UR #28: Photometry and Analysis of a Sample of Quasars on Archival Yerkes Observatory Photographic Plates

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Buduka Ogonor

University of Chicago

Buduka Ogonor is an undergraduate student studying physics at the University of Chicago. He completed this research alongside the Yerkes Plate Digitization Group, under the supervision of Dr. Richard Kron. The group presented their work at AAS 236 and hope to publish a paper by the end of the year.

Astronomical photographic plates are a growing tool for time-domain astronomy, a field interested in how astronomical objects change in time. This project demonstrated how the archival plates of the Yerkes Observatory plate vault could be used to study specifically the long-term time variation of quasars.

The Ritchey 24 inch reflector telescope

Quasars are a subclass of Active Galactic Nuclei, and are distance sources of radiation powered by accretion around supermassive black holes at the center of galaxies. These objects are known to vary in brightness over both small (days and weeks) and large timescales (decades). A better understanding of the mechanism behind variation could set constraints on the size of quasars, and overall improve the general model of AGNs. Leading theories chalk long term variation to unsteadiness in black hole accretion, but without data points older than ~50 years, this is difficult to conclude observationally. This is where the Yerkes Plate collection comes in handy, with its collection of photographic plates taken on the Ritchey 24’’reflector telescope at Yerkes Observatory in the early 1900s.

The histogram plots the difference between QSO magnitude measurements from SDSS and magnitudes of observations taken in the early 20th century. The spread of the histogram is wider than non-variable stellar observations on the same axes, demonstrating that the quasars varied in magnitude above uncertainty.

To find said quasar observations on the archival plates, our group scanned and digitized plates that could a) better detect light on the bluer, or more energetic, end of the light spectrum, b) depict fields of objects most likely in the area of the sky cataloged in the Sloan Digital Sky Survey database, and c) detect fuzzy, distant, and potentially dim objects. For the latter qualification, our plates were capable of detecting objects of at least ~18 magnitude in the SDSS measurement system. We then queried the SDSS database for quasars in the area of sky covered in each selected plate, and after calibrating the Yerkes plate magnitudes into the SDSS system, compared plate magnitudes to SDSS magnitudes. We found statistically significant variation in magnitude between the 1910 plate observations and the 21st century SDSS observations, solidifying the Yerkes plates as a tool with which to study long-term variation. In the future, we hope to use these measurements to evaluate theories for long-term variation. 

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