Master's thesis defense by Kasper Elm Heintz

Title: Exploring New Methods of Selecting Quasars

Supervisor: Johan P.U. Fynbo
Censor: Maximillian Stritzinger, Aarhus University

Abstract: About 50 years ago, in the early 1960's, it was discovered that some galaxies
harbored an active nuclei, systems in which the light observed was not originating from the stellar population of the galaxy but from the inner most region. The energy source is gravitational potential energy released from matter falling onto super massive black holes located in the galaxy centres. The most luminous of these active galactic nuclei obtained the name of quasi-stellar radio sources (quasars for short) due to their optical stellar-like morphology and the emitting radio lobes of which these were first detected. After this
discovery, the focus has been on further classifying this peculiar type of object and although more than a hundred thousand sources have been observed to this day still more and more are being classified, some more spectacular than the other. Due to the complex nature of quasars it has so far been impossible to account for all the various sub-types, which is required to have a full understanding of the true nature and the cosmic evolution of quasars.

As an attempt of further illuminating parts of these issues in modern day astrophysics this thesis aims at exploring the incompleteness of current quasar samples. Most quasars are selected using methods that rely on photometric properties of the sources, e.g., the high UV excess of quasars compared to that of stars. This implies biases in the selection whereby some quasars evade selection in large photometric surveys and are hence underrepresented in the samples. Two well-known examples are quasars reddened by dust in the host galaxies harboring the active galactic nuclei or quasars reddened by dust-rich, intervening galaxies in the line-of-sight to the background source.

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