysis. Dr. Sehgal will also explore the feasibility of measuring cluster peculiar velocities using millimeter-wave simulations and thereby assess their utility as an alternative probe of structure growth.

Dr. Sehgal will also work with the New Jersey Astronomy Center for Education to develop and support several series of class modules that teach high-school students about key concepts in cosmology. She will also train high-school teachers on the use of these modules and assess their effectiveness through surveys. These modules will be distributed to high-schools nationwide.

Project Report

PI: Neelima Sehgal The PI was supported by an NSF Astronomy and Astrophysics Postdoctoral Fellowship from Sept. 2011 to Sept. 2012, at Princeton University. During that year as an NSF Fellow, the PI led an analysis for the Atacama Cosmology Telescope (ACT) collaboration, stacking optically-selected and X-ray-selected galaxy clusters in ACT millimeter-wave maps to measure their thermal Sunyaev-Zel’dovich (tSZ) signals. This was done to explore the relation between the cluster tSZ signal and the cluster mass, as proposed by the PI in the NSF Postdoc Proposal. Understanding this relation is vital to being able to exploit catalogs of tSZ-detected galaxy clusters in order to measure the growth of cosmic structure and probe the nature of dark energy. This work was published in the Astrophysical Journal in 2013 (Sehgal et al., ApJ, Vol 767, Issue 1, article id. 38, 10 pp. (2013)). This publication currently has 17 citations according to NASA ADS. The main result of this publication was that a discrepancy was found between the expected and measured tSZ signals when stacking the optically-selected clusters from the MaxBCG cluster sample (Koester et al. 2007a, ApJ 660,239 and Koester et al. 2007b, ApJ 660, 221). A similar discrepancy was also found stacking these optical clusters in Planck millimeter-wave maps (Planck Collaboration et al. 2011, A&A 536, A12) as well as in WMAP millimeter-wave maps (Draper et al. 2011, Phys. Rev. D. 85, 023005). These discrepancies suggest that either the relation between the tSZ signal and mass, as implemented in this work via an X-ray - mass relation from Arnaud et al 2010, A&A 517, A92, needs to be reconsidered, or the relation between richness and mass, as given by MaxBCG publications, needs to be readdressed. Alternatively, both relations, or possibly contaminants in the millimeter-wave maps, such as dusty sources could explain the discrepancies. In addition to the discrepancies found by all three millimeter-wave experiments, this new work found an additional discrepancy between the measured Planck tSZ signal and the measured ACT tSZ signal, when stacking the MaxBCG clusters. This last discrepancy can be explained with some degree of mis-centering of the MaxBCG clusters. In addition to this work, the PI was also co-author of 9 other publications, with a combined citation record of 288 citations, during this year as an NSF Postdoctoral Fellow. As part of the PI's outreach, the PI facilitated 7 teacher training workshops at Raritan Valley Community College on Moon phases, lunar geology, Earth seasons, the formation of the Solar System, and the use of Starlabs. The PI also participated in Project ASTRO, partnering with a 6th grade teacher (Ms. Donna Blum) at West Amwell Township Elementary School in Lambertville, NJ. The PI visited two 6th grade classes at West Amwell, four times each throughout the school year, and led astronomy activities with the students on the Solar System, Moon phases, lunar and solar eclipses, and drawing the Universe to scale. The PI has continued to partner with Ms. Donna Blum at West Amwell, even after the completion of the NSF Postdoctoral Fellowship, and has visited her two 6th grade classes four times each year for two more subsequent years.

National Science Foundation (NSF)
Division of Astronomical Sciences (AST)
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Harshal Gupta
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Sehgal Neelima
United States
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