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Black Holes: Finding Them, Building Instruments that Help You Look, and how Systemic Racism can Prevent You From Doing Any of That


This work aims to connect the disparate elements of `doing astronomy’ involved in the production of knowledge, highlighting the advantages of an interdisciplinary approach and emphasizing the importance of each component to the overall success of the field. Broadly dividing the work into ideas, tools, and people, each area will be exemplified to play a critical role in the pursuit of a scientific result. The science case used to illustrate each components’ contribution will be the detection of black holes in our galaxy. First, a method of black hole detection is demonstrated, relying on the theory of relativity and astronomical observations (Chapter 2). Second, a technology that makes such observations possible is described, with a novel variation making improvements on former versions (Chapter 3). Finally, social dynamics that limit the entry and progression (and by extension, the potential scientific contributions in the aforementioned areas) of minoritized groups in the field are analyzed in the context of graduate programs (Chapter 4).

1 - Ideas: Theory and Observations

Though stellar-mass black holes (BHs) are likely abundant in the Milky Way (N=10^8--10^9), only ~20 have been detected to date, all in accreting binary systems. Gravitational microlensing is a proposed technique to search for isolated BHs, which to date have not been detected. Two microlensing events, MACHO-1996-BLG-5 (M96-B5) and MACHO-1998-BLG-6 (M98-B6), initially observed near the lens-source minimum angular separation in 1996 and 1998, respectively, have long Einstein crossing times (>300 days), identifying the lenses as candidate black holes. Twenty years have elapsed since the time of lens-source closest approach for each of these events, indicating that if the lens and source are both luminous, and if their relative proper motion is sufficiently large, the two components should be spatially resolvable. We attempt to eliminate the possibility of a stellar lens for these events by: (1) using Keck near-infrared adaptive optics images to search for a potentially now-resolved, luminous lens; and (2) examining multi-band photometry of the source to search for flux contributions from a potentially unresolved, luminous lens.

We combine detection limits from NIRC2 images with light curve data to eliminate all non-BH lenses for relative lens-source proper motions above 0.81 mas/yr for M96-B5 and 2.48 mas/yr for M98-B6. Further, we use WFPC2 broadband images to eliminate the possibility of stellar lenses at any proper motion. We present the narrow range of non-BH possibilities allowed by our varied analyses. Finally, we suggest future observations that would constrain the remaining parameter space with the methods developed in this work.

2 - Tools: Optics and Instrumentation

`Imaka is a ground layer adaptive optics (GLAO) demonstrator on the University of Hawaii 2.2m telescope with a 24'x18' field-of-view, nearly an order of magnitude larger than previous AO instruments. In 15 nights of observing with natural guide star asterisms ~16' in diameter, we measure median AO-off and AO-on empirical full-widths at half-maximum (FWHM) of 0.95 and 0.64 in R-band, 0.81'' and 0.48'' in I-band, and 0.76'' and 0.44'' at 1 micron. This factor of 1.5-1.7 reduction in the size of the point spread function (PSF) results from correcting both the atmosphere and telescope tracking errors. The AO-on PSF is uniform out to field positions ~5' off-axis, with a typical standard deviation in the FWHM of 0.018''. Images exhibit variation in FWMM by 4.5% across the field, which has been applied as a correction to the aforementioned quantities. The AO-on PSF is also 10x more stable in time compared to the AO-off PSF.

In comparing the delivered image quality to proxy measurements, we find that in both AO-off and AO-on data, delivered image quality is correlated with `imaka's telemetry, with R-band correlation coefficients of 0.68 and 0.70, respectively. At the same wavelength, the data are correlated to DIMM and MASS seeing with coefficients of 0.45 and 0.55. Our results are an essential first step to implementing facility-class, wide-field GLAO on Maunakea telescopes, enabling new opportunities to study extended astronomical sources, such as deep galaxy fields, nearby galaxies or star clusters, at high angular resolution.

3 - People: Society and Culture

Despite decades of interest in promoting diversity and inclusion in the field and higher education more broadly, physics and astronomy departments in American universities remain predominantly white and male. While some STEM fields have made progress in recent years towards correcting the historic overrepresentation of white men, 75% of physics and astronomy PhDs awarded in 2019 went to this demographic that constitutes only 30% of the US population, higher than chemistry, math, computer science, and engineering. This work seeks to understand the barriers to creating a more diverse field by examining how its cultural beliefs can work to maintain inequity in graduate programs.

We use the lived experiences of 12 female and gender-non conforming students of color to construct a model outlining how physics and astronomy graduate programs in American predominantly white institutions maintain equity gaps between majoritized and minoritized students. This model connects participant’s observations of attitudes, policies, and behaviors that relate to two foundational cultural beliefs in Western physics and astronomy: 1) that the field functions as a meritocracy where only the fittest survive, and 2) that physicists comprise an objective, cultureless, and apolitical community, impervious to social influence. We contextualize our findings with literature to demonstrate the negative outcomes such systems create for minoritized students, including alienation from their departments, mental health problems, and lower retention in the field. In outlining this system, we emphasize the design of program structures as a critical point of intervention. To this end, we conclude the work with recommendations based on participant input.

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