2016 Undergraduate Awards

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The region immediately surrounding an actively accreting supermassive black hole at the centre of a massive galaxy, the accretion disk, produces an enormous amount of radiation resulting in a luminous, short-lived phenomenon called a quasar. About 20% of quasars show broad, blue-shifted absorption features in their UV spectra, indicative of an outflowing wind from the accretion disk. These winds can remove angular momentum from the accretion disk, thereby contributing to the growth of the central black hole. Understanding these winds will help us to better constrain the details of how black holes grow during the quasar phase. The structures of the absorption features are sensitive to the properties (ionization state, velocity profile, and thickness) of the winds. Consequently, the broad absorption line profiles of these objects show great diversity in depth and velocity width. Using a sample of 1,084 broad absorption line quasar spectra from the Sloan Digital Sky Survey, we apply an agglomerative hierarchical clustering algorithm to group spectra by similar C IV absorption line shapes. For each cluster, we compose median spectra and compare the shapes of the C IV broad absorption lines with the properties of prominent, broad emission lines. In agreement with results in the literature, low-velocity, deep troughs are found preferentially in objects for which the radiation from the accretion disk is more energetic. The link between broad absorption line properties and those of emission lines holds promise for allowing us to constrain the structure and dynamics of the outflowing winds.


Image: "ULAS J1120+0641" by ESO/M. Kornmesser. Licensed under CC BY 4.0.